Crystalline/amorphous Raman markers of hole-transport material NPD in organic light-emitting diodes

Raman marker bands characteristic of solid-state structure have been found for N,N′-di-1-naphthaleyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD), which is used as a hole-transport material in organic light-emitting diodes. The widths of the marker bands observed for an amorphous state at 1607, 1290, and 1192 cm−1 are broader than those for the crystalline state observed at 1609, 1288, and 1198 cm−1. These Raman bands are found to be useful for detecting the crystallization, which may cause degradation of organic light emitting diodes, of amorphous NPD films.