AM: Length-dependent nonradiative energy loss of donor-terminated alkyl chains in all-small-molecule organic solar cells

A fatal bottleneck for further breakthroughs in the efficiency of organic solar cells (OSCs) is to minimize non-radiative energy loss (eΔVnr) while maximizing charge generation. With the development of high-emitting low-bandgap nonfulvene acceptors, the design of high-performance donors has become critical. By shortening the end-capped alkyl chains of small molecule donors from hexyl (MPhS-C6) to ethyl (MPhS-C2), thermal annealing (TA)-insensitive aggregation and densely packed.

Aggregation that is insensitive to thermal annealing (TA) leads to small phase separation and suppresses the upshift 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, which is the smallest energy loss (0.497 eV) and ΔVnr ​​(0.192) among ASM OSCs due to almost unchanged high Jsc (26.6mA cm2) and Voc (0.888 V) and improved FF V) record efficiency. Our results highlight a potential material design direction to simultaneously obtain TA-insensitive aggregation and dense packing to maximize device performance with ultra-low ΔVnr.