Small energy loss in ternary organic solar cells with a blend of cascade energy levels: two fullerene-free acceptors as the electron acceptor

Fullerene-free organic solar cells (OSCs) were fabricated using a blend of wide bandgap donor PBDB-T and ultranarrow bandgap acceptor IEICO-4F as binary photoactive layers. The ultranarrow bandgap acceptor 4TIC was selected as the third component due to the shallow lowest unoccupied molecular orbital (LUMO) energy levels, the good compatibility compared to that of IEICO-4F and the wide energy levels offset between the donor and acceptor. The power conversion efficiency (PCE) of ternary OSCs increased from 10.02% to 11.63% when 40% IEICO-4F was replaced with 4TIC, with an enhanced open-circuit voltage (V-OC) of 0.82 V and without sacrificing the short-circuit current (J(SC)) or fill factor (FF). The main contribution of 4TIC can be summarized as the elevated LUMO level of the acceptor blended and enhanced exciton utilization of the ternary photoactive layer due to the second light absorption of 4TIC and efficient energy transfer from PBDB-T to the blended acceptor. Simultaneously, 4TIC can also act as a regulator to adjust the PBDB-T molecular arrangement for weak charge carrier recombination and optimizing charge carrier transport resulting in the lack of any decrease in the J(SC) and FF values of ternary OSCs even when the absorption spectrum of the ternary photoactive layer does not match the AM 1.5G solar simulator spectrum. Thus, approximately 15% PCE improvement was achieved by adopting the ternary strategy based on PBDB-T-IEICO-4F binary OSCs with a PCE of 10.02%.