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KINETIC BEHAVIOUR OF ION
INTERCALATION ELECTRODES AT
ELEVATED TEMPERATURES
PhD Thesis
Jeremy P. Matthews B. App. Sc. (Hons1)
School of Mechanical, Medical and Manufacturing Engineering
Queensland University of Technology
2001
i
Abstract
Electrochromic films undergo a colour change when small ions and electrons are
inserted into them, under the influence of an applied electric field. These films are also
known as ion-intercalation electrodes, and may be incorporated into glazing structures
more commonly known as smart windows. Smart windows are that which may be
used to control the amount of heat and light entering a building and may therefore be
used to minimise the energy consumption associated with heating, cooling and lighting.
The commercial success of smart windows, requires that they operate reproducibly at
temperatures up to approximately 70ºC, for many tens of thousands of colouring and
bleaching cycles. An understanding of the underlying kinetic processes over a wide
temperature range is therefore needed, in order to determine suitable control strategies
and switching conditions capable of fulfilling these requirements.
The research detailed in this thesis has involved an investigation into the kinetic
behaviour of ion-intercalation electrodes, and simulation of the electrical response as a
means of developing a tool for predicting and then optimising electrochromic switching.
More specifically, the electrical and optical properties of electrochromic thin films of
WO3/TiO2 have been studied over a wide temperature range, appropriate for the
operation of electrochromic windows. The magnitude of the voltages required for
coloration and bleaching significantly reduces as temperature increases. Some
irreversibility was observed at high temperature, as well as a reduction in coloration
efficiency. Further investigation revealed that self-bleaching and irreversibility effects
ii
were caused by the presence of water, and this problem was exacerbated at high
temperature. Post-experiment chemical analysis of a film sample revealed that some
trapping of the inserted ions had occurred, however the amount of ions remaining in the
film was much smaller than expected. The results suggested that a large quantity of the
lithium ions injected into the film were lost to the electrolyte after many cycles, possibly
accompanied by some film dissolution.
Experimental work carried out in a dry-box showed that films may be cycled reversibly
in a very dry environment, and the optical properties were independent of temperature
under these conditions. Unfortunately, the conditions which led to reversible cycling
and good electrochromic memory, also resulted in very long response times for film
bleaching. This result implies that a good electrochromic memory and a fast response
are mutually competitive aims.
Data from high temperature experiments was simulated with a mathematical model and
the mobility of lithium ions inside the electrochromic films was estimated in the
process. The estimated diffusion coefficients agreed well with published values, and
exhibited an Arrhenius dependence on temperature. Activation energies for diffusion
were calculated and the results were very reasonable. Some deviation from ideal
Arrhenius behaviour was observed for the estimated diffusion coefficients at high
temperature. It is likely that the rate limiting mechanism changes from diffusive motion
of ions at low temperature, to charge transfer at high temperature.
iii
Keywords: Electrochromic thin films; temperature effects; reversibility; kinetic
behaviour; self-bleaching
v
LIST OF PUBLICATIONS
1. J.P. Matthews, J.M. Bell and I.L. Skryabin, "Effect of temperature on electrochromic
device switching voltages", Electrochimica Acta, 44(18), 3245-3250 (1999).
2. J.P. Matthews, J.M. Bell and I.L. Skryabin, "High temperature behaviour of
electrochromics", Renewables: The Energy for the 21st Century
Proceedings of the World Renewable Energy Congress VI, Brighton, UK (A.A.M.
Sayigh Ed.), 230-235 (2000).
3. J.M. Bell and J.P. Matthews, "Temperature dependence of kinetic properties of sol-
gel deposited electrochromics", Solar Energy Materials and Solar Cells, 68, 249-263 (2001).
4. J.P. Matthews, J.M. Bell and I.L. Skryabin, Simulation of electrochromic switching
voltages at elevated temperatures, Electrochimica Acta, 46, 1957-1961 (2001).
5. J.P. Matthews and J.M. Bell, Self-bleaching, memory effect and reversibility of
electrochromism at elevated temperatures, submitted to Solar Energy Materials and
Solar Cells.
APPENDICES:
1. J.M. Bell and J.P. Matthews, "Glazing materials", Materials Forum, 22, 1-24(1998).
2. J.M. Bell, J.P. Matthews, I.L. Skryabin, J. Wang and B.G. Monsma, "Sol-gel
deposited electrochromic devices", Renewable Energy, 15(1-4), 312-317(1998)
3. J.M. Bell, J.P. Matthews and I.L. Skryabin, "Smart Windows", entry in Encyclopedia
of Smart Materials, accepted for publication.
vii
TABLE OF CONTENTS
ABSTRACT....................................................................................................................... i
KEYWORDS ...................................................................................................................iii
LIST OF PUBLICATIONS .............................................................................................. v
TABLE OF CONTENTS................................................................................................vii
STATEMENT OF ORIGINAL AUTHORSHIP ............................................................ xv
LIST OF ABBREVIATIONS........................................................................................xvi
LIST OF FIGURES ......................................................................................................xvii
ACKNOWLEDGEMENTS .......................................................................................xxviii
Chapter 1 INTRODUCTION ........................................................................................... 1
1.1 DESCRIPTION OF RESEARCH PROBLEM INVESTIGATED............................................. 2
1.2 A BASIC INTRODUCTION TO ELECTROCHROMICS....................................................... 4
1.3 ION INTERCALATION MECHANISM............................................................................. 7
1.4 ION INTERCALATION ELECTROCHEMISTRY ............................................................... 9
1.5 OPTICAL CHARACTERISTICS ................................................................................... 13
1.6 OVERALL OBJECTIVES OF THE STUDY.................................................................... 17
1.7 SPECIFIC AIMS OF THE STUDY ................................................................................ 17
1.8 RESEARCH HISTORY............................................................................................... 18
REFERENCES ................................................................................................................ 24
viii
Chapter 2 LITERATURE REVIEW............................................................................... 27
2.1 ELECTROCHROMISM A BRIEF HISTORY ............................................................... 28
2.2 THE ELECTROCHROMIC REACTION IN WO3 FILMS ................................................. 30
2.3 ELECTROCHROMIC CHARACTERISTICS AT ELEVATED TEMPERATURES .................. 32
2.4 LIFETIME, IRREVERSIBILITY AND SELF-BLEACHING............................................... 36
2.5 MODELS FOR SIMULATION OF ELECTROCHROMIC SWITCHING CHARACTERISTICS. 46
2.6 SUMMARY .............................................................................................................. 58
REFERENCES ................................................................................................................ 59
Chapter 3 EFFECT OF TEMPERATURE ON ELECTROCHROMIC DEVICE
SWITCHING VOLTAGES. ........................................................................................... 63
ABSTRACT.................................................................................................................... 65
3.1 INTRODUCTION....................................................................................................... 66
3.2 EXPERIMENTAL ...................................................................................................... 67
3.2.1 Electrode preparation .................................................................................... 67
3.2.2 Electrochemical Testing................................................................................. 68
3.3 RESULTS AND DISCUSSION...................................................................................... 69
3.3.1 Effect of temperature on applied voltage, Va(t) ............................................. 69
3.3.2 Effect of temperature on colouring efficiency................................................73
3.3.3 Effect of temperature on coloured state electromotive force, emfc, and
maximum colouring voltage, Vc max. ........................................................................ 75
3.4 CONCLUSION .......................................................................................................... 80
ACKNOWLEDGEMENTS................................................................................................. 81
ix
REFERENCES ................................................................................................................ 82
Chapter 4 HIGH TEMPERATURE BEHAVIOUR OF ELECTROCHROMICS ......... 83
ABSTRACT.................................................................................................................... 85
4.1 INTRODUCTION....................................................................................................... 86
4.2 EXPERIMENTAL ...................................................................................................... 88
4.2.1 Electrode preparation .................................................................................... 88
4.2.2 Electrochemical testing.................................................................................. 88
4.2.3 Chemical analysis .......................................................................................... 90
4.3 RESULTS AND DISCUSSION ..................................................................................... 91
4.3.1 Effect of temperature on coloration efficiency...............................................91
4.3.2 Observation of self-bleaching ........................................................................ 95
4.3.3 Determination of trapped lithium in WO3 film by ICP-AES .......................... 98
4.4 CONCLUSIONS ...................................................................................................... 100
ACKNOWLEDGEMENTS............................................................................................... 101
REFERENCES .............................................................................................................. 101
Chapter 5 TEMPERATURE DEPENDENCE OF KINETIC BEHAVIOUR OF SOL-
GEL DEPOSITED ELECTROCHROMICS ................................................................ 103
ABSTRACT.................................................................................................................. 105
5.1 INTRODUCTION..................................................................................................... 106
5.2 THEORY................................................................................................................ 108
5.2.1 Temperature effects on kinetic behaviour ....................................................108
x
5.2.2 Thermodynamics of coloration .................................................................... 111
5.2.3 Modelling of concentration profile .............................................................. 114
5.3 EXPERIMENTAL .................................................................................................... 117
5.3.1 Film preparation .......................................................................................... 117
5.3.2 Electrochemical testing................................................................................ 117
5.4 RESULTS............................................................................................................... 119
5.4.1 Variation in switching voltage with temperature.........................................119
5.4.2 Simulation of Voltage Response of Films ....................................................121
5.5 DISCUSSION.......................................................................................................... 123
5.6 CONCLUSION ........................................................................................................ 127
ACKNOWLEDGEMENTS............................................................................................... 127
REFERENCES .............................................................................................................. 128
Chapter 6 SIMULATION OF ELECTROCHROMIC SWITCHING VOLTAGES AT
ELEVATED TEMPERATURES. ................................................................................ 131
ABSTRACT.................................................................................................................. 133
6.1 INTRODUCTION..................................................................................................... 134
6.2 EXPERIMENTAL .................................................................................................... 135
6.2.1 Film preparation .......................................................................................... 135
6.2.2 Electrochemical testing................................................................................ 135
6.2.3 Simulation of EC film coloration voltage .................................................... 136
6.3 RESULTS AND DISCUSSION ................................................................................... 138
6.3.1 Voltage characteristics ................................................................................ 138
xi
6.4 CONCLUSION ........................................................................................................ 143
ACKNOWLEDGEMENTS............................................................................................... 144
REFERENCES .............................................................................................................. 145
Chapter 7 SELF-BLEACHING, MEMORY EFFECT AND REVERSIBILITY OF
ELECTROCHROMISM AT ELEVATED TEMPERATURES. ................................. 147
ABSTRACT.................................................................................................................. 149
7.1 INTRODUCTION..................................................................................................... 150
7.2 EXPERIMENTAL .................................................................................................... 152
7.2.1 Film preparation .......................................................................................... 152
7.2.2 Electrochemical Testing............................................................................... 152
7.3 RESULTS AND DISCUSSION ................................................................................... 154
7.3.1 Observation of self-bleaching at elevated temperatures..............................154
7.3.2 Effects of water on self-bleaching rates at elevated temperatures .............. 157
7.3.3 Effects of water on voltage characteristics ..................................................159
7.4 CONCLUSION ........................................................................................................ 163
ACKNOWLEDGEMENTS............................................................................................... 164
REFERENCES .............................................................................................................. 165
Chapter 8 GENERAL DISCUSSION........................................................................... 167
8.1 INTRODUCTION AND IDENTIFICATION OF KNOWLEDGE GAPS............................... 168
8.2 INITIAL CHARACTERISATION IN THE AMBIENT LABORATORY ENVIRONMENT...... 169
8.3 DRY-BOX EXPERIMENTS ...................................................................................... 173
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8.4 SELF-BLEACHING EXPERIMENTS.......................................................................... 176
8.5 SIMULATION AND ESTIMATION OF IONIC MOBILITY............................................. 186
8.6 CONCLUSIONS ...................................................................................................... 192
8.7 FUTURE RESEARCH .............................................................................................. 193
REFERENCES .............................................................................................................. 195
Appendix 1 GLAZING MATERIALS. ........................................................................ 197
SYNOPSIS ................................................................................................................... 199
1 INTRODUCTION........................................................................................................ 200
2 REQUIREMENTS FOR GLAZING MATERIALS............................................................. 203
3 COATING SYSTEMS FOR WINDOW GLAZING............................................................ 209
3.1 Spectrally Selective Glazings ..........................................................................209
3.2 Angular Selective Glazings .............................................................................215
3.3 Switchable Glazings........................................................................................ 220
4 CASE STUDIES ......................................................................................................... 227
4.1 Spray pyrolysed fluorine doped tin oxide films............................................... 227
4.2 Angular selectivity of obliquely sputtered chromium films.............................233
4.3 Design and performance of an electrochromic device ...................................238
4.3.1 Film deposition ...................................................................................... 239
4.3.2 Electrode cycling ................................................................................... 240
4.3.3 Polymer electrolyte ................................................................................ 241
4.3.4 Device testing......................................................................................... 242
4.3.5 Device modelling ................................................................................... 244
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5 SUMMARY ............................................................................................................... 246
REFERENCES .............................................................................................................. 247
Appendix 2 SOL-GEL DEPOSITED ELECTROCHROMIC DEVICES. ................... 257
ABSTRACT.................................................................................................................. 259
1 INTRODUCTION........................................................................................................ 260
2 ELECTROCHROMIC FILM DEPOSITION...................................................................... 261
3 ELECTROCHROMIC FILM PERFORMANCE................................................................. 262
4 ELECTROCHROMIC DEVICE FABRICATION AND PERFORMANCE............................... 264
5 ENERGY PERFORMANCE OF ELECTROCHROMIC DEVICES ........................................ 267
6 CONCLUSIONS ......................................................................................................... 269
REFERENCES .............................................................................................................. 270
Appendix 3 SMART WINDOWS. ............................................................................... 273
1 OUTLINE.................................................................................................................. 275
2 INTRODUCTION........................................................................................................ 276
3 ARCHITECTURALGLAZING APPLICATIONS FOR SMART WINDOWS .......................... 277
3.1 Physics of Windows......................................................................................... 278
4 SURVEY OF SMART WINDOWS................................................................................. 283
4.1 Electrochromic Smart Windows...................................................................... 286
4.1.1 Inorganic Electrochromic Smart Windows............................................ 286
4.1.2 Organic Electrochromic Smart Windows .............................................. 290
4.2 Thermochromic Devices.................................................................................. 291
xiv
4.3 Thermotropic Devices ..................................................................................... 293
4.3.1 Hydrogels............................................................................................... 294
4.3.2 Polymer Blends...................................................................................... 295
4.3.3 Applications ........................................................................................... 296
4.4 Polymer Dispersed Liquid Crystal Devices ....................................................296
4.5 Suspended Particle Devices ............................................................................299
4.6 Gasochromic Devices ..................................................................................... 300
4.6.1 Gasochromic Technology ............................................................................ 302
4.6.2 Applications ................................................................................................. 303
5 ELECTROCHROMIC SMART WINDOWS ..................................................................... 304
5.1 Electrochromic Smart Window Structures......................................................305
5.1.1 Type 1 - Ion Conducting Layer and Passive Counterelectrode ............. 305
f5.1.2 Type 2 Combined Ion Conducting Layer and Counterelectrode....... 307
5.1.3 Type 3 Ion Transport Layer and Complimentary Counter-electrode . 307
5.2 Materials used inelectrochromic devices........................................................308
5.2.1 Electrochromic Materials....................................................................... 308
5.2.2 Counter-electrode materials ................................................................... 310
5.2.3 Ion Transport Layer ............................................................................... 310
5.2.4 Transparent Electronic Conductors.............................................................. 311
5.3 Control of Electrochromic Smart Window......................................................312
5.4 Future Directions ............................................................................................ 316
REFERENCES .............................................................................................................. 317
xv
STATEMENT OF ORIGINAL AUTHORSHIP
The work contained in this thesis has not been previously submitted for a degree or
diploma at any other higher education institution. To the best of my knowledge and
belief, the thesis contains no material previously published or written by another person
except where due reference is made.
Signed:...................................
Date: ...................................
xvi
LIST OF ABBREVIATIONS
CE Counter electrode
EC Electrochromic
emf electromotive force
ICP-MS Inductively-Coupled-Plasma-Mass Spectrometry
ITO Indium-tin oxide
PC Propylene carbonate
PECVD Plasma enhanced chemical vapour deposition
PET Polyethylene terephthalate
RE Reference electrode
STA Sustainable Technologies Australia LTD
SIMS Secondary-Ion Masss Spectrometry
TCO Transparent conducting oxide
WE Working electrode
η Coloration efficiency
XRD X-Ray Diffraction
xvii
LIST OF FIGURES
Chapter 1 - INTRODUCTION
Figure 1.1 Transmittance spectra for coloured and bleached states of an STA
electrochromic device. .............................................................................................. 5
Figure 1.2 Typical structure of an electrochromic device............................................... 6
Figure 1.3 Simulated concentration profile for t=200s (Qinj=20mC/cm2). ................... 11
Figure 1.4 Determination of coloration efficiency from a plot of change in optical
density versus injected charge density. Coloration efficiency = 45.5cm2/C.......... 16
Chapter 3 - EFFECT OF TEMPERATURE ON ELECTROCHROMIC DEVICE
SWITCHING VOLTAGES
Figure 3.1 Curves of applied voltage versus time, measured during coloration and
bleaching of WO3 thin film electrode and plotted for four temperatures, for an
injected charge density of 15mC/cm2. .................................................................... 70
Figure 3.2 Maximum voltages required to colour WO3 thin film electrode to 15mC/cm2
at elevated temperatures.......................................................................................... 71
Figure 3.3 Photocell voltage versus injected charge for WO3 thin film electrode at four
temperatures. ........................................................................................................... 73
Figure 3.4 Change in optical density versus injected charge for WO3 thin film electrode
at four temperatures................................................................................................. 74
xviii
Figure 3.5 Maximum voltages required for coloration of WO3 thin film electrode to
5mC/cm2, and corresponding emf values measured between 20°C and 50°C........ 76
Figure 3.6 WO3 film maximum coloration voltage versus temperature, for an injected
charge density of 5mC/cm2. .................................................................................... 78
Figure 3.7 Log of coloration voltage versus reciprocal temperature for WO3 film with
an injected charge density of 5mC/cm2................................................................... 79
Chapter 4 - HIGH TEMPERATURE BEHAVIOUR OF ELECTROCHROMICS
Figure 4.1 Change in optical density versus injected charge for WO3 films cycled to
15mC/cm2 at elevated temperatures. The results shown in (a) are for an experiment
carried out in the ambient environment, while the results shown for (b) are for an
experiment carried out in a dry-box. ....................................................................... 91
Figure 4.2 Reversibility of cycling at elevated temperatures, represented as the
percentage of the injected charge density trapped per cycle................................... 93
Figure 4.3 Change in (a) photocell voltage and (b) emf of WO3 electrode during self-
bleaching experiment. ............................................................................................. 96
Figure 4.4 Correlation of estimated and measured quantities of charge lost during self-
bleaching. ................................................................................................................ 97
xix
Chapter 5 - TEMPERATURE DEPENDENCE OF KINETIC BEHAVIOUR OF
SOL-GEL DEPOSITED ELECTROCHROMICS
Figure 5.1 Simulated concentration profile for t=1s (Qinj=0.1mC/cm2). .................... 115
Figure 5.2 Simulated concentration profile for t=200s (Qinj=20mC/cm2). ................. 116
Figure 5.3 Applied voltage for colouration and bleaching of a sol-gel WO3 film to
15mC/cm2.............................................................................................................. 119
Figure 5.4 Dependence of emf on temperature and surface lithium concentration
predicted using equation 5.8. ................................................................................ 120
Figure 5.5 Experimental and simulated voltages during charge injection of a sol-gel
electrochromic film. (a) Temperature = 20.1ºC, D = 2.07x10-12cm2,
(b) temperature = 30.3ºC, D = 6.33x10-12cm2, (c) temperature = 40.3ºC,
D = 1.33x10-11cm2 and (d) temperature = 50.0ºC, D = 1.71x10-11cm2. ................ 122
Figure 5.6 Variation in estimated diffusion coefficients with temperature plotted for
range (a) 20.1 < T < 50.0ºC and (b) 20.1 < T < 40.3ºC. ....................................... 124
Chapter 6 - SIMULATION OF ELECTROCHROMIC SWITCHING
VOLTAGES AT ELEVATED TEMPERATURES.
Figure 6.1 Applied voltage during colouration and bleaching of a sol-gel WO3/TiO2
film to 15mC/cm2/s. .............................................................................................. 138
xx
Figure 6.2 Experimental and simulated voltages during charge injection of a sol-gel
electrochromic film. (a) T = 35.8ºC, D = 8.68x10-13cm2/s, (b) T = 46.5ºC,
D = 1.54x10-12cm2/s, (c) T = 56.2ºC, D = 4.02x10-12cm2/s, (d) T = 65.3ºC,
D = 1.48x10-11cm2/s and (e) T = 76.4ºC, D = 6.00x10-11cm2/s............................. 139
Figure 6.3 Arrhenius plot showing the variation in estimated diffusion coefficients with
temperature............................................................................................................ 141
Chapter 7 - SELF-BLEACHING, MEMORY EFFECT AND REVERSIBILITY
OF ELECTROCHROMISM AT ELEVATED TEMPERATURES.
Figure 7.1 Change in optical density (a) and measured voltage (b) during self-bleaching
of a WO3/TiO2 film in dry electrolyte................................................................... 154
Figure 7.2 Change in optical density (a) and measured voltage (b) during self-bleaching
of a WO3/TiO2 film, after firing at 250ºC for hours. ............................................ 155
Figure 7.3 Rate of self-bleaching for WO3/TiO2 films under various conditions....... 158
Figure 7.4 Voltage characteristics during cycling of WO3/TiO2 films under various
conditions. ............................................................................................................. 159
Figure 7.5 Dependence of electrochromic cycling characteristics on electrolyte water
concentration. ........................................................................................................ 161
xxi
Chapter 8 - GENERAL DISCUSSION
Figure 8.1 Curves of applied voltage versus time, measured during coloration and
bleaching of WO3 thin film electrode and plotted for four temperatures, for an
injected charge density of 15mC/cm2. .................................................................. 170
Figure 8.2 Log of absolute value of maximum coloration voltage versus reciprocal
temperature for WO3 film, for an injected charge density of 5mC/cm2................ 171
Figure 8.3 Change in optical density versus injected charge for WO3 thin film electrode
at four temperatures............................................................................................... 172
Figure 8.4 Reversibility of cycling at elevated temperatures, represented as the
percentage of the injected charge density trapped per cycle................................. 174
Figure 8.5 Change in optical density with time for a film coloured to 15mC/cm2 at
various temperatures. ............................................................................................ 177
Figure 8.6 Change in optical density during self-bleaching of a WO3/TiO2 film in dry
electrolyte.............................................................................................................. 179
Figure 8.7 Change in optical density (a) and measured voltage (b) during self-bleaching
of a WO3/TiO2 film, after firing at 250ºC for hours. ............................................ 180
Figure 8.8 Rate of self-bleaching for WO3/TiO2 films under various conditions and
temperatures. ......................................................................................................... 181
Figure 8.9 Voltage characteristics during cycling of WO3/TiO2 films to 15mC/cm2
under various conditions. ...................................................................................... 182
Figure 8.10 Dependence of electrochromic cycling characteristics on electrolyte water
concentration. ........................................................................................................ 184
xxii
Figure 8.11 Experimental and simulated voltages during charge injection of a sol-gel
electrochromic film. (a) Temperature = 20.1ºC, D = 2.07x10-12cm2 and
(b) temperature = 50.0ºC, D = 1.71x10-11cm2....................................................... 188
Figure 8.12 Arrhenius plot showing the variation in estimated diffusion coefficients
with temperature between 20.1ºC and 50.0ºC....................................................... 189
Figure 8.13 Arrhenius plot showing the variation in estimated diffusion coefficients
with temperature between 35.6ºC and 76.4ºC....................................................... 190
Appendix 1 - GLAZING MATERIALS
Figure 1 The three components related to the ambient radiation environment which
need to be considered for design of windows. The three distinct spectral regions
correspond to the wavelengths 0.3<λ<2.5 µm (solar radiation), λ>3µm (thermal
IR), and 0.37<λ<0.77µm corresponding to the visible response of the human eye.
............................................................................................................................... 205
Figure 2 The instantaneous energy balance in a double pane window. The radiative
heat transfer between the panes is dependent on the absorption of solar radiation in
the outer pane (inward) and by the temperature of the inside pane relative to the
outside pane (outward). Both are reduced by using low emittance coatings on
surfaces 2 and/or 3. ............................................................................................... 208
Figure 3 Transmittance spectra for a solar control coating based on ZrN. Note the high
reflectivity in the IR and narrow band transmittance in the visible. From Roos and
Karlsson [18]. The dotted line shows a theoretical calculation of the spectrum
xxiii
based on material optical properties...................................................................... 212
Figure 4 One of the principles of angular selectivity, with different levels of
transmittance for radiation incident on either side of the normal. ........................ 216
Figure 5 The relationship between deposition geometry (atom flux direction with
respect to substrate) and the column orientation. Adapted from Smith [75] ....... 217
Figure 6 Typical structure of an electrochromic device.............................................. 221
Figure 7 Development of an absorption band in WO3 deposited by sputtering onto
substrates held at various temperatures. The graph shows the coloration efficiency
as a function of wavelength, which is directly proportional to absorption. Note the
shift in the absorption band peak position as the substrate temperature increases and
the film becomes more crystalline. From Wang and Bell [88]. ........................... 222
Figure 8 Schematic of the 'in-line' spray pyrolysis process. ....................................... 228
Figure 9 Transmittance (solid lines) and Reflectance (dashed lines) spectra for four
TCO samples: - LOF TEC 8 SnO2:F; - LOF TEC10 SnO2:F ; and -
sputtered ITO (Donnelly Applied Films, 15Ω/square); and - LOF TEC20
SnO2:F. Note the very sharp spectral selectivity and the very low visible
absorption of the sputtered ITO film compared to the spray pyrolysed FTO. This is
characteristic of the smoother and finer grain structure of the sputtered films (see
figure 10)............................................................................................................... 230
Figure 10 Micrographs of three different TCO films. (a) LOF TEC8 (b) LOF TEC10
and (c) Donnelly Applied Films, sputtered ITO, nominally 15Ω/square. ............ 231
Figure 11 Thickness of the layers in LOF TEC15 and TEC20 glass products according
the ellipsometric analysis by von Rottkay and Rubin [153]. ................................ 232
xxiv
Figure 12 Spectral transmittance of a 90nm Cr film, for s-polarised and p-polarised
light at various angles of incidence....................................................................... 234
Figure 13 The voltage non-uniformity ratio along a device for two different electrolyte
resistivities. ........................................................................................................... 242
Figure 14 Transmittance spectra for coloured and bleached states of an STA
electrochromic device. The ratio of Tvis/Tsol in both states are shown on the graph,
indicating the increase in spectral selectivity in the coloured state. The dynamic
range for the device is Tsol=51.5% to Tsol=20.6%, with injected charge of
10mC/cm2. From Bright [151]. ............................................................................ 243
Appendix 2 - SOL-GEL DEPOSITED ELECTROCHROMIC DEVICES
Figure 1 (a) Transmittance spectra of sputtered (solid lines) and sol-gel (dashed lines)
in the coloured and bleached states, and (b) the electrical characteristics of the sol-
gel deposited film, obtained using constant current charge injection. .................. 262
Figure 2 The variation of the injected and extracted charge for a sol-gel deposited film
cycled in 1 M LiClO4 electrolyte, and the difference between the maximum
injected (extracted) charge and the quantity Qvmax (Qvmin), which is the value of
injected (extracted) charge when the preset voltage limit is reached. The change in
characteristics after cycle 1000 is attributed to a long break in cycling (nearly 1
month), which disappears after 300 cycles. .......................................................... 263
Figure 3 The transmittance spectra for a sol-gel deposited electrochromic device in
both coloured and bleached states......................................................................... 266
xxv
Figure 4 The relative coloration measured as the ratio of a photocell voltage at time t to
the photocell voltage in the bleached state. The inserts show the initial coloration
and bleaching of the device................................................................................... 267
Figure 5 The energy savings calculated from a simulation (DOE2.1E) of a prototypical
office building in three locations in Australia resulting from using an
electrochromic glazing instead of a bronze tint glazing........................................ 268
Appendix 3 - SMART WINDOWS
Figure 1 The three components related to the ambient radiation environment which
need to be considered for design of windows. The three distinct spectral regions
correspond to the wavelengths 0.3<λ<2.5 µm (solar radiation - ϕsol(λ)), λ>3µm
(thermal IR), shown here as ϕth(λ) for T=300K, and 0.37<λ<0.77µm corresponding
to the visible response of the human eye, ϕvis(λ). ................................................. 279
Figure 2 Schematic illustration of the different types of switching of transmitted
radiation which are possible with different switchable window systems. ............ 280
Figure 3 The variation in optical density for four differently electrically activated smart
window devices. • - a sol-gel deposited device manufactured by Sustainable Technologies
Australia Ltd, and based on WO3; ×- a sputtered device manufactured by Asahi Glass Co,
and based on a WO3-NiO complementary device (see Section 4.2); +- an organic
electrochromic from Gentex Corporation, based on viologens; - a polymer dispersed
liquid crystal device manufactured by 3M Corporation. The different curves represent
different spectra used in calculating the optical density: a broad band transmittance;
a− − − the visible transmittance Tvis; and the solar transmittance Tsol. The
xxvi
divergence in the optical density for visible and solar transmittance for the Gentex
viologen-based device is a reflection of the type of spectral change (Type D in figure 2).
