Spectroscopic investigation of Fermi level pinning at metal oxide/polymer interfaces and implications for built-in voltage

The built-in potential (Vbi) critically governs charge injection and transport in organic and hybrid optoelectronic devices. While self-assembled monolayers (SAMs) are commonly used to tune electrode work functions, whether this tuning directly translates into controllable Vbi remains unresolved due to possible Fermi level pinning. This study systematically investigates how interface pinning influences the energy level landscape and Vbi in poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD) diodes fabricated on SAM-modified NiOx substrates. By tailoring the substrate work function (Φsub) with SAMs, we achieve precise tuning over a wide range (4.3–5.7 eV). Combining electro-modulated absorption spectroscopy, Kelvin probe measurements, and numerical simulations, we find that pinning emerges in poly-TPD when Φsub ≥ 4.7 eV and quantify how Vbi evolves in both pinned and unpinned regimes. The simulations reproduce the observed transition, which is triggered when Φsub exceeds a threshold of approximately 4.9 eV, corresponding to the hole-accepting level in poly-TPD with a state density of ∼2 × 1020 cm−3. Furthermore, both the interfacial potential bending in the pinned state and the polaron signal intensity increase monotonically with Φsub. This work reveals that charge transfer between the substrate and polymer pins Vbi, an insight that provides a mechanistic basis for interface designing in oxide/SAM-based organic electronic devices.