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

Abstract

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