............................................................................................................................... 282
Figure 4 A Schematic illustration of the structure of a typical inorganic electrochromic
device, showing the five layer structure................................................................ 287
Figure 5 The spectral coloration efficiency, which directly proportional to change in
optical density in sputtered WO3. This shows the development of an absorption
band in WO3 during ion injection. Note the shift in the absorption band peak
position as the substrate temperature increases and the film becomes more
crystalline, representing the change from absorption to reflection modulation.... 289
Figure 6 The transmittance of (a) VO2 and (b) W-doped VO2 at temperatures above
(80°C) and below (20°C) the thermochormic transition. The spectra are of type C
in both cases (see Figure 2). Reprinted from Solar Energy Materials and Solar
Cells, Volume 44, M.A. Sobhan, R.T. Kivaisi, B. Stjerna and C-G. Granqvist,
Thermochromism of Sputter Deposited WxV1-xO2 films, 1996, pages 451-455, with
permission from Elsevier Science......................................................................... 292
Figure 7 The structure of a themortropic laminate. In the low temperature state (a), the
device is fully transparent, but in the high temperature state the thermotropic state
separates into discreet particles and the layer becomes scattering........................ 294
Figure 8 The normal-hemispherical reflectance of a thermotropic hydrogel laminate at
a range of temperatures below (30°C) and above (>35°C) the thermotropic
transition temperature. The spectra are of type B, demonstrating a high degree of
selectivity. ............................................................................................................. 295
Figure 9 The structure of a PDLC device. Unlike the electrochromic device, it is not a
xxvii
conducting device, with the switching dependent on the electric field across the
PDLC layer. .......................................................................................................... 297
Figure 10 Normal-hemispherical transmittance spectra for the PDLC device for applied
voltages of 0, 10, 20, 30 and 100 V. The device is opaque (in the sense of being
unable to perceive images through the device in the off state (0V), but still
transmits significant energy. Reproduced with permission of Dr Arne Roos. .... 298
Figure 11 A typical structure of a gasochromic device............................................... 301
Figure 12 Transmittance spectra for coloured and bleached states of an STA
electrochromic device. The selectivity in both states are shown on the graph,
indicating the increase in selectivity in the coloured state. The dynamic range for
the device is Tsol=51.5% to Tsol=20.6%, with injected charge of 10mC/cm2. ...... 306
Figure 13 Transmittance spectra of an Asahi electrochromic device in the bleached
state and for 3 colored states, with 5mC/cm2, 10mC/cm2 and 15mC/cm2 injected
into the WO3 layer at a constant potential of 1.5V. .............................................. 308
Figure 14 Electrochromic device performance at 50°C (+) and 18°C (×). The solid
lines represent the device voltage and the dashed lines the relative transmittance of
the device. In both cycles the operational parameters are: current
density=0.1mA/cm2 , Qin=13mC/cm2; Qout=-13mC/cm2, preset switching voltages
are: Vmax= 1.8V, Vmin=-0.7V. Note that at 50°C voltage does not reach Vmax. The
high temperature cycle period is shorter and lower voltages are required, however,
transmittance change in the two cycles is almost identical................................... 313
xxviii
ACKNOWLEDGEMENTS
This work was supported by an Australian Postgraduate Award (Industry) scholarship
from the Australian Research Council, and Sustainable Technologies Australia Limited
(STA). The work described in this thesis has been supported by the Australian
Cooperative Research Centre for Renewable Energy (ACRE). ACREs activities are
funded by the Commonwealths Cooperative Research Centres Program.
The author would also like to thank Mr Graeme Evans for technical assistance and for
provision of WO3/TiO2 films for use in this project, Mr Mark Hayne for technical
assistance and Mr Pat Stevens for his assistance with chemical analyses.
CHAPTER 1
INTRODUCTION
2
INTRODUCTION
1.1 Description of Research Problem Investigated
This thesis is concerned with the operation of electrochromic (EC) windows under
conditions expected during their normal operation. Owing to the absorption of tungsten
oxide in the coloured state, the windows are expected to heat up to between 50 and
70ºC. Smart windows operate by the movement of charge between the electrodes and
the coloration mechanism involves a redox process in the tungsten oxide electrode (See
section 1.3), therefore it is expected that the kinetics of coloration and bleaching will be
affected as the temperature increases. The research problem described in this thesis
therefore involves the electrochemical and optical properties of electrochromic films at
elevated temperatures. The goal of the work is to develop a detailed understanding of
the switching voltage and current, and reversibility of the electrochromic reaction as a
function of temperature. This should enable control strategies to be developed for smart
windows to increase device lifetime and permit successful commercialisation of
electrochromic systems. The work was performed using sol-gel deposited films of
WO3-TiO2 produced by STA (Sustainable Technologies Australia) as a part of their
development of smart window systems.
The kinetics of the individual reaction mechanisms will change with temperature, and
the switching regime should reflect these changes. The application of high voltages or
currents leads to fast coloration but also increases the possibility of side reactions
occurring, thereby degrading device performance and shortening the useful lifetime.
3
Low voltages and currents make switching safer thereby increasing lifetime, but the
slower response may be perceived as a loss in product value by the end user.
It is therefore important to optimise the device-switching regime to achieve maximum
lifetime, while still achieving reasonable response times. Such an optimisation requires
detailed knowledge of the kinetic properties of electrochromic films over a range of
conditions applicable to real electrochromic device application. Simulation of the
device response with a mathematical model may then be used as a valuable tool for
predicting how a device will function under specific conditions. The ability to predict
how the kinetics of an electrochromic device will change in response to real conditions
is extremely useful in terms of device design and optimisation.
The research problem has therefore involved determining the effects of temperature on
the kinetic properties of the electrochromic reaction in mixed oxide WO3/TiO2 films.
Specifically, this has involved measurement of the optical and electrical responses of EC
films during cycling over a wide temperature range from room temperature to
approximately 70ºC. A large portion of this work has involved specific investigation
into the causes of detrimental effects such as self-bleaching and charge insertion
irreversibility. This research work has also involved the use of a mathematic model to
simulate device-switching characteristics during the coloration process, and to estimate
the dependence of ionic mobility on temperature.
4
1.2 A basic introduction to electrochromics
An ion intercalation electrode is a solid substrate, into which ions can be inserted by the
application of an electric field. Considerable research has been conducted on the
properties of tungsten oxide (WO3) as an ion intercalation electrode since the early
1970s. In 1969 Deb published results showing that colour centers could be formed in
thin films of tungsten oxide by the application of an electric field [1]. Deb called the
tungsten oxide film electrochromic, a term previously only associated with some
organic compounds. This work revealed the potential of inorganic electrochromic thin
films, and sparked much interest in electrochromic research.
Inorganic electrochromic materials such as tungsten oxide (WO3) change their
transmittance when ions and electrons are injected into the material under the influence
of an electric field. This is schematically illustrated in the reaction scheme below, using
WO3 as the electrochromic material, V2O5 as a counter-electrode material and Li+ ions
as the mobile ionic species:
WO xLi xe Li WO working electrode
transparent deep blue
Li V O Li V O xLi xe counter electrode
V
Vx
y
V
Vy x
3 3
2 5 2 5
+ + →←
→← + +
+ −−
+
+
−−
+ −
(1.1)
WO3 is transparent while LixWO3 is a deep blue colour, therefore the colour of the WO3
film changes when the above reaction occurs. Figure 1 shows the transmittance
properties of an electrochromic device in the coloured and bleached states. The
transmittance of the infrared radiation component (λ>1000nm) is inherently lower than
5
the visible part of the spectrum (370nm<λ<780nm) which means that these films may
be used to reduce solar gain entering a building through the windows, while still
allowing significant daylighting to improve comfort and reduce the requirement for
lighting energy.
0
10
20
30
40
50
60
70
500 1000 1500 2000
trans
mitt
ance
(%)
λ (nm)
coloured T
bleached
Figure 1 Transmittance spectra for coloured and bleached states of an STA
electrochromic device.
A major application of electrochromic materials is in the fabrication of smart
windows. Smart windows are active glazing devices that can be used to control the
amount of visible and solar radiation entering a building, in order to minimise the
buildings energy load normally associated with heating, cooling and lighting. Thin
films of tungsten and vanadium oxide can be incorporated into a smart window structure
with transparent conducting oxides (TCOs) and a polymer electrolyte, as shown
6
schematically in Figure 2. The TCO layers allow electron current to flow between the
working and counter electrodes while maintaining transparency, a property mandatory
for window applications. The polymer electrolyte serves as an electrical insulator and
ionic conductor, as well as adding mechanical strength and physically holding the
device together.
The charge that is used to colour the WO3 film (ie, Li+, H+ etc) must be stored outside
the working electrode to achieve bleaching, and this is accomplished using a suitable
counter electrode. Vanadium oxide is a good counter electrode material because a
relatively large amount of lithium ions may be intercalated into it, without any
significant colour change.
Figure 2 Typical structure of an electrochromic device.
7
The voltages associated with the redox reactions outlined in equation (1.1) depend on
several factors including:
o value of x in LixWO3 (ie. film emf)
o Li+ diffusion coefficients in the films
o charge transfer at WO3/electrolyte interface
o series resistance of the cell
o temperature
As well as the electrochromic reactions, there are a number of undesirable side reactions
that ultimately lead to degradation of a device. EC devices absorb a proportion of the
incident radiation, and so will heat up considerably under normal operating conditions.
At elevated temperatures the voltages required to colour and bleach the smart windows
are considerably reduced, due primarily to increased diffusion coefficients and normal
Nernstian behaviour. This means that excessive overvoltages may be applied at high
temperatures if the room temperature voltage limits are applied. Thus maximum
lifetime of electrochromic devices can only be obtained if the kinetic behaviour of the
electrodes is understood and the control strategy is optimised over a wide temperature
range.
1.3 Ion intercalation mechanism
The response of electrochromic systems to an applied electrical signal is very complex,
but the process may be broken down into a number of simpler stages. When a current or
voltage is applied between the working and counter electrodes, an electric field is
8
established across the electrolyte. During coloration the mobile ions in the electrolyte
move towards the working electrode under the influence of this electric field, and
electrons enter the electrode from the TCO contact. The ions in solution must lose their
solvation sheaths and then combine with an electron at a tungsten site on the film
surface. The WO3 lattice expands slightly as the metal ion is intercalated into an
interstitial position. The electron is localised on a tungsten site, according to equation
(1.1) and an optical transition occurs. There is some controversy regarding the exact
nature of the initial species formed when the ion migrates from the electrolyte to the
electrode surface [2], but it is commonly accepted that the process involves the double
injection of the metal ion and an electron [3,4].
As coloration proceeds, the concentration of metal ions just below the surface of the
WO3 electrode will increase, saturating the surface sites available for intercalation.
Ion/electron pairs must migrate deeper into the film if more ions are to be intercalated.
The conductivity of the cycled WO3 film is quite high, and it is generally assumed that
there is no drop in electrical potential internal to the electrode. This means that there is
no electric field present to drive the ions deeper into the film, and so the ions must
diffuse into the film solely under the influence of the concentration gradient. Upon
bleaching the ions must migrate from within the film to the electrode/electrolyte
interface, before they can be extracted. The research presented in this thesis focuses on
the coloration process, so the mechanics of bleaching will not be discussed further here.
9
1.4 Ion intercalation electrochemistry
Experimental measurements made on EC films usually involve a three-electrode cell
consisting of a working electrode (WO3 film), a counter electrode (in this case an inert
platinum electrode) and a reference electrode, all immersed in some electrolyte solution.
A current or voltage is applied across the working and counter electrodes to colour or
bleach the film. The potential of the working electrode is measured relative to the
reference electrode, as current flow between these electrodes is negligible and the
reference electrode potential is constant.
The measured voltage during coloration of an EC film in an electrolyte solution is the
sum of all of the potential drops between the TCO (of the working electrode) and the
reference electrode. The conductivity of the transparent conductors and the electrolyte
is relatively high, which means that the electrical potential drop across these layers will
be small. Assuming that there is no internal resistance for the WO3 film, the large
majority of the potential will be applied across the electrolyte/electrode interface.
The voltage measured during an experiment depends on several factors, one being the
chemical composition of the film. If no voltage or current is applied to the film and
significant time has passed allowing diffusion to occur (ie. the system is at equilibrium)
then the measured voltage is the difference between the electromotive force (emf) of the
film and the electrical potential of the reference electrode. The emf of a film of
composition MyWO3 is related to the free energy change associated with introducing y
moles of the atom M into the solid lattice of WO3. This emf is given by equation (1.2)
10
−
−+=y
yFRTbyaemf
1lnν (1.2)
where a, b and ν are constants which are discussed in detail in section 2.5, and y is the
stoichiometric lithium coefficient (ie. y in MyWO3). This equation is similar to the
Nernst equation for redox reactions in solution, except that in this case we are
considering the concentration of a species in a solid solution. Cell potentials are related
to the free energy change (∆G) of reaction by equation (1.3)
nEFG −=∆ (1.3)
where n is the number of electrons in the process, E is the cell potential and F is
Faradays constant. Spontaneous reactions occur when the free energy change is
negative (ie. the reaction will tend to move in the direction of the energetically more
stable state) and therefore when the sign of the emf is positive. As y in MyWO3
increases and the emf of the working electrode (relative to the reference electrode)
becomes more negative, indicating that the film is being reduced and is therefore
moving cathodically further from equilibrium.
During switching of an EC film, the system is displaced from equilibrium and the
voltage response is considerably more complex. Unless ionic diffusion is extremely
fast, the surface concentration of ions will be greater than the average concentration.
Assuming no internal potential drop in the electrode, the voltage during switching is
related to the concentration of injected ions at the electrode surface so we need to
investigate the distribution of ions within the film. If the diffusion coefficient is high,
the surface concentration of ions will be low because the atoms move very quickly into
11
the film. This will mean that the emf and consequently the measured voltage are
expected to decrease as ionic mobility increases. Increases in temperature should result
in faster diffusion and hence lower voltages are expected at higher temperatures.
Figure 3 shows a graph of stoichiometric lithium concentration (ie. y in MyWO3) versus
film depth (x), for different values of the chemical diffusion coefficient. The data on
this graph was calculated using a model reported by Wang [5] which is detailed in
section 2.5. It is evident from this graph that the surface concentration will be
considerably higher then the average concentration inside the film, or alternately, the
greater the diffusion coefficient, the flatter the concentration profile will be.
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 50 100 150 200
D = 1x10-12 cm2/s
D = 2x10-12 cm2/s
D = 5x10-12 cm2/s
D = 1x10-11 cm2/s
Con
cent
ratio
n (
)
Distance x (nm)
y
Figure 3 Simulated concentration profile for t=200s (Qinj=20mC/cm2)
Various models have been proposed to describe the electrical response of EC films and
devices according to different rate limiting mechanisms. Many rate-limiting
12
mechanisms have been proposed including the diffusion limited motion of ions in the
film [6], the presence of a barrier to charge transfer at the electrolyte/electrode interface
[7] and the series resistance of the complete cell. Several of these rate-limiting
mechanisms are discussed in more detail in section 2.2.
Regardless of the limiting mechanism for a particular EC system, processes such as
diffusion and charge transfer will occur more readily at high temperatures. It is
therefore likely that the electrical potential required to achieve a specific charge density
will depend heavily on temperature. EC films and devices for use in smart window
applications are known to heat up considerably in the coloured state [8], because they
change their optical constants by a principle known as absorption modulation (See
section 1.5), and therefore absorb a significant proportion of the incoming radiation.
The way in which temperature affects the kinetic behaviour of electrochromic systems is
therefore of great interest to anyone wishing to control devices in a manner which will
reduce the possibility of degradation and maximise device lifetime. The effects of
temperature in EC cycling are discussed in more detail in sections 2.3 and 2.4, and
various models describing the electrical response are discussed in section 2.5.
13
1.5 Optical characteristics
Electrochromic films undergo a colour change by either reflectance modulation or
absorption modulation [9]. Reflectance modulation results from an increase in the free
electron density in the material [10] and is observed in crystalline electrochromic
materials. Amorphous electrochromic materials exhibit absorption modulation, caused
by the development of an absorption band [11]. Crystalline electrochromic films have
significantly lower ionic mobility than amorphous films, which limits the maximum
switching speed, and these films are also difficult to produce in combination with other
layers. Amorphous electrochromic layers are therefore preferable for smart window
components. The sol-gel deposited films used in this work are all amorphous and
therefore colour by absorption modulation. The following discussion of optical
characteristics is then limited to amorphous films, and the optical properties of
crystalline films are not considered further in this thesis.
There is some controversy as to the exact nature of the colour centre formed in EC
materials, but it is commonly accepted that the coloration process involves the double
injection or extraction of ions and electrons [4]. Several models have been proposed to
describe the coloration process including the formation of colour centers by electron
trapping at oxygen vacancies [12] (analogous to the formation of F centers in alkali
halides), an interband transition model and a small polaron model [13,14].
14
Perhaps the most widely accepted model describes the colouration process as arising
from an intervalence charge transfer between a tungsten(VI) atom and a neighbouring
tungsten(V) atom according to equation (1.4) [3]
W5+(A) + W6+(B) + hν ⇔ W6+(A) + W5+(B) (1.4)
According to this explanation, an electron trapped at a tungsten(V) site is transferred to
a neighbouring tungsten(VI) site when incident radiation is absorbed. It is this
absorption which is thought to give rise to the colour change of the EC material.
Several parameters are used to discuss the optical properties of EC materials. The
optical density (OD) is a parameter that describes the level of coloration, and is defined
for a particular wavelength (λ) by
( )λλλ xTTOD /log 0= (1.5)
where T0λ and Txλ are the transmittances of a reference sample and measured sample
respectively, at wavelength λ. The optical density is therefore a comparison between a
measured transmittance and some reference transmittance. The change in optical
density (∆ODλ) is frequently used to describe the optical state of EC systems and it is
defined by equation (1.6).
( )λλλ cb TTOD /log=∆ (1.6)
where Tcλ and Tbλ are the sample transmittances in the bleached and coloured states
respectively, at wavelength λ. The coloration efficiency (ηλ) is a measure of the
intensity of the colour change per injected ion concentration, and is defined as
15
( )QOD /λλη ∆= (1.7)
where Q is the injected charge density per unit area. In reality the optical density is only
proportional to the injected charge density over a limited region. The optical transition
as denoted by equation 1.4 requires the presence of both W5+ and W6+ sites. If charge is
injected until the stoichiometric lithium coefficient (y) is 0.5, there will be equal
numbers of W5+ and W6+ sites. Further charge injection will decrease the number of
transitions possible because there will be too few W5+ sites available. This site
saturation model predicts that the coloration efficiency will be zero at y=0.5 and then
become negative for higher charge densities, and this behaviour has been observed
experimentally [15].
In practise the coloration efficiency may be determined experimentally as the slope of
the linear region of a ∆ODλ versus Q plot. Figure 4 shows such a plot for a tungsten
oxide film coloured with lithium ions. Linear regression of the data shown in Figure 4
gives a coloration efficiency of 45.5cm2/C.
16
0.00
0.20
0.40
0.60
0.80
0.0 5.0 10.0 15.0 20.0 25.0
OD
Charge injected (mC/cm2)
∆
Figure 4 Determination of coloration efficiency from a plot of change in optical density
versus injected charge density. Coloration efficiency = 45.5cm2/C.
Some electrochromic devices have the ability to maintain a given level of coloration for
several hours, when disconnected from the external circuit. This ability of a film to
maintain coloration is called electrochromic memory and is a very desirable attribute for
a real EC system. If an EC device does not have a good electrochromic memory, the
optical density will decrease with time and the device will consume more power if it is
forced to maintain constant optical density. In the past, some coloured EC films have
been observed to self-bleach when exposed to various environmental conditions, and
self-bleaching has been observed to be heavily dependent on humidity.
ηλ=45.5cm2/C
17
In this work, self-bleaching was observed to be associated with irreversible charge
injection. The amount of charge available for transfer between the electrodes of a real
EC device is determined during manufacture. If self-bleaching occurs in a real device,
the slow reduction in the amount of mobile charge available for switching, may
eventually render it useless. The ability to produce an electrochromic device with an
excellent memory, very reversible charge injection and fast response times is the holy
grail for EC researchers.
1.6 Overall Objectives of the Study
The overall objectives of this study were to provide detailed information about the
kinetic processes occurring in sol-gel electrochromic films and the ways that these
processes are affected by increasing temperature. This information is required in order
to improve device design, and to make progress towards successful electrochromic
device commercialisation. The effects of temperature on the optical and electrical
properties of electrochromic films were determined, and the results modelled over a
wide temperature range. The effects of water and temperature on the reversibility and
response time of the EC process were also investigated.
1.7 Specific Aims of the Study
The specific aims of the study are:
o To investigate the effect of temperature on the voltage response of
electrochromic films
o To investigate the effects of temperature on the optical response of
electrochromic films
18
o To determine the effect of temperature on reversibility of the electrochromic
reaction
o To determine the effect of water on the reversibility of the electrochromic
reaction
o To validate and extend an existing model [5] by simulating experimental data
collected at elevated temperatures, thereby assessing the validity of this model.
o To demonstrate that information about the ionic mobility may be extracted from
the process of simulating experimental data obtained at elevated temperatures
o To gain information regarding the causes of self-bleaching, and the effect of
water and temperature on this phenomenon.
1.8 Research History
The research reported in this thesis took place in several stages, each culminating in a
published journal article or the presentation of a conference paper. This section is a
brief historical account of the research progress in my PhD program, to give the reader a
sense of the context and linkage of each paper. The experimental component of the
research is discussed in much more detail in Chapter 8.
The initial experimental work in this research project was focussed on simply
determining the voltage response of an EC film cycled in a liquid electrolyte over a
range of temperatures. A WO3 film was cycled in the ambient laboratory environment
at temperatures between 20 and 50ºC and the electrical and optical properties were
observed. The films required significantly smaller voltages for coloration and bleaching
when cycled at high temperature. The reversibility of the electrochromic reaction was
19
observed to decrease, with almost 4% of the injected charge unable to be extracted per
cycle at 50ºC. The coloration efficiency was also seen to decrease slightly at the higher
temperatures and this research was published in Electrochimica Acta with the title
Effect of temperature on electrochromic device switching voltages (Thesis Chapter 3)
in 1999. The decrease in coloration efficiency at high temperatures was anomalous and
experiments were planned to investigate the effect further.
It seemed plausible that the charge trapping observed in initial experiments was due to
the presence of water, and so the experimental apparatus was moved into a nitrogen-
filled dry-box of approximately 1ppm absolute humidity. Experiments in a dry-box
showed the same voltage-temperature behaviour as seen previously, however the
coloration efficiency and cycling reversibility were now independent of temperature. It
was then evident that both temperature and moisture play a significant role in the
reversibility of the EC process.
A set of self-bleaching experiments was carried out in the ambient laboratory
environment in order to better determine the effects of temperature and water on cycling
reversibility. A film was coloured to a specific charge density and then the counter
electrode was disconnected. The electrodes remained in the electrolyte solution for a
half-hour period, and the films were observed to slowly bleach. The emf shifted towards
more anodic potentials and the optical density decreased with time. The self-bleaching
rate was observed to increase with temperature, as did the amount of irreversibly
inserted charge (Qin-Qout). At 50ºC approximately 4.5mC/cm2 of the injected charge
20
(Qin=20mC/cm2) could not be extracted. Optical and electrical measurements were used
to estimate the rate of self-bleaching, and the estimates correlated well with the
measured values. This work showed that charge trapping is a real phenomenon that will
rapidly reduce the useful lifetime of an EC device and was published in Renewable
Energy, in a paper titled High temperature behaviour of electrochromics (Thesis
Chapter 4).
A major objective of this work was to simulate high temperature experimental data
using a mathematical model, and extend the work of Wang [5]. The data from
irreversible cycling was not suitable for modelling because it was very difficult to
estimate the amount of charge in the film at a given time. Film cycling in the dry-box
was very reversible and hence provided data to use in the simulation process. The
modelling work involved the simulation of experimental data obtained during coloration
at temperatures ranging from 20ºC to 50ºC. Estimates were made for the chemical
diffusion coefficient of lithium, and the variation in surface lithium concentration with
time was used to model the voltage during the charge injection process. Good fits were
obtained for data collected between 20ºC and 50ºC, and the estimated diffusion
coefficients increased with temperature, obeying Arrhenius type activation behaviour.
This allowed for the extrapolation of the activation energy for diffusion of lithium ions,
of 0.73eV. This work was published in Solar Materials and Solar Cells in a paper titled
Temperature Dependence of kinetic behaviour of sol-gel deposited electrochromics
(Thesis Chapter 5) and was the first publication to report the estimation of ion mobility
by simulation of voltage-time data. This was very significant because the direct
21
measurement of diffusion coefficients is a process that usually involves very time
consuming experimental work, often requiring costly equipment.
It was desirable to increase the temperature range at which the films were cycled, so an
experiment was carried out from 36ºC to 76ºC. The data was again simulated and
reasonable fits to the data were obtained over the wider temperature range. An
activation energy of 0.99eV was calculated, and although this was significantly larger
than the activation energy determined previously, it was in accord with the voltage
characteristics and indicated lower ionic mobility within the film. The results of this
modelling work suggested that the EC process was limited by diffusional motion of
atoms inside the EC film at low temperatures, and by the charge transfer step at high
temperatures. This work has been accepted for publication in Electrochimica Acta in a
paper titled Simulation of electrochromic switching voltages at elevated temperatures
(Thesis Chapter 6).
The earlier work on self-bleaching showed that films would lose their coloration when
exposed to an electrolyte in a moist environment for an extended period of time,
however no results were collected in a very dry environment. The same experiments
were then repeated in very dry conditions, in order to support the hypothesis that water
causes irreversibility and self-bleaching. Films in the dry-box were observed to slowly
self-bleach but at a rate much slower than in the ambient environment. A fresh film was
fired at 250ºC for several hours in an attempt to drive off any adsorbed water, and then
used in a similar self-bleaching experiment. This time the film did not self-bleach
22
significantly over a half-hour period. The cause of the self-bleaching effect was then
ascribed to the presence of water in different states. The presence of water in the
electrolyte led to relatively high irreversibility, and when the electrolyte was dry,
reversibility improved considerably. Removal of adsorbed water from the film itself
resulted in very reversible cycling of the film however the response time was very long.
Some cycles required in excess of 15 minutes in order to extract all of the injected
charge, thereby bleaching the film. This final experimental work on self-bleaching and
reversibility represented the end of the experimental work in this project, and
culminated in the paper Self-bleaching, memory effect and reversibility of
electrochromism at elevated temperatures (Thesis Chapter 7), which has been
submitted to Solar Energy Materials and Solar Cells.
Three other papers have also been published as a result of this PhD project, and these
form the Appendices of this thesis. These papers are more general than those described
above, and give the reader a more fundamental understanding of not only
electrochromic materials and systems, but advanced optical systems in general.
Appendix 1, Glazing Materials, was published in Materials Forum and is a review of
glazing materials, including but not limited to electrochromic systems. Other systems
such as liquid crystal, thermotropic, and angular selective devices are discussed here,
giving the reader an overview of the state-of-the-art in advanced glazing systems.
Appendix 2, Sol-Gel Deposited Electrochromic Devices has been published in
Renewable Energy and describes results of devices produced from sol-gel deposited
electrochromic films. The results presented include electrical and optical characteristics,
23
and also an analysis of the potential energy saving benefits of these devices. Appendix
3, entitled Smart Windows is an encyclopedia entry in the Encyclopedia of Smart
Materials, and discusses several types of glazing technologies. This paper also gives a
discussion of the physics appropriate to window glazings, and defines several of the
parameters used to characterise advanced glazing systems. If the reader is not familiar
with the technology described in the heavily research oriented papers discussed above,
then reading the appendices will provide much of the background necessary for a more
complete understanding of this research work.
24
REFERENCES
[1] S.K. Deb, Appl. Optics, Suppl. 3 on Electrophotography, 192-195 (1969).
[2] S.K. Deb, Solar Energy Materials and Solar Cells, 25, 327-338 (1992).
[3] B.W. Faughnan, R.S. Crandall and P.M. Heyman, R. C. A. Rev., 36, 177-197
(1975).
[4] B.W. Faughnan and R.S. Crandall. Electrochromic Display Based on WO3 in J.I.
Pankove, ed., Topics in Applied Physics V40, Display Devices, Spring-Verlag Berlag,
New York, Ch. 5. (1980)
[5] J. Wang, PhD Thesis, University of Technology, Sydney (1998).
[6] C. Ho, I.D. Raistrick and R.A. Huggins, J. Electrochem. Soc., 127(2), 343-350
(1980).
[7] S.K. Mohapatra, J. Electrochem. Soc., 125(2), 284-288 (1978).
[8] C.M. Lampert, A. Agrawal, C. Baertlien and J. Nagai, Solar energy Materials and
Solar Cells, 56, 449-463 (1999).
[9] C.G. Granqvist. Introduction to Materials Science for Solar Energy Conversion
Systems in A. A. M. Sayigh, ed., Materials Science for Solar Energy Conversion
Systems, Pergamon Press, New York, 1 (1991).
[10] J.S.E.M. Svensson and C.G. Granqvist, Appl. Phys.Lett., 45(8), 828-830 (1984).
[11] C. Wang and J.M. Bell, Solar Energy Materials and Solar Cells, 43(4), 377-391
(1996).
[12] S.K. Deb, Phil. Mag., 27, 801(1973).
[13] O.F. Schirmer, V. Wittwer and G. Bauer, J. Electrochem. Soc., 124, 749(1977).
[14] V. Wittwer, O.F. Schirmer and P. Schlotter, Solid State Commun., 25, 977(1978).
25
[15] M. Denesuk and D.R. Uhlmann, J. Electrochem. Soc., 143(9), L168-188 (1996).
26
CHAPTER 2
LITERATURE REVIEW
28
LITERATURE REVIEW
2.1 Electrochromism A Brief History
Between the late fifties and mid-nineteen sixties there was considerable interest in the
properties of tungsten bronzes [1-6]. It was observed that metal ions could be
preparatively incorporated into the lattice structure of tungsten oxide (WO3), and the
resulting non-stoichiometric compounds exhibited strong colour centres and metallic
properties.
In 1969 Deb reported that an electric field could be used to form colour centres in thin
films of tungsten oxide, producing colours similar to those of the tungsten bronzes [7].
Deb reported his results as A Novel Electrophotographic System, where a WO3 film
was incorporated with a photoconductive (CdS) layer. A picture could be reversibly
recorded on the WO3 film by the application of an electric potential across the film
combined with the projection of light (through a photographic negative) onto the film.
The light projected through the negative increased the conductivity of the CdS layer
allowing colouration of the tungsten oxide under the applied potential.
In the following ten years, the potential of electrochromic films in devices such as
smart windows was realised, and interest in the field grew rapidly [8-21]. Initial
research into electrochromics concentrated primarily on the optical and electrical
properties of HxWO3 [12,14], while other research investigated a number of other
possible electrochromic and counter electrode materials [15-18]. Metal oxides which
29
have been shown to exhibit electrochromism include NiO, TiO2, CeO2-TiO2 and IrO2.
NiO and V2O5 have been studied extensively as counter electrode materials. NiO is a
particularly attractive counter electrode material as is colours anodically (upon extraction
of ions), as opposed to WO3 which colours cathodically (upon ion insertion). Devices
which use both anodically and cathodically coloured layers exhibit a greater
transmittance range between the coloured and bleached states and are known as
complementary devices.
In recent years the developments in processing and production of transparent conducting
oxides (TCO) have increased the potential for application of electrochromic films in large
area smart windows. The commercialisation of on-line spray pyrolysis coating of
fluorine doped tin oxide (SnO2:F) [19] has led to cheaper TCOs and hence more
economically viable large-area devices.
In these devices it is undesirable to have liquid components and much research has been
carried out searching for suitable all solid state devices. The electrolyte layers used today
generally either consist of an organic polymer (doped with the ion inserted in the EC
reaction), or an inorganic fast-ion conductor such as M-β-alumina, (where M = Na, Li,
etc) [20,21]. Numerous reviews of electrochromic systems have been published recently,
and examples are given [22,23].
