Scanning Kelvin probe microscope images have been used to optimize the performance of both organic and inorganic photovoltaic materials and understand their operation at the 10’s of nm scale [1]. While most studies have examined the spatial distribution of the potential, photopotential, capacitance, photocapacitance, current, and photocurrent, there is much to be learned by measuring and mapping the time-dependence of these quantities [2]. In organic bulk heterojunction solar cells, Ginger et al. have established that the microscopic photocapacitance charging rate in a film, measured by time-resolved electrostatic force microscopy (tr-EFM), is directly proportional to the external quantum efficiency (EQE) measured in a completed device. Single point measurements with sub-microsecond time resolution have been collected, but acquiring these transients required frustratingly long signal-averaging times [2]. Building on the insight of Moore, Marohn, and co-workers [3], we are developing a method to rapidly acquire transients of photocapacitance indirectly, in a stepped-time experiment, by encoding and measuring the photocapacitance as a change in the phase of a vibrating cantilever. Our new approach allows us to measure and image photocapacitance on the microsecond to nanosecond time scale. This capability would represent an exciting new route to understanding geminate recombination in heterogeneous hybrid organic/inorganic photovoltaic materials.
[1] O’Dea, J. R.; Brown, L. M.; Hoepker, N.; Marohn, J. A. & Sadewasser, S. Scanned Probe Microscopy of Solar Cells: From Inorganic Thin Films to Organic Photovoltaics. Mater. Res. Soc. Bulletin, 2012, 37: 642 - 650 [http://dx.doi.org/10.1557/mrs.2012.143]. [2] Giridharagopal, R.; Rayermann, G. E.; Shao, G.; Moore, D. T.; Reid, O. G.; Tillack, A. F.; Masiello, D. J. & Ginger, D. S. Submicrosecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics. Nano Lett., 2012, 12: 893 - 898 [http://dx.doi.org/10.1021/nl203956q]. [3] Moore, E. W.; Lee, S.-G.; Hickman, S. A.; Wright, S. J.; Harrell, L. E.; Borbat, P. P.; Freed, J. H. & Marohn, J. A. Scanned-Probe Detection of Electron Spin Resonance from a Nitroxide Spin Probe. Proc. Natl. Acad. Sci. U.S.A., 2009, 106: 22251 - 22256 [http://dx.doi.org/10.1073/pnas.0908120106].