derek j. hollman undergraduate physics symposium interfacial charge transfer in solar cells: a...
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Derek J. HollmanUndergraduate Physics Symposium
Interfacial Charge Transfer in Solar Interfacial Charge Transfer in Solar Cells: A Single Molecule PerspectiveCells: A Single Molecule Perspective
8 May 08
Understanding the DSSC
• Understanding interfacial charge transfer in DSSC complicated by heterogeneity
• Necessitates well-defined model system with controlled interface
• Bulk properties do not reveal complete dynamics in heterogeneous systems such as DSSC
• Must observe single molecules to address rates and mechanisms of charge transfer
Experimental Realization
We may observe:• Electron transfer rates
• Distance dependence
• Influence of interband states
• Influence of surface states
• Orientation dependence
System:• Perylene bisimide dye
• Gallium Nitride (GaN)
• Scandium Oxide (Sc2O3)
• Ultra-high vacuum
• Confocal Microscopy
• Thickness from 5 Å-1000 Å to slow charge transfer• Near-perfect, abrupt interface• Sc2O3 (111) grown heteroepitaxially on GaN (0001)
The Choice of Sc2O3/GaN
Chang Liu et al., APL 88 (2006), 222113
Single Molecule CT Reporter
R = -C4H9 or -C13H27
• Strong absorber ( = 75000 M-1cm-1) with unity quantum yield• Low intersystem crossing rates and short triplet lifetime• Perylene/TiO2 used in DSSC• Electronic properties tunable by bay-substitution
-1.80
Towards Single Molecule Spectroscopy in UHV
27.57
0
kcps
Photoblinking
Photobleaching
• Distinct “on” and “off” states only seen at single molecule level
Photoblinking
Objective
Histograms/distributions: P(τ)
Autocorrelation function: g(2)(τ)
• From these analyses, information about CT kinetics can be elucidated
• Simulate 2-state system, develop statistical analyses to recover rate information
MechanismMechanism!!
With kf >> kex >> kfct, 3-state system effectively becomes a 2-state system
Simulation: Signal Generation
,kefffct bctk ton, toff exp. deviate
repeat
on/offcounts
On/off Time Distributions
• On/off transitions may be Poissonian processes; on/off times are exponentially distributed
• CT kinetics may also be power-law distributed• Observing fluorescence intermittency provides information on CT kinetics• Distribution contains information on mechanism
0 20 400
2
4
6
ln(
#occ
uren
ces)
Time (ms)
Dependence on Bin Size
Ambiguity of on/off state
0 100000 2000000
3
6
cou
nts
pe
r tim
e b
in (
1/1
0
s)
Time (s)0 100000 200000
0
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20
30
cou
nts
pe
r tim
e b
in (
1/1
00
s)
Time (s)
0 100000 2000000
60
120
cou
nts
pe
r tim
e b
in (
1/1
ms)
Time (s)
0 30 60 900
70
140
# O
ccu
ren
ces
Time (ms)
0 20 400
2
4
6
ln(
#o
ccu
ren
ces)
Time (ms)
Drawing the Line
0 10 20 30 40 500
50
100
150
200
# O
ccu
ren
ces
Time (ms)
kfct = 100Hz
krecovered = 97 ± 5 Hzoff-time histogram on-time histogram
kbct = 100Hz
Analysis:• Start clock; measure time molecule was “on” or
“off”
• When a transition occurs, record time, bin it, reset clock
• Repeat
Autocorrelation
1000 10000 1000001000000
0.0
0.5
1.0
g(2) ()
- 1
Time (s)
2)2(
)(
)()()(
tI
tItIg
• Determine correlation between pairs of photons at arbitrarily long times
Conclusions
• CT kinetics of a DSSC can be understood by analyzing single molecule fluorescence intermittency trajectories
• Experimental design allows for a good model and control of many parameters
• Simulation provides a framework for developing analyses
• Analyses can recover rates for a 2-state system
Future Simulation Work
• Fit autocorrelation functions• Power-law kinetics• Multiple dark states• Photon arrival times for additional information• Use analyses on real data!
University of ArizonaUniversity of Arizona
Dr. Oliver L. A. MontiDr. Oliver L. A. Monti
Dr. Brandon S. TackettDr. Brandon S. Tackett
Michael L. BlumenfeldMichael L. Blumenfeld
Laura K. SchirraLaura K. Schirra
Mary P. SteeleMary P. Steele
Jason M. TylerJason M. Tyler
Stefan Kreitmeier (TU MStefan Kreitmeier (TU Müünchen)nchen)
University of FloridaUniversity of Florida
Dr. Brent P. GilaDr. Brent P. Gila
Dr. Stephen J. PeartonDr. Stephen J. Pearton
DSSC – A Complex Structure
SEM micrograph of titanium oxide films. M. Grätzel et al., J. Am. Ceram. Soc. 80, 3157.
