Efficient ternary polymer solar cells with a parallel-linkage structure

A series of ternary polymer solar cells (PSCs) were fabricated based on two narrow bandgap polymers PTB7 and PBDT-TS1 as the donors and PC₇₁BM as the acceptor. The performances of the ternary PSCs monotonically increased along with the PBDT-TS1 doping ratio, up to 80 wt% in the donors. The optimum power conversion efficiency (PCE) achieved for the ternary PSCs was 7.91% with an open circuit voltage (V_OC) of 0.76 V, a short circuit current density (J_SC) of 18.85 mA cm^-2 and a fill factor (FF) of 55.2% for the active layers with an 80 wt% PBDT-TS1 doping ratio in the donors. The optimized ternary PSCs show a 12.8% improvement compared to the optimum PCE of 7.01% for the binary PSCs with PBDT-TS1 as the donor and a 28.2% improvement compared with the optimum PCE of 6.17% for the binary PSCs with PTB7 as the donor. The FFs of all the ternary PSCs are larger than 54%, indicating efficient charge carrier transport channels in the ternary active layers. The energy or charge transfer between PTB7 and PBDT-TS1 should be neglected according to the investigation of the photoluminescence spectra of the blend films and the current density-voltage (J-V) curves of devices without the acceptor in the active layers. The ternary PSCs should be parallel-linkage structures with the donors independently working with the acceptor, which may be the most promising strategy for obtaining highly efficient ternary PSCs.