ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 94 (2010) 668–674

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

$This

(EMRS)

tion Soln Corr

Taiwan

fax: +8

E-m

journal homepage: www.elsevier.com/locate/solmat

Enhancing the performance of dye-sensitized solar cells by incorporatingnanomica in gel electrolytes$

Yi-Hsuan Lai a, Chia-Yu Lin a, Jian-Ging Chen a, Chun-Chieh Wang a, Kuan-Chieh Huang a, Ken-Yen Liu b,King-Fu Lin b,c, Jiang-Jen Lin c, Kuo-Chuan Ho a,c,n

a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwanb Department of Materials and Science Engineering, National Taiwan University, Taipei 10617, Taiwanc Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

a r t i c l e i n f o

Article history:

Received 5 November 2009

Received in revised form

23 November 2009

Accepted 24 November 2009

Keywords:

Dye-sensitized solar cells

Gel electrolytes

Nanomica

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.solmat.2009.11.027

paper (AP3-42) was presented at the Europea

2009 Spring Meeting, Mesoscopic Dye Sensiti

ar Cells, June 8 to 12, Strasbourg, France.

esponding author at: Department of Chem

University, Taipei 10617, Taiwan. Tel.: +886

86 2 2362 3040.

ail address: kcho@ntu.edu.tw (K.-C. Ho).

a b s t r a c t

Gel-type dye-sensitized solar cells (DSSCs) were fabricated with 5.0 wt% polyvinyidene fluoride-co-

hexafluoro propylene (PVDF-HFP) in methoxy propionitrile (MPN) as gel polymer electrolyte (GPE),

1-butyl-3-methylimidazolium iodide (BMII)/iodine (I2) as redox couple, 4-tertiary butyl pyridine (TBP)

and guanidine thiocyanate as additives.

The incorporation of alkyl-modified nanomica (AMNM) in the PVDF-HFP gel electrolytes caused the

reduction of crystallization of PVDF-HFP, which was confirmed by X-ray diffraction (XRD) analysis. The

short-circuit current density (JSC) of the cell increased due to the decrease of diffusion resistance, as

judged by the electrochemical impedance spectra (EIS) analysis, while the open-circuit voltage (VOC)

remained almost the same. As the loading of AMNM in the PVDF-HFP gel electrolyte was increased to

3.0 wt%, the JSC and power conversion efficiency (Z) of the cells increased from 8.3 to 13.6 mA/cm2 and

3.5% to 5.7%, respectively. However, the JSC decreased as the loading of AMNM exceeded 3.0 wt%. At

higher AMNM loadings, nanomica acted as a barrier interface between the electrolyte and the dye

molecules to hinder electron transfer, and thus reducing the cell’s photocurrent density.

Furthermore, the DSSCs fabricated by dispersing polymethyl methacrylate (PMMA) microspheres in

the TiO2 electrode with the GPE containing 3.0 wt% AMNM improved the Z to 6.70%. The TiO2 films

would exhibit larger porosity by blending with PMMA, leading the penetration of GPEs into the porous

TiO2 easier, thus improving the contact between the dye-adsorbed TiO2 surfaces and the GPEs, as

characterized by EIS. Moreover, the Z of gel-type DSSCs with a 25 mm thickness of surlyn reached 7.96%

as compared with 6.70% for the DSSCs with a 60 mm surlyn.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Although classical DSSCs, composed of liquid electrolytes,reach rather high energy conversion efficiencies, the vaporizationand leakage of the liquid electrolytes result in low durability [1,2].In order to solve the problems, many efforts have been made toreplace liquid electrolytes, including p-type semiconductors [3,4],organic hole-transport materials [5,6] and gel electrolytes [7–13].As for the gel electrolytes investigated, they were preparedeither by incorporating liquid electrolyte into a polymer matrix,

ll rights reserved.

n Materials Research Society

zed and Organic Heterojunc-

ical Engineering, National

2 2366 0739;

including polyvinyidene fluoride-co-hexafluoro propylene (PVDF-HFP) [7], polyacrylonitrile (PAN) [8], polyethylene oxide (PEO)[8,9], polyacrylonitrile-co-methyl methacrylate [10] and poly(methylmethacrylate) (PMMA) [11,12] or by in situ polymeriza-tion [13]. However, due to lower ionic conductivity and poorcontact between TiO2 electrode and electrolytes of solid state-type or gel-type DSSCs, they showed poor cell performancecompared with liquid-type.