Some recent work closely related to this project involves the modelling of the
current/voltage characteristics of electrochromic films and devices. Bell et al [24] have
modelled the electrical characteristics of small and large area electrochromics based on
30
an equivalent network of distributed electrical devices, in order to predict the behaviour
of devices under given conditions. The small device model is applicable to the samples
used in this work, while the large device model is applicable to devices of size suitable
for commercial window application. Zhang et al [25,26] reported a model which
predicted WO3 film behaviour under constant voltage cycling, given the cell series
resistance, the lithium diffusion impedance and the emf (electromotive force) of the film.
This model has been modified recently [27] for constant current cycling of WO3 films
and also has the advantage of not assuming the semi-infinite approximation. This means
that this model can be applied to films of low thickness (typical of the films dealt with in
this work), and should describe the electrical characteristics of devices at high
temperatures. One of the aims of this work is to validate this model over a broad
temperature range, and future work is aimed at modification of this model to further
describe the behaviour of V2O5 counter electrodes and eventually complete devices.
2.2 The Electrochromic Reaction in WO3 Films
Electrochromic reactions occur when ions and electrons are simultaneously inserted into
the host lattice of an ion-intercalation electrode under the influence of an electric field.
The process occurs in several steps [28] as outlined below:
(i) ions in the electrolyte migrate towards the surface of the WO3 film under the
influences of diffusion and the applied electric field, and electrons from the
external circuit are inserted into the WO3 film via the ITO conductor
31
(ii) charge transfer occurs between the ion and electron at the WO3/electrolyte
interface. The cation is inserted into an interstitial position in the host lattice
between WO6 octahedra [29, 30]
(iii) the ion/electron couple diffuses into the oxide film (ie. towards the ITO)
under the influence of the concentration gradient
(iv) an intervalence charge transition takes place between a pair of neighbouring
tungsten atoms and an adjacent ion/electron pair resulting in a change in the
materials optical properties according to equation (2.1)
W5+(A) + W6+(B) + hν ⇔ W6+(A) + W5+(B) (1.1)
where (A) and (B) are neighbouring tungsten sites, the W6+ site is reduced
when it receives an intercalated electron from a W5+ site and light of energy
hν is absorbed [12].
The EC reaction therefore combines charge transfer (step (iii)) above and diffusion (steps
(iii) and (i) above) processes and the rates of these will depend on the way in which the
system is switched. There has been much discussion regarding the particular steps which
limit the rate of the EC reaction, and these will be discussed more fully below.
Failure of electrochromic systems may occur in a variety of ways, resulting in loss of the
ability to make the EC material undergo a significant optical transition. Several
degradation mechanisms have been proposed for EC systems, but there is no single
generally accepted mechanism that applies to EC WO3 films. Faughnan and Crandall
illustrated that lifetime will be greatest for switching conditions involving smaller
changes in optical density, and hence the application of lower electrical potentials.
32
Conversely, the higher the change in optical density achieved during cycling, the lower
the lifetime [12].
It is commonly believed that water plays an important role in the lifetime of EC systems.
One of the first investigations into the effects of water on EC cycling was published by
Arnoldussen [31] in 1981. Arnoldussen reported that WO3 films will dissolve in water by
forming tungstate species. The small size of the water molecule allows it to penetrate the
WO3 lattice and hydrolyse W-O-W bonds to form two W-OH bonds. It has been
suggested that these W-OH bonds are potential sites for irreversible lithium intercalation
[32], which supports the notion of water being a major cause of EC film and device
degradation. Svensson and Granqvist [33] concluded in 1984 that a long electrochromic
life for an EC device could only be achieved if water was carefully excluded from the
system. The degradation issues due to the presence of water in an electrochromic system
have resulted in a search for more aprotic solvents. The effects of water on the cycling
characteristics are discussed in more detail in section 2.4.
2.3 Electrochromic Characteristics at Elevated Temperatures
Electrochromic devices reduce the transmission of light by absorbing a significant
proportion of the incoming radiation. The absorption of this energy is associated with an
increase in the temperature of the device and hence EC devices may typically reach
temperatures exceeding 65ºC [34,35] during their normal operation. It is therefore
essential to understand the ways in which the kinetic properties of EC devices are
affected by temperature, so that the control regime for switching these devices may be
optimised for a wide temperature range. The response time for EC devices slows down
33
as temperature decreases [22], and conversely the mobility of ions within EC systems
increases with increasing temperature. It should then be possible to switch EC devices
with the application of smaller voltages at high temperatures, as predicted by Nernstian
behaviour (see section 1.4)
, or alternately higher switching currents (and faster switching) may be possible without
device degradation. There is little published information in the literature regarding the
effects of temperature on electrochromic device switching properties and this is the
primary knowledge gap that this work aims to address.
Hackwood et al [17] reported a study in 1980 that illustrated the dependence of switching
speed of electrochromic iridium oxide films on temperature. The EC iridium oxide films
were cycled by insertion and extraction of protons (in aqueous media), and colouration
and bleaching speeds were measured at temperatures ranging from +20 to 43ºC. In this
work it was observed that the switching speed of these films increased with increasing
temperature and the response was explained by an activation-controlled scheme, with an
activation energy of approximately 0.25eV. The colouration efficiency at +20ºC was not
significantly different from that at 30ºC and was therefore observed to be independent of
temperature. The iridium oxide films discussed in this paper have application in fast
switching (switching speeds <0.5s) display devices, where it is unlikely that their
operational temperature range will approach that of ECs for smart window applications.
Although the temperature range, film material and inserted species are dissimilar to the
tungsten oxide EC films common to smart window research, the paper is important as the
first reported experimental work on temperature effects on EC systems and the reported
34
trends in kinetic behaviour may reasonably be expected to be observed in other EC
systems.
In 1996 Badding et al [34] reported results from durability testing of a monolithic EC
device over a wide temperature range. The multilayer device utilised a solid-state ionic
conductor as the electrolyte and used a constant voltage step to colour and bleach the
device, applying the same voltage limits for all temperatures. Switching speed was
defined as the time taken to reach 90% of the full range of visible coloration and this was
measured from 27 to +70ºC. At 27ºC the device was observed to switch very slowly,
taking almost 25 minutes to reach 90% of its full coloration. Switching speed increased
rapidly as temperature was increased, taking less than five minutes to colour the device
for all temperatures above 0ºC. At 25ºC and 70ºC the switching times were
approximately 2 minutes and no significant increases in speed were observed over this
temperature range. The paper does not report any attempt to calculate an activation
energy for diffusion from the results of the switching speeds, or the effect of faster
switching speeds on device degradation. As switching currents were not measured during
cycling, the amount of charge injected and extracted per cycle was not calculated and no
conclusions may be made regarding the cycling reversibility at high temperature. The
increases in switching speed indicate that kinetic processes such as ion transport are
occurring faster at high temperatures, which is in agreement with Hackwood et als work
on iridium oxide films.
Another study by Tulloch et al [36] reported the cycling voltages of a sol-gel EC film at
18ºC and at 50ºC, under constant current operation. In this work a complete device using
sol-gel deposited electrodes was cycled in an oil bath and electrical and optical
35
measurements made. The device was cycled using a constant current technique in order
to control the quantity of charge injected and hence the colouration level achieved. The
switching algorithm involved cycling of the device under a constant current step until a
specific injected charge density was reached. If the voltage reached a preset safe limit
during coloration or bleaching, the mode of operation changed to constant voltage at the
preset voltage limit in order to protect the device from high overpotentials. The voltage
required to colour the device to 10mC/cm2 at 18ºC was approximately 1.6V while at 50ºC
only 1.0V was required to achieve the same injected charge density. A total of
13mC/cm2 was injected in the cycles reported and the maximum voltage limit on
coloration was set to +1.8V. The device cycled at 18ºC reached this limit after
approximately 13.1mC/cm2 was injected, but never reached this limit when cycled at
50ºC. This work demonstrated that significantly lower voltages are required to colour
and bleach EC devices at elevated temperatures and hence the way in which we switch
them is very important. The paper also showed that a voltage-limited constant current
technique has the advantage (over constant voltage techniques) that the amount of charge
injected and hence the level of coloration is the same each cycle regardless of the
temperature. Tulloch et al did not however elaborate on the kinetic mechanisms
responsible for the lower voltages except to comment that the result was expected
because diffusion coefficients and polymer conductance increase with temperature.
It is evident from the papers discussed above that the general effect of temperature is to
increase the ease by which EC reactions occur. Identification of the mechanism or
mechanisms responsible for this increased reaction rate (constant voltage cycling) or
reduction in switching voltage (constant current cycling) is not possible from the previous
36
work reported in the literature, and more work is needed to clarify this. Nernstian
behaviour (see section 1.3) dictates that the magnitude of the electromotive force (emf)
for a given state of charge injection (or film composition) will decrease with increasing
temperature. The result is that lower switching voltages are required at high
temperatures. A concern however, is that side reactions leading to device degradation
will also proceed more easily and so the application of the same switching voltages over a
wide temperature range may adversely affect device lifetime. A better understanding of
the kinetic properties and rate limiting mechanisms of ECs at elevated temperatures will
allow us control EC devices in a fashion that will use knowledge of material properties to
maximise device lifetime.
2.4 Lifetime, Irreversibility and Self-Bleaching
Ideally EC films and devices may be cycled reversibly between coloured and bleached
states, so that the charge injected into the working electrode is always fully recovered
upon bleaching. In practice this is not always possible and the reversibility is dependent
upon various combinations of deposition conditions, microstructure and cycling
conditions. Irreversibility has been observed many times over the last three decades and
several papers outline the anomalous effects observed.
In 1978 Randin [37] reported work whereby the stability of EC WO3 films was studied in
10:1 glycerin/sulfuric acid (H2SO4) mixtures and in various non-aqueous electrolytes.
The WO3 films were observed to slowly dissolve in glycerin/H2SO4 regardless of whether
the films were actively cycled or simply stored in the electrolyte solution, however the
37
dissolution rate was significantly increased by film cycling. WO3 films cycled in non-
aqueous electrolytes were observed to dissolve much more slowly, presumably due to the
lack of available water. Randin proposed that the tungsten oxide dissolved in water from
the electrolyte to form a soluble polytungstic acid [38] species. Randin also suggested
that films cycled in lithium salt/organic electrolytes with small amounts of water present
would undergo dual insertion of protons and lithium ions, because proton migration
occurs much faster than for lithium. Randin observed one anodic peak in the cyclic
voltammograms of films cycled in aqueous media, but two peaks for films cycled in non-
aqueous electrolytes. These two peaks were attributed the to solid-state diffusion of
protons and lithium ions as described by the space-charge limited bleaching model of
Faughnan and Crandal[14]. Randins paper was the first to investigate the stability of
tungsten oxide electrodes in different solvents and to report the effects of various solvents
on the reversibility of electrochromic cycling. This paper also showed that cycling
lifetime is dependent on switching frequency and to discuss the implications of this in
terms of validation of accelerated tests.
In 1990 Zhong et al [39] reported work where WO3 films were coloured and bleached at
room temperature and significant amounts of lithium were observed to remain inside the
films, even after the bleaching process. The crystallographic structure of the tungsten
oxide changed upon insertion of either H+ or Li+ ions, and Zhong et al monitored these
changes using x-ray diffraction techniques. The x-ray diffraction studies showed
characteristic peaks attributable to the formation of the tungsten bronzes HxWO3 and
LixWO3 when the WO3 films were coloured with H+ and Li+ ions respectively. When the
films were bleached, only the peaks from tungsten oxide were present. The crystal
38
structure was therefore observed to revert back to its original form after bleaching, even
though a significant proportion of the injected ions still remained inside the films.
Secondary ion mass spectroscopy (SIMS) and inductively coupled plasma-mass
spectrometry (ICP-MS) experiments revealed that on average, approximately 50% of the
ions injected during the coloration process were still inside the films after bleaching.
Using this combination of XRD, SIMS and ICP-MS techniques, Zhong was able to show
that a proportion of injected ions remained inside the WO3 EC films after bleaching,
however these ions were not held in interstitial positions in the tungsten oxide lattice and
they did not contribute to coloration.
Zhong et al hypothesised that during the bleaching process, some of the ions were trapped
in optically inactive sites outside the WO3 lattice such as on grain boundaries. An ion
trapped at a grain boundary would not participate in the EC reaction because its electron
would also be trapped and therefore would not be available for donation to a tungsten(VI)
atom. Zhong et al observed that the conductivity of bleached samples was the same as
that for as-deposited samples, which showed that the electrons (associated with the
remaining injected cations) were not available for electrical conduction. This supports
the hypothesis that the cations that remain inside the films are not in interstitial lattice
positions and so cannot contribute to coloration.
In 1991 Hashimoto and Matsuoka [32] reported a study on the electrochromic lifetime of
mixed oxide WO3-TiO2 films. The films were prepared by electron beam deposition with
TiO2 concentrations ranging from zero to 30 mole percent. The longest lifetime was
observed for a film with 15.6 mol % TiO2 and the lifetime was five times greater than for
39
a pure WO3 film. Hashimoto and Matsuoka used SIMS to measure the amount of lithium
in as-deposited, bleached and coloured films and found that there was a significant
amount of lithium in all of the bleached samples. The bleached WO3 sample contained
approximately 44% of the injected lithium ions, while the bleached 15.6 mol% TiO2
sample only contained about 20% of the injected lithium ions. This was the first report of
evidence relating cycling lifetime to the reversibility of individual cycles. Hashimoto and
Matsuoka used X-ray photoelectron spectroscopy (XPS) to obtain information on the
binding of lithium ions in coloured and bleached films and showed that the lithium in the
bleached films is not located in the body centred position in the WO3 lattice, as it is for a
tungsten bronze. They suggested that the lithium is present in the bleached film as O-Li
after exchange between a proton and lithium ion of an O-H group. The XPS experiments
also showed that the number of lithium ions present as O-Li in the bleached samples was
smallest for those doped with TiO2. Hashimoto and Matsuoka investigated the cause for
increased lifetime of the mixed oxide films by using low loss region EELS and showed
that the electrons in WO3-TiO2 are more tightly bound than in pure WO3, and used
Raman spectroscopy to show that the W-O bond length is decreased by TiO2 doping.
Hashimoto and Matsuoka proposed that there are several defects present in pure WO3,
including W-O-H and W=O sites, which may act as lithium trapping sites. These bonds
may be broken by adding small amounts of TiO2 to the structure, the Ti atoms bonding to
oxygens (W-O-Ti-O-W) to remove the trapping sites. Hashimoto and Matsuoka
proposed in earlier work [40] that crystalline lithium tungstate was irreversibly formed
when relatively large amounts of lithium were intercalated into films of amorphous WO3
40
thereby degrading the amorphous film. The increase in lifetime upon addition of TiO2 to
WO3 films was therefore explained as a result reducing the number of defect bonds in the
lattice, and thereby preventing the accumulation of lithium and the subsequent formation
of lithium tungstate.
In 1992 Duffy et al [41] reported on a series of experiments in which several
electrochromic devices were fabricated with dry or damp polymer electrolytes. A
polymer electrolyte was prepared by adding solid orthophosphoric oxide to polyethylene
oxide, and plasticising with pre-dried acetonitrile. A portion of the resulting polymer was
used to fabricate dry devices, while the remainder was exposed to the ambient
laboratory environment for several hours before being used to fabricate wet devices.
Duffy et al found that after coloration the dry EC devices retained their colour almost
indefinitely (when disconnected from the external circuit) and therefore had an excellent
electrochromic memory, while the damp devices lost all colour within one week under
the same conditions. When a damp device had lost its colour it could no longer be cycled
unless the WO3 was first discharged (cycled with WO3 as the anode) or if the counter
electrode was replaced with a fresh piece of protonated ITO. Duffy concluded that the
loss of colour in devices with damp electrolyte was not caused by loss of the working
electrode or by deactivation of the WO3.
Duffy et al investigated the mechanism of self-bleaching by using impedance
spectroscopy and electron microscopy on devices with dry and damp electrolytes. The
impedance spectra for the dry and damp devices were very different, the latter exhibiting
41
blocking electrode behaviour and had no charge transfer semicircle in its spectrum. The
high frequency resistance was the same for coloured and self-bleached samples, which
rules out the possible explanation for self-bleaching of protons migrating into the ITO
layer. The electron micrograph of a film that was cycled in a damp device for one day
showed signs of swelling and buckling of the film. Another film was cycled for one
month under the same conditions and the electron micrograph clearly showed the film
cracking and breaking away from the substrate.
Duffy et al believed that the mechanism for self-bleaching involved selective dissolution
of tungsten(V) atoms so that the intervalence charge transfer which gives rise to
coloration (see section 2.2) is unable to occur. Duffy et al suggested that protons and
electrons migrate to internal surfaces within the electrode where WV species dissolve into
the electrolyte with the hydrogen and oxygen atoms. They also reasoned that this
dissolution would lead to an increase in film porosity which in turn would lead to water
uptake and swelling of the film.
In 1993 Zhang et al[42] reported what is currently the most complete and systematic
study into self-bleaching of electrochromic tungsten oxide films. Zhang et al studied the
effects of preparation conditions, the injected ion, environmental conditions, substrate
and film surface roughness. In this work WO3 films were coloured either 1M
LiClO4/propylene carbonate (PC) or dilute H2SO4 electrolyte, then washed and dried and
transferred to a sealed chamber. Into this chamber they introduced several different gas
mixtures to study their effects. The coloured films were exposed to pure argon, nitrogen
42
or oxygen, and the ambient laboratory environment of ~21%O2, ~79%N2 and 33%
relative humidity at 23ºC.
Sputtered WO3 films coloured with lithium ions were observed to bleach almost
completely (transmittance increased from 30% to 81%) over an 8 hour period, when
exposed to the ambient environment, whereas the same films only increased in
transmittance by 10% and 3% in dry oxygen and nitrogen atmospheres respectively.
When exposed to pure argon atmosphere or a 10-6 torr vacuum, negligible self-bleaching
was observed and Zhang et al concluded that the self-bleaching of EC films coloured
with lithium ions is dominated by the reaction of Li+ ions with water.
The same sputtered films coloured by proton injection were also exposed to environments
of ambient, oxygen and argon and generally exhibited much less self-bleaching than the
films coloured with Li+ ions. These films did not self-bleach significantly in argon, but
increased in transmittance from 30% to approximately 50% when exposed to either
oxygen or the ambient environment for an 8 hour period, so Zhang et al concluded that
self-bleaching in EC films coloured with protons is dominated by the reaction between
the inserted protons and oxygen gas.
Electrochromic WO3 films coloured with lithium ions and protons were exposed to
ambient and argon environments in order to examine the effect of deposition conditions
on the self-bleaching process. The films coloured by proton injection exhibited slower
self-bleaching than those coloured with lithium ions, and the evaporated WO3 films self-
bleached much more slowly than the sputtered films under the same exposure conditions.
43
The authors also mentioned that they have films deposited by plasma enhanced chemical
vapour deposition (PECVD) which were coloured and then stored in air for four years,
and have retained much of their original coloration. The differences in the self-bleaching
rates of films deposited by different techniques illustrates that self-bleaching behaviour of
a film is strongly dependent on the morphology.
Zhang et al also studied the effect of surface thickness and observed that self-bleaching
rate decreased monotonically with increasing film thickness. They found that the self-
bleaching rate was related more specifically to the ratio of the surface roughness to film
thickness, which is a measure of the amount of surface area relative to the thickness of
the film. Zhang et al used the results from all of these experiments to hypothesise a
model for the mechanism of self-bleaching in WO3 EC films. The model describes the
process occurring in two stages. Initially gases from the environment react with injected
protons or lithium ions on the surfaces of internal pores according to equations (2.2) and
(2.3) respectively.
2H+ + 2e- + 1/ 2O2 ⇒ H2O (1.2)
2Li+ + 2e- + 2H2O ⇒ 2LiOH + H2 (1.3)
Due to the ready availability of these surface ions, this step will be reaction limited and
will result in an initial fast decrease in the colouration centers formed by the protons and
lithium ions. Films with large surface roughness will therefore self-bleach faster and
films with high thickness will bleach more slowlyslower because a smaller portion of the
injected ions is at the surface. The second process involves diffusion of protons/lithium
44
ions to the surface or pores and diffusion of oxygen/water into the film. This process is
therefore diffusion limited and explains why the self-bleaching rate at long time periods
(>2 hours) increases with film thickness.
In 1993 Burdis and Siddle [43] published their work on the effect of sputtering conditions
on the reversibility of ionic insertion into EC WO3 films. Burdis and Siddle produced
films with a wide range of sputtering conditions including various pressures, reactive gas
mixtures and choice of metal or metal oxide targets. They grouped the films produced
into the broad classifications of polycrystalline and amorphous materials. They observed
that the polycrystalline materials obeyed Beer-Lambert law [44] upon ionic insertion, so
the optical density increased linearly with injected charge density. The amorphous
materials exhibited different behaviour in which large amounts of charge were inserted
before there was a linear increase in the optical density of the film. In one case
75mC/cm2 of lithium ions was injected into a film in order to achieve a solar optical
density of approximately 0.27 and this optical density returned to zero when only
15mC/cm2 was extracted from the film. The remaining 60mC/cm2 was injected
irreversibly, as seen by the excessive voltage (6V vs. Li) required to remove further
charge. The amount of charge irreversibly intercalated per cycle was observed to
decrease rapidly with progressive cycling but the total amount of irreversible charge was
as large as 100mC/cm2. Burdis and Siddle dissolved the WO3 film off the substrate and
carried out atomic absorption spectroscopy (AAS) on the solution to check whether the
lithium was actually still inside the film. Approximately 97% of the expected lithium
concentration was recovered from the film, proving that irreversible charge intercalation
45
is a real phenomenon and that lithium may be intercalated into sites which are either
optically active or inactive.
Burdis and Siddle also carried out coulometric titrations on both polycrystalline and
amorphous samples, and showed that the amorphous film started to precipitate out a
second phase upon intercalation of lithium ions. They postulated that the second phase
(possibly Li2O) was a result of reaction between inserted lithium ions and oxygen gas
trapped within the structure from the sputtering process. Evidence supporting this
includes the fact that sputtering conditions that increase the amount of trapped gas also
increase the irreversibility, and the irreversible effect is much smaller for evaporated
films.
In 1996 Michalak and Owen [45] reported a study regarding the observation of parasitic
currents during cycling of electrochromic WO3 films in non-aqueous electrolytes.
Michalak and Owen defined the parasitic charge as the difference between the inserted
and extracted charge for a cycle that begins and ends at the same transmission. The
parasitic current is an electrical current due to a reaction other than the reversible
electrochromic reaction, and Michalak and Owen outlined four possible effects of these
parasitic currents:
(1) Gas evolution
(2) Formation of passivating layers of reaction product
(3) Corrosion of electrode, losing host ions into the electrolyte
(4) Charge imbalance between the colouring and bleaching cycles
46
Michalak and Owen integrated the applied current while using cyclic voltammetry to
colour and bleach a WO3 electrode, then calculated the total amount of charge inserted
and extracted. Approximately 15mC/cm2 of lithium ions was irreversibly inserted on the
first cycle. The initial insertion of this 15mC/cm2 during the first cycle was not
accompanied by an optical change of the film, and so the authors concluded that the
injected lithium ions were reacting with some impurities in the film, possibly peroxide or
O- species present in the virgin electrode. After the first cycle, a constant coloration
efficiency was then observed for any further cycles. Michalak and Owen reported that
the anodic parasitic current was negligible up to 4.5V vs Li+/Li and that the cathodic
parasitic current was approximately 0.1µA/cm2 at 2V vs Li+/Li.
These papers address the issues of lifetime, reversibility and self-bleaching. It is evident
that WO3 films coloured with lithium ions often deviate from ideal reversible behaviour.
When cycling EC films (as opposed to complete devices) in a liquid electrolyte this
problem has a minor effect on lifetime because there is an almost infinite ion source
available. In a complete electrochromic device however, there is a finite ion source, and
the lifetime of the device may be dictated by the total amount of charge incorporated into
the device during fabrication. When devices are made it is common to incorporate more
charge than will be injected and extracted from the working electrode per cycle, and the
excess charge is stored in the counter electrode. Any irreversibility or inability to extract
charge from the working electrode upon bleaching will deplete this excess charge. Once
the sum of the irreversible charge (over all cycles) exceeds the initial excess charge that
was incorporated into the device, there will be a reduction in the amount of charge able to
47
be transferred between the working and counter electrodes. This effect will be observed
as a reduction in the transmittance range of the device and this change may well signal
the end of the life of the device. Minimising irreversible charge injection is therefore
very important in maximising device lifetime.
The papers discussed here tell us that reversibility/self-bleaching and hence lifetime are
heavily dependent on factors such as deposition conditions, water content, the injected
ion and electrolyte composition. These papers do not address these issues in sol-gel films
nor do they address the issue of the high temperatures that devices will attain in real
operation. A major aim of this work is therefore to start to fill the knowledge gap in
terms of lifetime, reversibility and self-bleaching at high temperatures, specifically in sol-
gel deposited electrochromic WO3-TiO2 films.
2.5 Models for Simulation of Electrochromic Switching Characteristics
Models are mathematical tools that may be used to simulate or interpret an observed
response, or to predict a response under a certain set of conditions. Modelling of
electrochromic phenomena allow us to extract useful information about the kinetic
processes occurring on a molecular scale, but several assumptions must be made in order
to simplify the modelling process. The rate of the EC reaction will be limited by the
slowest step in the overall EC process, so we must consider all steps including diffusion
and charge transfer processes as well as electrical processes such as resistances across
interfaces between components.
48
In 1975 Faughnan et al[12] published a review paper which included a qualitative model
for the colouration and bleaching of electrochromic WO3 films and for the optical
transition which results. According to the coloration model, electrons are injected from
the TCO and ions are injected from the electrolyte, under the influence of the applied
electric field. The electrons and ions will migrate into the film, where they combine to
form HxWO3. The ion is accommodated inside the lattice of the WO3, and the electron is
localised on a nearby tungsten site thereby reducing W6+ to W5+. Faughnan et al
explained the optical transition as an intervalence charge transfer occurring according to
equation (2.1). An electron localised on a W5+ atom reacts with an adjacent W6+ atom,
after absorbing sufficient energy to cross a potential barrier. The simultaneous oxidation
and reduction reactions that occur are accompanied by a radiationless transition of
energy, sometimes called a phonon emission.
The electrical conductivity of a HxWO3 film was observed to change from insulating in
the bleached, and approach metallic conductivity when highly coloured and the authors
estimated proton mobility to be in the range of 10-10-10-6cm2/s. Faughnan et al described
the bleaching process as being dominated by a space-charge limited current flow of
electrons and ions. The surface of the WO3 film will bleach first, and remaining ions
must move across a region of decreasing charge density in order to reach the electrolyte
interface.
Later in 1975 Faughnan et al [13] presented a quantitative model for the current response
during bleaching of WO3 films, when extracting protons under a constant voltage step.
The model elaborated on the notion of a space-charge limited current being the rate-
49
limiting step and the authors developed mathematical equations which described the
experimental data very well. When a voltage is applied to bleach a WO3 film electrons
leave the film via the TCO and ions migrate back into the electrolyte. This results in a
region of film adjacent to the electrolyte being depleted of electrons and so any current in
this region is purely a proton current. There is also a similar region at the TCO interface
that is depleted of protons, hence only an electron current flows. The third region is the
coloured part inside the film and it has both proton and electron currents flowing through
it during bleaching.
Faughnan et al reasoned that the voltage drop across the film is equal to the sum of the
voltage drops across each of the three regions described above. The potential drop across
the neutral plasma region (the coloured part) will be negligible because the conductivity
of this region is very high. Faughnan et al measured that the electron mobility was
several orders of magnitude higher than proton mobility, which means that the voltage
drop across the region of electron current will be very small. The entire switching
voltage is therefore applied to the neutral plasma region, which will shrink with time.
The mathematical description of this process defines the bleaching current density as a
function of time under constant voltage, as follows:
( )( )4
3
21
41
03
4)(
t
VtJ pµκερ= (1.4)
where )(tJ is current density, ρ is the volume charge density of protons in the neutral
plasma region, κ is the relative dielectric constant, 0ε is the permittivity of free space,
50
pµ is the mobility of the protons, V is the voltage and t is time. The 43
−t dependence of
the current was confirmed experimentally and the model simulated the data very well.
Crandall and Faughnan [14] also published a quantitative model for the coloration
process when protons are inserted into WO3 films. This model assumed that a barrier to
charge transfer at the electrolyte/electrode interface limited the current flow. The barrier
is associated with charge passing the electrolyte double layer at the electrode surface,
losing a solvation sheath and attaching to the electrode surface. The equation for
coloration current density (as a function of time) described the experimental data very
well for short time periods, except for at the highest voltage (0.5V). The authors
concluded that it was likely that other mechanisms limited current flow at voltages in
excess of 0.5V. The inherent limitation of this model is that the process of diffusion is
not considered, and so it is only applicable for small voltages or short time periods and
hence low colouration densities.
In 1976 Crandall et al [46] derived an expression for the emf of a tungsten film after the
electrochemical insertion of protons and electrons. Crandall et al derived the model
using a first principles approach to the thermodynamic properties of the species present in
the EC reaction and the model was used to simulate experimental measurements of film
emf for various injected charge densities. Crandall et al used the general equation
E=∆G/F to relate the emf (E) to the free energy change (∆G) that occurs when x moles
of protons are transferred from the electrolyte to the solid lattice of composition HxWO3
(F is Faradays constant). Free energy is a function of the chemical potential of the
51
reactants and products, so Crandall et al used electrical measurements to obtain
information about the free energy changes upon ion insertion. Information about the free
energy change was then used to ascertain the dependence of the chemical potential on the
composition of the HxWO3 film.
Crandall et al constructed a simplified expression for the free energy of HxWO3, by
grouping the possible contributions by their dependence on the compositional parameter,
x. The free energy of formation of pure WO3 (G0) was assumed to be independent of
x. Terms with a linear dependence on x arise from the free energy changes of reducing
x moles of W6+ to W5+, and the associated changes in interactions between W5+-O and
H-O pairs. Interactions between pairs such as W5+- W5+, W5+- OH-, OH-OH- were
assumed to have a quadratic dependence on x, and contributions to free energy of a
higher order in x were neglected. Finally the energy of distribution (Gd) was included
to describe the entropy changes which occur when x moles of hydrogen is distributed in
one mole of WO3, and all the terms were combined to give the the theoretical expression
for the molar free energy WO3 as
dGBxAxGG +++= 20 (1.5)
where A and B are constants. The chemical potential of hydrogen in HxWO3 (µH) was
then determined by partially differentiating equation (3.5) with respect to x to give
−
++=x
xnRTBxAH 1ln2µ (1.6)
where A and B are constants and n is a factor which describes the spatial correlation of
the injected proton and its associated electron. An expression for emf was also given as
52
−
−+=x
xF
nRTbxaemf1
ln (1.7)
where a combines all constant terms and F
Bb 2−= .
