A simple medium-bandgap quinoidal A–D–A non-fullerene acceptor for ternary organic solar cells†

Abstract

The composition of the bulk heterojunction active layers in organic solar cells is crucial to their photovoltaic performance. Highly efficient organic solar cells have been constructed by the so-called ternary approach due to its process simplicity and diverse donor and acceptor materials. In the study described here, the simple medium-bandgap closed-shell quinoidal A–D–A non-fullerene small molecule acceptor QDT1, i.e., 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT), was used as a guest acceptor in the PBDB-T:Y6 host binary active layer to fabricate ternary organic solar cells. QDT1 has an absorption profile that is complementary to those of the host active materials (PBDB-T and Y6). This matching of profiles is beneficial for light-harvesting and ultimately enhances the photocurrent in the OSC devices. Ternary organic solar cells fabricated in ambient conditions with the optimized PBDB-T : QDT1 : Y6 (1.0 : 0.2 : 1.0) active layer gave an overall power conversion efficiency of 13.31%, which is better than that of the PBDB-T:Y6 counterpart (11.17%). The higher PCE in the ternary system is mainly attributed to the higher short circuit photocurrent and fill factor along with a decreased energy loss. The increases in photocurrent and fill factor can both be attributed to faster exciton dissociation, energy transfer from QDT1 to Y6, more rapid charge extraction, extended charge carrier lifetime and lower charge recombination. The organic solar cells based on a PBDB-T:QDT1 active layer provided a PCE greater than 23% under indoor illumination (white LED).