To improve the cell performance for practical application,some researchers incorporated nanoparticles into gel electrolytesto enhance the efficiency of DSSCs [14–17]. The improved cellperformance by adding nanoparticles can be attributed toreduction of crystallinity [14], building a transfer channel[14,17] and reducing the charge recombination [16].

Mica is natural clay and belongs to the structural family knownas the 2:1 phyllosilicates (Fig. 1(a)). Compared with nanoparticles,such as TiO2 and SiO2, nanomicas have high aspect ratio due totheir thin platelet structure, resulted from exfoliated process.

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Fig. 1. (a) The structure of 2:1 phyllosilicates and (b) the structure of AMNM.

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 94 (2010) 668–674 669

Their crystal lattice consists of two-dimensional layers, where acentral octahedral sheet of alumina or magnesia is fused to twoexternal silica tetrahedron by the tip so that the oxygen ions ofthe octahedral sheet do also belong to the tetrahedral sheets [18].The layer thickness and lateral dimension of nanomica used hereare around 1 and 300–600 nm, respectively. Stacking of layersilicate platelets create a regular van der Waals gap between theplatelets, which called the interlayer. Even though nature micais hydrophilic, it would become hydrophobic and has goodcompatibility with organic electrolytes, if modified with organ-ogroups.

In this study, we incorporated AMNM (Fig. 1(b)) into PVDF-HFP gel electrolytes to form nanocomposite gel electrolytes. Theeffect of the AMNM content on the cell performance wasinvestigated and the AMNM content was optimized. Besides, tofurther improve the cell performance, a porous TiO2 photoanodewas prepared. The porosity of the photoanode was controlled byadjusting various volume ratios of PMMA microsphere suspensionto TiO2 paste (denoted as VPMMA=TiO2

). Furthermore, the effect ofthe cell gap between the photoanode and the counter electrodewas also discussed.

2. Experimental

2.1. Materials

Anhydrous 1-butyl-3-methylimidazolium iodide (BMII),iodine (I2), poly(ethylene glycol) (PEG) (M.W.=20,000) and4-tert-butylpyridine (TBP), 3-methoxypropionitrile (MPN) were

obtained from Merck. Titanium (IV) isopropoxide (TTIP) (+98%)and guanidine thiocyanate (GUSCN) were bought from Acros.Water molecules of MPN were removed by putting molecularsieves (4 A) into the solvent. The N3 dye (Ruthenium 535),fluorine-doped SnO2 conducting glass (FTO), 15 O/& and surlyn(25 and 60 mm) were the commercial products obtained fromSolaronix S.A., Aubonne, Switzerland. PVDF-HFP (molecularweight=400,000) was purchased from Aldrich. AMNM wasoffered by NanoMica Technology Co., Ltd., Taiwan.

2.2. Preparation of TiO2 thin films and the cell assembly

The TiO2 nanoparticles were prepared according to theliterature [19]. After synthesis, the TiO2 dispersion was concen-trated to 13 and 30 wt% poly(ethylene glycol) (M.W.=20,000,PEG) was added to the TiO2 paste to prevent the film fromcracking during drying. To form the TiO2 electrode, the TiO2 pastewas coated on a fluorine-doped tin oxide (FTO) glass plate withthe glass rod method and repeat this process for three times.

