Interplay between charge separation and hole back transfer determines the efficiency of non-fullerene organic solar cells with low energy level offset

Organic bulk heterojunction solar cells with electron acceptors based on small donor-acceptor type molecules show record efficiencies mainly due to their long wavelength absorption, which enables efficient harvesting of solar light and, thus, causes high current density. Meanwhile, relative positions of HOMO and LUMO levels of donor and acceptor materials determine the open circuit voltage. Here, we apply ultrafast transient absorption and transient luminescence techniques together with specially-designed modelling technique to address charge carrier generation and recombination dynamics in detail. We demonstrate the importance of careful adjustment of the HOMO and LUMO levels, as their positions determine formation and recombination rates of interfacial charge transfer (CT) states. An insufficient donor and acceptor LUMO level offset, lower than ∼300 meV, leads to slow and inefficient CT state formation, while an offset of the HOMO level below ∼100 meV leads to fast CT state recombination, which we attribute to the back transfer of a hole from the donor to the acceptor.