Crandall et al measured the variation in emf for compositions ranging from x=0.002 to
x=0.5 and were able to use equation (2.7) to successfully simulate the experimental data.
In 1980 Faughnan and Crandall [30] outlined the possible mechanisms that may limit the
dynamics of the coloration process as:
1) transport of the electrons and cations through the bulk of the WO3 film
2) a barrier at the TCO/WO3 interface
3) a barrier at the electrolyte/WO3 interface
4) a barrier at the counter electrode
5) charge transport in the electrolyte
Faughnan and Crandall assumed that mechanism 2) is not significant in real systems
because the TCO and WO3 are in electrical contact, and mechanism 5) may also be
neglected if we choose an electrolyte of high conductivity. Mechanism 5) may also be
neglected if we choose a suitable counter electrode material so mechanisms 1) and 3)
may be considered the most likely rate limiting mechanisms for electrochromic reactions.
Mechanism 1) describes the diffusional motion of ions and electrons within the EC film.
Ions are inserted at the WO3/electrolyte interface at vacant interstitial positions, and
hence migrate deeper into the film if more ions are to be intercalated. The rate of this
process depends on how fast the ions may diffuse through the film and this will also
53
depend on temperature. Increases in the thermal energy of mobile ions as well as the host
lattice expansion will allow ionic diffusion to occur more readily at higher temperature.
Diffusion is often considered to be one of the most important rate limiting mechanism
because this behaviour is frequently observed in experimental results [26,28,47,48,49].
Mechanism 3) describes the charge transfer process across the electrode/electrolyte
interface. The applied electrical potential provides energy to assist the ions in the
electrolyte to lose their solvation sheaths and combine with an electron inside the host
lattice, so the greater the electrical potential, the faster this step will be [14].
In 1980 Reichman et al [47] proposed a digital simulation model for the electrical current
during electrochromic insertion of protons in WO3 films. Reichman et al assumed that
the current was limited by charge transfer across the electrolyte/electrode interface,
combined with the diffusive motion of protons within the film and accumulation of
protons inside the film approaching some saturation level. The model was used to
simulate the current-voltage and current-time relationships observed experimentally for
anodically deposited and evaporated WO3 films. Good fits to the experimental data were
obtained for longer time periods of up to 60 seconds, but fits were poor for very short
times(t~5s). The model was also used to successfully predict the dependence of the i-V
characteristics of cyclic voltammetry on the scan rate. The modelling procedure involved
estimating a charge transfer rate constant, the diffusion coefficient of hydrogen atoms
inside the film and it was also necessary to experimentally measure the electrochemical
isotherm in the modelling process. Hydrogen diffusion coefficients estimated from the
54
modelling process ranged from 1x10-9 - 2x10-10cm2/s for evaporated films to
5x10-8cm2/s for anodic films.
Nagai and Kamimori [48] reported an improved kinetic model for WO3 electrochromism
in 1983, which explained the results of emf , chronoamperometry, voltammetry and AC
impedance measurements. The model assumed that the overall impedance of the cell and
the diffusion of atoms inside the film limit the current flowing during coloration or
bleaching. Nagai and Kamimori used the following formula for emf as a function of x (in
LixWO3)
−
−+=x
xFRTbaxemf
1ln)( ν (1.8)
where a, b, and ν are all constants which are used to fit experimental data to the model.
Equation (2.8) simulated the experimental data for emf (x) very well using a = -0.66V, b
= -0.87V and ν = 5.76, when the film was cycled in 1M LiClO4/PC electrolyte.
Nagai and Kamimori carried out chronoamperometric experiments and found that the
current was nearly constant for short time periods (t<0.5s) immediately after the start of
coloration and bleaching. This current was described by the equation
RxemfEi app /))(( −= (1.9)
The resistance in equation (2.9) was determined from impedance measurements to be the
sum of the resistances of the solution, electrode, charge transfer and mass transfer. The
authors assumed that charge transfer occurred at the electrolyte/ WO3 interface and
estimated a diffusion coefficient of 5x10-10cm2/s. Nagai and Kamimori concluded that
55
the cell impedance was a combination of the series cell resistance (~100Ωcm2) and a
capacitance (~104-105µF/cm2), and therefore viewed the electrical characteristics of the
EC system as being similar to the charging and discharging of a capacitor in series with a
resistor. This analogy with electrical components explained the initial dependence of
current on R and the dependence of rate on the cells time constant and diffusion
coefficient.
Raistrick and Huggins [50] reported a model in 1982 which described the transient
electrical response of solid-solution electrodes such as EC WO3 films. The authors
theoretically calculated an electrode impedance by using Laplace transforms to solve
diffusion equations for solid solution electrodes and then used this impedance to
determine the response of the system to an applied voltage or current. The model is very
useful because it can be used to model electrical characteristics for either constant current
or voltage switching, but it does not consider the effect of electrode emf on switching
dynamics.
Bohnke et al [49] reported a model in 1992 that described the way in which the
colouration current for WO3 films is limited by the chemical potential of the HxWO3 film,
the applied overpotential and a series resistance. They observed that current was
proportional to voltage for low levels of charge injection and hypothesised that the
current was limited by a resistance, rather than being limited purely by the charge transfer
process. Vuillemin and Bohnke attributed this resistance to the conductivities of the
electrolyte, the HxWO3 bronze and substrate, and a charge transfer resistance. Impedance
56
spectroscopy was used to measure this resistance experimentally for various WO3 films.
The charge transfer resistance was observed to decrease at the beginning of insertion for
very small time periods (<100ms). Vuillemin and Bohnke [28] later reported the use of
this model to simulate experimental measurements of current versus time, and estimated a
diffusion coefficient of 5x10-10cm2/s for lithium ions in WO3 film.
In 1993 Zhang et al [25] improved on the work of Raistrick and Huggins [50] by
extending the previous model to describe the effects of the cell series resistance, the
lithium diffusion impedance and the emf of the film. The authors reported that each of
these factors may limit the current under certain experimental conditions, such as the film
thickness, applied potential, etc. The model described by Zhang et al therefore
encompassed the previous models and under certain conditions, may be mathematically
reduced to the forms reported previously. A limitation of this model is that it assumed
the semi-infinite approximation (ie for t << L2/D where t is time, L is film thickness and
D is the average diffusion coefficient), which means that it is only applicable for
relatively thick films (L > ~100nm). Zhang et als model describes the switching current
during coloration by a constant voltage step and hence is of limited use in describing EC
processes under constant current switching.
Wang [27] improved on the modelling work of Raistrick and Huggins [50] and Zhang et
al [25,26] in 1998 by adapting it to describe the voltage response to constant current
charge injection and extraction. The model reported by Wang also did not assume the
semi-infinite approximation, and hence is applicable for relatively thin films (~100 nm).
57
Wangs model for the voltage during the coloration process is described by equation
(2.10)
−
−++=),0(1
),0(ln),0('.)(ty
tyFRTtybaRitV ccaν (1.10)
where y(0,t) is the stoichiometric coefficient of lithium (in LiyWO3) at the
electrode/electrolyte boundary (ie y(0,t) = Vm.c(0,t) where Vm is the molar volume of the
film and c(0,t) is the surface lithium concentration per unit volume).
−=
dydEbb'
where dE/dy is the slope of the coulometric titration curve, and a, b, and ν are the same
constants as in Nagai and Kamimoris model (see above). Rc is the series resistance
during coloration and ic is the colouring current. This model requires the determination
of the surface concentration of lithium ions, and Wang calculated this using equation
(2.11)
Γ=DnF
jtc 2),0( (0.11(a))
and ππt
Dtlkt
Dtklerfc
Dkl
kk
−−−=Γ ∑∑∞
=
∞
= 1
22
1
)][exp(2]2[ (2.11(b))
where n is the number of electrons in the process, l is film thickness, j is current density,
F is Faradays constant and D is the chemical diffusion coefficient of lithium ions. The
diffusion coefficient must be estimated in order to determine the surface concentration of
lithium ions, and then calculate the voltage. Information regarding the mobility of
lithium ions inside the WO3 film may then be gained from the simulation process. Wang
used this model to simulate the voltage response of sol-gel deposited electrochromic WO3
58
films, coloured under various current densities. Although the model described the data
very well, it was only tested at room temperature.
All of the modelling work on EC systems has involved making at least one assumption
about the identity of the rate limiting mechanisms and attempting to validate the model
by demonstrating its ability to simulate experimental data. Analysis of the literature
presented above reveals that there is still much ambiguity regarding the rate limiting
process. If a model is to be of some use, it must accurately describe the cycling
characteristics over some useful range of conditions such as injected charge density,
voltage and temperature. To date no models have been used to describe the
electrochemical and optical properties of electrochromics over a wide temperature range.
The aim of this PhD research is to validate the modelling work of Wang [27] by
simulating voltage responses at elevated temperatures. This should allow the estimation
of diffusion coefficients at various temperatures, and therefore provide an insight into the
underlying kinetic processes that are occurring at high temperature.
59
2.6 Summary
It is evident from the literature reviewed above, that the electrical characteristics of
electrochromic films and devices are very complex and depend on many factors.
Faughnan et al [13] found in 1977 that evaporated WO3 films prepared by similar
techniques had different colouring and bleaching response times. Other papers have
reported that films prepared by different techniques (eg. evaporation, sputtering,
electrolytic) have large differences in response times and electrochemical characteristics
[51,52]. Key issues affecting the EC characteristics of various systems have been
identified as the presence of water and film porosity [37-52]. It is also evident that the
rate limiting mechanisms will depend on factors including film composition, applied
voltage or current, film thickness, etc[26].
At elevated temperatures the switching characteristics of electrochromics change
considerably. Although there is little published material regarding the high temperature
behaviour of electrochromics, it is evident that switching occurs more readily at high
temperature. An in-depth knowledge of the dependence of switching characteristics on
temperature is required in order to choose a suitable control strategy which will ensure
maximum device lifetime.
REFERENCES
[1] M.E. Straumanis, J. Am. Chem. Soc., 71, 679-683 (1949).
[2] M.E. Straumanis and A. Dravnieks, J. Am. Chem. Soc., 71, 683-687 (1949).
[3] A.S. Ribnick, B. Post and E. Banks, Advances in Chemistry Series, No. 139, American Chemical Society (1963).
60
[4] H.R. Shanks, P.H. Sidles and G.C. Danielson, in Electrical Properties of the Tungsten Bronzes, Advances in Chemistry Series, No. 139, American Chemical Society, (1963)
[5] M.J. Sienko, in Non-Stoichiometric Compounds, Advances in Chemistry, Series No. 139, American Chemical Society, (1963).
[6] B.O. Loopstra and P. Boldrini, Acta Cryst., 21, 158-162 (1966).
[7] S.K. Deb, Appl. Optics, Suppl. 3 on Electrophotography, 192-195 (1969).
[8] J.M. Berak and M.J. Sienko, J. Solid State Chem., 2, 109-133 (1970).
[9] R.S. McEwen, J. Phys. Chem., 75, 1782-1789 (1971).
[10] S.K. Deb, Phil. Mag., 27, 801-822 (1973).
[11] D.W. Lynch, R. Rosei, J.H. Weaver and C.G. Olson, J. Solid State Chem., 8, 242-252 (1973).
[12] B.W. Faughnan, R.S. Crandall and P.M. Heyman, R. C. A. Rev., 36, 177-197 (1975).
[13] B.W. Faughnan, R.S. Crandall and M.A. Lampert, Appl. Phys. Lett., 27, 275-277 (1975).
[14] R.S. Crandall and B.W. Faughnan, Appl. Phys. Lett., 28, 95-97 (1976).
[15] S.K. Deb, Proc. Roy. Soc. A, 304, 211-231 (1968).
[16] B.W. Faughnan and R.S. Crandall, Appl. Phys. Lett., 31, 834-836 (1977).
[17] S. Hackwood, G. Beni, W.C. Dautremont-Smith, L.M. Schiavone and J.L. Shay, Appl. Phys. Lett., 37, 965-967 (1980).
[18] H.-T. Zhang, P. Subramanian, O. Fussa-Rydel, J.C. Bebel and J.T. Hupp, Solar Energy Materials and Solar Cells, 25, 315-325 (1992).
[19] P.F. Gerhardinger and R.J. McCurdy, MRS, 426, 503 (1996).
[20] M. Green and K.S. Kang, Thin Solid Films, 40, L19-L21 (1977).
[21] W.C. Dautremont-Smith, M. Green and K.S. Kang, Electrochim. Acta, 22, 751-759 (1977).
[22] C.M. Lampert, Solar Energy Materials, 11, 1-27 (1984).
[23] S.K. Deb, Solar Energy Materials and Solar Cells, 25, 327-338 (1992).
[24] J.M. Bell, I.L. Skryabin and G. Vogelmann, Proceedings of the 3rd Symposium on Electrochromic Materials, International Electrochemical Society, 96-24, 396 (1996).
61
[25] J.-G. Zhang, C. Edwin Tracy, D.K. Benson and S.K. Deb, J. Mater. Res., 8, 2649-2656 (1993).
[26] J.-G. Zhang, D.K. Benson, C. Edwin Tracy and S.K. Deb, J. Mater. Res., 8, 2657-2667 (1993).
[27] J. Wang, PhD Thesis, University of Technology, Sydney (1998).
[28]. B. Vuillemin and O. Bohnke, Solid State Ionics, 68, 257-267 (1994).
[29] C.G.Granqvist, Solar Energy Materials and Solar Cells, 32, 369 (1994).
[30] B.W. Faughnan and R.S. Crandall, Electrochromic Displays Based on WO3in Topics in Applied Physics, Display Devices (Edited by J. I. Pankove), Vol. 40, Springer-Verlag, New York (1980).
[31] T.C. Arnoldussen, J. Electrochem. Soc., 128, 119 (1981).
[32] S. Hashimoto and H. Matsuoka, J. Electrochem. Soc., 138(8), 2403-2408 (1991).
[33] J.S.E.M. Svensson and C.G. Granqvist, Solar Energy Materials, 11, 29 (1984).
[34] M.E. Badding, S.C. Schulz, L.A. Michalski and R. Budziak, Electrochemical Society Proceedings, 96-24, 369-384 (1996).
[35] C.M. Lampert, A. Agrawal, C. Baertlien and J. Nagai, Solar energy Materials and Solar Cells, 56, 449-463 (1999).
[36] G. Tulloch, I. Skryabin, G. Evans and J. Bell, Proceedings of the SPIE, Vol 3136, 426-432 (1997).
[37] J.P. Randin, J. Electron. Mater., 7, 47-63 (1978).
[38] D.L. Kepert, in Progress in Inorganic Chemistry (Edited by F.A. Cotton), Vol. 4, Interscience, New York, 199-274(1962).
[39] Q. Zhong, S.A. Wessel, B. Heinrich and K. Colbow, Solar Energy Materials, 20, 289-296 (1990).
[40] S. Hashimoto, H. Matsuoka, H. Kagechika, M. Susa and K.S. Goto, ibid., 137, 1300 (1990).
[41] J.A. Duffy, M.D. Ingram and P.M.S. Monk, Solid State Ionics, 58, 109-114(1992).
[42] J.-G. Zhang, D.K. Benson, C.Edwin Tracy, J. Webb and S. Deb, Proceedings of the SPIE, Vol 2017, 104-112 (1993).
[43] M. Burdis and J.R.Siddle, Thin Solid Films, 237, 320-325 (1993).
[44] P.W. Atkins, Physical Chemistry, Chapter 18, Oxford University Press, London (1978).
62
[45] F.M. Michalak and J.R. Owen, Solid State Ionics, 86-88, 965-970 (1996).
[46] R.S. Crandall, P.J. Wojtowicz and B.W. Faughnan, Solid State Comm., 18, 1409-1411 (1976).
[47] B. Reichman, A.J. Bard and D. Laser, J. Electrochem.Soc., 127(3), 647-654 (1980).
[48] J. Nagai and T. Kamimori, Jap. J. Appl. Phys., 22, 681-687 (1983).
[49] O. Bohnke, M. Rezrazi, B. Vuillemin, C. Bohnke and P.A. Gillet, Solar Energy Materials and Solar Cells, 25, 361-374 (1992).
[50 I.D. Raistrick and R.A. Huggins, Solid State Ionics, 7, 213-218 (1982).
[51] H.R. Zeller and H.U. Beyeler, Appl. Phys., 13, 231-237 (1977).
[52] B. Reichman and A.J. Bard, J. Electrochem. Soc., 126, 583- (1979).
CHAPTER 3
EFFECT OF TEMPERATURE ON ELECTROCHROMIC DEVICE
SWITCHING VOLTAGES.
J.P. Matthews, J.M. Bell and I.L.Skryabin
Published: Electrochimica Acta, 44, 3245-3250 (1999).
64
Contributions of Authors
This paper presents the results of experimental work carried out by J.P. Matthews, under
the supervision of J.M. Bell and I.L. Skryabin. The paper was written by J.P. Matthews
and revised by J.M. Bell before final submission of the manuscript.
65
EFFECT OF TEMPERATURE ON ELECTROCHROMIC DEVICE
SWITCHING VOLTAGES.
J.P. Matthews1, *, J.M. Bell1 and I.L.Skryabin2
1 School of Mechanical, Manufacturing and Medical Engineering,
Queensland University of Technology, Australia
2 Sustainable Technologies Australia,
11 Aurora Ave, Queanbeyan, NSW, Australia
* Author to whom correspondence should be addressed.
Abstract
Excessive switching voltages in electrochromic devices cause rapid degradation in
performance. Optimisation of switching voltages is therefore critical in order to realise
the maximum possible device lifetime, and to produce a commercially reliable product.
The magnitude of the voltages required to colour and bleach a device are temperature
dependent, with lower voltages being required at higher temperatures. In real
applications, electrochromic devices may attain temperatures as high as 70°C. Use of
the room temperature switching regime at elevated temperatures may impose an
overvoltage on the device, which can significantly reduce both lifetime and optical
performance.
A voltage limited constant current charge injection technique was used to cycle sol-gel
66
deposited WO3 films at elevated temperatures. The voltages required for colouring and
bleaching at these temperatures were determined and correlated with the level of
coloration achieved. The results show that the variation in switching voltages is
significant, and therefore inclusion of temperature in the switching algorithms is
necessary to achieve maximum lifetime for electrochromic devices.
Keywords: Electrochromic thin films; switchable glazing; temperature effects; lifetime;
switching voltages
3.1 Introduction
The lifetime of electrochromic (EC) devices is an issue of critical importance, and is a
parameter that must be maximised in order for smart windows to become
commercially viable [1]. The useful lifetime of EC devices depends on many factors
including device composition, switching algorithm, current density during switching, the
level of charge injected and extracted and operating temperature. The use of excessive
switching voltages provides an overpotential which enables potentially damaging side
reactions to occur in EC devices [2]. The choice of safe switching voltages is
therefore paramount for successful device operation over a timescale of several decades.
Electrochromic devices based on the LixWO3 system absorb a significant proportion of
incoming radiation in the coloured state, so it is foreseeable that these devices will attain
temperatures in excess of 60°C. The effect of temperature on EC device switching
voltages is complex, and published literature on this topic is scarce.
67
This paper reports initial findings of a study aimed at determining the effects of
temperature on electrochromic device switching voltages, with the long term goal of
optimising the switching regime to achieve maximum device lifetime. The films and
devices reported in this study have been cycled under a voltage limited constant-current
charge injection method [3]. The measured or applied voltage during coloration, Va(t),
can be simply described by the equation [4]
Va(t) = emf(t) + Vc(t) (3.1)
where emf is the electromotive force of the cell, and Vc(t) is the overvoltage associated
with the ion insertion. The effect of temperature on the applied voltage Va, therefore
depends on the individual responses of both the electrochromic film emf and colouring
voltage Vc to temperature. Equation 3.1 somewhat simplifies the behaviour of real cells
because it does not consider other possible parameters such as the iR drop of the
electrolyte and the conducting glass substrate. The effects of temperature on these
parameters in experimental situations is however small enough for the model to be of
use, in terms of describing the change in switching potentials with temperature.
Experiments were carried out in order to evaluate these responses. The maximum
colouring voltages required for a specific level of coloration were measured over an
extended temperature range, and the emf of the film was also determined for the fully
coloured state. This has enabled the observation of trends in the emf and colouring
voltage Vc with temperature, and the implications of the results are discussed in the
context of a switching method which enables maximum lifetime to be achieved.
68
3.2 Experimental
3.2.1 Electrode preparation
The WO3 electrochromic films were deposited onto 10cm x 10cm substrates of LOF
TEC8/3 glass using the sol-gel dip coating method [5]. The alkoxide precursor solutions
used in the sol-gel dipping have been described previously [6].
3.2.2 Electrochemical Testing
Electrochemical measurements were made using a three electrode cell. The counter
electrode was a platinum foil (Area=64cm2) and the reference electrode was a Ag/AgCl
cell filled with an ethanolic solution of KCL, saturated with AgCl. The electrolyte used
was 1M LiClO4 in propylene carbonate. The electrolyte solution was dried over
molecular sieves and maintained under a nitrogen atmosphere. Experiments were
carried out in a glass tank (filled with electrolyte solution), which was partly submersed
in a larger glass tank of heating oil. An electrical heater/stirrer unit was used to control
the temperature.
The WO3 films were cycled using a voltage-limited constant current technique,
described previously [3,7]. The experiments were carried out using the convention that
coloration was due to positive currents, hence the corresponding electrical potentials
were also positive. The currents and potentials measured during bleaching are therefore
negative. This convention has been used throughout this paper. Under this convention,
the emf values reported for coloured films are therefore positive, and the emf increases
69
with increasing coloration, and hence with increasing x.
The results reported in sections 1 and 2 are for a film cycled to 15mC/cm2 (Current
density = 0.05mC/cm2, film area = 100cm2). The temperature was ramped from 20° to
50°C at approximately 1°C per cycle. Measurements were performed as the temperature
increased, but were not made as the system cooled.
The results reported in section 3 are for a film cycled to 5mC/cm2 (Current
density = 0.05mC/cm2, film area = 100cm2). The temperature was ramped from 20° to
50°C at approximately 1°C per cycle. Measurements were performed during both
increasing and decreasing temperature.
3.3 Results and discussion
3.3.1 Effect of temperature on applied voltage, Va(t)
Figure 3.1 shows a graph of applied voltage, Va(t), versus time for a sol-gel deposited
WO3 film, cycled at four different temperatures, with a charge injection level of Qin/out =
15mC/cm2. The coloration and bleaching were carried out using a voltage limited
constant-current technique as described previously [3, 7].
70
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500
20.6oC
28.4oC32.3oC
50.0oC
E/V
t/s
Figure 3.1 Curves of applied voltage versus time, measured during coloration and
bleaching of WO3 thin film electrode and plotted for four temperatures, for an injected
charge density of 15mC/cm2.
During coloration the voltage limit was not reached, hence the maximum applied
colouring voltage is readily observed. During the bleaching process the voltage limit (-
0.1V) was reached every cycle, and therefore the true minimum voltage reached during
the bleaching process was not observed. The bleaching data must therefore be discussed
somewhat qualitatively, based on the relative curves of Va(t) for each temperature.
It is evident from Figure 3.1 that the magnitude of the applied voltage decreases as
temperature is increased, and this behaviour is observed throughout both colouring and
bleaching stages. This behaviour is expected, considering the effects of temperature on
diffusion and charge transfer processes. The diffusion coefficients will increase with
71
increasing temperature so that charge is transported more readily within the WO3
electrode, aiding the colouring and bleaching processes. The extra thermal energy at
elevated temperatures also helps overcome the charge transfer activation energy, and
thus reduces the voltages required for coloration and bleaching [8]. Given these results,
it is possible that higher current densities may be used to switch these devices at higher
temperatures without adverse effects, thereby enabling faster response times.
Figure 3.2 shows the variation in applied voltage, Va(t), with temperature, and shows
that variation in switching voltage is greater for lower temperatures. The consequence
of this is that small deviations from room temperature will significantly alter the
electrical characteristics of electrochromic films and devices.
0.95
1
1.05
1.1
1.15
1.2
20 25 30 35 40 45 50 55
E/V
T/oC
Figure 3.2 Maximum voltages required to colour WO3 thin film electrode to 15mC/cm2
at elevated temperatures.
72
The implications of the effect of elevated temperatures on film and device cycling are
dependent on the switching methods used, and constant voltage cycling is likely to be
affected most by these results. Constant voltage coloration of films at various
temperatures while using the same voltage limits, would result in higher injected charge
densities at higher temperatures (and hence progressively darker films), with the
possibility of exceeding the reversible limit of x=0.4 in LixWO3 [4]. The consistent
application of potentials exceeding those necessary to colour the film will also promote
gas generation and film decomposition reactions and undoubtedly decrease the useful
lifetime of the film.
Constant current cycling on the other hand is less sensitive to the effects of temperature,
as the charge injected is measured (by integration of current with respect to time),
during the cycle and hence the same amount of charge is injected and extracted each
cycle. The same level of coloration should therefore be attained at all temperatures (see
section 3.3.2).
The applied or measured voltage, Va(t), can be written as the sum of the electromotive
force (emf) and the voltage associated with the ion insertion (see equation 3.1 above).
Temperature is a variable in Nernstian expressions for the emf of LixWO3 films so this
is one source of the voltage variation. The effects of temperature on both the colouring
voltage and the emf are considered below.
73
3.3.2 Effect of temperature on colouring efficiency
Figure 3.3 shows the relative transmittance (as measured by a photocell) versus injected
charge for cycles at four different temperatures. The response is essentially the same at
all four temperatures, indicating that the same amount of charge has been injected and
extracted each cycle, as we anticipate when using the constant current charge injection
technique. (Note that the curves have each been offset by 0.5V for visual clarity).
0
1
2
3
4
5
6
0 2 4 6 8 10 12 14 16
20.6oC
28.4oC32.3oC
50.0oC
E/V
Q/mCcm-2
Figure 3.3 Photocell voltage versus injected charge for WO3 thin film electrode at four
temperatures.
74
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 2 4 6 8 10 12 14 16
20.6oC
28.4oC
32.3oC
50.0oC
OD
∆
Q/mCcm-2
Figure 3.4 Change in optical density versus injected charge for WO3 thin film electrode
at four temperatures.
The change in optical density, ∆OD, has also been plotted against injected charge, in
Figure 3.4. The slope of these lines defines a useful characterisation parameter called
the coloration efficiency [9] for each of the curves shown. The coloration efficiency
appears to be approximately constant at the lower temperatures, (CE=43.9cm2/C,
R2=0.93) but decreases slightly to CE=38.1C/cm2, (R2=0.98) at 50°C (These values
were determined from simple linear regresion of the data for both the coloration and
bleaching processes). It is unclear at this time, whether this difference is due to a
change in optical response of the tungsten oxide film, or is due to errors in the optical
measurements introduced at high temperatures.
A particular problem associated with high temperature experiments is the changing
75
refractive indices of components between the laser and the optical detector, as this is a
single beam measurement. As the temperature rises, the refractive indices of the heating
oil, glass tanks, and LiClO4/PC electrolyte all increase. If the light transmission path is
even slightly off normal incidence, the laser beam will deviate as the temperature rises,
which would introduce a systematic change in photocell response with temperature.
This problem can be corrected by ensuring that the laser beam is normally incident on
all phase boundaries (heating tank walls, WO3 and counter electrode film surfaces, etc).
However this is experimentally difficult and was not ensured at the time of this work.
A further series of experiments is being carried out to determine firstly the normal
photocell response with respect to temperature, and then to establish whether indeed the
coloration efficiency does change with temperature (See section 4.3.1). If the coloration
efficiency is found to decrease with temperature, it may be necessary to modify the
switching to vary the charge injection level to compensate for changing coloration
efficiency at different temperatures.
3.3.3 Effect of temperature on coloured state electromotive force, emfc, and maximum
colouring voltage, Vc max.
Figure 3.5 shows the temperature dependence of the maximum applied voltage
(Va max) and coloured film emf (emfc). The emf was measured by disconnecting the film
and measuring the potential after sixty seconds had elapsed, thereby allowing time for
the system to equilibrate. The maximum colouring voltage was also calculated using
equation (3.1). The experiment was carried out by cycling a film to 5mC/cm2 while
76
slowly ramping the temperature from 22 to 50°C, and then allowing the system to cool.
The lower part of the curves correspond to increasing temperature, and the upper parts
were recorded as the system cooled.
0
0.1
0.2
0.3
0.4
0.5
0.6
20 25 30 35 40 45 50 55
emfc
Va max
E/V
T/oC
Increasing T
Decreasing T
Figure 3.5 Maximum voltages required for coloration of WO3 thin film electrode to
5mC/cm2, and corresponding emf values measured between 20°C and 50°C.
The hysteresis associated with the data in Figure 3.5 is believed to be due to charge
trapping (incomplete charge extraction) during cycling. If the amount of charge
extracted is less than the amount injected per cycle, lithium will accumulate in the film.
This will have the effect of gradually increasing the value of x corresponding to the
coloured state, and hence increasing the films emf. The emf of a LixWO3 film can be
described by the equation [10]
−
++=x
xFRTbxaxemf
1ln)( ν (3.2)
77
where a, b and ν are parameters which can be determined by fitting to experimental
data, and x (in LixWO3) is calculated from the amount of charge injected, the molar
volume and film thickness. If the intercalated charge of the coloured film was the same
each cycle (ie. constant x, no charge trapping), we would expect a graph of emf versus
temperature to yield a straight line with negative slope, proportional to (νR/F), with no
hysteresis. If we assume some fraction of the injected charge to remain in the films after
bleaching, we expect the emf to steadily increase. The difference between the emf
values for increasing and decreasing temperature is therefore attributed to the amount of
charge trapped in the film during the intervening cycles. The hypothesis of charge
trapping is supported by the optical data, which shows a slightly decreasing
transmittance for both coloured and bleached states as the experiment progresses. The
level of charge trapped between the first and last cycle was determined (from the optical
and electrical results) to be 1.25mC/cm2.
The relationship between applied voltage and emf is described by equation (3.1), where
the applied or measured voltage is the sum of the film emf and colouring voltage at any
given time. The applied voltage therefore incorporates the emf, so it is not surprising to
observe the same hysteresis in both graphs of Figure 3.5. When the maximum colouring
voltage, Vc max, is calculated from equation 3.1 and plotted against temperature (Figure
3.6) the hysteresis effect is removed, and the maximum colouring voltages are
essentially the same for both increasing and decreasing temperature, within the error of
the experimental noise. This fact supports the hypothesis that the hysteresis seen in
Figure 3.5 is purely associated with the film emf.
