Charge Generation In Polymer: Perylene Diimide Blends Probed By Vis-Nir Broadband Transient Absorption Pump-Probe Spectroscopy

We report the photovoltaic performance of a low-bandgap polymer: perylene diimide (PDI) photovoltaic blend and study the exciton to charge carrier conversion in the photoactive layer by Vis-NIR broadband transient absorption spectroscopy over a dynamic range from pico-to microseconds. Power conversion efficiencies of 1.2 % are obtained from the polymer: PDI blends with a maximum EQE of about 30 %, which is significantly below the performance of the same polymer with fullerene as acceptor indicating that severe loss processes exist that limit the photocurrent. From the evolution of the transient absorption spectra we conclude that the photovoltaic performance of the polymer: PDI blends is mainly limited by inefficient exciton harvesting and dissociation at the interface. However, once free charge carriers are generated in the blend they can be extracted as photocurrent as their recombination occurs on a timescale similar to the time typically needed for charge extraction from the photoactive layer. Hence, strategies to improve the efficiency of polymer: PDI blends should aim at increasing exciton harvesting at the heterojunction and the dissociation efficiency into free charges at the interface.