Intensity-Modulated Scanning Kelvin Probe Microscopy for Probing Recombination in Organic Photovoltaics

Abstract

We study surface photovoltage decays on sub-millisecond time scales in organic solar cells using intensity-modulated scanning Kelvin probe microscopy (SKPM). Using polymer/fullerene (poly[N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]/[6,6]-phenyl C71-butyric acid methyl ester, PCDTBT/PC71BM) bulk heterojunction devices as a test case, we show that the decay lifetimes measured by SKPM depend on the intensity of the background illumination. We propose that this intensity dependence is related to the well-known carrier-density-dependent recombination kinetics in organic bulk heterojunction materials. We perform transient photovoltage (TPV) and charge extraction (CE) measurements on the PCDTBT/PC71BM blends to extract the carrier-density dependence of the recombination lifetime in our samples, and we find that the device TPV and CE data are in good agreement with the intensity and frequency dependence observed via SKPM. Finally, we demonstrate the capability of intensity-modulated SKPM to probe local recombination rates due to buried interfaces in organic photovoltaics (OPVs). We measure the differences in photovoltage decay lifetimes over regions of an OPV cell fabricated on an indium tin oxide electrode patterned with two different phosphonic acid monolayers known to affect carrier lifetime.

Nanoscale Polarization‐Resolved Surface Photovoltage of a Pleochroic Squaraine Thin Film

“…It approaches the initial level with a time constant on the several tens of minutes to hours scale. In general, the SPV scales logarithmically with the illumination power over the investigated illumination range, similar to conventional organic photovoltaic devices …”

Probing of Local Multifield Coupling Phenomena of Advanced Materials by Scanning Probe Microscopy Techniques

“…The horizontal lines are the time‐averaged values recorded by the instrument. i) Reproduced with permission . Copyright 2014, American Chemical Society.…”

The limit of time resolution in frequency modulation atomic force microscopy by a pump-probe approach

“…Currently, most AFM measurements of structureproperty relations are slow, on time scales of millisecondshours [9,10]. Much effort has been put into adding fast time-resolution to understand dynamics and kinetics on time scales of nano-to milliseconds [10][11][12][13]. A common assumption in AFM is that the slow time resolution of AFM is related to the mechanical oscillating probe with resonant frequencies of 0.1-1 MHz and slow detection and feedback electronics [14,15].…”