Sulfur-fused perylene diimides (PDIs) having the formula 2PDI-nS, wherein n is an integer. Such sulfur-fused PDIs (e.g., 2PDI-2S, 2PDI-3S, and 2PDI-4S) are incorporated as electron acceptors in an active region of a bulk heterojunction solar cell and/or as an electron transport layer. Example solar cells exhibit a power conversion efficiency above 5% and a fill factor above 70% (a record high for non-fullerene bulk heterojunction solar cell devices) when 2PDI-nS is used as the electron acceptor. In addition, the solar cells exhibit low open circuit voltage (Voc) loss.