A fatal bottleneck for further breakthroughs in the efficiency of organic solar cells (OSCs) is to minimize the non-radiative energy loss (eΔVnr) while maximizing charge generation. With the development of highly emissive low-bandgap nonfulvene acceptors, the design of high-performance donors becomes crucial. By shortening the capped alkyl chain of the small molecule donor from hexyl (MPhS-C6) to ethyl (MPhS-C2), thermal annealing (TA)-insensitive aggregation and dense packing were simultaneously obtained.
Agglomeration insensitive to thermal annealing (TA) leads to small phase separation and suppresses the upward shift of the HOMO level during TA, while dense packing facilitates its efficient charge transport under aggregation-free stacking. Therefore, ΔVnr decreased from 0.242 to 0.182 V from MPhS-C6 to MPhS-C2 OSCs.
An excellent PCE of 17.11% was obtained by adding DIO due to almost unchanged high Jsc (26.6mA cm2) and Voc (0.888 V) and improved FF, which is the smallest energy loss (0.497 eV) and ΔVnr (0.192 V) record efficiency. Our results highlight potential material design directions to simultaneously obtain TA-insensitive aggregation and dense packing to maximize device performance with ultralow ΔVnr.