78
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
20 25 30 35 40 45 50 55
Vc max
E/V
T/oC
Figure 3.6 WO3 film maximum coloration voltage versus temperature, for an injected
charge density of 5mC/cm2.
If charge is indeed being trapped in the films, the emf will steadily increase each cycle
and so the maximum applied voltage Va will soon approach the safe colouring voltage
limit. A reduction in the amount of trapped charge is therefore critical for a long
lifetime, and must be minimised by adjusting cycling conditions (eg. injected charge
density or current density), or other film characteristics.
The data for the maximum colouring voltage Vc max as a function of temperature can be
linearised by plotting log Vc max versus 1/T. This graph is shown in Figure 3.7, together
with the equation for the line of best fit. It is foreseeable that this linear relationship
may be used to determine kinetic information regarding the ion injection process (eg.
diffusion coefficients, charge transfer resistance). This information is also useful in
79
determining the required voltage to achieve a particular state of coloration, given the
emf of the film. If the effect of temperature on the emf of the film can be successfully
modelled, this can be incorporated with the temperature dependence of the coloration
voltage in order to predict safe switching voltages over a wide range of operating
temperatures.
-0.95
-0.9
-0.85
-0.8
-0.75
-0.7
-0.65
-0.6
0.003 0.0031 0.0032 0.0033 0.0034 0.0035
log Vc
E/V
T-1/oC-1
y = 771.59 x - 3.3194R2 = 0.8916
Figure 3.7 Log of coloration voltage versus reciprocal temperature for WO3 film with
an injected charge density of 5mC/cm2.
80
3.4 Conclusion
The magnitudes of the voltages required to colour and bleach electrochromic thin films
of tungsten oxide decrease with increasing temperature. The response of the applied
voltage to temperature was dependent on the individual responses of electromotive force
(emf) and colouring voltage. The emf of the coloured state of these films decreased
with increasing temperature, however the observation of this effect was hindered by
charge trapping during repeated cycling. This charge trapping resulted in a hysteresis
effect for the emf data recorded during increasing and then decreasing temperature. The
maximum colouring voltage was found to decrease with increasing temperature, and this
relationship was described by a linear plot of logVc vs 1/T.
The voltage limited constant current charge injection method used, resulted in the same
level of charge injection and extraction into the WO3 film at high temperatures. This
allowed the WO3 films to be coloured to approximately the same optical density each
cycle over a wide temperature range, even though the voltages required for coloration
decreased with increasing temperature.
Knowledge of the emfc and Vc response to temperature enables one to predict the
maximum applied voltages required to colour electrochromic films under specific sets of
conditions. If these voltages are exceeded, the extra potential provided will promote
side reactions and reduce device lifetime. An understanding of the temperature
dependence of electrochromic device switching voltages can therefore be used to
determine safe switching conditions.
81
Planned future research into this behaviour focuses largely on the modelling of the
electrical characteristics of tungsten oxide EC films and devices as a function of
temperature, and the eventual utilisation of the model as a tool for optimising the
switching algorithm to achieve the maximum possible device lifetime.
Acknowledgements
This work is supported by an Australian Postgraduate Award (Industry) scholarship
from the Australian Research Council, and Sustainable Technologies Australia (STA).
82
REFERENCES [1] C. M. Lampert, IEEE Circuits and Devices, 8, 19-26 (1992).
[2] B.W. Faughnan and R.S. Crandall, Electrochromic Displays Based on WO3in
Topics in Applied Physics, Display Devices (Edited by J. I. Pankove), Vol. 40, Springer-
Verlag, New York (1980).
[3] J. M. Bell and I. L. Skryabin, Solar Energy Materials and Solar Cells, 56 (1999)
437.
[4] J.-G. Zhang, D.K. Benson, C. Edwin Tracy and S.K. Deb, J. Mater. Res., 8, 2657-
2667 (1993).
[5] J. M. Bell, G. B. Smith, I. L. Skryabin, B. G. Monsma, N. C. Ruck and T. Dinh,
Sol-gel Deposited Electrochromic Devices, in Proceedings of Windows Innovations
Conference WIC 95, 383-391, Minister of Supply and Services, Canada.
[6] A. Koplik, Australian Patent Application, PP0274 (1997).
[7] I. L. Skryabin and J. M. Bell, Control of Electrochromic Devices, International
Patent Application PCT/AU97/00697.
[8] A. J. Bard and L. R. Faulkner, in Electrochemical Methods: Fundamentals and
Applications, Wiley, New York (1980).
[9] C.M. Lampert, V-V. Truong, J. Nagai and M.G. Hutchins, in Characterization
Parameters and Test Methods for Electrochromic Devices in Glazing Applications,
International Energy Agency Task X-C Final Report, University of California (1994).
[10] J. Nagai and T. Kamimori, Jap. J. Appl. Phys., 22, 681-687 (1983).
CHAPTER 4
HIGH TEMPERATURE BEHAVIOUR OF ELECTROCHROMICS
J.P. Matthews, J.M. Bell and I.L.Skryabin
Published: Renewables: The Energy for the 21st Century
Proceedings of the World Renewable Energy Congress VI, Brighton, UK (A.A.M.
Sayigh Ed.), 230-235 (2000).
84
Contributions of Authors
This paper presents the results of experimental work carried out by J.P. Matthews, under
the supervision of J.M. Bell and I.L. Skryabin. The paper was written by J.P. Matthews
and revised by J.M. Bell before final submission of the manuscript. This paper was
presented by John Bell as an invited paper at the World Renewable Energy Congress VI,
Brighton, UK, 2000.
85
HIGH TEMPERATURE BEHAVIOUR OF ELECTROCHROMICS
J.P. Matthews1, J.M. Bell1 * and I.L.Skryabin2
1 Research Concentration in Materials Technology,
School of Mechanical, Manufacturing and Medical Engineering,
Queensland University of Technology, Australia
2 Sustainable Technologies, Australia,
11 Aurora Ave, Queanbeyan, NSW, Australia
* Author to whom correspondence should be addressed.
Abstract
Sol-gel deposited electrochromic films have been cycled at elevated temperatures under
various environmental conditions. Significant irreversibility was observed during
cycling of films when moisture was present in the electrolyte, especially at high
temperatures. A proportion of the injected charge did not cause colouration under these
conditions, which caused an apparent decrease in colouration efficiency at high
temperatures. An experiment was carried out which enabled the observation of the slow
bleaching of these films in an electrolyte solution, even though the working electrode
was electrically isolated from the external circuit. This self-bleaching was associated
with irreversible charge injection under conditions where moisture was present. Films
cycled under very dry conditions exhibited very reversible behaviour, and the
colouration efficiency was found to be independent of temperature.
86
Keywords: Electrochromic thin films; switchable glazing; colouration efficiency; self-
bleaching; temperature effects
4.1 Introduction
Previous experiments investigating the effects of temperature on electrochromic device
switching voltages [1] have shown that the magnitude of the voltages that are required to
colour and bleach electrochromic (EC) films decreases with increasing temperature.
When the films were cycled at temperatures exceeding approximately 30ºC some
irreversibility was observed in the charge injection/extraction process. Some of the
charge injected during colouration was unable to be extracted during the bleaching
process however the optical density of the bleached film was consistent with cycles that
were totally reversible. The amount of charge unable to be extracted each cycle
increased with temperature and this trapped charge apparently did not contribute to the
colouration of the film. Although the amount of charge trapped per cycle was relatively
small, the cumulative effect over many cycles is very significant. The amount of charge
available for transfer between the working and counter electrodes of an EC device is
limited by the amount of charge incorporated during device fabrication. After device
fabrication no more charge can be introduced so a reduction in the reversibility of the
EC process limits the maximum possible change in optical density and ultimately the
device lifetime.
In order to better understand the effect of this charge trapping phenomenon, a series of
self-bleaching experiments were carried out. These experiments involved colouring of
87
EC films to a specific charge density, and disconnecting the counter electrode thereby
electrically isolating the working electrode from the external circuit. These coloured
films were observed to undergo a slow self-bleaching process and this change was
monitored by continual measurement of the electromotive force (emf) and optical
density of the working electrode.
After the self-bleaching experiment the WO3 substrate was dissolved off the glass/FTO
substrate with an alkaline solution. Chemical analysis of this solution revealed that
there was a large amount of lithium still present in the film, confirming that there
actually was lithium still inside the film, and that some of this lithium did not contribute
to colouration.
The reversibility problems encountered at high temperatures hindered simulation of the
experimental data because the amount of charge extracted varied for each cycle. In
order to try and establish reversible cycling at elevated temperatures, experiments were
repeated in a nitrogen filled dry-box, in which very low levels of humidity were
stringently maintained. It was found that the EC reaction was reversible over the entire
experimental temperature range and that the colouration efficiency was linear and
independent of temperature.
88
4.2 Experimental
4.2.1 Electrode preparation
Mixed tungsten-titanium oxide electrochromic films were deposited using sol-gel
processing from organic precursors onto 10cm × 10cm substrates of LOF TEC8/3 glass
using the sol-gel dip coating method [2]. The alkoxide precursor solutions used in the
sol-gel dipping have been described previously [3].
4.2.2 Electrochemical testing
Electrochemical measurements were made using a three electrode cell. The counter
electrode was a platinum foil (Area=64cm2) and the reference electrode was a Ag/AgCl
cell filled with an ethanolic solution of KCl, saturated with AgCl. The electrolyte used
was 1M LiClO4 in propylene carbonate, which was stored over molecular sieves after
preparation. Experiments were carried out in a glass tank (filled with electrolyte
solution) partly submersed in a larger heating tank filled with mineral oil. An electrical
heater/stirrer unit was used to control the temperature of the oil bath, and hence the
electrolyte solution. The electrolyte solution was also stirred during the experiments to
minimise the temperature difference between the heating oil and the electrolyte solution.
The WO3 films were cycled using a voltage-limited constant current technique,
described previously [4,5]. Films were cycled to 15mC/cm2 (Current density =
0.1mA/cm2, film area = 100cm2). Optical measurements were made by directing a
1mW, 670nm laser beam through the electrodes, and onto a silicon photodiode. The
89
photocell voltages reported in the results are the output voltages of the silicon
photodiode. As the film is coloured, the intensity of the laser beam reaching the
photodiode is reduced, hence photocell voltages decrease with increasing optical density
of the film.
Experiments were carried out either in the ambient environment of the laboratory, or
inside a dry-box. The experiments carried out in the ambient laboratory environment
were carried out while slowly bubbling dried nitrogen through the electrolyte solution
before and during testing to maintain a slight positive pressure and minimise the mount
of moisture in the electrolyte. During the dry-box experiments the complete
experimental apparatus including electrolyte tank, oil bath, heater unit and optical bench
remained in a nitrogen-filled dry glovebox. The atmosphere inside the glovebox was
kept dry by exposing it to P2O5 desiccant, and recirculating the nitrogen through a
column of dried molecular sieves. The humidity level inside the glovebox was
monitored with a HMP235 humidity and temperature transmitter, manufactured by
Vaisala. The humidity during the dry-box experiment was maintained at 1.05ppm
absolute humidity (Relative humidity = 5.3% and temperature = 22.4ºC immediately
prior to experiment).
The self-bleaching experiment was performed under the ambient laboratory conditions
described above. Films were coloured to 20mC/cm2 (Current density = 0.1mA /cm2,
film area = 100cm2), and the counter electrode was disconnected from the electrical
circuit immediately after completion of the coloration. The counter electrode was
90
disconnected for 30 minutes and the electrolyte solution was maintained at the
appropriate temperature during this time. At the end of the 30 minute self-bleaching
period, the counter electrode was reconnected, the film was bleached and the
temperature was increased for the next data set.
4.2.3 Chemical analysis
The amount of lithium and tungsten in a bleached film was determined, after the self-
bleaching experiment, by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES). The film was washed off the glass/FTO substrate with aqueous sodium
hydroxide, and the solution was diluted to 50mL. Tungsten and lithium stock standard
solutions were used prepared a series dilution of calibration standards. Calibration
graphs were prepared and used to determine the concentrations of lithium and tungsten
in the sample solution. The ICP-AES measurements were performed on a Spectroflame
spectrometer, manufactured by Spectro Analytical Instruments, West Germany.
91
4.3 Results and Discussion
4.3.1 Effect of temperature on coloration efficiency
Figure 4.1 shows the changing optical properties of two sol-gel deposited WO3 films
during colouration and bleaching at various temperatures. The experiment shown in
Figure 4.1(a) was carried out in ambient laboratory conditions, and nitrogen was
bubbled through the electrolyte solution in an attempt to keep water (from the air) out of
the system. The results shown in Figure 4.1(b) are for an experiment carried out in an
extremely dry environment, inside a nitrogen filled glove box. Plots of change in optical
density versus injected charge are expected to be linear for EC films and devices, and
the slope is defined as the colouration efficiency (CE) [6]. It is evident that this
behaviour is observed only in the very dry case (Figure 4.1(b)) where the plots at
different temperatures are virtually the same with an average CE of 38.6cm2/C
(R2=0.99).
Figure 4.1 Change in optical density versus injected charge for WO3 films cycled to
15mC/cm2 at elevated temperatures. The results shown in (a) are for an experiment
carried out in the ambient environment, while the results shown for (b) are for an
92
experiment carried out in a dry-box.
The colouration efficiency for the first experiment (Figure 4.1(a)) is approximately
linear for the room temperature cycle (20.6ºC), and the colouration efficiency is
determined from a linear regression to be 43.9cm2/C (R2=0.93). As temperature
increases the optical density, for each level of injected charge density, decreases
indicating that some of the injected charge is not contributing to colouration. The
reversibility of the charge injection process in this experiment was also affected by
temperature, with significant irreversibility noticeable for temperatures above 30ºC.
The film was cycled using a constant current charge injection/extraction technique (as
described in the experimental section, above), and the bleaching process was terminated
when the voltage reached some safe limit, predetermined to prevent damage to the film.
Any charge remaining in the film after bleaching was therefore unable to be extracted
without applying larger voltages which would have damaged the film. Figure 4.2 shows
the relative amount of charge unable to be extracted from the films each cycle, during
cycling at elevated temperatures, for both of the experiments discussed above.
93
Figure 4.2 Reversibility of cycling at elevated temperatures, represented as the
percentage of the injected charge density trapped per cycle.
It is evident that the ion injection process for the film cycled in ambient conditions was
much less reversible than for the film cycled in the dry-box. The relative amount of
charge not extracted each cycle in the ambient case increases with temperature, and at
50ºC approximately 3.5% of the charge injected was unable to be extracted during the
bleaching process. The reversible limit of x = 0.4 (in LixWO3) [7] was not exceeded
during these cycles so the irreversibility must be accounted to some other reaction
involving the lithium ions.
At room temperature the EC reaction of the film cycled in the dry-box was very
reversible, and the percentage of charge trapped is very close to zero, within the limits
of experimental error. As temperature increases, the amount of charge extracted
actually exceeds the level of charge injected for that cycle, which would suggest some
94
experimental errors. This may be due to the combination of the switching regime used,
and the reduction in switching voltages which occurs at elevated temperatures. Before
the experiment was carried out, the film was pre-loaded with charge. The films do not
cycle reversibly for the first few cycles, so 20 cycles were performed where it is
common for a significant proportion of the injected charge to remain in the films after
bleaching, even though the films appear to be bleached as normal.
As temperature rises, the magnitude of the voltage required to achieve a given charge
density decreases and so applying a set voltage limit for the bleaching cycle, we are
driving the bleaching process further at high temperature. In this experiment the same
voltage limit was applied to the bleaching process at all temperatures, so it is possible
that some of the pre-loaded lithium was removed at higher temperatures. The fact that
the amount of charge extracted increases with temperature for the dry-box experiment
supports this proposition. It is also possible that there is a small experimental error
associated with the measurement of the currents, such as a bias towards the
measurement of the bleaching current. An electrical calibration error of this kind would
be expected to be independent of temperature, and this would also mean that the amount
of charge trapped in the ambient conditions was even greater than that shown in Figure
4.2. The fact that there are some negative results for the charge trapped during the
drybox experiments therefore does not affect the conclusions made about the experiment
carried out in the ambient environment.
The large differences between the results observed from films cycled in ambient and
95
very dry conditions suggests that the problems associated with irreversibility and charge
not causing colouration may be ascribed to water present in the system. Although an
attempt was made to keep the ambient experiment dry by bubbling dry nitrogen
through the electrolyte, the high humidity of the experimental location (Brisbane,
Ausralia) combined with the highly hygroscopic nature of the propylene carbonate
electrolyte means that it is unlikely that there was no water present in the electrolyte.
Inside the drybox, it is relatively easy to ensure that there is very little water present and
so the presence of water is thought to be the major difference in the conditions of the
two experiments described above. In order to further investigate the cycling
irreversibility and the proportion of injected lithium not causing colouration at high
temperature, a self bleaching experiment was carried out using the ambient environment
conditions described in the experimental.
4.3.2 Observation of self-bleaching
Figure 4.3(a) shows the change in photocell voltage of the WO3 film during self-
bleaching at elevated temperatures and Figure 4.3(b) shows emf measurements taken
over the same time period. The WO3 electrode was electrically isolated at 150seconds
(after the end of colouration) and then reconnected after a 30 minute period, just prior to
the bleaching half-cycle. The increase in photocell voltage observed in Figure 4.3(a)
indicates a reduction in the optical density and suggests that the concentration of lithium
in the film is decreasing or that some of the lithium is being converted to an optically
non-active form. The drift in emf observed in Figure 4.3 indicates a changing chemical
96
potential of the film, and the drift towards more positive potentials is consistent with a
decreasing lithium concentration over time and at higher temperature. These results
suggest that the film is bleaching as per the normal EC reaction but with the counter
electrode disconnected there is no path for electron flow from the back of the working
electrode. Any lithium reaction must therefore be with some species already present in
the system which also supports the theory that water in the system is responsible for
some of the injected ions not causing colouration and for the irreversibility observed.
Figure 4.3 Change in (a) photocell voltage and (b) emf of WO3 electrode during self-
bleaching experiment.
The currents measured during colouration and bleaching were integrated with respect to
time to determine the amount of charge injected and extracted respectively. These
values were used to calculate the measured amount of charge which was trapped per
cycle. Calibration curves of photocell voltage and emf versus injected charge density
were constructed for each temperature, by interpolation of measurements made at the
highest and lowest temperatures. These calibration curves were used in conjunction
with photocell and emf values at the start and end of the self-bleaching period (ie. from
97
(a) and (b)), in order to estimate the amount of charge apparently lost during the 30
minute self-bleaching period.
These estimated values of charge loss were then correlated with the measured values
obtained from the difference between the integration of the colouration and bleaching
currents.
Figure 4.4 shows a plot of the estimates of charge lost versus the measured charge loss,
during the self-bleaching period. If the lithium ion concentration in the film was
decreasing (eg. lithium was reacting at the electrode surface to form a new species
outside the film) we would expect the plots of estimated versus measured charge loss to
be linear with a slope of one and intercept of zero.
Figure 4.4 Correlation of estimated and measured quantities of charge lost during self-
bleaching.
98
These plots are indeed linear however the slope is not one and the intercept is not zero.
The photocell measurements are an indication of the number of lithium ions contributing
to colouration, and the charge loss estimated from photocell measurements is therefore
an indicator of the amount of lithium no longer causing colouration at the end of the
self-bleaching period. The measured amount of charge remaining in the film after
bleaching at each temperature is smaller than the estimates, which suggests that some of
the charge that was extracted was not contributing to colouration of the film. For
example, at the end of the self-bleaching period at 50.5ºC, the amount of injected charge
no longer causing colouration is estimated from the photocell voltage (immediately prior
to bleaching) to be 4.7mC/cm2. The amount of charge remaining in the film after the
subsequent bleaching cycle was 4.3mC/cm2. Approximately 4.7mC/cm2 of lithium ions
therefore were not causing colouration after the half hour self bleaching period, and
0.4mC/cm2 of this was later electrochemically extracted from the film. The remaining
4.3mC/cm2 of lithium ions either stayed in the film, was lost into the electrolyte solution
to a side reaction or a combination of both.
4.3.3 Determination of trapped lithium in WO3 film by ICP-AES
In order to answer some of the questions regarding the location of lithium ions which
could not be extracted from the WO3 film by the bleaching process, a portion of a film
used in another self-bleaching experiment was subjected to further chemical analysis.
The WO3 film was washed off the glass/FTO substrate with a sodium hydroxide
solution, and then diluted to 50mL. The film area used was 55.6cm2, and inductively
99
coupled plasma-atomic emission spectroscopy (ICP-AES) was used to determine the
lithium and tungsten concentrations of this solution. The ICP-AES analysis revealed
that there was approximately 100µg of lithium and 8mg of tungsten in the 50mL
solution, which corresponds to x=0.33 in LixWO3 or an injected charge density of
approximately 15mC/cm2 (for a 200nm thick film, with molar volume of 42cm2/mol).
The total measured charge lost during this experiment was approximately 130mC/cm2, a
value clearly very much larger than the amount of ions recovered from the film.
The fact that a large proportion of the measured injected charge was not recovered
suggests that a large amount of the injected ions were either lost to side reactions or that
the ion injection process was not 100% efficient.
Considering that the charge measurement was made by integration of the electron
current with respect to time, any side reactions occurring simultaneously along with the
normal ion intercalation would contribute to the measured current and hence the
measured charge. If the measured current resulted solely from ion injection, the
remaining charge which was not recovered was presumably lost to side-reaction(s) to
form a new species, which then dissolved into the electrolyte. Another possibility is that
side reactions such as gas evolution occurred during ion injection and made a significant
contribution to the measured current, however no evidence of gas evolution was
observed during the experiment.
100
4.4 Conclusions
Sol-gel deposited EC films were cycled under various conditions and a range of
temperatures. Significant irreversibility was observed for films cycled with moisture
present, especially at high temperature. This irreversibility was associated with a
proportion of the injected charge not causing colouration, and consequently there was an
apparent reduction in colouration efficiency at high temperatures. In very dry
conditions, films cycled very reversibly and colouration efficiency was independent of
temperature.
Coloured films were observed to slowly self-bleach in an electrolyte which was not
completely dry, even though the counter electrode was disconnected. After leaving the
film in this electrolyte for 30 minutes, some of the charge remained in the film even
after the bleaching process, and this charge did not give rise to colouration.
Measurements of the photocell voltage and emf of the film during the 30 minute period
were used to estimate the amount of charge trapped during the self-bleaching period.
These estimates were compared to the measured values of charge remaining in the film
after bleaching, and a reasonable correlation was attained.
ICP-AES was used to confirm that there was actually lithium trapped in the film after
self-bleaching, but only a small proportion of the expected amount of lithium was found.
This implied that some charge was trapped in the film while a much larger proportion
was lost to a side reaction, probably reaction of lithium ions with water present in the
electrolyte.
101
Acknowledgements
This work is supported by an Australian Postgraduate Award (Industry) scholarship
from the Australian Research Council, and Sustainable Technologies Australia (STA).
The work described in this paper has been supported by the Australian Cooperative
Research Centre for Renewable Energy (ACRE). ACREs activities are funded by the
Commonwealths Cooperative Research Centres Program. We would also like to thank
Pat Stevens for his advice and assistance with the ICP-AES measurements.
REFERENCES [1] J.P. Matthews, J.M. Bell and I.L. Skryabin, Electrochimica Acta 44 (1999) 3245.
[2] J. M. Bell, G. B. Smith, I. L. Skryabin, B. G. Monsma, N. C. Ruck and T. Dinh,
Sol-gel Deposited Electrochromic Devices, in Proceedings of Windows Innovations
Conference WIC 95, 383-391, Minister of Supply and Services, Canada.
[3] A. Koplik, Australian Patent Application, PP0274 (1997).
[4] J. M. Bell and I. L. Skryabin, Solar Energy Materials and Solar Cells, 56 (1999)
437.
[5] I. L. Skryabin and J. M. Bell, Control of Electrochromic Devices, International
Patent Application PCT/AU97/00697.
[6] C.M. Lampert, V-V. Truong, J. Nagai and M.G. Hutchins, in Characterization
Parameters and Test Methods for Electrochromic Devices in Glazing Applications,
International Energy Agency Task X-C Final Report, University of California (1994).
[7] J.-G. Zhang, D.K. Benson, C. Edwin Tracy and S.K. Deb, J. Mater. Res., 8, 2657-
2667 (1993).
102
CHAPTER 5
TEMPERATURE DEPENDENCE OF KINETIC BEHAVIOUR OF
SOL-GEL DEPOSITED ELECTROCHROMICS
J.M. Bell and J.P. Matthews
Published: Solar Energy Materials and Solar Cells, 68, 249-263 (2001).
104
Contributions of Authors
This paper presents the results of experimental work carried out by J.P. Matthews, under
the supervision of J.M. Bell and I.L. Skryabin. The paper was written by J.P. Matthews
and revised by J.M. Bell before final submission of the manuscript. This journal article
was an invited paper for a special edition of Solar Energy Materials and Solar Cells, on
sol-gel technologies.
105
TEMPERATURE DEPENDENCE
KINETIC BEHAVIOUR OF SOL-GEL DEPOSITED ELECTROCHROMICS
J.M. Bell* and J.P. Matthews
Research Concentration in Materials Technology
School of Mechanical, Manufacturing and Medical Engineering,
Queensland University of Technology, GPO Box 2434, Brisbane, Qld, 4001, Australia
* Author to whom correspondence should be addressed. E-mail [email protected]
Abstract
The kinetic behaviour of sol-gel deposited electrochromic films is affected by
temperature in a complex manner and may be modelled by considering the reaction
mechanism, and in particular the rate limiting steps. If assumptions are made about the
rate limiting steps in a reaction, a model may be formed which can be used to provide
information about the kinetic parameters such as diffusion coefficient and charge
transfer resistance. Changes in the free energy of a reaction are observed as changes in
the electrical potential associated with the cells and electrodes. We have measured
changes in the switching characteristics of a sol-gel deposited electrochromic film and
modelled these results in order to extract information about the change in lithium
chemical diffusion coefficient (D) with temperature. Values of D estimated using the
model described in this paper are in close agreement with those determined by other
means, however there are some anomalies at high temperatures.
Keywords: Electrochromic film; kinetic behaviour; temperature dependence; diffusion
coefficient;
106
5.1 Introduction
Electrochromic materials undergo a change in their transmittance of heat and visible
light when a small voltage or current is passed through them [1,2]. The cycling of
electrochromic films and devices between coloured and bleached states involves the
injection and extraction of small cations and electrons into the EC material. The guest
cations move through the electrolyte under the influence of an applied electric field, to
the surface of the electrode. The cations must then combine with an electron (provided
by the external circuit) at the electrode surface and lose their solvation sheaths, (thereby
overcoming an associated charge transfer resistance) in order to intercalate into the
crystal structure of the host substrate. After intercalation the ions and electrons diffuse
into the film under the influence of the concentration gradient which results from the
injection of charge at the electrode surface. The bleaching of EC films involves the
extraction of these ions from the electrode (back into the electrolyte) and they must
therefore diffuse from a position within the electrode to the electrode/electrolyte
interface. The ease with which ions can cross the electrode/electrolyte interface and
diffuse within the host substrate is strongly dependent on the microstructure of the film
and also on temperature.
Electrochromic Smart Windows, are glazings that enable the amount of heat and light
entering a building to be controlled to optimise energy efficiency [3]. The operation of
these devices, and also electrochromic display devices, depends critically on
understanding the kinetics of ion injection in these materials. The expense of film
deposition in smart window manufacture is a drawback to commercial viability
107
especially when considering the large area of some glazings (from 1m2 upwards).
Several techniques exist for the deposition of electrochromic films onto glass substrates,
however the sol-gel route has significant cost advantages for large area coatings over
other more energy intensive and high capital-cost processes such as sputtering,
electrolytic deposition and vacuum evaporation [4]. The microstructures of the films
produced by these routes differ significantly, and hence the kinetic behaviour of the
various electrochromic films is also different [5,6,7]. In order to be able to optimise
processes such as manufacturing conditions, device switching and lifetime it is
necessary to understand the kinetics of the electrochromic reaction mechanism.
The description of kinetic behaviour includes information about the reaction mechanism
itself, rates of reaction, rate limiting steps and measurement of parameters such as
diffusion coefficients and charge transfer resistances. Smart windows will be required
to operate at temperatures up to 60ºC (and potentially higher) owing to absorption of
solar radiation in the electrochromic (EC) film in the coloured state [8], and hence it is
useful to understand how changes in temperature affect the kinetic behaviour of
electrochromics. Various models have been proposed to describe the kinetic behaviour
of sol-gel deposited ECs, and several well-defined experiments exist for the
determination of kinetic parameters.
This paper will outline one current kinetic model for sol-gel ECs, and the experimental
techniques used for the determination of relevant parameters. Measurements of the
diffusion coefficient of sol-gel deposited films have been made at various temperatures
from 20°C to 50°C, and they show behaviour close to the activated behaviour expected
108
for diffusion, although it appears that there may be some deviation from this at the
higher temperatures.
The theoretical basis of temperature dependence of ion intercalation in electrochromic
films is discussed in section 5.2, and the experimental work is described in section 5.3.
The analysis of the results is outlined in section 5.4, and the final section is a discussion
of the significance of these results and how these experiments and their analysis can be
improved.
5.2 Theory
5.2.1 Temperature effects on kinetic behaviour
Temperature affects the kinetic behaviour of electrochromic films in several ways,
depending on the mechanism of the reaction and in particular the rate limiting step for a
given process. Generally accepted rate limiting mechanisms include charge transfer at
the electrode/electrolyte interface [9,10], diffusion limited mass transfer inside the
electrode [11,12] and the series resistance of the cell [13,14]. The mobility of the ions
in the electrolyte is very high compared to the mobility within the electrode, so it is
unlikely that ionic diffusion in the electrolyte will limit the reaction rate.
Ions in the electrolyte will move towards the electrode under the effect of the applied
electric field. In order for an ion in solution to intercalate into the host lattice of the
electrode, it must lose its solvation sheath and also overcome the activation energy
109
associated with crossing the electrolyte/electrode boundary, and combine with an
electron at a host site. The energy required for this can come from the applied electric
field and also has a temperature contribution. The larger the potential difference across
the electrode/electrolyte interface, the larger the electrostatic force drawing ions across
the boundary. At high temperatures the ions will have more thermal energy which will
contribute to the ease with which they can be intercalated. Temperature therefore
provides extra driving force for the charge transfer step and lowers the electrical
potential difference required for intercalation. At elevated temperatures we therefore
expect the magnitude of the voltages required for colouration and bleaching of an
electrochromic film to be lower, if the charge transfer process is the rate limiting step.
At high voltages, the charge transfer process will be fast and hence this process is often
considered the rate-limiting step when small switching voltages are applied [15].
