Interfacial electronic structure of ClSubPc non-fullerene acceptors in organic photovoltaics using soft X-ray spectroscopies.

In organic photovoltaics (OPVs), determining the energy-level alignment of a donor and an acceptor is particularly important since the interfacial energy gap between the highest occupied molecular orbital (HOMO) level of a donor and the lowest unoccupied molecular orbital (LUMO) level of an acceptor (E-E) gives the theoretical maximum value of the open-circuit voltage (V). To increase the E-E, non-fullerene acceptors, which have a lower electron affinity (EA) than C, are receiving increasing attention. In this study, we investigated the energy-level alignment at the interface of a boron chloride subphthalocyanine (SubPc) donor and a halogenated SubPc (ClSubPc) acceptor using soft X-ray spectroscopy techniques. The estimated E-E of ClSubPc/SubPc was 1.95 eV, which was significantly higher than that of 1.51 eV found at the interface of C/SubPc. This increased E-E was the origin of the enhanced V in OPVs. Additionally, we studied the molecular orientation of ClSubPc using angle-dependent X-ray absorption spectroscopy. The highly disordered ClSubPc molecules result in low carrier mobility, which contributes to the lower short-circuit current density of the ClSubPc acceptor OPVs than the C acceptor OPVs.