Ternary polymer solar cells by employing two well-compatible donors with cascade energy levels

In this study, a ternary polymer solar cell (PSCs) are fabricated by using two well-compatible donors as a blend donor (D18 as the host donor and PBDB-TF as the guest donor) and Y6 as an acceptor. The D18:Y6 and PBDB-TF:Y6 binary PSCs exhibit 13.96% and 13.42% power conversion efficiencies (PCEs), respectively. D18 and PBDB-TF have similar highest occupied molecular orbital (HOMO) energy levels, which prevents the formation of charge recombination centers within photoactive layer. The increased short-circuit current density (JSC) is mainly attributed to the complementary absorption spectra of PBDB-TF to that of D18 and Y6. In addition, the cascade lowest unoccupied molecular orbital (LUMO) energy levels of D18 and PBDB-TF lead to improved exciton dissociation and charge transport at the D18/PBDB-TF interface. The optimized ternary PSCs show highest PCE of 14.85%. Such ternary strategy with two well-compatible donors with similar HOMO energy levels and cascade LUMO energy levels is an efficient method to realize performance improvement of PSCs.