Real-space Observation of Charge Separation in Mixed Dimensional InSe-C₇₀ van der Waals Heterojunctions

Two-dimensional InSe is a direct bandgap semiconductor promising for high performance photodetectors and solar cells. Mixed dimensional p-n heterojunction between InSe and organic molecules integrates desirable properties of both. Electronic structure of surface states is critical for optimal band offsets at the heterojunction. Although there is no consensus on the type of intrinsic doping, few-layered InSe field-effect transistors showed exclusively n-type behavior. Here, we report that the atomically clean surface of mechanically exfoliated InSe is unintentionally p-doped. STS taken over the native surface selenium vacancies resolves occupied defect states near the valance band edge. These defect states lead to an upward band bending near the InSe surface, rendering InSe surface p-type. In prototype InSe/C₇₀ heterojunction, PL spectroscopy and STS both indicate formation of a type-II heterojunction. Electrons can transfer between InSe and C₇₀ evidence from mutual PL quenching of both InSe and C₇₀. STS in vacuum and KPFM in the ambient consistently reveal the electron transfer from InSe to C₇₀ resulting in a photovoltage of ~0.25 eV. This study reveals the unusual electron donor behavior of InSe in an organic-inorganic heterostructure.