Electronic states of organic semiconductors and their interfaces by using photoelectron spectroscopy

This article describes recent progress in organic semiconductor research with ultraviolet photoelectron spectroscopy (UPS). Origins of the energy level alignment at organic-conductor interfaces and the charge mobility in organic semiconductor systems are the most important subjects, which have been left to be elusive. Direct experimental detection of very low electronic density-of-states in the band gap is necessary to unravel mysteries of the energy level alignment. Experimental measurements of the energy band dispersion give information about the intermolecular interaction and the effective mass of transporting hole, and experimental detection of electron-phonon coupling at the highest occupied molecular orbital (HOMO) is indispensable to comprehend impacts of the coupling on the hole transport. We summarize our recent challenges on UPS measurements of organic semiconductors, which give the band dispersion of the HOMO, the HOMO hole-vibration coupling and the band-gap states appearing near the interface.