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 hybridoptoelectronic devices. While self-assembled monolayers (SAMs) are commonly used to tuneelectrode work functions, whether this tuning directly translates into controllable Vbi remainsunresolved due to possible Fermi-level pinning. This study systematically investigates how interfacepinning 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 tailoringthe substrate work function (Φsub) with SAMs, we achieve precise tuning over a wide range (4.3-5.7eV). Combining electro-modulated absorption spectroscopy, Kelvin probe measurements, andnumerical simulations, we find that pinning emerges in poly-TPD when Φsub {greater than or equal to} 4.7 eV and quantifyhow Vbi evolves in both pinned and unpinned regimes. Simulations reproduce the observed transitionwhen pinning is triggered as Φsub exceeds a defect level at around 4.9 eV, assuming an interface statedensity of ~2×1020 cm−3. Furthermore, both the interfacial potential bending in the pinned state andthe polaron signal intensity increase monotonically with Φsub. These results clarify the interplaybetween SAM-induced work-function shifts and Fermi-level pinning, providing a physical basis forinterface engineering in oxide/SAM-based organic electronics.