This article describes recent progress in organic semiconductor research with ultraviolet photoelectron spectroscopy (UPS). Origins of the energy level alignment at organic-conductor/dielectrics interfaces and the charge mobility in organic semiconductor systems are the most important subjects to be elucidated. 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

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... ergy band dispersion give information about the intermolecular interaction and the effective mass of transporting hole, and experimental detection of the highest occupied molecular orbital (HOMO) holemolecular vibration coupling is indispensable to comprehend impacts of the coupling on the hole transport. In this article, 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.