1. Derek J. Hollman Undergraduate Physics Symposium Interfacial Charge Transfer in Solar Cells: A Single Molecule Perspective 8 May 08

2. Dye-Sensitized Solar Cells (DSSC) Interfacial Dynamics Essential to Device Performance!

3. 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

4. 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 (Sc 2 O 3 ) Ultra-high vacuum Confocal Microscopy

5. Thickness from 5 Å-1000 Å to slow charge transfer Near-perfect, abrupt interface Sc 2 O 3 (111) grown heteroepitaxially on GaN (0001) The Choice of Sc 2 O 3 /GaN Chang Liu et al., APL 88 (2006), 222113

6. Single Molecule CT Reporter R = -C 4 H 9 or -C 13 H 27 Strong absorber (  = 75000 M -1 cm -1 ) with unity quantum yield Low intersystem crossing rates and short triplet lifetime Perylene/TiO 2 used in DSSC Electronic properties tunable by bay-substitution

7. Towards Single Molecule Spectroscopy in UHV 27.57 0 kcps Photoblinking Photobleaching Distinct “on” and “off” states only seen at single molecule level Photoblinking

8. 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 Mechanism!

9. With k f >> k ex >> k fct, 3-state system effectively becomes a 2-state system Simulation: Signal Generation t on, t off exp. deviate repeat on/off counts

10. 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 Basche, et. al

11. Dependence on Bin Size Ambiguity of on/off state

12. Drawing the Line k fct = 100Hz k recovered = 97 ± 5 Hzoff-time histogramon-time histogram k bct = 100Hz Analysis: Start clock; measure time molecule was “on” or “off” When a transition occurs, record time, bin it, reset clock Repeat

13. Autocorrelation Determine correlation between pairs of photons at arbitrarily long times

14. 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

15. Future Simulation Work Fit autocorrelation functions Power-law kinetics Multiple dark states Photon arrival times for additional information Use analyses on real data!

16. University of Arizona Dr. Oliver L. A. Monti Dr. Brandon S. Tackett Michael L. Blumenfeld Laura K. Schirra Mary P. Steele Jason M. Tyler Stefan Kreitmeier (TU München) University of Florida Dr. Brent P. Gila Dr. Stephen J. Pearton

17. 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

18. 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

19. 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

20. 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 Sc 2 O 3 / GaN … one molecule at a time!

21. Single Molecule CT Reporter R = -C 4 H 9 or -C 13 H 27 Strong absorber (  = 75000 M -1 cm -1 ) with unity quantum yield Low intersystem crossing rates and short triplet lifetime Perylene/TiO 2 used in DSSC Electronic properties tunable by bay-substitution

22. PTCDI/Sc 2 O 3 /GaN so far E LUMO (PTCDI) = 0±100 meV vs. Sc 2 O 3 /GaN CBM

23. Excitation/Emission GaN There are states within the bandgap!

24. 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 t on, “on-time”: period of continual excitation/fluorescing until a single molecule ISC or CT event t off, “off-time”: period until a charge recombination or reverse ISC event

25. 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

26. What it looks like Distinct visible states, on and off, only seen at single molecule level t on t off

27. Model System With k f >> k ex >> k fct, 3-state system effectively becomes a 2-state system Experimental acquisition rate: 10 3 - 10 4 Hz k f ~ 10 9 Hz, k ex ~ 10 6 Hz, k fct ~ 10 3 Hz

28. 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

29. 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 kinetics Basche, et. al

30. 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

31. Simulated Fluorescence Trajectory Signal generated at rate much faster than real acquisition rate, then re-binned

32. Re-binning Simulated Trace Simulated data generated on 1µs time step Real data acquisition rate closer to 0.1-1ms

33. On/off Histograms Will investigate dependence on threshold, bin size off times histogram on times histogram

34. k recovered = 97 ± 5 Hz Recovery Fit histograms to exponential; decay rate should be input rate Recovery! k fct = 100Hz m = -0.097 ± 0.005 off times histogram

35. Autocorrelation Determine correlation between pairs of photons at arbitrarily long times Shape of autocorrelation contains kinetics of system Algorithm implemented: