Electronic States of Pentacene Thin Films at Interfaces with Ionic-Liquid Layers Probed by Photoelectron Yield Spectroscopy

Understanding semiconductor–ionic-liquid (IL) interfaces is essential for achieving high-performance electronic devices. However, it is difficult to analyze the interfacial electronic states, particularly in the highest occupied molecular orbital (HOMO), because the probing depth of conventional photoelectron spectroscopy is limited to ∼1 nm. Here, we employ photoelectron yield spectroscopy to probe the interfacial electronic states of pentacene submonolayer thin films underneath IL layers and investigate the impact of the IL layers on the shapes and energy levels of the HOMO for the pentacene submonolayer thin films. With increasing the thickness of the IL layers, the density of states on the HOMO peak broadened, indicating that the IL layers induced an energetic disorder in the density of states of the pentacene submonolayer thin films. In addition, the peak top energy of the HOMO peak and the ionization energy gradually shifted toward higher energies as the IL thickness increased, which is attributed to the shifts in the vacuum level and the increase in the polarization energy at the interfaces. This work demonstrates that the photoelectron yield spectroscopy (PYS) technique can adequately access the electronic states at heterointerfaces, highlighting its significant potential for application in the interfacial characterization of devices.