To fabricate porous TiO2 photoelectrode, the PMMA/TiO2 pastewas prepared first by mixing the certain volume ratio of TiO2 pastwith PMMA suspension. PMMA suspension was prepared bydispersing 6.5 wt% PMMA microspheres (�300 nm) and 0.5 wt%sodium dodecyl sulfate (SDS) in H2O and ethanol solution(volume ratio=9:1). After the deposition of the first TiO2 layerby using TiO2 paste, PMMA/TiO2 paste was used for the depositionof the second and third TiO2 layers. Finally, after sintering at500 1C for 30 min, the porous TiO2 photoanode was done.

Thereafter, the TiO2 or porous TiO2 photoanodes with an activesurface area of 0.16 cm2 was immersed in an acetonitrile and tert-butanol solution (volume ratio=1:1) containing 2�10�4 M N3

dye overnight. Then, the photoanode was assembled with theplatinized counter electrode (�25 nm) and sealed by surlynpolymer film upon heating.

2.3. Preparation of the electrolytes

The composition of liquid electrolyte is as follows: 0.6 M BMII,0.1 M I2, 0.5 M TBP and 0.1 M GUSCN in MPN. PVDF-HFP gelelectrolyte was prepared by adding 5 wt% PVDF-HFP (vs. liquidelectrolyte) into liquid electrolyte and heated under stirring untilthe polymer melted. The AMNM composite PVDF-HFP gelelectrolytes were prepared as following: 5 wt% PVDF-HFP anddifferent weight ratios (0–3 wt%) of AMNM were added to liquidelectrolyte, and heated to melt the polymer under vigorousstirring for 4 h.

2.4. Fabrication of DSSCs

The gel electrolytes were heated to 100 1C under stirring untilthe gels completely melted. Then the electrolytes (hot solution)were injected into the internal space of the cell through the holemade on the platinized counter electrode, and the hole was sealedby heating the hot-melt surlyn film between the thin glass coverand the platinized counter electrode. After cooling down to roomtemperature, a motionless gel layer was formed in cell.

2.5. Instrumentation

The cells were illuminated by a PECCELL solar simulator (PEC-L11) equipped with a water-based IR filter under AM 1.5 G. Tovalidate our data, the photocurrent–voltage characteristics of ourDSSCs were also carried out independently by using an Oriel ClassA solar simulator (Oriel 91195A, Newport Corporation) installedat the Institute of Chemistry, Academia Sinica, Nankang, Taiwan.

ARTICLE IN PRESS

20

Inte

nsity

(a.u

.)

2 Theta (degree)

0 wt% AMNM0.5 wt% AMNM1 wt% AMNM3 wt% AMNM5 wt% AMNM

40

Fig. 3. The XRD patterns of PVDF-HFP/AMNM with various AMNM loadings (0–

0.5 wt%).

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 94 (2010) 668–674670

Electrochemical impedance spectra (EIS) were obtained with aPGSTAT 30 (Autolab, Eco-Chemie, The Netherlands) potentiostat/galvanostat equipped with an FRA2 module under constant lightillumination of 100 mW/cm2. The frequency range explored was10 mHz to 65 kHz. The applied bias voltage and ac amplitudewere set at open-circuit voltage and 10 mV, respectively, betweenthe counter electrode and the working electrode, starting from theshort-circuit condition in the illumination state. The impedancespectra were analyzed by an equivalent circuit model, interpret-ing the characteristics of the DSSCs [20]. The impedanceparameters were determined by fitting of impedance spectrumusing Z-view software. The lifetime of injected electron in theTiO2 film before recombination with oxidized dye can bemeasured by transient photovoltage under open-circuit condi-tions. The transient photovoltage of assembled devices wererecorded with a digital oscilloscope (model LT322, LeCroy, USA).Pulsed laser excitation was applied by a frequency-doubled Q-switched Nd:YAG laser (Spectra-Physics laser, model Quanta-RayGCR-3-10) with 2-Hz repetition rate at 532 nm, and 7-ns pulsewidth at half-height. The beam size was slightly larger than0.25 cm2 to cover the area of the device with an incident energy of1 mJ/cm2. The average electron lifetime can be approximatelyestimated by fitting a decay of the open-circuit voltage transientwith exp (�t/te), where t is time and te is an average timeconstant before recombination [21].

