Enhancement in Open-Circuit Voltage in Organic Solar Cells by Using Ladder-Type Nonfullerene Acceptors

The open-circuit voltage (V_oc) loss has always been a major factor in lowering power conversion efficiencies (PCEs) in bulk heterojunction organic photovoltaic cells (OPVs). A method to improve the V_oc is indispensable to achieve high PCEs. In this paper, we investigated a series of perylene diimide-based ladder-type molecules as electron acceptors in nonfullerene OPVs. The D-A ladder-type structures described here lock our π-systems into a planar structure and eliminate bond twisting associated with linear conjugated systems. This enlarges the interface energy gap (ΔE_DA), extends electronic delocalization, and hence improves the V_oc. More importantly, these devices showed an increase in V_oc without compromising either the J_sc or the FF. C5r exhibited a strong intermolecular interaction and a PCE value of 6.1%. Moreover, grazing-incident wide-angle X-ray scattering analysis and atomic force microscopy images suggested that our fused-ring acceptors showed a suitable domain size and uniform blend films, which were not affected by their rigid molecular structures.