Abstract

Carrier injection, which is a key factor in controlling and improving organic device properties, has been predominantly studied using the injection barrier height derived from HOMO and LUMO positions. The weak density of states (DOS) within the HOMO–LUMO energy gap is also important to understand the practical injection properties. In this study, the DOS of the α-NPD/electrode model interfaces are investigated using high-sensitivity UV photoemission spectroscopy. The nature of hole injection is discussed based on the observed DOS and a simple simulation. The results indicate that the weak DOS close to the Fermi level is critical for carrier injection.