Interface limited charge extraction and recombination in organic photovoltaics

Abstract

We report that the transit-time of photogenerated holes within an OPV device is significantly controlled by the interface between the photoactive layer and the electrode. We used time-resolved optical absorption spectroscopy to track the evolution and transport of photogenerated holes in situ within efficient PTB7:PC₇₀BM (thieno [3,4-b] thiophene/benzodithiophene:[6,6]-phenyl C₇₁-butyric acid methyl ester) devices. Our results demonstrate that the time required for 50% hole extraction is reduced from 2.3 μs for the standard devices to 0.9 μs either by introducing a thin layer of conjugated polyelectrolyte (CPE) layer at the electron-extracting cathode or simply by exposing the device to a polar solvent such as methanol which is used for spin coating the CPEs. We consider that the CPE modifies the interface between the photoactive layer and the cathode whereas the exposure of the device to associated polar solvent modifies the buried PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) hole extracting layer.