Multi-Sulfur-Annulated Fused Perylene Diimides for Organic Solar Cells with Low Open-Circuit Voltage Loss

Nonfullerene acceptors are important for increasing the power conversion efficiency of organic bulk heterojunction (BHJ) solar cells. The number of well performing nonfullerene acceptors is still relatively limited compared to the wide range of donor materials making it difficult to find donor-acceptor pairs with matching optical and electronic properties. We report the synthesis of three sulfur-annulated fused perylene diimide-based compounds (2PDI-2S, 2PDI-3S, and 2PDI-4S) that have varying electron affinity. BHJ solar cells with these acceptors and the donor polymer PTB7-Th have solar power conversion efficiencies above 5% with relatively high fill factors (>60%) for nonfullerene acceptors. The origin of the performance of the BHJs was studied using a combination of physical and optoelectronic characterization methods. X-ray scattering revealed that the domains of 2PDI-nS acceptors are disordered in neat films and in BHJs. The carrier mobility was highest for 2PDI-4S leading to the highest fill factor for the series of acceptors. The open-circuit voltage loss was modeled using two approaches and was found to be low for the series relative to many BHJs. This work demonstrates the utility of fused-sulfur atoms to tune the electron affinity of this class of nonfullerene acceptors.