RdiffRct2Rct1

Rs

10

20

30

40

50 0 wt% AMNM 0.5 wt% AMNM 1 wt% AMNM 3 wt% AMNM 5 wt% AMNM

-Z" (

ohm

)

020

25

30

35

40

45

50

55

Rct

2 (o

hm)

AMNM content (wt %)

Rdi

ff (o

hm)

20

25

30

35

40

45

50

55

2 4 6

3. Results and discussion

3.1. Influence of AMNM on the cell performance

Fig. 2 shows the I–V curves of PVDF-HFP gel-types DSSCs usingAMNM as an additive, and the inset shows the calculated cellefficiencies at different AMNM loadings in the gel electrolytes. Itcan be found that both the current density and the cell efficiencyincreased from 8.33 to 13.61 mA/cm2 and 3.5% to 5.7%,respectively, as the content of the AMNM in gel electrolytes wasincreased up to 3 wt%, although the open-circuit voltage (VOC)remained almost the same. However, as the content of AMNMexceeded 3 wt%, the cell efficiency decreased. Fig. 3 shows theXRD patterns of the gel electrolyte with various contents ofAMNM. The peaks at 2y=18.2, 20, 26.6 and 38 correspond to

0.00

4

8

12

16

20

24

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

0 wt% AMNM 0.5 wt% AMNM 1 wt% AMNM 3 wt% AMNM 5 wt% AMNM

02.02.53.03.54.04.55.05.56.06.5

Effi

cien

cy (%

)

AMNM content (wt %)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1 2 3 4 5

Fig. 2. The I–V curves for the PVDF-HFP gel-type DSSCs at different AMNM

loadings in the electrolytes. The inset shows the calculated cell efficiencies at

different AMNM loadings in the electrolytes.

400

Z' (ohm)60 80 100 120 140 160 180 200

Fig. 4. (a) The equivalent circuit of the DSSCs. (b) The Nyquist plot for the PVDF-

HFP gel electrolytes with various AMNM loadings. The inset is a plot of the charge

transfer resistances (Rct2) and the diffusion resistances (Rdiff) for the PVDF-HFP gel-

type DSSCs with different AMNM loadings in the electrolytes.

(1 0 0), (0 2 0), (1 1 0) and (0 2 1) crystalline peaks of PVDF,respectively [22]. It can be seen that after adding AMNM, thecrystallinity of PVDF-HFP gel electrolyte decreased, implying thatthe amorphous region increases, thus decreasing the I�/I3

�

diffusion resistance. It is also worth noting that too muchAMNM incorporation (e.g. 5 wt%) would cause aggregation ofnanomica and phase separation with PVDF-HFP, increasingfurther the crystallinity of the copolymer, thus increasing theI�/I3

� diffusion resistance.The same behavior can also be seen from the EIS analysis.

Fig. 4(a) shows the equivalent electrical circuit for the DSSCs.Fig. 4(b) is the Nyquist plot for PVDF-HFP gel electrolytes systemwith various amount of AMNM loading. The inset is a plot of thecharge transfer resistances (Rct2) and the diffusion resistances

ARTICLE IN PRESS

Table 1The fitted impedance parameters for the PVDF-HFP gel-type DSSCs at various

AMNM loadings in the electrolytes.

AMNM (wt%) Rs (O) Rct1 (O) Rct2 (O) Rdiff. (O)

0 59.1 38.7 53.1 52.1

0.5 59.0 40.8 38.7 40.8

1 59.3 41.1 38.1 28.1

3 59.0 42.5 38.0 22.3

5 58.0 44.9 45.0 37.7

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 94 (2010) 668–674 671

(Rdiff) for the PVDF-HFP gel-type DSSCs with different loadings ofthe AMNM in the electrolytes.

Furthermore, the fitted parameters are summarized in Table 1.In general, the EIS spectrum of the DSSC shows three semicirclesin the measured frequency range of 10 mHz to 65 kHz. The Ohmicserial resistance, Rs, is associated with the series resistance ofthe electrolytes and electric contacts in the DSSCs. Rct1, Rct2 andRdiff correspond to the charge transfer processes occurringat the Pt counter electrode (corresponding the first arc), theTiO2/dye/electrolyte interface (corresponding the second arc)and the Warburg diffusion process of I�=I�3 in the electrolyte(corresponding the third arc), respectively. It can be seen that thediffusion resistance of I�=I�3 (Rdiff) decreased from 52.1 to 22.3 Oas the content of AMNM was increased to 3 wt%. However,the charge transfer resistance at the TiO2/dye/electrolyte (Rct2)interface increased when the AMNM amount exceeded 3 wt%,which can be attributed to the hindered ionic path between theTiO2 and electrolyte, thus reducing the electron transfer rate fromthe electrolyte to the dye molecules.

Fig. 5. The top-view of SEM images for the TiO2 electrodes with (a) VPMMA=TiO2=0

and (b) VPMMA=TiO2=0.5.

0.00

4

8

12

16

20

24

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

VPMMA/TiO2

0.00 0.250.330.501.00

0.02

4

6

8

Cur

rent

den

sity

(mA

/cm

2 )

VPMMA/TiO2

Cel

l effi

cien

cy (%

)

8

10

12

14

16

18

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.2 0.4 0.6 0.8 1.0

Fig. 6. The I–V curves for PVDF-HFP gel type (with 3 wt% AMNM) DSSCs using TiO2

photoelectrodes with various VPMMA=TiO2values. The inset shows the calculated cell

efficiencies at different volume ratios of PMMA/TiO2.

3.2. Using porous TiO2 electrode to improve the performance of

PVDF-HFP/AMNM nanocomposite gel-type DSSCs

Because GPEs have higher viscosity compared with that ofthe liquid electrolytes, it is difficult for the gel electrolytes topenetrate into the underside of TiO2 to interact with the dyemolecules. To facilitate the penetration of GPE, a porous TiO2

was prepared by annealing the PMMA/TiO2 electrode. Fig. 5 isthe SEM images of the TiO2 electrodes with VPMMA=TiO2

=0 andVPMMA=TiO2

=0.5. It could be seen from the result that the TiO2

electrodes with VPMMA=TiO2=0.5 has many micropores, and the

diameter is around 300 nm.Fig. 6 shows the I–V curves for PVDF-HFP gel-type (with 3 wt%

nanomica) DSSCs by using TiO2 photoanodes prepared by usingvarious volume ratios of PMMA suspension to TiO2 solution. It canbe found that the cell efficiency was improved from 5.5% to 6.7%as the volume ratio was increased from 0 to 0.5.

The inset of Fig. 7 is the Nyquist plot of the photoelectrodesprepared by using various VPMMA=TiO2

values. The resistance,including Rct1, Rct2, is almost constant, except for Rct2, whichstands for the charge transfer resistance at the TiO2/dye/electrolyte interface. It can be concluded, by comparing with thepure TiO2 electrode system, that the porous TiO2 electrodes havesmaller values of Rct2. Especially for the porous TiO2 photo-electrode with VPMMA=TiO2

of 0.5, the value of Rct2 decreased to18.4 O (from 30.1 O).