Once ions are intercalated into the film, they combine with an electron at a host atom
site. For example, consider the reaction
yLi+ + WO3 + ye- ⇔ LiyWO3 (5.1)
The tungsten atom is reduced from a +6 oxidation state to a +5 state, accompanied by an
optical transition (colour change) of the film. As ion intercalation proceeds the tungsten
sites at the electrode surface will soon become saturated with lithium ions, and
ion/electron couples will need to migrate further into the film if we are to continue to
inject ions into vacant tungsten sites. The concentration of ions at the electrode surface
is much higher than the concentration of ions within the film, and so the ions will move
according to Ficks laws of diffusion [16]. The ions will experience a force to move
110
under the concentration gradient formed upon intercalation. The greater the
concentration gradient and the greater the diffusion coefficient, the faster the ions will
move within the film. If the ions cannot move easily from the surface of the film, the
electrical potential required to intercalate further ions will increase. Large diffusion
coefficients will therefore lead to lower voltages for continued ion intercalation.
Diffusion is a thermally activated process which may be described by the equation [17]
)(
0RTQd
eDD−
= (5.2)
where Qd is the activation energy for diffusion, R is the ideal gas constant, T is absolute
temperature and D0 is a pre-exponential term. The exponential relationship between
diffusion coefficient and temperature means that relatively small increases in
temperature are accompanied by significant increases in the diffusion coefficient and
hence reductions in the switching voltages required for diffusion controlled processes at
elevated temperatures. It has been proposed that diffusion of ions within the
electrochromic electrode is the rate limiting step when switching voltages are relatively
large (ie. charge transfer is fast) [15].
The series resistance of a cell is the sum of the resistances of all components in the
circuit. This includes the external circuit, the transparent conducting oxide (TCO)
electrodes, the films themselves and the electrolyte. Temperature only has a small effect
on the series resistance of the cell, so we expect the series resistance to be fairly
constant. There will be a slight increase in the electrical conductivity of the electrolyte
with increasing temperature, and therefore a slight reduction in the ohmic drop across
the electrolyte, but this is small when compared to the voltages applied for colouration
111
and bleaching and can be minimised by reducing the electrode separation.
The voltage required to switch an electrochromic film is also dependent on the emf
(electromotive force) of the film, which in turn is related to the chemical composition of
the film at a given time by the Nernst equation [16] and is discussed below. The effect
of temperature on the kinetic behaviour of electrochromics is therefore a complex
combination of the temperature dependencies of diffusion, charge transfer and emf.
Generally however, we expect reactions to proceed more easily at elevated temperatures
and hence switching voltages should be lower, regardless of the rate limiting mechanism
for a particular system. This qualitative understanding helps explain the general trends
we observe when switching electrochromic films at elevated temperatures, but if we are
to optimise factors such as electrical and optical efficiency and lifetime hence improving
device design, we require more quantitative information.
5.2.2 Thermodynamics of coloration
The basic quantity governing these processes and reactions is free energy. The electrical
potential (E) associated with an electrochemical reaction is related to the free energy
change of the reaction (∆G) by
nEFG −=∆ (5.3)
where n is the number of electrons involved in the reaction, F is Faradays constant and
E is the potential in volts [16]. A cell emf (electromotive force) is therefore a way of
describing the free energy change of the reaction and a positive emf indicates a negative
free energy change, and hence a spontaneous reaction. The sign of the emf tells us
112
which direction a reaction will spontaneously proceed in. The standard free energy
change is given by ∆Go = -nEoF. The standard free energy change is also related to the
equilibrium constant (K) by
)ln(KRTG −=∆ (5.4)
where K = products
reactants
activityactivity when the system is at equilibrium.
Combining these two equations gives an expression for the standard cell potential of a
system in equilibrium,
)ln(º KnFRTE = (5.5)
The standard electrode potential is the potential of the electrode when the activities of all
species are defined by the equilibrium constant (ie. activityproducts =
K×activityreactants when expressed in some standard unit (eg Molality or mol/Kg). The
potential of a system not at equilibrium (ie E ≠ E0) is described by the Nernst equation
0
0
ln( )
ln( )
red
ox
ox
red
aRTnF a
aRTnF a
E E
E E
= −
= + (5.6)
The entropy change of a reaction describes the way in which temperature affects the free
energy change, and can be described mathematically by [16]
( )ET P
S nF δδ∆ = (5.7)
The change in cell emf with temperature is therefore directly related to the entropy
change of reaction and by measuring the change in emf with temperature, we can
113
determine this entropy change. When considering electrochromic reactions occurring in
thin film electrodes, the emf is related to the concentration (or more strictly, the activity)
of the injected ion at the electrode/electrolyte interface (c(0,t)). The emf associated with
the WO3 electrode (relative to some reference electrode) in the electrochromic reaction
described by equation 5.1 is given by the equation [18,19]
−
−+=),0(.1
),0(.ln),0(..
tcVtcV
FRTtcVbaemf
m
mm
ν (5.8)
where Vm is the molar volume of the WO3 and c(0,t) is the charge density in moles per
unit volume. a and b are constants related to the free energy of formation of WO3 and
free energy changes upon reduction of W6+ to W5+ and the resultant changes in
interactions between the atomic centres within the film, and ν is related to the entropy
change associated with the reaction [18]. In practise these parameters are used to fit
experimental data to equation 5.8. As this reaction proceeds to the right, the electrode
potential decreases (becomes more negative) indicating that the system is being
cathodically pushed further from equilibrium.
When colouring an electrochromic cell, the measured or applied voltage is related to the
emf and the colouration voltage (Vc(t)) by [6]
)()()( tVtemftV ca += (5.9)
and Vc(t) is given by [20]
),0(..)( tcVdydERitV mccc
−= (5.10)
114
where dydE is the slope of the coulometric titration curve, which gives the cell emf versus
stoichiometric coefficient for lithium (y). The total applied voltage observed during
colouration in an experiment is then [21]
−
−++=),0(1
),0(ln),0('.)(ty
tyFRTtybaRitV ccaν (5.11)
where y(0,t) is the stoichiometric coefficient of lithium (in LiyWO3) at the
electrode/electrolyte boundary (ie y(0,t) = Vm.c(0,t)) and
−=
dydEbb' . Equation 5.11
therefore provides a quantitative measure of the free energy change for a reaction in
terms of the measured cell voltage. We can extract information about the underlying
processes and reaction mechanisms occurring by measuring the cell voltages under
various conditions, such as varying temperature. Changes in the free energy of reaction
and hence changes in the measured potential can then be used to gather information
about the rate limiting mechanisms by modelling the voltage behaviour.
5.2.3 Modelling of concentration profile
The way in which the ions are distributed within the film can be described by the
equation [21]
Ψ=DnF
jtxc ),( (5.12(a))
and
( ) ( )
)4
)1(2)1(24
224
²)1(2exp24
²2exp2(0
Dtxlkerfc
Dxlk
Dtxklerfc
Dxkl
Dtxlkt
Dtxklt
k
−+
−+−
+
+−
−+−+
+−=Ψ ∑
∞
= ππ (5.12(b))
115
where n is the number of electrons in the process, l is film thickness, j is current density,
F is Faradays constant and D is the chemical diffusion coefficient of lithium ions. We
can use equation 5.12 to determine the concentration profile during ion injection for
various values of diffusion coefficient and time.
Figure 5.1 and Figure 5.2 show concentration profiles for t=1s and t=200s respectively,
simulated using equation (5.12). These results are for a 200nm thick film, coloured at
0.1mA/cm2. The concentration (y) is expressed as the stoichiometric coefficient of
lithium (in LiyWO3) and is therefore equal to c(x,t).Vm.
-0.01
0
0.01
0.02
0.03
0.04
0.05
0 50 100 150 200
D = 1x10-12 cm2/s
D = 2x10-12 cm2/s
D = 5x10-12 cm2/s
D = 1x10-11 cm2/s
Con
cent
ratio
n (y
)
Distance x (nm)
Figure 5.1 Simulated concentration profile for t=1s (Qinj=0.1mC/cm2)
116
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 50 100 150 200
D = 1x10-12 cm2/s
D = 2x10-12 cm2/s
D = 5x10-12 cm2/s
D = 1x10-11 cm2/s
Con
cent
ratio
n (y
)
Distance x (nm)
Figure 5.2 Simulated concentration profile for t=200s (Qinj=20mC/cm2)
It is evident from Figure 5.1 and Figure 5.2 that the surface lithium concentration is
strongly dependent on the diffusion coefficient, and hence on temperature. As the
diffusion coefficient increases, the concentration profile becomes flatter and hence the
concentration of ions at the electrode surface is decreased. The emf of a cell at high
temperature will therefore be lower (Equation 5.8) than that for low temperatures, for a
specific injected charge density, since the surface charge density will be lower owing to
an increase in the rate of diffusion away from the surface of the film.
The surface concentration of lithium ions for a diffusion coefficient of 5 x 10-12cm2/s
(Qin = 20mC/cm2) is approximately y = 0.49 (for y in LiyWO3). If the diffusion
coefficient is doubled to 1 x 10-11cm2/s for the same injected charge density, the surface
lithium concentration is approximately y = 0.46. This difference in surface
concentrations of lithium is the major cause of the observed differences in switching
characteristics of electrochromic films at elevated temperatures.
117
The voltage calculated with Equation 5.8 depends on the surface concentration of
lithium ions, because the potential drop occurs solely across the electrolyte/electrode
interface (assuming there is no internal potential drop in the electrode). The surface
lithium concentration is then calculated by solving equation 5.12 for the special case
where x = 0. Solving these, we obtain [21]
Γ=DnF
jtc 2),0( (5.13 (a))
and ππt
Dtlkt
Dtklerfc
Dkl
kk
−−−=Γ ∑∑∞
=
∞
= 1
22
1)][exp(2]2[ (5.13(b))
5.3 Experimental
5.3.1 Film preparation
Mixed tungsten-titanium oxide electrochromic films were deposited using sol-gel
processing from organic precursors onto 10cm × 10cm substrates of LOF TEC8/3 glass
using the sol-gel dip coating method [22]. The alkoxide precursor solutions used in the
sol-gel dipping have been described previously [23].
5.3.2 Electrochemical testing
Electrochemical measurements were made using a three electrode cell. The counter
electrode was a platinum foil (Area=64cm2) and the reference electrode was a Ag/AgCl
cell filled with an ethanolic solution of KCl, saturated with AgCl. The electrolyte used
was a solution of 1M LiClO4 in propylene carbonate. The electrolyte solution was dried
118
over molecular sieves prior to use and stored and used inside a dry glovebox, which was
maintained with a nitrogen atmosphere under a slight positive pressure. Experiments
were carried out in a glass tank (filled with electrolyte solution) partly submersed in a
larger heating tank filled with mineral oil. An electrical heater/stirrer unit was used to
control the temperature of the oil bath, and hence the electrolyte solution. The
electrolyte solution was also stirred during the experiments to minimise the temperature
difference between the heating oil and electrolyte solution. The complete apparatus
including electrolyte tank, oil bath and heater unit was kept in the dry-box during all
experiments.
The atmosphere inside the glovebox was kept dry by exposing it to P2O5 dessicant, and
recirculating the nitrogen through a column of dried molecular sieves. The humidity
level inside the glovebox was monitored with a HMP235 humidity and temperature
transmitter, manufactured by Vaisala. The humidity during the experiment was
maintained at 1.05ppm absolute humidity (Relative humidity = 5.3% and temperature =
22.4ºC immediately prior to experiment).
The WO3 films were cycled using a voltage-limited constant current technique,
described previously [24,25]. The results reported are for a film cycled to 15mC/cm2
(Current density = 0.1mA/cm2, film area = 100cm2) hence the first 150s correspond to
the colouration cycle. The film was bleached after a 2s delay, with a current density of
0.1mA/cm2. During the experiment the temperature was ramped from 20° to 50°C at
approximately 1°C per cycle. Measurements were performed as the temperature
119
increased, but were not made as the system cooled.
5.4 Results
5.4.1 Variation in switching voltage with temperature
Figure 5.3 shows the results obtained from measurement of a sol-gel deposited
electrochromic film maintained under rigorously dry conditions before and during
cycling, as described above.
-1
-0.5
0
0.5
1
0 50 100 150 200 250 300 350
20.1oC
30.3oC
40.3oC
50.0oC
Volta
ge (V
)
Time (s)
Figure 5.3 Applied voltage for colouration and bleaching of a sol-gel WO3 film to
15mC/cm2
It is observed that the magnitude of the applied voltages decreases with increasing
temperature as predicted from considerations discussed in section 5.2.1. The decrease in
applied voltage with increasing temperature is greatest at low temperatures, which
indicates that small increases from room temperature result in significant decreases in
the voltages required for colouration and bleaching as predicted by equation 5.2.
120
Equation 5.11 describes this variation in voltage with temperature, in terms of the
surface concentration of lithium ions (y(0,t)). The effect of temperature on the cell emf
(for a given lithium concentration) is relatively small, and arises purely from the
Nernstian contribution to equation 5.11.
Figure 5.4 predicts the cell emf versus injected charge density for the same temperatures
as the experiment in Figure 5.3, using equation 5.8 and a=-0.66V, b=-0.87V, ν=5.76
[19]. The variation in emf with temperature for a given surface lithium concentration is
very small, and does not explain the large variation in switching voltages seen in Figure
5.3.
Figure 5.4 Dependence of emf on temperature and surface lithium concentration
predicted using equation 5.8.
A more marked effect of temperature arises from the change in diffusion coefficient, and
hence the way in which the ions are distributed within the films. Ions are injected into
121
the electrochromic film at the interface with the electrolyte (x = 0) and then diffuse into
the film under the influence of the concentration gradient. The concentration of ions in
the film is therefore greatest at x = 0, and decreases with increasing film depth. At
elevated temperatures the diffusion coefficient of lithium ions will be greater, and hence
the surface concentration of lithium ions (for a specific injected charge density) will be
lower. We must therefore take the concentration profile into account, in order to
determine the surface concentration of lithium ions (y(0,t)) and hence simulate the
experimental data using equation 5.11.
5.4.2 Simulation of Voltage Response of Films
We can now use equation 5.13 to simulate the variation of c(0,t) with time during charge
injection, and therefore indirectly determine the diffusion coefficient for each
temperature. The 20.1ºC voltage/time data for colouration of the film in Figure 5.5 was
simulated using equation 5.11, using a least squares method to obtain the best fit by
adjusting the parameters a, b, ν, Rc and D. The constants a, b, ν and Rc were assumed
to be intrinsic to the electrochromic film being studied, and hence only the diffusion
coefficient varied with temperature. The best fit for the 20.1ºC data was obtained with
the following constants:
a = -1.06V,
b' = -0.65V,
ν= 4.2,
Rc = 40Ω and
D = 2.1 x 10-12cm2/s
122
Figure 5.5 Experimental and simulated voltages during charge injection of a sol-gel
electrochromic film. (a) Temperature = 20.1ºC, D = 2.07x10-12cm2,
(b) temperature = 30.3ºC, D = 6.33x10-12cm2, (c) temperature = 40.3ºC,
D = 1.33x10-11cm2 and (d) temperature = 50.0ºC, D = 1.71x10-11cm2.
The voltage/time data for the colouration at the other temperatures was then simulated
using the above values for a, b, ν and Rc, and only changing the diffusion coefficient.
The experimental and simulated data for four temperatures are shown together in and
the values estimated for the diffusion coefficients and the least squares sum in the
simulations are shown in Table 6.1.
123
T (ºC) T (K) 1/T(K-1) D (cm2/s) ln D Σ(Vsim-Vexp)2
20.1 293.3 0.00341 2.07E-12 -26.90 0.0061
30.3 303.5 0.00330 6.33E-12 -25.79 0.0077
40.3 313.5 0.00319 1.33E-11 -25.04 0.0058
50.0 323.2 0.00309 1.71E-11 -24.79 0.0062
Table 5.1 Diffusion coefficients used to simulate experimental voltage/time data for
sol-gel WO3 film coloured to 15mC/cm2, and least squares sum from data fitting.
5.5 Discussion
It is evident from the theoretical fits (See Figure 5.5(a)-(d)), using equation 5.13 to
estimate the surface lithium concentration during ion injection, and equation 5.11 to
calculate the corresponding voltages, to the experimental data is very good. We observe
that the diffusion coefficients estimated for each data set increase with temperature, so
the trend is as we predicted in section 5.2.1. There is a small deviation between
experimental and simulated data for long time periods, but in general the model
describes the data well considering that the diffusion coefficient was the only parameter
changed for the simulations. The chemical diffusion coefficients estimated from the
simulations are within the range of those reported in the literature for lithium in WO3
[5,19,26]. The room temperature (20.1ºC) diffusion coefficient of 2.01x10-12 cm2/s
seems a little low and would imply that the surface lithium concentration exceeded the
124
reversible limit during colouration, however this may be due to a number of limitations
and assumptions made in the model which are discussed below.
We would expect the values of diffusion coefficients shown in Table 5.1 to follow the
temperature dependence of equation 5.2. An Arrhenius plot of ln D versus 1/T should
then yield a straight line of slope R
Qd− and intercept equal to ln(D0). Figure 5.6(a)
shows one such plot for the diffusion coefficient values extrapolated from the
experimental data simulations. We see that equation 5.2 is satisfied for low temperature
data, but the plot deviates from linearity as temperature increases past approximately
40ºC. Figure 5.6(b) shows the same information plotted for temperature values between
20.1 and 40.3ºC and the data very closely approximates a linear relationship, with a
Pearson moment correlation coefficient, R2 = 0.98.
Figure 5.6 Variation in estimated diffusion coefficients with temperature plotted for
range (a) 20.1 < T < 50.0ºC and (b) 20.1 < T < 40.3ºC.
The calculated diffusion activation energy using the slope of the regression line from
125
Figure 5.6(a) is 0.58eV and the pre-exponential term (D0) is 2.8x10-2 cm2/s. We have
been unable to find published values in the literature for activation energies of diffusion
of Li+ ions in electrochromic films. We may however reasonably expect a small ion
moving within a lattice of relatively large atoms (like lithium in WO3) to have a
diffusion activation energy of approximately the same order of magnitude as, for
example, carbon atoms in α-iron. The activation energy for carbon atoms in α-iron is
0.83eV/atom, and so compares reasonably well with the result determined from this
experimental work. The pre-exponential (D0) for carbon atoms in α-iron is 1x10-1
cm2/s and so the value determined in this work is also in an appropriate range.
The diffusion activation energy determined from the regression line of Figure 5.6(b) is
0.73eV and the pre-exponential (D0) is 7.3x10 cm2/s. The activation energy therefore
seems reasonable, but the pre-exponential term is unusually high and would seem
erroneous. The fact that the data in Figure 5.6 appears more linear at low temperatures
may suggest that the low temperature data is more accurate. A closer analysis of the
assumptions and limitations used in this modelling provides some possible explanations
of this observation.
The model assumes that there is no voltage drop internal to the electrode (WO3 film), so
the measured potential drop occurs solely across the electrode/electrolyte interface and
in the external circuit. In reality the conductivity of the WO3 layer increases as charge is
injected and approaches metallic conductivity as the free electron density approaches the
Mott critical density [11,27,28]. This means that there is a real potential drop internal to
126
the electrode during the early stages of colouration that will provide an electrostatic
force to drive ions in the same direction as their diffusive motion (ie. away from the
electrode surface). The effect of this is to increase the speed at which ions move for low
values of the stoichiometric lithium coefficient (y) and the diffusion coefficient will
appear larger at the start of colouration.
The model also assumes that the slope of the coulometric titration curve dydE is constant
throughout the charge injection, and also for changing temperature. In reality, this slope
is larger for low values of y and decreases with charge injection and will decrease
significantly as temperature increases. The effect of this assumption is to underestimate
the values of Va(t) for low injected charge density and for high temperatures and so
introduces another error in this method of analysis of the data.
The series resistance during coloration, Rc, is also assumed to be independent of
temperature, but in reality will decrease with increasing temperature due to the presence
of more electrons in the conduction bands of the various components in the circuit. The
error introduced by this assumption is however small, as changes in resistance of the
circuit components is very small in relation to the voltages being measured. The
experimental voltages were not corrected for iR drop of the electrolyte, but this effect
will also be small as the working-reference electrode separation was minimal (approx 5-
10mm) and the electrolyte conductivity is relatively high.
The assumptions made in order to simulate the data with this model make complete
interpretation of the results more difficult, however the trend of the results is in
127
accordance with kinetic theory. Future experiments are planned to take into account
these assumptions and produce a better model capable of accurately predicting the
measured voltages during ion insertion and extraction, and explaining the observed
changes in kinetic behaviour with temperature.
5.6 Conclusion
A simulation model based on a combination of diffusion-limited motion and series
colouration resistance has been used to simulate the voltage-time characteristics of
lithium injection into sol-gel deposited tungsten-titanium mixed oxide films. The model
was successfully used to simulate experimental data over an extended temperature
range, by changing only the chemical diffusion coefficient. The diffusion coefficients
estimated from the simulations increased with temperature in accordance with
Arrhenius-like activation behaviour, however there was some deviation from this
behaviour at temperatures above 40ºC. It has been proposed that deviations from ideal
diffusion-temperature behaviour are due to some of the assumptions made in the
modelling process, and future experiments have been planned to minimise these
limitations. Diffusion coefficients in the range of 10-11-10-12cm2/s were estimated with
this model, which are in good agreement with values published in the literature.
Acknowledgements
This work is supported by an Australian Postgraduate Award (Industry) scholarship
from the Australian Research Council, and Sustainable Technologies Australia (STA).
The work described in this paper has been supported by the Australian Cooperative
128
Research Centre for Renewable Energy (ACRE). ACREs activities are funded by the
Commonwealths Cooperative Research Centres Program.
REFERENCES [1] B.W. Faughnan, R.S. Crandall and P.M. Heyman, R. C. A. Rev., 36, 177-197 (1975).
[2] C.M. Lampert, Introduction to Chromogenics in: C.M. Lampert and C.G. Granqvist
(Eds.), Large-Area Chromogenics: Materials and Devices for Transmittance Control,
Optical Engineering Press-SPIE, Bellingham, WA, 1990, 378.
[3] C. M. Lampert, IEEE Circuits and Devices, 8, 19-26 (1992).
[4] N. Ozer and C.M. Lampert, Solar Energy Materials and Solar Cells 54 (1998) 147.
[5] J.-G. Zhang, C. Edwin Tracy, D.K. Benson and S.K. Deb, J. Mater. Res., 8, 2649-
2656 (1993).
[6] J.-G. Zhang, D.K. Benson, C. Edwin Tracy and S.K. Deb, J. Mater. Res., 8, 2657-
2667 (1993).
[7] C.G. Granqvist, Appl. Phys. A 57 (1993) 3.
[8] C.M. Lampert, A. Agrawal, C. Baertlien and J. Nagai, Solar energy Materials and
Solar Cells, 56, 449-463 (1999).
[9] S.K. Mohapatra, J. Electrochem. Soc., 125(2), 284-288 (1978).
[10] B.W. Faughnan and R.S. Crandall, Electrochromic Displays Based on WO3in
Topics in Applied Physics, Display Devices (Edited by J. I. Pankove), Vol. 40, Springer-
Verlag, New York (1980).
[11] C. Ho, I.D. Raistrick and R.A. Huggins, J. Electrochem. Soc., 127(2), 343-350
(1980).
[12] B. Reichman, A.J. Bard and D. Laser, J. Electrochem.Soc., 127(3), 647-654
129
(1980).
[13] B. Vuillemin and O. Bohnke, Solid State Ionics, 68, 257-267 (1994).
[14] O. Bohnke, M. Rezrazi, B. Vuillemin, C. Bohnke and P.A. Gillet, Solar Energy
Materials and Solar Cells, 25, 361-374 (1992).
[15] R.S. Crandall and B.W. Faughnan, Appl. Phys. Lett., 28, 95-97 (1976).
[16] A. J. Bard and L. R. Faulkner, in Electrochemical Methods: Fundamentals and
Applications, Wiley, New York (1980).
[17] W.D. Callister, Materials Science and Engineering: An Introduction, John Wiley
and Sons, Inc., New York (1985).
[18] R.S. Crandall, P.J. Wojtowicz and B.W. Faughnan, Solid State Comm., 18, 1409-
1411 (1976).
[19] J. Nagai and T. Kamimori, Jap. J. Appl. Phys., 22, 681-687 (1983).
[20] I.D. Raistrick and R.A. Huggins, Solid State Ionics, 7, 213-218 (1982).
[21] J. Wang, PhD Thesis, University of Technology, Sydney (1998)..
[22] J. M. Bell, G. B. Smith, I. L. Skryabin, B. G. Monsma, N. C. Ruck and T. Dinh,
Sol-gel Deposited Electrochromic Devices, in Proceedings of Windows Innovations
Conference WIC 95, 383-391, Minister of Supply and Services, Canada.
[23] A. Koplik, Australian Patent Application, PP0274 (1997).
[24] J. M. Bell and I. L. Skryabin, Solar Energy Materials and Solar Cells, 56 (1999)
437.
[25] I. L. Skryabin and J. M. Bell, Control of Electrochromic Devices, International
Patent Application PCT/AU97/00697.
[26] M. Green, W.C. Smith and J.A. Weener, Thin Solid Films 38 (1976) 89.
130
[27] N.F. Mott, J. Non-Cryst. Solids 1 (1968) 1.
[28] V. Wittwer, O.F. Schirmer and P. Schlotter, Solid State Comm. 25 (1978) 977.
CHAPTER 6
SIMULATION OF ELECTROCHROMIC SWITCHING VOLTAGES
AT ELEVATED TEMPERATURES.
J.P. Matthews, J.M. Bell and I.L. Skryabin
Published: Electrochimica Acta, 46, 1957-1961 (2001).
132
Contributions of Authors
This paper presents the results of experimental work carried out by J.P. Matthews, under
the supervision of J.M. Bell and I.L. Skryabin. The paper was written by J.P. Matthews
and revised by J.M. Bell before final submission of the manuscript.
133
SIMULATION OF ELECTROCHROMIC SWITCHING VOLTAGES AT
ELEVATED TEMPERATURES.
J.P. Matthews1, J.M. Bell1 and I.L.Skryabin2
1 School of Mechanical, Manufacturing and Medical Engineering,
Queensland University of Technology, Australia
2 Sustainable Technologies, Australia,
11 Aurora Ave, Queanbeyan, NSW, Australia
Abstract
Sol-gel deposited electrochromic WO3/TiO2 films have been reversibly cycled at
temperatures up to 70ºC using a constant-current charge injection technique, in a
stringently dry environment. The resultant switching voltages have been simulated with
a model involving several parameters including the chemical diffusion coefficient (D) of
lithium in the films. The experimental data at various elevated temperatures has been
fitted with the model by varying the diffusion coefficient at each temperature and
holding other parameters constant. The values of D estimated from the simulation of
experimental data are within the range of published values for similar films, however
there are some limitations due to assumptions made in the model.
This paper discusses the application of the model for the prediction of cycling voltages
at high temperatures and the suitability of the model in the estimation of the chemical
diffusion coefficient (D).
Keywords: Electrochromic switching; kinetic behaviour; temperature dependence;
diffusion coefficient; voltage simulation
134
6.1 Introduction
Smart windows based on amorphous thin films of tungsten oxide (WO3) may be used to
reduce the transmittance of both heat and light by changing from a clear state to a deep
blue coloured state. In the coloured state the amount of heat and light transmitted is
significantly reduced because the electrochromic film absorbs a large proportion of the
incoming radiation. This absorption of radiation is accompanied by an increase in the
temperature of the material and it is foreseeable that these devices may be required to
operate at temperatures up to 70ºC [1,2]. The switching characteristics of
electrochromic (EC) devices are dependent on temperature, so it is desirable to predict
this behaviour in order to prevent the application of excessive voltages. The ability to
simulate EC switching voltages over an extended temperature range therefore has
application in the determination of suitable control strategies. An optimal control
strategy would ensure maximum device lifetime and also uniform and consistent
coloration of the devices, regardless of temperature.
The kinetic behaviour of EC materials is affected by many factors including
microstructure [3,4], composition [5], ion mobility [6] (specifically diffusion rates) and
charge transfer resistance [7]. Temperature significantly affects ionic mobility, and the
diffusion rate of the mobile ions within the EC host lattice is observed to increase with
temperature [8]. The increased ease with which ions may move in the EC material at
high temperatures is one reason why the magnitude of the voltages required to colour
and bleach EC materials decreases at high temperatures. An increased diffusion rate
also means that the concentration profile of ions inside an EC film will be flatter, and
135
the surface concentration will then be lower than for the same injected charge density at
a lower temperature. These effects of temperature on ionic diffusion may be described
by a mathematical model, which in turn may be used to simulate the voltages required to
colour and bleach these films.
In this paper the simulation model developed by Wang [9] has been used to model
experimental data between 35ºC and 76ºC and the chemical diffusion coefficient of
lithium in the films has been extracted from the simulation process. The diffusion is
observed to closely follow the expected Arrhenius type activation behaviour, and the
activation energy for diffusion of lithium ions in the films has been calculated.
6.2 Experimental
6.2.1 Film preparation
Mixed tungsten-titanium oxide electrochromic films (mole ratio W:Ti of 4:1), were
deposited onto 10cm x 10cm substrates of LOF TEC8/3 glass. The films were coated
using the sol-gel dip coating method [10], with solutions of tungsten and titanium oxy-
butoxides as the organic precursors. The details of the film preparation have been
described previously [11].
6.2.2 Electrochemical testing
Electrochemical measurements were made using a three electrode cell, the counter and
reference electrodes being a sheet of copper coated platinum (area=85cm2), and a
Ag/AgCl wire respectively. The electrolyte was 1M LiClO4/propylene carbonate and
was dried over molecular sieves prior to use. The complete electrochemical cell was
136
contained inside a dry glovebox, and maintained with a nitrogen atmosphere under a
slight positive pressure. A glass tank was used to hold the electrodes and electrolyte of
the cell, and this was partially submerged in a temperature controlled oil-bath. An
electrical heater/stirrer unit was used to control the temperature of the oil bath, and
hence the electrolyte solution. The electrolyte solution was also stirred during the
experiments to minimise any thermal lag. The atmosphere inside the dry-box was
maintained at less than 1ppm absolute humidity by exposure to phosphorus pentoxide
(P2O5) and recirculation through pre-dried molecular sieves.
The WO3/TiO2 films were cycled using a voltage-limited constant current technique,
described previously [12,13]. The only difference between this method and that
described previously is that the maximum voltage limit for bleaching was not fixed, but
was determined during cycling by measuring the rate of change of voltage. The
constant current bleaching step was terminated when the rate of voltage increase
exceeded 0.5V/s, which ensured that excessive bleaching voltages were not applied at
elevated temperatures.
