VUV photoemission using synchrotron light: a tool for characterising surfaces and interfaces occurring in OLEDs

Abstract

Interfaces between an organic material and another organic material or a metal play an essential role in organic light-emitting diodes (OLEDs). Photoelectron spectroscopy in the vacuum-ultraviolet range has been used to characterise the alignment of energy levels and to discover possible chemical interaction at such interfaces. The systems investigated here are the interfaces of active conjugated organic molecules (pentacene, para-sexiphenyl, and N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1biphenyl1-4,4′-diamine) with materials used for carrier injection. Low work function metals such as samarium or alkali are used for electron injection. Materials with high work function such as gold or poly(3,4-ethylenedioxythiophene)/polystyrenosulfonate are used for hole injection. In this paper, we will discuss the differences in metal-organic interaction (physisorption versus stronger bonding) and energy level alignment at organic–metal and organic–organic interfaces.