Ultraviolet photoelectron spectroscopy (UPS) III: Direct study of "invisible" band gap states by ultrahigh-sensitivity UPS

Abstract

We developed ultrahigh sensitivity UPS with very low background, which does not introduce detectable radiation damages into organic materials. The UPS detects density of states of the order of ~1016 states eV-1 cm-3 even for radiation-sensitive organic films, which is comparable to electrical measurements of charge trapping centers. In this chapter we introduce the method of ultrahigh sensitivity measurement and some results on the energy distribution of gap states in pentacene films. Striking effects on the gap states and the energy level alignment due to (i) gas exposure (one atmosphere N2, O2 and Ar) and (ii) hole doping are discussed.