Ternary organic solar cells with 16.88% efficiency enabled by a twisted perylene diimide derivative to enhance the open-circuit voltage

Abstract

For the high-efficiency PM6:Y6 binary system, there is still space for narrowing the lowest unoccupied molecular orbital (LUMO) level offset (0.36 eV) between PM6 and Y6 to further mitigate energy loss through introducing the second acceptor constructing ternary organic solar cells (OSCs). In this study, we designed and synthesized a new slightly twisted PDI-tetramer with a large, planar coronene unit as the core, Cor-T-PDI, which has four quasi molecular orbitals with little energy difference. Although Cor-T-PDI pairing with PM6 gave fairly low photovoltaic performance, interestingly, Cor-T-PDI as the second acceptor, an obvious increase in open-circuit voltage (Voc) was achieved for PM6:Y6 base ternary system due to creating cascade energy level alignment by Cor-T-PDI. After tuning the PM6:Y6:Cor-T-PDI ratio to 1:1.0:0.1, the Voc was significantly increased to 0.854 V without sacrificing short-circuit current density (Jsc) and fill factor (FF), resulting in a highest PCE of 16.88% with a low energy loss (0.47 eV), which is among the best values for single-junction OSCs. This preliminary study showed that for the binary system with high Jsc but low or moderate Voc, such as PM6:Y6 blend, producing a cascade energy level alignment was a more efficient strategy to enhance PCE by introducing the third component with appropriate structure. It is believed that lowering the HOMO level of this PDI derivative to fall between that of Y6 and PM6 or tailoring the absorption to achieve complementary absorption profile with that of PM6:Y6 blend will further improve the efficiency.