Discussion on hole traps of amorphous films of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine ( α- NPD) deposited at different substrate temperatures

Abstract

The hole current in amorphous films of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) strongly depends on substrate temperature during vacuum deposition (Tsub) and is the highest at a Tsub value of around 275 K. However, the reason for this enhancement of hole current at this Tsub is not clearly understood. In this study, we performed thermally stimulated current (TSC) measurements, which is a versatile method used to obtain information about carrier traps, on α-NPD films. The TSC results revealed that hole traps were uniformly distributed throughout the films and that hole traps were the shallowest for films fabricated at a Tsub value of around 275 K. Thus, the shallowest hole traps at this Tsub are believed to be one reason for the highest hole current for α-NPD films. This is the demonstration of how Tsub affects carrier traps, contributing to a better understanding of the underlying physics in organic amorphous films.