The results reported are for a film cycled to 15mC/cm2 (Current density = 0.1mA/cm2,
film area = 75cm2) hence the first 150s correspond to the coloration cycle. The film was
bleached after a 2s delay, with a current density of 0.1mA/cm2. The cycles used for the
data simulations were number 42(35.8ºC), 69(46.5ºC), 86(56.2ºC), 100(65.3ºC) and
118(76.4ºC).
6.2.3 Simulation of EC film coloration voltage
The coloration voltage simulations were carried out by fitting experimental data to the
137
equation [9]
−
−++=),0(.1
),0(.ln),0('.)(
tcVtcV
FRTtcVbaRitV
m
mmcca
ν (6.1)
which is a modified form of equations first reported by Nagai et al [14] and Crandall et
al [15]. Rc is the series resistance of coloration, ic is the switching current and a is a
constant related to the free energy of formation of WO3 and free energy changes upon
reduction of W6+ to W5+ and the resultant changes in interactions between the atomic
centres within the film. ν is related to the entropy change associated with the EC
reaction [15]. Vm is the molar volume of the film and
−=
dydEbb' where dE/dy is the
slope of the coulometric titration curve. In practice the parameters a, b' and ν are used
to fit experimental data to equation (6.1). The surface lithium concentration c(0,t) is the
charge density in moles per unit volume at the electrode/electrolyte interface and is
determined from the equations [8,9]
Γ=DnF
jtc 2),0( (6.2(a))
and ππt
Dtlkt
Dtklerfc
Dkl
kk−−−=Γ ∑∑
∞
=
∞
= 1
22
1)][exp(2]2[ (6.2(b))
where n is the number of electrons in the process, l is film thickness, j is current density,
F is Faradays constant and D is the chemical diffusion coefficient of lithium ions. We
can use Eq. (7.2) to determine the electrode surface lithium concentration during ion
injection for various values of current density, diffusion coefficient and time.
138
6.3 Results and Discussion
6.3.1 Voltage characteristics
Figure 6.1 shows the voltage (relative to a reference electrode) of the mixed oxide EC
film during coloration and bleaching at temperatures from 35.8ºC to 76.4ºC. The
magnitude of the voltages was observed to decrease as temperature increased primarily
due to an increase in the lithium ion mobility and therefore a reduction in the surface
lithium concentration [8].
-1.5
-1
-0.5
0
0.5
1
1.5
0 50 100 150 200 250 300 350 400
35.8oC
46.5oC
56.2oC
65.3oC
76.4oC
Volta
ge (V
)
Time (s)
Figure 6.1 Applied voltage during colouration and bleaching of a sol-gel WO3/TiO2
film to 15mC/cm2/s.
The data for the 35.8ºC cycle was simulated using Eq. (6.1) and (6.2) using a least
squares method to obtain the best fit by adjusting the parameters a, b, ν, Rc and D. The
constants a, b', ν and Rc were assumed to be intrinsic to the electrochromic film being
studied, and hence only the diffusion coefficient varied with temperature. A close fit for
the 35.8ºC data was obtained using the constants a = -1.3V, b' = -0.3V,
139
ν= 4.2, Rc = 40Ω and D = 8.7 x 10-13cm2/s and Figure 6.2(a) shows the experimental and
simulated data at this temperature. The voltage/time data for the coloration at the other
temperatures was then simulated (See Figure 6.2(b)-(e)) using the above values for a, b,
ν and Rc, and only changing the diffusion coefficients.
-1.3
-1.2
-1.1
-1
-0.9
-0.8
-0.7
-0.6
0 40 80 120 160
Exp 35.8 oC
Sim 35.8 oC
Volta
ge (V
)
Time (s)
(a)
-1.2
-1.1
-1
-0.9
-0.8
-0.7
-0.6
-0.5
0 40 80 120 160
Exp 46.5 oC
Sim 46.5 oC
Volta
ge (V
)
Time (s)
(b)
-1.1
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
0 40 80 120 160
Exp 56.2 oC
Sim 56.2 oC
Volta
ge (V
)
Time (s)
(c)
-1.1
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
0 40 80 120 160
Exp 65.3 oC
Sim 65.3 oC
Volta
ge (V
)
Time (s)
(d)
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
0 40 80 120 160
Exp 76.4 oC
Sim 76.4 oC
Volta
ge (V
)
Time (s)
(e)
Figure 6.2 Experimental and simulated voltages during charge injection of a sol-gel
electrochromic film. (a) T = 35.8ºC, D = 8.68x10-13cm2/s, (b) T = 46.5ºC,
D = 1.54x10-12cm2/s, (c) T = 56.2ºC, D = 4.02x10-12cm2/s, (d) T = 65.3ºC,
D = 1.48x10-11cm2/s and (e) T = 76.4ºC, D = 6.00x10-11cm2/s.
140
It is evident from the data simulations shown in Figure 6.2 that the experimental data is
quite well simulated by Eq. (7.1) and (7.2) with a single fitting parameter. The lithium
chemical diffusion coefficients determined from the data simulations were observed to
increase with temperature, as expected from kinetic and thermodynamic considerations
[8]. The diffusion coefficients estimated from simulation of the experimental data (For
values see Figure 6.2) are within the approximate range of values reported in the
literature [3,14,16] for other EC films. The relatively low diffusion coefficients
estimated at the lower temperatures (approximately 1x10-12cm2/s) would indicate a fairly
low ionic mobility, and this conclusion is supported by the fact that relatively large
voltages were required for switching at these temperatures. The difference in the
maximum coloration and bleaching voltage at 35.8ºC was 2.64V, whereas in a previous
experiment [8] this voltage range was only 1.44V at 20.1ºC (D=2.07x10-12cm2/s
extrapolated from data simulation), for the same injected charge density. This behaviour
suggests that the switching voltages are strongly dependent on ionic mobility, with high
mobility allowing EC films to be switched with the application of quite low electrical
potentials.
We would expect the values of the chemical diffusion coefficient (determined from the
simulations) to follow the temperature dependence of
)(
0RTQd
eDD−
= (6.3)
where Qd is the activation energy for diffusion, R is the ideal gas constant, T is absolute
temperature and D0 is a pre-exponential term. An Arrhenius plot of ln D versus 1/T
should then yield a straight line of slope R
Qd− and intercept equal to ln(D0). Figure 6.3
141
shows one such plot for the diffusion coefficients extrapolated from the data simulations
and we can see that equation (6.3) is reasonably well satisfied (R2=0.961) over the
experimental temperature range.
-28
-27
-26
-25
-24
-23
2.85 10-3 2.95 10-3 3.05 10-3 3.15 10-3 3.25 10-3
y = -11497x + 9.052
R2= 0.961
ln D
1/T (K-1)
Figure 6.3 Arrhenius plot showing the variation in estimated diffusion coefficients with
temperature.
The plot begins to deviate from linearity as temperature increases, with the trend that the
diffusion coefficient is becoming very large. This behaviour may be due to a shift from
a diffusion-limited motion to a mechanism limited by charge transfer as temperature
increases, as discussed below.
The activation energy for diffusion, calculated from the slope of the regression line in
Figure 6.3, is 0.99eV, which is approximately twice the value extrapolated from
previous experimental work [8], in which the switching voltages were also found to be
142
significantly smaller. This result is in agreement with the conclusion that the lithium ion
mobility in the films in this experiment was quite low. We have been unable to find
published values of the diffusion activation energy of lithium ions for comparison with
this result (excluding our own previous experimental work). We may however
reasonably expect this value to be of a similar order of magnitude to that for diffusion of
carbon in
α-iron, because this system also involves the movement of relatively small ions within a
lattice of much larger host atoms. The activation energy for the diffusion of carbon
atoms within α-iron is 0.83eV/atom [17], and so compares reasonably well with the
result determined from this experimental work.
The simulation model presented here is based primarily on diffusion limited motion of
ions in the EC film. The diffusion coefficient estimated at 76.4ºC was 6x10-11cm2/s,
which implies that diffusion was very fast at this temperature. At this temperature, the
applied voltages are also quite low which will decrease the rate of the charge transfer
step [18]. It is possible that the EC reaction is limited by diffusion at lower
temperatures (when diffusion is relatively slow and voltages are large) and limited by
charge transfer at the electrode surface at high temperatures (when diffusion is fast and
voltages are low). The absence of a term describing the charge transfer process is
therefore an inherent limitation to the current simulation model presented here, and a
possible explanation for the deviation from linearity of Figure 6.3 towards high
temperatures.
The incorporation of a term describing the rate of the charge transfer step in this
simulation model may help better describe experimental results, and lead to a better
143
understanding of the underlying mechanisms occurring during coloration and bleaching
of EC films and devices.
6.4 Conclusion
The voltage-time characteristics for the coloration of a mixed tungsten/titanium oxide
EC film have been simulated over a wide temperature range, using a model based on the
diffusion limited motion of ions and a series coloration resistance. A good
approximation of the experimental data was made for temperatures between 36º-76ºC by
only changing the diffusion coefficient for each temperature. The relationship between
the extrapolated diffusion coefficients and temperature was close to Arrhenius activation
behaviour, with an activation energy for diffusion of 0.99eV. It has been proposed that
the charge transfer process increasingly becomes a limiting mechanism at high
temperatures and the inclusion of a term describing this in the simulation model will
enable better prediction and understanding of the EC reaction. Lithium ion chemical
diffusion coefficients in the range of 10-11-10-13cm2/s were estimated with this model,
which are in reasonable agreement with values published in the literature.
144
Acknowledgements
This work is supported by an Australian Postgraduate Award (Industry) scholarship
from the Australian Research Council, and Sustainable Technologies Australia (STA).
The work described in this paper has been supported by the Australian Cooperative
Research Centre for Renewable Energy (ACRE). ACREs activities are funded by the
Commonwealths Cooperative Research Centres Program.
145
REFERENCES
[1] C.M. Lampert, A. Agrawal, C. Baertlien and J. Nagai, Solar Energy Materials and
Solar Cells, 56, 449-463 (1999).
[2] M.E. Badding, S.C. Schulz, L.A. Michalski and R. Budziak, Electrochemical
Society Proceedings, 96-24, 369-384 (1996).
[3] J.-G. Zhang, C. Edwin Tracy, D.K. Benson and S.K. Deb, J. Mater. Res., 8, 2649-
2656 (1993).
[4] J.-G. Zhang, D.K. Benson, C. Edwin Tracy and S.K. Deb, J. Mater. Res., 8, 2657-
2667 (1993).
[5] S. Hashimoto and H. Matsuoka, J. Electrochem. Soc., 138(8), 2403-2408 (1991).
[6] C. Ho, I.D. Raistrick and R.A. Huggins, J. Electrochem. Soc., 127(2), 343-350
(1980).
[7] S.K. Mohapatra, J. Electrochem. Soc., 125(2), 284-288 (1978).
[8] J.M. Bell and J.P. Matthews, Temperature Dependence of Kinetic Behaviour of Sol-
Gel Deposited Electrochromics, Solar Energy Materials and Solar Cells (2000),
accepted for publication.
[9] J. Wang, PhD Thesis, University of Technology, Sydney (1998).
[10] J. M. Bell, G. B. Smith, I. L. Skryabin, B. G. Monsma, N. C. Ruck and T. Dinh,
Sol-gel Deposited Electrochromic Devices, in Proceedings of Windows Innovations
Conference WIC 95, 383-391, Minister of Supply and Services, Canada.
[11] A. Koplik, Australian Patent Application, PP0274 (1997).
[12] J. M. Bell and I. L. Skryabin, Solar Energy Materials and Solar Cells, 56 (1999)
437.
146
[13] I. L. Skryabin and J. M. Bell, Control of Electrochromic Devices, International
Patent Application PCT/AU97/00697.
[14] J. Nagai and T. Kamimori, Jap. J. Appl. Phys., 22, 681-687 (1983).
[15] R.S. Crandall, P.J. Wojtowicz and B.W. Faughnan, Solid State Comm., 18, 1409-
1411 (1976).
[16] M. Green, W.C. Smith and J.A. Weener, Thin Solid Films 38 (1976) 89.
[17] W.D. Callister, Materials Science and Engineering: An Introduction, John Wiley
and Sons, Inc., New York (1985).
[18] R.S. Crandall and B.W. Faughnan, Appl. Phys. Lett., 28, 95-97 (1976).
CHAPTER 7
SELF-BLEACHING, MEMORY EFFECT AND REVERSIBILITY OF
ELECTROCHROMISM AT ELEVATED TEMPERATURES.
J.P. Matthews and J.M. Bell
Status: Submitted to Solar Energy Materials and Solar Cells.
148
Contributions of Authors
This paper presents the results of experimental work carried out by J.P. Matthews, under
the supervision of J.M. Bell. The paper was written by J.P. Matthews and revised by
J.M. Bell before final submission of the manuscript.
149
SELF-BLEACHING, MEMORY EFFECT AND REVERSIBILITY OF
ELECTROCHROMISM AT ELEVATED TEMPERATURES.
J.P. Matthews1, J.M. Bell1
1 School of Mechanical, Manufacturing and Medical Engineering,
Queensland University of Technology, Australia
Abstract
Sol-gel deposited WO3/TiO2 electrochromic films have been cycled under various
conditions of moisture and temperature. Films that were coloured and left in a liquid
electrolyte were observed to self-bleach with time, even with the counter electrode
disconnected from the cell. The rate of this self-bleaching was observed to increase
with moisture content and temperature. Charge was able to be injected and extracted
very reversibly under stringently dry conditions, however the time required to bleach the
film under these conditions was unacceptably long.
The results suggest that the water present in the electrolyte and the water present inside
the film play different roles in the self-bleaching mechanism. The addition of water to a
dry electrolyte during film cycling, resulted in significantly lower voltages being
required for bleaching, however the coloration voltage was unaffected. The overall
result is that an electrochromic film has been demonstrated to have an excellent
electrochromic memory and reversibility over a wide temperature range, however this
was accompanied by very long switching times and high voltages required to colour and
bleach.
150
Keywords: Electrochromic memory; reversibility; self-bleaching; water; temperature
dependence
7.1 Introduction
Electrochromic materials undergoes a colour change when ions and electrons are
inserted into them under the influence of an applied electric field or current.
Electrochromic materials have application in Smart Windows, glazings that enable the
amount of heat and light entering a building to be controlled, in order to optimise energy
efficiency [1]. Smart windows will be required to operate at temperatures exceeding
60ºC due to the absorption of solar radiation, inherent to the coloured state [2]. It is then
useful to understand how changes in temperature may affect the electrical and optical
properties of electrochromic materials, especially in terms of reversibility and
electrochromic memory. Several papers have described self-bleaching of
electrochromic films and the way this behaviour is affected by various conditions
including water content [3,4], the type of injected ion [5], film composition [6]
preparation conditions [7] and film roughness [8].
Recent experimental work [9] has shown that the reversibility of the electrochromic
reaction is largely dependent on environmental conditions such as moisture level and
temperature. Irreversible charge injection was observed for temperatures over 30ºC,
whereby a small proportion of the charge injected during colouration was unable to be
extracted during the bleaching process. Chemical analysis revealed that a significant
proportion of the injected ions did indeed remain inside the film after bleaching. Films
that were coloured in a liquid electrolyte in the ambient laboratory environment were
151
also observed to slowly self-bleach with time [9] even when the working electrode was
electrically isolated from the cell. The self-bleaching rate was found to increase
monotonically with temperature, and the authors speculated that the irreversibility
observed was due to the presence of water in the electrolyte. Similar films showed very
reversible behaviour when cycled in a stringently dry environment, at temperatures up to
50ºC, however self-bleaching experiments using a very dry electrolyte have not been
reported.
This paper describes the observation of self-bleaching in electrochromic WO3/TiO2
films cycled under various conditions of moisture and temperature. Films cycled in a
very dry electrolyte were observed to self-bleach, however the rate was much slower
than for a moist electrolyte. A film was fired in a furnace at 250ºC for 8 hours and then
used in self-bleaching experiments in very dry conditions. The film did not self-bleach
significantly over a 30-minute period even at 75ºC, however it was electrochemically
very difficult to bleach the film. The voltage characteristics of the bleaching process
were found to be highly dependent on the level of moisture in the electrolyte.
The electrochromic memory and reversibility of the fired film cycled in a dry electrolyte
were very good, however the slow bleaching response and large voltages required for
switching largely limit the application of such a system.
152
7.2 Experimental
7.2.1 Film preparation
Mixed tungsten-titanium oxide electrochromic films (mole ratio W:Ti of 4:1), were
deposited onto 10cm x 10cm substrates of LOF TEC8/3 glass. The films were coated
using the sol-gel dip coating method [10], with solutions of tungsten and titanium oxy-
butoxides as the organic precursors. The details of the film preparation have been
described previously [11].
7.2.2 Electrochemical Testing
Electrochemical measurements were made using a three electrode cell, the counter and
reference electrodes being a sheet of copper coated platinum (area=85cm2), and a
Ag/AgCl wire respectively. The electrolyte solution was 1M LiClO4/propylene
carbonate, which was dried over molecular sieves prior to use. The complete
electrochemical cell was contained inside a dry glovebox, and maintained with a
nitrogen atmosphere under a slight positive pressure. A glass tank was used to hold the
electrodes and electrolyte of the cell, and this was partially submerged in a temperature
controlled oil-bath. An electrical heater/stirrer unit was used to control the temperature
of the oil bath, and hence the electrolyte solution. The electrolyte solution was also
stirred during the experiments to minimise any thermal lag. The atmosphere inside the
dry-box was maintained at less than 1ppm absolute humidity by exposure to phosphorus
pentoxide (P2O5) and recirculation through pre-dried molecular sieves.
The WO3/TiO2 films were cycled using a voltage-limited constant current technique,
described previously [12,13]. The only difference between this method and that
153
described previously is that the maximum voltage limit for bleaching was not fixed, but
instead was determined in-situ during cycling by measuring the rate of change of
voltage. The constant current bleaching step was terminated when the rate of voltage
increase exceeded 0.5V/s, which ensured that excessive bleaching voltages were not
applied at elevated temperatures.
The self-bleaching experiments involved colouring of films to 15mC/cm2 using a
current density of 0.1mA /cm2 and a film area of 85cm2. The counter electrode was
disconnected from the electrical circuit immediately after completion of the coloration.
Optical and electrical measurements were made at a frequency of 1Hz while the
electrolyte solution was maintained at the appropriate temperature. At the end of the 30
minute self-bleaching period, the counter electrode was reconnected, the film was
bleached and the temperature was changed for the next data set.
The cycling data reported in section 3.3 was recorded in the same fashion as the
experiment described above, however there was no self-bleaching period and the
counter electrode was permanently connected to the circuit.
Optical measurements were made by passing a 1mW, 670nm laser beam through the
working electrode onto a silicon photodiode, and the photocell voltages were used to
calculate the change in optical density (∆OD) of the films during cycling.
154
7.3 Results and Discussion
7.3.1 Observation of self-bleaching at elevated temperatures
Self-bleaching was previously observed when experiments were carried out in the
ambient laboratory environment, and it was proposed that water present in the
electrolyte was reacting with lithium ions at the electrode surface [9]. It was therefore
expected that there would be no self-bleaching when the experiment was repeated in a
very dry electrolyte, but this result was not observed here.
Figure 7.1(a) shows the change in optical density (∆OD) as a function of time during a
self-bleaching experiment carried out under stringently dry conditions. In this
experiment a WO3/TiO2 was coloured by injection of 15mC/cm2 of lithium ions, and the
counter electrode was immediately disconnected from the electrochemical cell. Any
reaction observed is therefore separate to the normal electrochromic process, which
requires an electron current to flow from the back contact of the working electrode to
the counter electrode.
0.45
0.50
0.55
0.60
0.65
0.70
0 5 10 15 20 25 30 35 40
24.3oC
34.8oC
45.2oC
55.1oC
65.3oC
75.3oC
OD
Time (min)
∆
(a)
-0.85
-0.80
-0.75
-0.70
-0.65
-0.60
-0.55
0 5 10 15 20 25 30 35 40
24.3
34.8
45.2
55.1
65.3
75.3
Volta
ge (V
)
Time (min)
(b)
Figure 7.1 Change in optical density (a) and measured voltage (b) during self-bleaching
of a WO3/TiO2 film in dry electrolyte.
155
The optical density of the coloured film is seen to decrease with time as the film slowly
bleaches in the liquid electrolyte. The rate of change in ∆OD increases monotonically
with temperature, although the rates are generally slower than for similar experiments
previously carried out in the ambient environment where significant amounts of
moisture were present. Figure 7.1(b) shows the change in film voltage (measured
relative to the reference electrode) with time. The measured voltage is a function of the
surface composition of the film, and this result indicates that the composition of the
electrode surface is still changing even after 30 minutes at 75ºC.
The fact that self-bleaching was observed even in a very dry electrolyte suggested that
there was some moisture present inside the film, even though they had been stored over
P2O5, so a film was fired in a furnace at 250ºC for eight hours to further dry it. This
fired film was then used for a similar self-bleaching experiment, the results of which are
shown in Figure 7.2(a) and (b).
0.35
0.40
0.45
0.50
0.55
0.60
0 5 10 15 20 25 30 35 40
28.6
35.5
43.8
50
58.1
71.7
Time (min)
OD
∆
(a)-1.20
-1.10
-1.00
-0.90
-0.80
-0.70
0 5 10 15 20 25 30 35 40
28.635.5
43.850
58.171.7
Volta
ge (V
)
Time (min)
(b)
Figure 7.2 Change in optical density (a) and measured voltage (b) during self-bleaching
of a WO3/TiO2 film, after firing at 250ºC for 8 hours.
156
After firing the film, the rate of self-bleaching decreased dramatically, and very little
change in the optical density is observed with time, even at 58ºC. The large amount of
noise in the optical data at 71.7ºC is a result of maintaining the constant temperature
during this cycle. The heater thermostat switched the heating element on and off
intermittently, resulting in changing refractive indices of the oil bath, and hence
interfering with the optical measurements. The rate of self-bleaching is seen to have
almost no temperature dependence, as the lines of ∆OD as a function of time are not
observed to diverge at long times, as they do in Figure 7.1(a).
Figure 7.2(b) shows the electrical response of the working electrode, which is much
flatter than for the un-fired film, especially at the higher temperatures. The voltage data
for the 28.6ºC cycle appears to significantly deviate from linearity, but it is likely that
insufficient time has passed for diffusion processes to occur. This is supported by the
fact that the slopes of the lines of Figure 7.2(b) become approximately the same for high
temperatures and long times. There is little difference between the slopes of the
voltage-time lines between 35.5 and 71.1ºC also indicating that there is little
temperature dependence of the self-bleaching and that the memory of the fired film is
significantly better than of the un-fired film.
The optical density of the coloured state of the film was not uniform as expected but it is
believed that this is due to incomplete charge extraction under the experimental
conditions used. The optical density of the fired film in the bleached states was
observed to increase with progressive cycling however this was not necessarily due to
charge trapping. There is experimental evidence to suggest that not enough time was
allowed to completely bleach the film even though the bleaching current was very small
157
(~1µA/cm2) when the cycle was terminated. When the same film was repeatedly cycled
at room temperature after the self-bleaching experiment, charge extracted during
successive bleaching cycles was greater than that injected during coloration and the
optical density for the bleached states of the film was observed to decrease again,
approaching its original value. Stated quite simply, the fired film cycled in dry
electrolyte was unable to be bleached effectively within a reasonable time frame. The
difficulty in bleaching the film was exacerbated by leaving the film in the coloured state
for a prolonged period of time. Even though the film was not bleached completely
during cycling, the data trends are the same and the significance of the differences seen
when cycling a fired film is still obvious.
7.3.2 Effects of water on self-bleaching rates at elevated temperatures
The change in optical density is related to the injected charge density by the coloration
efficiency (η) hence the slopes of the ∆OD-time plots are proportional to the rates of
self-bleaching. Linear regression was carried out on the data from Figure 7.1 and Figure
7.2, as well as on previously reported self-bleaching data [9] in order to investigate the
effects of moisture and temperature on self-bleaching rate. The curves are
approximately linear from 5 minutes after the end of coloration, until the initialisation of
bleaching, and so this data was used for the determination of self-bleaching rates.
Figure 7.3 shows the change in self-bleaching rate as a function of temperature under
various conditions, where the rate is equal in magnitude but negative in sign to the slope
of the ∆OD-time plots.
158
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
20 30 40 50 60 70 80
unfired film/moist electrolyteunfired film/dry electrolytefired film/dry electrolyte
Self-
blea
chin
g ra
te (-
d O
D/d
t)
Temperature (oC)
∆
Figure 7.3 Rate of self-bleaching for WO3/TiO2 films under various conditions.
It is evident from Figure 7.3 that the self-bleaching rate increases with temperature and
also with the amount of moisture present in the system. It also appears that water
present in the electrolyte plays a much larger role in the self-bleaching mechanism, than
the water inside the film which is removed upon firing. The fired film in the dry
electrolyte does appear to undergo some self-bleaching, however the rate is much lower
than for an unfired film in moist or dry electrolytes, and the temperature dependence is
minimal. Although the fired film cycles very reversibly even up to 70ºC and has a good
electrochromic memory, the rate at which the film bleaches is unacceptably slow unless
very high switching voltages are applied.
159
7.3.3 Effects of water on voltage characteristics
Figure 7.4 shows the voltage-time characteristics during room temperature cycling of
WO3/TiO2 films under various experimental conditions.
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
0 2 4 6 8 10 12 14 16
unfired film/moist electrolyteunfired film/dry electrolytefired film/dry electrolyte
Volta
ge (V
)
Time (min)
Figure 7.4 Voltage characteristics during cycling of WO3/TiO2 films under various
conditions.
The films were cycled using a voltage-limited constant-current injection and extraction
technique described previously [12,13] whereby a constant current was used to colour
and bleach the films, until some voltage limit was reached. The cycling characteristics
of the films shown in Figure 7.4 are all very different, owing primarily to the different
environments used to conduct the experiments. The unfired film cycled in a moist
electrolyte [9] only required relatively small voltages to colour and bleach the film, and
was bleached in approximately 3 minutes. When a similar film was cycled in a very dry
160
electrolyte it took only 30 seconds longer to bleach, however the voltages required for
coloration and bleaching were much higher. The voltage limit during bleaching was
reached when approximately 12mC/cm2 was extracted, and the remainder of the inserted
charge was extracted at this constant voltage. The fired film cycled in the dry
electrolyte required very large voltages to insert and extract charge, and the voltage limit
on bleaching was reached after only 5mC/cm2 of the injected charge was extracted.
This meant that the remaining 10mC/cm2 of lithium ions was extracted under constant
voltage, taking 13 minutes to bleach the film while the current decayed exponentially.
Although these conditions enabled the film to be cycled very reversibly (ie. all of the
injected charge was eventually able to be extracted), the slow response time for
bleaching and the high voltages required for switching make them unpractical for use in
a commercial device.
The lower magnitude of the voltages required to colour and bleach the films when
moisture is present indicates that water plays a key role in the mechanism of charge
injection and extraction. It is interesting to note that the difference in switching
characteristics are more marked for the bleaching process, indicating that water plays a
more significant role in the bleaching mechanism than in the coloration mechanism. It
also seems that the water present in the electrolyte and the water inside the film play
quite separate roles in the electrochromic process.
It is possible that water inside the film assists in the diffusion of ions through the
substrate lattice, by providing a site for charge transfer and by causing swelling and
therefore lattice expansion. Another possibility is that adsorbed water may cation
exchange a proton for a lithium ion, thus forming LiOH and inserting a proton into the
161
lattice. As the protons are much more mobile than the lithium ions, we may reasonably
expect a reduction in the voltage required to maintain a constant current. It is likely that
mixed diffusion of protons and lithium ions is a major cause of the large variation in
switching characteristics between moist and dry conditions.
In order to further investigate the effects of water in the electrolyte on the device
switching characteristics, an unfired film was continuously cycled at room temperature
and water was periodically added to an initially dry electrolyte. Figure 7.5 shows the
way that the cycling characteristics changed when water was added in increments of
approximately 0.01%v/v at 30 minute intervals.
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
0 50 100 150 200 250 300 350
no water
0.01% water
0.02% water
0.03% water
Volta
ge (V
)
Time (s)
Figure 7.5 Dependence of electrochromic cycling characteristics on electrolyte water
concentration.
162
The addition of water to the dry electrolyte had little effect on the coloration voltage, but
considerably changed the bleaching characteristics. This implies that the large
difference between the coloration voltages when cycling unfired and fired films in a dry
electrolyte (Figure 7.4), is due to the presence of water inside the unfired film. We may
reasonably expect water in the electrolyte to change the properties of charge transfer at
the electrode surface. We may also expect water inside the film to affect the diffusion
properties of the lithium ions. As the addition of water to the electrolyte does not affect
the coloration voltage, it is likely that the diffusive motion of ions inside the electrode
limits the electrochromic system discussed here.
The bleaching voltage changes considerably when water is added to the electrolyte,
possibly by reacting with lithium ions at the electrode surface. This hypothesis is
supported by the fact that self-bleaching occurs even when it is not possible for an
electron current to flow, and that some of the injected charge cannot be extracted when
cycling in conditions where moisture is present. It is possible that by the time bleaching
is initiated, some of the charge injected during coloration has already been lost to a
reaction with water in the electrolyte, thereby reducing the optical density of the film
and reducing the quantity of charge remaining to be extracted.
163
7.4 Conclusion
Self-bleaching has been observed in electrochromic films of WO3/TiO2, in a liquid
electrolyte. The rate of self-bleaching was found to increase significantly with
temperature and with increased moisture in the system. Water present in the electrolyte
solution reduced the electrochromic memory of the film, and led to significant
irreversibility of the ion injection process. Water present in the film before cycling was
found to increase the ease with which coloration and bleaching occurred, possibly due to
the mixed diffusion of lithium ions and protons, but also led to some small irreversibility
of the electrochromic process. A fired film was cycled very reversibly in a dry
electrolyte and had an excellent memory, however the voltages required to switch were
very high under these conditions. The time required to bleach the film was also very
excessive, at around 15 minutes.
The results suggest that good electrochromic memory and reversibility are achieved at
the expense of response time, and vice versa. This information may be used in order to
design a device with enough water present to enable reasonably fast switching, while
minimising irreversibility enough to achieve a specific device lifetime.
164
Acknowledgements
This work is supported by an Australian Postgraduate Award (Industry) scholarship
from the Australian Research Council, and Sustainable Technologies Australia (STA).
The work described in this paper has been supported by the Australian Cooperative
Research Centre for Renewable Energy (ACRE). ACREs activities are funded by the
Commonwealths Cooperative Research Centres Program.
165
REFERENCES [1] C. M. Lampert, IEEE Circuits and Devices, 8, 19-26 (1992).
[2] C.M. Lampert, A. Agrawal, C. Baertlien and J. Nagai, Solar energy Materials and
Solar Cells, 56, 449-463 (1999).