These results could also be confirmed by the electron lifetimeanalyses. Laser pulse induced transient photovoltages obtainedfor both the pure TiO2 electrode and different volume ratios of thePMMA/TiO2 electrodes are shown in Fig. 8. The average electronlifetime can be approximately estimated by fitting a decay ofthe open-circuit voltage transient with exp (�t/te), where t is thetime and te is an average time constant before recombination. Theelectron lifetime increased from 2.22 to 3.40 ms when replacing

ARTICLE IN PRESS

0.0

15

20

25

30

35

40

Rct

2 (o

hm)

200

10

20

30

40 VPMMA/TiO2 = 0.00

VPMMA/TiO2 = 0.25

VPMMA/TiO2 = 0.33

VPMMA/TiO2 = 0.50

VPMMA/TiO2 = 1.00

Z" (o

hm)

Z' (ohm)

0.2 0.4 0.6 0.8 1.0PMMA/TiO2 (v/v)

30 40 50 60 70 80 90

Fig. 7. The charge transfer resistance at the TiO2/dye/electrolyte (Rct2) interface for

the PVDF-HFP gel-type (with 3 wt% AMNM) DSSCs using TiO2 photoelectrodes

with various VPMMA=TiO2values. The inset shows the Nyquist plot for PVDF-HFP gel

type (with 3 wt% AMNM) DSSCs using using TiO2 photoelectrodes with various

VPMMA=TiO2values.

0

0

40

80

120

160

200

2.22 ms

3.40 ms

2.80 ms

2.75 ms

2.49 ms

Pho

tovo

ltage

(mV

)

Time (ms)

VPMMA/TiO2 = 0.00, τe = 2.22 ms

VPMMA/TiO2 = 0.25, τe = 2.49 ms

VPMMA/TiO2 = 0.33, τe = 2.75 ms

VPMMA/TiO2 = 0.50, τe = 3.40 ms

VPMMA/TiO2 = 1.00, τe = 2.80 ms

2 4 6 8 10 12 14

Fig. 8. Electron lifetime analyses using TiO2 photoelectrodes with various

VPMMA=TiO2ratios.

400

0

20

40

60

80

IPC

E (%

)

Wavelength (nm)

VPMMA/TiO2 = 0.5

VPMMA/TiO2 = 0.0

500 600 700 800

Fig. 9. The IPCE spectra for the PVDF-HFP gel-type (with 3 wt% AMNM) DSSCs

using different volume ratios of PMMA/TiO2 as the photoelectrodes.

0.00

5

10

15C

urre

nt d

ensi

ty (m

A/c

m2 )

Voltage (V)

liquid (25 μm)liquid (60 μm)gel/3 wt% AMNM (25 μm)gel/3 wt% AMNM (60 μm)gel/0 wt% AMNM (25 μm)gel/0 wt% AMNM (60 μm)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fig. 10. The I–V curves of the DSSCs, with two cell gaps, based on liquid

electrolytes, PVDF-HFP gel electrolytes and PVDF-HFP/AMNM gel electrolytes.

Table 2The DSSC performance for liquid electrolytes, PVDF-HFP gel electrolytes, PVDF-

HFP containing 3 wt% AMNM gel electrolytes with two cell gaps.

Voc (V) Jsc (mA/cm2) Z (%) FF

Liquid electrolyte (25 mm) 0.74 16.28 8.11 0.66

Liquid electrolyte (60 mm) 0.75 16.00 7.33 0.62

Gel electrolyte/0 wt% AMNM (25 mm) 0.76 13.14 6.14 0.69

Gel electrolyte/0 wt% AMNM (60 mm) 0.77 12.57 4.90 0.63

Gel electrolyte/3 wt% AMNM (25 mm) 0.76 15.63 7.96 0.69

Gel electrolyte/3 wt% AMNM (60 mm) 0.76 15.20 6.70 0.58

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 94 (2010) 668–674672

with a PMMA/TiO2 (at a volume ratio of 0.5) electrode with thepure TiO2 electrode. This implies that the additional porositycreated would facilitate the gel electrolytes to penetrating moredeeply into the dye-absorbed TiO2 film and, thus increasingthe electron lifetime and lowering the charge resistance at theTiO2/dye/electrolyte interface.

