Growth mechanism of NPD crystal from amorphous thin film by observation of morphology and measuring growth rate

Abstract

Crystallization in organic amorphous thin films causes degradation of organic light-emitting devices. Crystallization of N′-di-1-naphthyl-N,N′-diphenylbenzidine (α-NPD) thin films occurs by the migration of NPD molecules on a substrate. In this study, we investigated the kinetic process of the crystallization of α-NPD in amorphous thin films by comparing the growth rate with the morphology of NPD crystals. The growth rate of NPD crystals was changed indicating different kinetic processes. The observation of a depression zone formed around a single fibril crystal revealed that the depression zone was formed only at the side of the fibril crystal, not at the tip of the fibril crystal in thinner films. This shows that NPD molecules were transferred directly from the amorphous region to the tip of the fibril crystal. After the crystalline region became dense by the growth and divergence of fibril crystals, the circular crystalline region was expanded by two-dimensional diffusion-limited growth.