Abstract
Small-molecule "nonfullerene" acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet, the efficiency limiting processes and their dependence on the acceptor structure are not clearly understood. Here, we investigate the impact of the acceptor core structure (cyclopenta-[2,1-b:3,4-b']dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, namely CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer, the IDTT-based acceptor achieves Power conversion efficiencies (8.4%) that are higher than those of the,CDT-based acceptor (5.6%) because of a concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage.