Electron spectroscopy of functional organic thin films: Deep insights into valence electronic structure in relation to charge transport property

We summarize both historical and recent challenges on angle-resolved and high-energy resolution ultraviolet photoelectron spectroscopy (UPS) of organic thin films. Topics selected for this article are mainly on electron spectroscopic study of the electronic states in relation to charge mobility of organic molecular thin films, especially of weakly interacting organic molecular solids. We describe intramolecular band dispersion in a quasi-one-dimensional molecular chain and intermolecular band dispersion measured with angle-resolved UPS. The latter offers a spectroscopic estimation of the drift hole mobility in organic semiconductors. Furthermore we describe briefly hole-vibration coupling in organic ultrathin films, which dominates the hopping hole mobility and has been recently measured with high resolution UPS. These experiments are thus considered to be a kind of the first-principle measurement of the mobility of organic thin films, which have not yet been realized with electrical measurements. Conduction band dispersion studied with low-energy electron transmission, which is needed in analyzing angle-resolved UPS, ultrafast phenomena appearing in conventional UPS measurements and other interesting work are also introduced.