[3] J.P. Randin, J. Electron. Mater., 7, 47-63 (1978).
[4] J.A. Duffy, M.D. Ingram and P.M.S. Monk, Solid State Ionics, 58, 109-114(1992).
[5] Q. Zhong, S.A. Wessel, B. Heinrich and K. Colbow, Solar Energy Materials, 20,
289-296 (1990).
[6] S. Hashimoto and H. Matsuoka, J. Electrochem. Soc., 138(8), 2403-2408 (1991).
[7] M. Burdis and J.R.Siddle, Thin Solid Films, 237, 320-325 (1993).
[8] J.-G. Zhang, D.K. Benson, C.Edwin Tracy, J. Webb and S. Deb, Proceedings of the
SPIE, Vol 2017, 104-112 (1993).
[9] J.P. Matthews, J.M. Bell and I.L. Skryabin, "High Temperature Behaviour of
Electrochromics", Renewables: The Energy for the 21st Century
Proceedings of the World Renewable Energy Congress VI, Brighton, UK (A.A.M.
Sayigh Ed.), 230-235 (2000).
[10] J. M. Bell, G. B. Smith, I. L. Skryabin, B. G. Monsma, N. C. Ruck and T. Dinh,
Sol-gel Deposited Electrochromic Devices, in Proceedings of Windows Innovations
Conference WIC 95, 383-391, Minister of Supply and Services, Canada.
[11] A. Koplik, Australian Patent Application, PP0274 (1997).
[12] J. M. Bell and I. L. Skryabin, Solar Energy Materials and Solar Cells, 56 (1999)
437.
[13] I. L. Skryabin and J. M. Bell, Control of Electrochromic Devices, International
166
Patent Application PCT/AU97/00697.
CHAPTER 8
GENERAL DISCUSSION
168
GENERAL DISCUSSION
8.1 Introduction and Identification of Knowledge Gaps
This chapter discusses the entire body of results collected in this PhD research as a
single collection, as opposed to as individual publications. The experimental research
was carried out in several stages, and each stage is discussed separately in sections 8.2
to 8.5. These sections are not necessarily presented in chronological order (as they are
in the publications), but are rather presented in the context of a cohesive research project
with specific aims and goals. Several knowledge gaps were identified in the Literature
Review (Chapter 2) and these can be summarised as follows:
1. How does temperature effect
a. Electrical response of EC films
b. Optical response of EC films
2. How is the reversibility of the EC process affected by
a. Temperature?
b. Water?
3. What are the causes of irreversibility and self-bleaching, and how can these
effects be minimised?
4. Can a single model be used to describe EC behaviour over a wide temperature
range (ie. 20ºC < T < 70ºC)?
169
Sections 8.2 to 8.5 therefore serve to discuss the research progress in the context of
addressing and systematically filling these four knowledge gaps in turn. Although this
chapter necessarily repeats discussion and conclusions made in previous chapters, it has
been more succinctly discussed to minimise repetition. The detail given here is then
more brief than for previous chapters, however reference is made to relevant chapters if
more detailed information is sought.
8.2 Initial Characterisation in the Ambient Laboratory Environment
Initial experiments involved cycling of WO3/TiO2 films using a constant current
technique between 20ºC and 50ºC, and Figure 8.1 shows the voltage response during
coloration and bleaching at elevated temperatures. The film was coloured with a
constant current for 150 seconds, and then bleached after a 30 second delay. The
magnitude of the voltages required for coloration and bleaching was reduced by
increasing the temperature, although the same amount of charge was injected each time.
Decreases in switching voltages were greatest for small deviations from room
temperature with the voltage data converging at high temperature. These results
illustrated that small increases from room temperature will result in significant changes
in the kinetic behaviour of the film, and a good switching regime should appropriately
reflect this.
170
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0 50 100 150 200 250 300 350 400
20.6oC
28.4oC
32.3oC
50.0oC
Volta
ge (V
)
Time (s)
Figure 8.1 Curves of applied voltage versus time, measured during coloration and
bleaching of WO3 thin film electrode and plotted for four temperatures, for an injected
charge density of 15mC/cm2.
Under these conditions some irreversibility was observed at high temperatures,
identified by the inability to extract all the injected charge. The amount of charge that
could not be extracted increased monotonically with temperature, however the same
optical density was achieved for the bleached states. The small amount of charge
remaining in the films after bleaching at high temperature therefore did not contribute to
coloration. As the reversibility was found to decrease with temperature, a separate
experiment was carried out involving cycling to 5mC/cm2, in an attempt to reduce the
amount of charge trapped at high temperature. The emf for coloured and bleached states
of the film was measured after allowing sufficient time for diffusion processes to occur.
171
The magnitude of the emf was found to decrease with temperature, however there was a
hysteresis effect for results obtained at increasing and decreasing temperatures.
The coloration voltage (Vc), calculated by subtracting the emf from the measured
voltage, did not exhibit this hysteresis, suggesting that the hysteresis effect was
associated purely with the film emf. The maximum coloration voltage (Vc max) was
found to be related to temperature by Arrhenius type behaviour, with log Vc max
approximately proportional to reciprocal temperature as shown in Figure 8.2.
-0.95
-0.90
-0.85
-0.80
-0.75
-0.70
0.0030 0.0031 0.0032 0.0033 0.0034 0.0035
y = -3.0546 + 688.62x
R2= 0.87498
log
|V
|
(V)
1/T (K-1)
c m
ax
Figure 8.2 Log of absolute value of maximum coloration voltage versus reciprocal
temperature for WO3 film, for an injected charge density of 5mC/cm2.
Figure 8.3 shows the change in optical density (∆OD) versus injected charge density for
cycles performed up to 50ºC. The coloration efficiency is observed to decrease slightly
at elevated temperatures and it was hypothesised that effect was related to the issue of
172
irreversible charge injection. As the injected charge density increases during coloration,
the emf decreases. If charge was irreversibly accumulating in the film with progressive
cycling we would expect the magnitude of the emf for a given charge density to slowly
decrease. This was observed in this experimental work, which supports the theory of
irreversible charge injection and helps explain the hysteresis associated with the voltage
data.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.0 4.0 8.0 12.0 16.0
20.6oC
28.4oC
32.3oC
50.0oC
OD
Charge injected (mC/cm2)
∆
Figure 8.3 Change in optical density versus injected charge for WO3 thin film electrode
at four temperatures.
This work showed that electrochromic voltage characteristics are heavily dependent on
temperature and also demonstrated that a constant current technique is superior to
constant voltage, for controlling electrochromics over a wide temperature range.
Constant current charge injection allows control of injected charge density and therefore
coloration level, regardless of temperature. Previous reports of electrochromic
173
experiments at high temperature focussed on response times, or only provided limited
data hence this research and paper 1 (Chapter 3) represent a significant contribution to
filing that particular knowledge gap (Knowledge gap 1 in section 8.1).
8.3 Dry-box Experiments
At the time of this initial research the causes of the charge injection irreversibility and
reduction in coloration efficiency were not known although it was suspected that water
may play a key role. All of the work discussed in section 8.2 was carried out in the
ambient laboratory environment. During this time, attempts were made to keep the
electrolyte solution free from water by using molecular sieves and a positive pressure of
dry nitrogen, however the high humidity of the location (Brisbane, QLD Australia)
combined with the highly hygroscopic nature of the electrolyte made this experimentally
difficult. In order to determine the effect of water on reversibility, the experimental
apparatus was moved inside a dry-box.
The dry-box was maintained in a stringently dry condition by exposing the internal
atmosphere to phosphorus pentoxide (P2O5) and recirculating nitrogen through
gas-drying units. Access to the experimental apparatus was made possible via three
glove ports and a load lock chamber. Solenoid valves controlled the flow of gas into
and out of the box in order to maintain a slight positive pressure inside the box at all
times and humidity was kept below 1 ppm absolute.
174
Initial experiments in the dry-box involved cycling of a WO3/TiO2 film to 15mC/cm2
between 20ºC and 50ºC. Films cycled in this fashion exhibited very reversible
behaviour and the coloration efficiency was found to be independent of temperature.
The coloration efficiency was determined to be 38.6cm2/mC, and the plot of ∆OD
(change in optical density) versus injected charge density was very linear with a Pearson
moment correlation coefficient (R2) of 0.99 (See Figure 4.1(b)). The large differences in
reversibility when cycling in ambient and dry conditions are illustrated by Figure 8.4,
which shows the percentage of injected charge unable to be extracted on bleaching. The
amount of irreversible charge clearly increases with temperature under ambient
conditions, and is significant even at 30ºC.
-3
-2
-1
0
1
2
3
4
0 10 20 30 40 50 60
Ambient conditionsDrybox experiment
Cha
rge
trapp
ed p
er c
ycle
(%)
Temperature (oC)
Figure 8.4 Reversibility of cycling at elevated temperatures, represented as the
percentage of the injected charge density trapped per cycle.
175
The amount of irreversible charge in dry conditions is very small, and at high
temperatures more charge was removed than inserted for most of the cycles. This was
because the same voltage limits were applied for bleaching at all temperatures, which
has the effect of pushing the bleaching process progressively further as temperature
rises. During pre-experiment training of the WO3/TiO2 film, large amounts of charge
were irreversibly inserted however reversibility quickly improved after a few (~10)
cycles. The fact that the quantity of charge extracted exceeds the injected charge at high
temperatures it likely due to removal of some of the lithium ions inserted during the film
training process.
This body of work showed that the trends in voltage reduction with temperature which
were first observed in the ambient laboratory environment, are the same for very dry
conditions. This work also showed that films may be cycled much more reversibly in
dry conditions, even up to 50ºC, and that the coloration efficiency and ion injection
irreversibility are not dependent on temperature under these conditions. These facts
support the theory that water is responsible for the irreversibility issues observed in
ambient conditions. This work then provides a valuable contribution to filling
knowledge gap 2 (Section 8.1).
176
8.4 Self-bleaching Experiments
The work described in the previous section has shown that irreversibility increases with
water content and temperature. It does not however show why the irreversible charge
did not contribute to coloration and why the coloration efficiency decreased at elevated
temperatures. A set of self-bleaching experiments was devised in an attempt to answer
these questions and fill knowledge gap 3 (Section 8.1). These experiments involved the
coloration of a film in the electrolyte, followed by immediate disconnection of the
counter electrode. Under these conditions the film should not undergo any redox
reaction because there is no path for electron flow from the working electrode. We then
ideally expect the film to maintain a constant level of coloration independent of time, an
effect known as electrochromic memory. Figure 8.5 shows the change in optical density
versus time for a film self-bleaching in electrolyte in the ambient environment.
It is evident from Figure 8.5 that the film was slowly self-bleaching with time, and the
rate increases at higher temperatures. Voltage measurements showed a similar trend
(See Figure 4.3(b)) towards more anodic potentials indicating that the surface
composition of the film was slowly changing with time.
177
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0 5 10 15 20 25 30 35 40
26.4oC
32.9oC
40.2oC
45.8oC
50.5oCO
D
Time (min)
∆
Figure 8.5 Change in optical density with time for a film coloured to 15mC/cm2 at
various temperatures.
After the 30 minute self-beaching period the counter electrode was connected and the
cell bleached as per usual. The amount of charge that could not be extracted from the
films was proportional to temperature and at 50.5ºC approximately 4.3mC/cm2 (of the
20mC/cm2 injected) could not be extracted from the film. This self-bleaching did not
occur by the normal electrochromic process because the electrode was isolated from the
cell, and so must be attributed to some side reaction. It is likely that this side reaction
involves water and lithium ions and this reaction would be at least partially responsible
for the observed irreversibility.
Chemical analyses were carried out to gain information about the self-bleaching
mechanism and to determine the location of the missing lithium ions. A WO3/TiO2 film
178
that had been used for a self-bleaching experiment was dissolved off the glass/ITO
substrate and the quantity of lithium and tungsten present was determined by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES). The results showed that the
amount of lithium present in the bleached film corresponded to approximately
15mC/cm2. The total accumulation of charge not extracted after many cycles was
almost 130mC/cm2 so only a small portion of the total irreversible charge remained
inside the film. The large remainder of the lithium was then presumably lost to the
electrolyte by reaction of water (in the electrolyte) with lithium at the film surface,
possibly causing some film dissolution. This work showed that both water and
temperature play key roles in the reversibility of the EC process, and that water
promotes self-bleaching and is closely associated with irreversibility.
At this stage, it was desirable to show that self-bleaching did not occur appreciably
under very dry conditions, thereby confirming the connection between water and the
reduction in coloration efficiency and hence electrochromic memory. Self-bleaching
experiments were then carried out in the dry-box with the expectation that no self-
bleaching would be observed, however this result was not seen. Figure 8.6 shows the
change in optical density versus time for a self-bleaching experiment in the dry-box.
179
0.45
0.50
0.55
0.60
0.65
0.70
0 5 10 15 20 25 30 35 40
24.3oC
34.8oC
45.2oC
55.1oC
65.3oC
75.3oCO
D
Time (min)
∆
Figure 8.6 Change in optical density during self-bleaching of a WO3/TiO2 film in dry
electrolyte.
It is obvious that self-bleaching still occurred even in the dry-box and the rate increased
with temperature, however the rates are considerably lower than for in the ambient
environment. The voltage-time data (See Figure 7.1(b)) confirmed that the surface
composition of the film was actually changing with time and the rate was also
proportional to temperature.
The fact that self-bleaching occurred even in a very dry electrolyte suggested that
perhaps some water was still present inside the film, so another film from the same
batch was fired in a furnace at 250ºC for eight hours, and then used in a similar
experiment. The results from self-bleaching of a fired film are shown in Figure 8.7.
After firing the self-bleaching characteristics of the film were very different. The rate of
180
self-bleaching was very small and the optical density remained essentially constant even
at 58ºC.
0.35
0.40
0.45
0.50
0.55
0.60
0 5 10 15 20 25 30 35 40
28.6
35.5
43.8
50
58.1
71.7
Time (min)
OD
∆
(a)-1.20
-1.10
-1.00
-0.90
-0.80
-0.70
0 5 10 15 20 25 30 35 40
28.635.5
43.850
58.171.7
Volta
ge (V
)Time (min)
(b)
Figure 8.7 Change in optical density (a) and measured voltage (b) during self-bleaching
of a WO3/TiO2 film, after firing at 250ºC for hours.
The large amount of noise present in the 71.7ºC optical data was due to the heater being
used to maintain the temperature (as described in section7.3.1), and does not accurately
represent the optical properties of the film. The voltage data is free from this noise as
seen in Figure 8.7(b) and the voltages are indeed very flat with time. The slight
curvature of the low temperature data was caused by not allowing sufficient time for
diffusion processes to occur (as discussed in section 7.3.1), a conclusion supported by
the fact that the voltage-time slopes are almost constant for long times. The slopes of
the voltage-time lines for 35.5ºC and 71.1ºC are almost the same, indicating that the rate
of self-bleaching for the fired film is almost independent of temperature. This
conclusion is also supported by the fact that the optical density values do not diverge for
181
long times, as they do for the unfired films (See for comparison Figure 8.5 and Figure
8.6).
Self-bleaching rates were calculated by linear regression of the optical density-time
curves for experiments carried out in the ambient environment, and for fired and unfired
films in the dry-box (See section7.3.2). The dependence of the self-bleaching rates on
water and temperature is illustrated by Figure 8.8.
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
20 30 40 50 60 70 80
unfired film/moist electrolyteunfired film/dry electrolytefired film/dry electrolyte
Self-
blea
chin
g ra
te (-
d O
D/d
t)
Temperature (oC)
∆
Figure 8.8 Rate of self-bleaching for WO3/TiO2 films under various conditions and
temperatures.
It is clear from Figure 8.8 that the rate of self-bleaching increases with both water and
temperature. It is also apparent that water present in the electrolyte plays a different role
182
in the self-bleaching mechanism, to water present in the film. Self-bleaching is
detrimental to the normal operation of an EC system, it would seem that the ideal
conditions include very dry electrodes cycled in very dry electrolytes. The fired film
cycled in the dry electrolyte does indeed have very good electrochromic memory and
reversibility even at high temperature, however the response times for bleaching are
excessively long. This effect is demonstrated more clearly by looking at the voltage-
time characteristics for cycles carried out at room temperature, under the various
conditions described above. Figure 8.9 shows the measured voltage during cycling of
three films coloured to 15mC/cm2, under the same set of conditions as for Figure 8.8.
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
0 2 4 6 8 10 12 14 16
unfired film/moist electrolyteunfired film/dry electrolytefired film/dry electrolyte
Volta
ge (V
)
Time (min)
Figure 8.9 Voltage characteristics during cycling of WO3/TiO2 films to 15mC/cm2
under various conditions.
The film cycled in the ambient laboratory environment only required very small
183
voltages for coloration and bleaching, and was bleached in approximately three minutes.
Cycling a similar film in a dry electrolyte significantly increased the voltages required
for coloration and bleaching, although the bleaching step only took 30 seconds longer.
Cycling the fired film in dry electrolyte required very high voltages for coloration and
bleaching and the bleaching step took approximately 13 minutes.
The constant current technique used for this cycling involved the application of
0.1mA/cm2 until some voltage limit was reached. In the bleaching step, the voltage
limit was determined in-situ by limiting the rate of change voltage with time, as
described in section 7.2.2. Only 5mC/cm2 of the injected charge was extracted before
the bleaching voltage limit was reached for the fired film in dry electrolyte. This means
that the remaining 10mC/cm2 of charge was extracted under a constant voltage, while
the current decayed exponentially. Cycling in this manner enabled the charge to be
extracted very reversibly (ie. all of the inserted charge could eventually be removed with
time), however the slow response and high switching voltages required, make this
electrochromic system of little practical use.
It is possible that water inside the film assists in diffusion of ions thereby reducing the
voltage required for cycling. It is also possible that water inside the film could cation
exchange a proton, thereby trapping a lithium ion by forming LiOH. The high mobility
of protons would allow much faster switching, thereby requiring smaller voltages to
maintain a given current density. It is then likely that mixed diffusion of protons and
184
lithium ions occurs in electrodes where water is present, contributing to the large
difference in switching voltage characteristics for moist and dry conditions.
Another experiment was carried out in order to determine the effect of electrolyte water
on cycling conditions. An unfired film was cycled continuously to 15mC/cm2 in dry
electrolyte and water was periodically added to the electrolyte in small increments (See
section 7.3.3). Figure 8.10 shows the voltage-time characteristics for the film, for
several additions of water to the electrolyte.
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
0 50 100 150 200 250 300 350
no water
0.01% water
0.02% water
0.03% water
Volta
ge (V
)
Time (s)
Figure 8.10 Dependence of electrochromic cycling characteristics on electrolyte water
concentration.
185
The coloration voltage is essentially unaffected by addition of water to the electrolyte,
however the ease with which bleaching occurs is considerably increased. This implies
that the significant difference in coloration voltages for fired and unfired films in a dry
electrolyte (Figure 8.9) is due to water inside the film, and not caused by electrolyte
water. It is therefore likely that the coloration process for the system investigated is
limited by diffusion, rather than charge transfer at the electrode/electrolyte interface.
It is also probable that the reduction in bleaching voltages observed in Figure 8.10 is due
to reaction of water with lithium ions at the film surface. Self-bleaching occurs even
when it is not possible for an electron current to flow, which further supports this
hypothesis and would also account for the ion injection irreversibility associated with
the self-bleaching process. According to this scheme, at the time when bleaching is
initiated, some of the injected charge has already been lost to reaction with electrolyte
water, thereby reducing the amount of charge available for extraction and reducing the
films optical density.
This work shows a clear correlation between the irreversibility, self-bleaching and the
presence of moisture. Films may be cycled very reversibly, with little self-bleaching
under very dry conditions, however this is achieved at the expense of response time.
This suggests that good electrochromic memory and fast response are mutually
competitive aims, and perhaps some compromise is needed, and this work goes a long
way towards filling knowledge gap 3 (Section 8.1).
186
8.5 Simulation and Estimation of Ionic Mobility
A major objective of this work was to simulate electrochromic voltage response to a
coloration current at elevated temperatures, to enable prediction of EC behaviour and
improve device design and control strategy and hence contribute to filling knowledge
gap 4 (Section 8.1). Simulation of data involves modelling of the ion distribution during
coloration but if the process is not reversible, charge may accumulate and there is no
simple way to tell how much charge is in the film for a given cycle. The results from
initial experiments in the ambient environment were therefore not suitable for
modelling, but dry-box experiments provided good data for this purpose.
The simulation model reported by Wang [1] elaborated on, and combined parts of
previous electrochromic models. Wang used this model to simulate voltage
characteristics for cycling of a film to various charge densities at various rates. The
model simulated the data well at room temperature, but no results were reported for
higher temperatures. The model is discussed in detail in Chapter 5 so this section will
focus more closely on the results of data simulations at high temperature. The
simulation model may be described as a function of the surface lithium concentration
(c(0,t)) by the equation
−
−++=),0(.1
),0(.ln),0('.)(
tcVtcV
FRTtcVbaRitV
m
mmcca
ν (8.1)
which is a modified form of equations first reported by Nagai et al [2] and Crandall et al
[3]. Rc is the series resistance of coloration, ic is the switching current and a, b' and ν
are constants discussed in section 5.2.3. Vm is the molar volume of the film and
187
−=
dydEbb' where dE/dy is the slope of the coulometric titration curve. The surface
lithium concentration c(0,t) is the charge density in moles per unit volume at the
electrode/electrolyte interface and is determined from the equations
Γ=DnF
jtc 2),0( (8.2(a))
and ππt
Dtlkt
Dtklerfc
Dkl
kk
−−−=Γ ∑∑∞
=
∞
= 1
22
1
)][exp(2]2[ (8.2(b))
where n is the number of electrons in the process, l is film thickness, j is current density,
F is Faradays constant and D is the chemical diffusion coefficient of lithium ions.
All the variables in equation 8.2 were known, except for diffusion coefficient. The
modelling process then involved estimation of diffusion coefficients for lithium at
various temperatures in order to solve equation 8.2(a) and (b) and calculate the surface
lithium concentration for given time (t). The variation in surface lithium concentration
c(0,t) was then used to solve equation (8.1) and estimate the applied voltage as a
function of time.
The experimental data for the modelling component was collected by cycling a
WO3/TiO2 film to 15mC/cm2 at temperatures ranging from 20ºC to 50ºC. A least
squares method was used to determine the best fit to the 20ºC data by adjusting the
constants a, b, ν, Rc and D. The voltage/time data colouration at the other temperatures
was then simulated by holding the values for a, b, ν and Rc constant and only changing
the diffusion coefficient. Figure 8.11 shows the experimental data and simulations for
188
the lowest and highest temperatures studied (Graphs for the other temperatures are
shown in Figure 5.5).
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
0 50 100 150
Exp 20.1oCSim 20.1oC
Volta
ge (V
)
Time (s)
(a)
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0 50 100 150
Exp 50.0oC
Sim 50.0oC
Volta
ge (V
)
Time (s)
(b)
Figure 8.11 Experimental and simulated voltages during charge injection of a sol-gel
electrochromic film. (a) Temperature = 20.1ºC, D = 2.07x10-12cm2 and
(b) temperature = 50.0ºC, D = 1.71x10-11cm2.
It is evident from Figure 8.11 that the theoretical voltage characteristics predicted with
the model described above are a very close approximation to the experimental data. The
diffusion coefficients estimated from the simulation process increased with temperature
as we may predict from thermodynamic and kinetic considerations, with an Arrhenius
dependence on temperature. Figure 8.12 shows an Arrhenius plot of lnD versus
reciprocal temperature, which was used to calculate the activation energy for diffusion
of lithium ions of 0.58eV.
189
-27.0
-26.5
-26.0
-25.5
-25.0
-24.5
0.003 0.0031 0.0032 0.0033 0.0034 0.0035
y = -3.585 -6767.8x
R2= 0.94663
ln D
1/T (K-1)
Figure 8.12 Arrhenius plot showing the variation in estimated diffusion coefficients
with temperature between 20.1ºC and 50.0ºC.
The plot shown in Figure 8.12 does appear to deviate from linearity at high temperature,
but the cause of this was unclear at the time of this work. The deviation from linearity
may be due to limitations and assumptions inherent to the model as discussed in section
5.5, and it was hoped that data recorded at higher temperature may help understand this
behaviour. This research work represents the first time a relatively simple model was
used to simulate voltage characteristics of an electrochromic system at elevated
temperatures. Other models have failed to describe the large change in switching
voltages when temperature increases, so this work was a significant improvement in
terms of electrochromic modelling. The paper resulting from this research was also the
first to report the estimation of ionic mobility from voltage-time data, which makes this
technique a valuable research tool.
190
Film cycling experiments were repeated between 36ºC and 76ºC in order to test the
model over a more realistic temperature range and also to attempt to resolve the issue of
high temperature deviation from Arrhenius behaviour. Films were again coloured to
15mC/cm2 and voltage characteristics recorded. The results were simulated in a similar
fashion to the experiment described above (See section 6.2.3 for details and plots).
Diffusion coefficients were again within the approximate range expected although the
35.6ºC diffusion coefficient of 2x10-12cm2/s was quite low. This result indicated that the
switching characteristics are very dependent on ionic mobility, with low voltages
required to switch films with high ionic mobility. The estimated diffusion coefficients
again increased with temperature and obeyed an Arrhenius law, as illustrated by
Figure 8.13.
-28.0
-27.0
-26.0
-25.0
-24.0
-23.0
0.0029 0.0029 0.0031 0.0031 0.0032
y = -11497x + 9.052
R2= 0.961
ln D
1/T (K-1)
Figure 8.13 Arrhenius plot showing the variation in estimated diffusion coefficients
with temperature between 35.6ºC and 76.4ºC.
191
Figure 8.13 was used to determine the activation energy for diffusion of lithium ions of
0.99eV. This value is significantly higher than that for the previous modelling
experiment, however this result is in accord with the smaller lithium diffusion rates for
the film used this experiment.
The Arrhenius plot shown in Figure 8.13 still deviates from linearity at high
temperature, however this time it is in the other direction. It is likely that the model
used is accurate for lower temperatures because under these conditions diffusion
processes are slower, and then more likely to be the limiting mechanism. The model
used assumes that the coloration process is controlled by the diffusion-limited motion of
ions and a series resistance. As temperature increases, diffusion becomes very fast and
the applied voltages are much lower. Under these conditions, it is likely that charge
transfer is a more significant rate limiting mechanism and this is a possible reason for
deviations from linearity observed in Figure 8.12 and Figure 8.13. It is likely that the
model could be improved by including a term to describe the charge transfer resistance
with temperature and this model would ideally describe the gradual change in rate-
limiting mechanisms as temperature increases.
This research work and resulting paper (Chapter 6) is significant because it is the first to
address the possibility that the rate limiting mechanism of the electrochromic process is
dependent on temperature, and also is an important contribution to knowledge gap 4
(Section 8.1).
192
8.6 Conclusions
The kinetic behaviour of electrochromic tungsten/titanium mixed oxide ion-insertion
electrodes is largely affected by both temperature and water. Temperature affects the
properties in several ways. Firstly it increases ionic diffusion rates, which makes faster
switching possible and requires lower switching potentials for a specific injected charge
density. Increased temperature also increases the ease with which side reactions occur
and therefore may cause detrimental effects such as self-bleaching, ion injection
irreversibility and decreased electrochromic memory, if water is present.
The presence of water promotes ion injection irreversibility and causes self-bleaching to
occur with time, thereby reducing the optical density and progressively lowering the
maximum contrast ratio that may be achieved. The presence of water may also increase
the ease with which bleaching is performed, possibly due to mixed diffusion of protons
as well as the injected lithium ions. Electrochromic films may be cycled very reversibly
at temperatures as high as 70ºC if very dry conditions are maintained, but the bleaching
rate is then significantly reduced.
These results suggest that achieving a good electrochromic memory and high
reversibility may come at the cost of increased response time. Conversely a device with
moisture present may require small voltages for switching and respond very quickly, but
reversibility will be lowered thereby limiting device lifetime. It is possible that a
successful commercial electrochromic device may only be produced when a fine
balance between reversibility and response time is achieved.
193
This work has also shown that a single mathematical model may be used to describe the
kinetics of the coloration process in sol-gel deposited tungsten-titanium oxide films. A
large advantage of this process is that ionic mobility can be estimated in the process.
The ability of the model to accurately describe the electrical response is poorer at
elevated temperatures (>50ºC) possibly due to a change in rate limiting mechanism from
one of diffusion-limited motion of ions, to being charge-transfer limited. Even though
there is some small discrepancy between real and simulated data at these temperatures,
the difference is small (ie. ~10mV). The model is therefore suitable for use in
determination of suitable control strategies for electrochromic devices, over a
temperature range suitable for real practical use.
8.7 Future Research
Several knowledge gaps have been at least partially filled as a result of this doctoral
project, however several more questions have arisen. A large area for future research
relates to specific information regarding the mechanisms of the degradation processes,
in particular irreversible ion insertion and self-bleaching.
The location of ions which may not be extracted is not known, as well as the reason for
these ions not contributing to the overall film coloration. It is possible that the ions
could for example be trapped at grain boundaries, and if this were so it may be possible
to improve the morphology of the film to reduce these detrimental effects. It would be
very useful to know the location of all lithium ions which can not be extracted after film
194
coloration. It is not possible to look for these ions in the electrolyte, because the
concentration of lithium is so high initially (ie. 1MLiClO4 in propylene carbonate). If
indeed the ions are lost to the electrolyte by reaction of water with surface LiWO3 sites,
we would reasonably expect to see some small concentration of tungsten in the
electrolyte solution. This experiment would be relatively simple, yet provide valuable
information regarding the mechanism of the irreversibility.
Perhaps some chemical modification of the film composition may also help stabilise the
film against dissolution, while still allowing reasonably fast ionic insertion and
extraction.
It would also be beneficial to extend the model described here to consider the bleaching
process as well as the charge transfer step at high temperatures. The situation of a
complete electrochromic cell is considerably more complex than for a single film, and a
large amount of work is required to adapt the model for this use.
A future goal would be to use the model to build an electrochromic controller which can
use cycling information and external parameters such as temperature and light levels, to
automatically control EC devices. Such a system could optimally control an EC device
in a manner so that excessive voltages are never applied, and some requirements such as
internal temperature (of a building) or light level are consistently met. It is likely that
EC device will always slowly degrade, but the rate of degradation need only be reduced
sufficiently to achieve a lifetime great enough to for allow successful commercialisation.
195
REFERENCES
[1] J. Wang, PhD Thesis, University of Technology, Sydney (1998).
[2] J. Nagai and T. Kamimori, Jap. J. Appl. Phys., 22, 681-687 (1983).
[3] R.S. Crandall, P.J. Wojtowicz and B.W. Faughnan, Solid State Comm., 18, 1409-
1411 (1976).
196