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.