Derek J. Hollman Undergraduate Physics Symposium

1. Interfacial Charge Transfer in Solar Cells: A Single Molecule Perspective Derek J. HollmanUndergraduate Physics Symposium 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 • System: • Perylenebisimide dye • Gallium Nitride (GaN) • Scandium Oxide (Sc2O3) • Ultra-high vacuum • Confocal Microscopy • We may observe: • Electron transfer rates • Distance dependence • Influence of interband states • Influence of surface states • Orientation dependence

5. The Choice of Sc2O3/GaN • Thickness from 5 Å-1000 Å to slow charge transfer • Near-perfect, abrupt interface • Sc2O3 (111) grown heteroepitaxially on GaN (0001) Chang Liu et al., APL 88 (2006), 222113

6. Single Molecule CT Reporter • Strong absorber (e = 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 R = -C4H9 or -C13H27

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

8. Objective Histograms/distributions: P(τ) Autocorrelation function: g(2)(τ) Mechanism! • From these analyses, information about CT kinetics can be elucidated • Simulate 2-state system, develop statistical analyses to recover rate information

9. Simulation: Signal Generation With kf>> kex >> kfct, 3-state system effectively becomes a 2-state system ton,toff exp. deviate on/off counts repeat

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 • Analysis: • Start clock; measure time molecule was “on” or “off” • When a transition occurs, record time, bin it, reset clock • Repeat off-time histogram on-time histogram krecovered = 97 ± 5 Hz kbct = 100Hz kfct = 100Hz

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 Florida Dr. Brent P. Gila Dr. Stephen J. Pearton 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)

17. DSSC – A Complex Structure • Charge transfer in heterogeneous environment • Crystal face- and structure-dependent device performance L. Kavan, M. Grätzel, S. E. Gilbert, C. Klemenz, H. J. Scheel, JACS 118, 6716 SEM micrograph of titanium oxide films. M. Grätzel et al., J. Am. Ceram. Soc. 80, 3157.

18. Kinetics in DSSC T. Hannappel, B. Burfeindt, W. Storck, F. Willig, JPCB 101, 6799 Result: Non-exponential charge transfer kinetics S.A. Haque, Y. Tachibana, D.L. Klug, J.R. Durrant, JPCB 102, 1745

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 System:Perylenebisimide on Sc2O3 / GaN • We may observe: • Forward and backward electron transfer rates • Distance dependence • Influence of interband states • Influence of surface states • Orientation dependence … one molecule at a time!

21. Single Molecule CT Reporter • Strong absorber (e = 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 R = -C4H9 or -C13H27

22. PTCDI/Sc2O3/GaN so far • ELUMO(PTCDI) = 0±100 meV vs. Sc2O3/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 • 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

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 ton toff • Distinct visible states, on and off, only seen at single molecule level

27. 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

28. Poissonian Processes • On/off transitions are Poissonian processes • On or off times may be characterized by Poisson distribution • Exponential because • Transfer of charge may be a tunneling process • Kinetics may follow well-defined rate constant ke-kt

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 off times histogram on times histogram • Will investigate dependence on threshold, bin size

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

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