Microscopic Origin of Charge Transfer at the Organic Semiconductor/MoO₃ Hybrid Interface

Molybdenum trioxide (MoO₃) is widely utilized as an interfacial layer in organic electronic devices due to its high work function and favorable energy level alignment with organic semiconductors. While its role in facilitating hole injection has been extensively studied, the microscopic mechanisms underlying charge transfer at MoO₃/organic interfaces remain elusive. Here, we investigate the interaction between 2H-phthalocyanine (2H-Pc) and ultrathin MoO₃ films grown on Pd(100) as a model system to explore the microscopic origin of charge transfer from the organic layer to the oxide substrate. Using a combination of scanning tunneling microscopy/spectroscopy, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure, work function measurements, and density functional theory, we find clear evidence for integer charge transfer from the molecules into the substrate, resulting in positively charged molecules in both upright and flat adsorption geometries. The electronic signatures of charging are accompanied by distinct SOMO-SUMO gaps, with upright molecules exhibiting a small gap (∼0.4 eV), while flat-lying molecules show a significantly larger gap (∼1.5 eV) owing to reduced electronic screening. These findings provide atomically resolved insight into charge transfer and highlight how adsorption geometry and local dielectric environment govern the electronic structure of hybrid interfaces.