Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells

Energy level alignment (ELA) at donor-acceptor heterojunctions is of vital importance yet largely undetermined in organic solar cells. Here, authors determine the heterojunction ELA with (mono) layer-by-layer precision to understand the co-existence of efficient charge. Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems.

Automated summary

Energy level alignment at donor-acceptor heterojunctions is crucial for charge generation and recombination in organic photovoltaic devices. This study systematically investigates the energy level alignment and its variation at different interfaces. Contrary to previous assumptions, significant vacuum level shifts are observed at the interfaces, resulting in reduced interfacial energetic offsets and increased charge transfer state energies.