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-C-61-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 V-OC of 1.20 V, whereas analogous device based on PC61BM only delivers a V-OC 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 V-OC in the devices; thus, the extent to which two factors limit V-OC can be quantified.