Simultaneously increasing open-circuit voltage and short-circuit current to minimize the energy loss in organic solar cells via designing asymmetrical non-fullerene acceptor

The inevitable energy loss (E-loss) in organic solar cells (OSCs) makes it a challenge to simultaneously enhance the open-circuit voltage (V-OC) and short-circuit current (J(SC)). Herein, we designed and synthesized an asymmetrical non-fullerene small molecule acceptor (SMA), namely a-BTTIC, with a novel eight-fused ladder-type donor unit. Compared to its symmetrical analogue (BTTIC), a-BTTIC shows a red-shifted absorption (by over 20 nm) and an elevated LUMO level (by 0.07 eV), which was beneficial for boosting V-OC and J(SC) concurrently. As a result, OSCs based on a-BTTIC achieved higher power conversion efficiencies (PCEs) of up to 13.60% with simultaneously increased V-OC and J(SC) and a significantly lower E-loss of 0.526 eV than the BTTIC-based OSCs. We noted that the 13.60% PCE and the 0.526 eV E-loss were the best values among OSCs based on commercialized PBDB-T. Through various electrical and morphological characterizations, we observed that the simultaneous enhancement of V-OC and J(SC) in a-BTTIC-based OSCs was attributed to the different molecular conformations and the small change in LUMO level from neutral to ion state of asymmetrical SMAs. Overall, our design route for asymmetrical SMAs serves the dual roles of minimizing the E-loss and promoting the PCE. These results shed light on how to further reduce E-loss for high-performance OSCs with E-loss below 0.6 eV, which thus provides a promising molecular design strategy for further PCE breakthroughs.