Fig. 9 shows the incident photo-to-current conversionefficiency (IPCE) obtained for the TiO2 photoelectrodes withVPMMA=TiO2

=0 and 0.5. It can be found that the TiO2 photo-electrode with VPMMA=TiO2

=0.5 showed higher IPCE than the onewith VPMMA=TiO2

=0, which can be attributed to the porosity effect.

3.3. DSSC efficiencies and the significance of cell gap in DSSC

fabricated with GPEs

Decreasing the cell gap would decrease the electrolyte resistance,hence raise the fill factor and the current density, especially for thegel electrolytes which have lower ionic conductivity compared with

that of the liquid electrolytes [23]. Two types of surlyn, 25 or 60 mm,were used to control the cell gap, along with the use of the PMMAmodified TiO2 electrode (volume ratio is 0.5). The I–V curves areshown in Fig. 10 and the cell performance parameters correspondingto these DSSCs. are summarized in Table 2. It could be observed thatby decreasing the cell gap, the cell efficiency of gel-type (with 3 wt%AMNM) DSSCs can be improved from 6.70% to 7.96%, which iscomparable with that of the liquid-type DSSCs (8.11%). This resultwas also confirmed by the AC impedance measurement, indicating amuch smaller resistance of Rdiff by using a smaller cell gap (Fig. 11).

ARTICLE IN PRESS

200

5

10

15

20

25

30

35

40

Z" (o

hm)

Z' (ohm)

liquid electrolyte (25 μm)gel electrolyte/3 wt% AMNM (25 μm)gel electrolyte/3 wt% AMNM (60 μm)

25 30 35 40 45 50 55 60 65 70

Fig. 11. The Nyquist plots for DSSCs based on liquid electrolyte, PVDF-HFP gel

electrolyte (with 3 wt% AMNM) with 25 mm cell gap, and PVDF-HFP gel electrolyte

(with 3 wt% AMNM) with 60 mm cell gap.

00

2

4

6

8

10

Effi

cien

cy (%

)

Time (h)

3 wt% AMNM0 wt% AMNM

200 400 600 800 1000

Fig. 12. At-rest stability for the PVDF-HFP gel-type DSSCs with different AMNM

loadings in the PVDF-HFP gel electrolytes.

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 94 (2010) 668–674 673

3.4. At-rest stability

Fig. 12 shows the at-rest stability of the cells with the PVDF-HFP based gel electrolyte, the porous TiO2 photoanode(VPMMA=TiO2

=0.5), and with a 25 mm cell gap. It could beobserved that in the early stage, the cell efficiencies of bothcells increased as the gel electrolyte penetrated more deeply intothe porous TiO2 film. The results show that the gel electrolytewith 3 wt% AMNM has better durability over a period of 1000 h(93.3% of its initial efficiency), as compared with that of the barePVDF-HFP gel-type DSSC (85.9% of its initial efficiency).

4. Conclusions

With the pure TiO2 electrode, by incorporating 3 wt% AMNMinto the PVDF-HFP gel electrolytes, the cell efficiency can beimproved from 3.5% to 5.7%, resulted from the decreasing I�=I�3diffusion resistance. The cell performance can be further im-proved by using the porous TiO2 photoelectrode and decreasing

the cell gap. The best cell efficiency of the gel-type DSSCs reached7.96%, which is comparable with that of the liquid-type.

Acknowledgements

This work was financially supported by the King AbdullahUniversity of Science and Technology (KAUST) through the GlobalResearch Partnership Centers-in-Development grant (KAUST GRP-CID). Some of the instruments used in this study were madeavailable through the support of the National Science Council(NSC) of Taiwan under Grant nos. NSC 96-2120-M-002-016 andNSC 97-2120-M-002-012.

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.solmat.2009.11.027.

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