L. Kavan, M. Grätzel, S. E. Gilbert, C. Klemenz, H. J. Scheel, JACS 118, 6716
• Charge transfer in heterogeneous environment• Crystal face- and structure-dependent device
performance
Kinetics in DSSC
T. Hannappel, B. Burfeindt, W. Storck, F. Willig, JPCB 101, 6799
S.A. Haque, Y. Tachibana, D.L. Klug, J.R. Durrant, JPCB 102, 1745
Result: Non-exponential charge transfer kinetics
Ideal Model System
• Donor: Single molecule to model excited state in solar cell
• Acceptor: Single-crystalline wide bandgap semiconductor
• Spacer Layer: – Heteroepitaxial single crystalline surface– Controllably vary donor-acceptor distance– Slow down charge transfer kinetics
• Conditions: Growth and measurement in ultra-high vacuum
Experimental Realization
We may observe:• Forward and backward
electron transfer rates
• Distance dependence
• Influence of interband states
• Influence of surface states
• Orientation dependence
System: Perylene bisimide on Sc2O3 / GaN
… one molecule at a time!
Single Molecule CT Reporter
R = -C4H9 or -C13H27
• Strong absorber ( = 75000 M-1cm-1) with unity quantum yield• Low intersystem crossing rates and short triplet lifetime• Perylene/TiO2 used in DSSC• Electronic properties tunable by bay-substitution
Excitation/Emission GaN
• There are states within the bandgap!
300
400
500
600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
340
360380
400420
440460
Flu
ore
scen
ce (
AU
)
Excita
tion (n
m)
Emission (nm)
300
400
500
600
340360
380400
420440
460
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Excitation (nm)
Fluorescence Intermittency
• Single molecules exhibit “blinking”• On/Bright state: continual excitation,
fluorescence cycling• Off/Dark state: non-fluorescing state resulting
from ISC or CT event• ton, “on-time”: period of continual
excitation/fluorescing until a single molecule ISC or CT event
• toff, “off-time”: period until a charge recombination or reverse ISC event
Time Scales
• ISC events occur with low transition rate and short lifetime, typically microsecond or shorter
• CT events occur with much longer lifetimes, millisecond to seconds, also tunable (insulator layer)
• Data acquisition rate much slower than ISC event rate
• ISC events only lower average cps
What it looks like
• Distinct visible states, on and off, only seen at single molecule level
0 500000 10000000
60
120
cou
nts
pe
r tim
e b
in (
1/1
ms)
Time (s)
ton toff
Model System
• With kf >> kex >> kfct, 3-state system effectively becomes a 2-state system
• Experimental acquisition rate: 103 - 104 Hz
• kf ~ 109 Hz, kex ~ 106 Hz, kfct ~ 103 Hz
Poissonian Processes• On/off transitions are Poissonian processes• On or off times may be characterized by Poisson
distribution
ke-kt
Exponential because• Transfer of charge may be a tunneling process• Kinetics may follow well-defined rate constant
Power-law Kinetics
• CT kinetics may be power-law distributed:
• Fluctuating rate constant; molecule sampling multiple surface sites
• Observing fluorescence intermittency provides information on CT kineticsBasche, et. al
mAttP )(
Motivation for a Simulation
• Shot-noise limited signals with low S/N, need sophisticated methods of analyzing data
• Simulation provides framework for developing various analyses
• Control of input rate parameters, want to recover them
• Do not know experimental rates a priori, can not verify analyses otherwise
Simulated Fluorescence Trajectory
• Signal generated at rate much faster than real acquisition rate, then re-binned
0 200000 400000
0
8
16
cou
nts
pe
r 0
.1 m
s
Time (s)
Re-binning Simulated Trace
• Simulated data generated on 1µs time step• Real data acquisition rate closer to 0.1-1ms
0 500 10000
1
2
cou
nts
pe
r tim
e b
in (
1/1
s)
Time (s)
0 400 8000
5
10
15
cou
nts
pe
r tim
e b
in (
1/1
00
s)
Time (s)
On/off Histograms
• Will investigate dependence on threshold, bin size
0 10 20 30 40 500
50
100
150
200
# O
ccu
ren
ces
Time (ms)
off times histogram on times histogram
0 30 60 900
70
140
# O
ccu
ren
ces
Time (ms)
0 20 400
2
4
6
ln(
#o
ccu
ren
ces)
Time (ms)
krecovered = 97 ± 5 Hz
0 10 20 30 40 500
50
100
150
200
# O
ccu
ren
ces
Time (ms)
Recovery
• Fit histograms to exponential; decay rate should be input rate
• Recovery!
kfct = 100Hz
m = -0.097 ± 0.005off times histogram
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