Interfacial electronic structure of Cl 6 SubPc 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 (Cl SubPc) acceptor using soft X-ray spectroscopy techniques. The estimated E-E of Cl SubPc/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 Cl SubPc using angle-dependent X-ray absorption spectroscopy. The highly disordered Cl SubPc molecules result in low carrier mobility, which contributes to the lower short-circuit current density of the Cl SubPc acceptor OPVs than the C acceptor OPVs.