The angle-resolved photoemission studies (ARPES) of organic semiconductors is the key to understand fundamental parameters in the carrier transport, such as transfer integrals, effective mass, and carrier-phonon coupling constant. The ARPES of organic semiconductors, however, requires well-defined, single-crystalline and highly conductive samples. Since organic semiconductors have extremely low conductivity with low symmetry unit cells, the number of ARPES research was limited, which was an obstruction to developing synthesis policy for high mobility materials. In this review, we will summarize one of the successful strategy for preparing quasi-singly-oriented and highly-conductive sample for ARPES. We will demonstrate that bismuth terminated silicon surface is an ideal surface to grow epitaxial organic films with practically important “standing” orientation. It was also confirmed that quasi-single crystalline film was grown by using vicinal surfaces and reducing surface symmetry. We will also present the band dispersion results with key parameters and discuss future prospects.

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  The electronic structure of organic thin films and interfaces plays a crucial role in the performance of optoelectronic devices using organic semiconductors, and is seriously dominated by the geometric film/interface structure due to the anisotropic spatial distribution of molecular orbitals. This paper briefly reviews the recent progress of the examination of correlating electronic structure and geometric structure of archetypal organic semiconductor thin films and interfaces by using spectroscopic experiments with synchrotron radiation such as angle-resolved photoelectron spectroscopy, x-ray absorption spectroscopy, and x-ray standing wave.

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  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.

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  The relation between the crystallographic and electronic structures is discussed for single crystals and thin films of pentacene. The polymorphs of pentacene reported so far are reviewed with particular emphasis on the structural analysis of thin films using X-ray diffraction techniques. For the three different polymorphs of which the precise crystallographic structures are available, the band calculations are made using density functional theory. Band structures, effective masses and transfer integrals depend strongly on the crystallographic structures indicating a large difference in the transport properties. Only three transfer integrals are needed to describe the band structures in the framework of tight-binding approximation. By considering the relation between the transfer integrals and the crystal structure, the difference in the band structures can simply be explained by the different orientation angles of molecular long axis.

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  Most research in the field of organic electronics has focused on π-conjugated molecules with a closed-shell configuration, which accommodate their π-electrons in only bonding orbitals and are consequently quite stable. In general, closed-shell molecules have wide energy gaps, and they have significantly weaker intermolecular interactions. In contrast, recently organic biradicals have attracted considerable attention, owing to their interesting physical properties, e.g., a small energy gap and strong intermolecular interactions. Organic biradical molecules are composed of a pair of organic radicals that possess an open-shell configuration. Owing to their radical nature, biradical molecules exhibit unusual physical properties such as those listed above. This article is a review of our recent study on the electronic structure of two different types of organic biradical molecules that have potential as new materials for manufacturing electronic devices. In particular, our results show that organic biradicals are expected to form a new class of materials for use in organic electronics as new building blocks for highly ordered organic semiconducting films with wide energy band dispersion widths and a narrow energy gap.

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