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

## TL;DR

This study reveals how charge moves at the interface between organic semiconductors and MoO3, showing how molecule orientation affects electronic properties.

## Contribution

The paper provides atomic-level insight into charge transfer mechanisms at hybrid organic-inorganic interfaces.

## Key findings

- Integer charge transfer occurs from 2H-Pc molecules to MoO3, resulting in positively charged molecules.
- Adsorption geometry affects the SOMO–SUMO gap, with upright molecules showing a smaller gap (∼0.4 eV) and flat molecules a larger gap (∼1.5 eV).
- Reduced electronic screening in flat-lying molecules leads to larger energy gaps.

## Abstract

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.