Energy band and electron-vibration coupling in organic thin films: photoelectron spectroscopy as a powerful tool for studying the charge transport

• Published in: Applied physics. A, Materials science & processing Vol. 92; no. 3; pp. 495 - 504
• Main Authors: Ueno, Nobuo, Kera, Satoshi, Sakamoto, Kazuyuki, Okudaira, Koji. K.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.08.2008; Springer
• Subjects: Characterization and Evaluation of Materials; Clean metal, semiconductor, and insulator surfaces; Condensed Matter Physics; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electron and ion emission by liquids and solids; impact phenomena; Electron states and collective excitations in thin films, multilayers, quantum wells, mesoscopic and nanoscale systems; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Exact sciences and technology; Machines; Manufacturing; Nanotechnology; Optical and Electronic Materials; Photoemission and photoelectron spectra; Physics; Physics and Astronomy; Processes; Surfaces and Interfaces; Thin Films; Thin films and multilayers; 73.50.-h; 72.80.Le; 72.20.-i; 73.20.-r; Thin films; Band structure; Frontier orbital; Dispersion relations; Organic semiconductors; Ultraviolet photoelectron spectra; Transport processes; Property structure relationship; Electron-phonon interactions; Microstructure
• ISSN: 0947-8396; 1432-0630
• DOI: 10.1007/s00339-008-4553-8

This article describes the origins of the width of the highest occupied molecular orbital (HOMO) state observed in the ultraviolet photoemission spectra (UPS) of thin organic semiconductor films. Although much research has been performed on the electrical properties of organic devices, a lot of crucial problems still remain. Among these problems, the charge mobility in organic semiconductor systems is one the most important subjects to be elucidated. In order to discuss the mobility, it is essential to understand both the intermolecular interaction and the electron-molecular vibration coupling. Experimental measurements of the energy band dispersion give information about the intermolecular interaction, and experimental detection of the HOMO hole-vibration coupling is indispensable to comprehend impacts of the electron-vibration coupling on the hole transport. Since most of the information is hidden behind the finite bandwidth of the HOMO, only careful UPS measurements can provide information on these important phenomena related to charge carrier dynamics. In this article, we summarize our recent challenges on UPS measurements of organic thin films, which give the band dispersion of the HOMO and the HOMO hole-vibration coupling, and discuss the origins of the UPS bandwidth that relates to the charge carrier dynamics.