Modeling of thickness-dependent energy level alignment at organic and inorganic semiconductor interfaces

We identify a universality in the Fermi level change of Van der Waals interacting semiconductor interfaces. We show that the disappearing of quasi-Fermi level pinning at a certain thickness of semiconductor films for both intrinsic (undoped) and extrinsic (doped) semiconductors over a wide range of bulk systems including inorganic, organic, and even organic–inorganic hybridized semiconductors. The Fermi level (EF) position located in the energy bandgap was dominated by not only the substrate work function (Φsub) but also the thickness of semiconductor films, in which the final EF shall be located at the position reflecting the thermal equilibrium of semiconductors themselves. Such universalities originate from the charge transfer between the substrate and semiconductor films after solving one-dimensional Poisson's equation. Our calculation resolves some of the conflicting results from experimental results determined by using ultraviolet photoelectron spectroscopy (UPS) and unifies the general rule on extracting EF positions in energy bandgaps from (i) inorganic semiconductors to organic semiconductors and (ii) intrinsic (undoped) to extrinsic (doped) semiconductors. Our findings shall provide a simple analytical scaling for obtaining the “quantitative energy diagram” in the real devices, thus paving the way for a fundamental understanding of interface physics and designing functional devices.