Interfacial electronic structure of Cl₆SubPc 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 (EDHOMO-EALUMO) gives the theoretical maximum value of the open-circuit voltage (V_OC). To increase the EDHOMO-EALUMO, non-fullerene acceptors, which have a lower electron affinity (EA) than C₆₀, 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 (Cl₆SubPc) acceptor using soft X-ray spectroscopy techniques. The estimated EDHOMO-EALUMO of Cl₆SubPc/SubPc was 1.95 eV, which was significantly higher than that of 1.51 eV found at the interface of C₆₀/SubPc. This increased EDHOMO-EALUMO was the origin of the enhanced V_OC in OPVs. Additionally, we studied the molecular orientation of Cl₆SubPc using angle-dependent X-ray absorption spectroscopy. The highly disordered Cl₆SubPc molecules result in low carrier mobility, which contributes to the lower short-circuit current density of the Cl₆SubPc acceptor OPVs than the C₆₀ acceptor OPVs.