Tuning Voc for high performance organic ternary solar cells with non-fullerene acceptor alloys

Open circuit voltage (V-oc) is a critical parameter for ternary organic solar cells, while its mechanism is obscure. Here we employed two non-fullerene molecules TPE-4PDI and FT-2PDI (perylenediimide, a PDI-based small molecule) to form acceptor alloys with ITIC-Th (indacenodithieno, an IDDT-based small molecule) for ternary systems. The results demonstrated that the experimental V-oc values fit the simulation data accurately based on the equation not only in our new ternary systems but also in other reported small molecular alloy based ternary systems. More importantly, TPE-4PDI is more efficient to enhance the V-oc of ternary solar cells as the third component than FT-2PDI, since TPE-4PDI possesses a larger quasi frontier orbital density (N-e) value. Thus, upon tuning the weight ratio of the TPE-4PDI: ITIC-Th acceptor alloy, high performance ternary solar cells with an efficiency over 11% were achieved. This contribution has shed light on understanding the mechanisms of ternary solar cells and demonstrated a method for enhancing V-oc efficiently to achieve high performance solar cells.