Understanding the Enhanced Open-Circuit Voltage of Polymer Solar Cells Based on a Diketopyrrolopyrrole Small Molecular Acceptor.

In this study, polymer solar cells employing poly(3-hexylthiophene) (P3HT) as a donor and fullerene derivative PC61BM (phenyl-C61-butyric acid methyl ester) or nonfullerene diketopyrrolopyrrole (DPP)-based small molecule (SF-DPPEH) as an acceptor are investigated. Device based on SF-DPPEH shows a remarkably high VOC of 1.20 V, whereas analogous device based on PC61BM only delivers a VOC of 0.64 V. Employing transient photovoltage/photocurrent techniques, we measure charge carrier lifetime and density and nongeminate recombination rate in the photoactive layer and correlate material energetics and charge recombination dynamics with the change of VOC in the devices; thus, the extent to which two factors limit VOC can be quantified.