Charge Recycling Mechanism Through a Triplet Charge-Transfer State in Ternary-Blend Organic Solar Cells Containing a Nonfullerene Acceptor

We investigated the side-group and backbone-extension effects of a push-pull conjugated polymer (CP) on the charge-generation mechanism in ternary-blend organic solar cells (OSCs) containing PC71BM and a nonfullerene acceptor (NFA, ITIC). Transient absorption spectroscopy results revealed that the CP extension increased the hole-transfer efficiency from ITIC to the CPs, inducing the formation of triplet (T-1) excitons by nongeminate recombination (via triplet charge-transfer states, 3CT) because of the finely mixed morphologies. Particularly, the (CT)-C-3 state relaxation to T-1 excitons was efficiently suppressed by PC71BM in the ternary-blend films based on the side-group and backbone extended CPs. This caused the recycling of the free carriers generated by the hole transfer from ITIC to the CPs, thereby increasing the efficiency of the OSC having the optimized ternary-blend morphology. Our findings provide new insights on the mechanism of charge generation through the photoinduced hole transfer from NFAs to CPs.