Glass Structure Controls Crystal Polymorph Selection in Vapor-Deposited Films of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl

Abstract

Glasses of a common OLED host material, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl or CBP, were prepared by vapor deposition at various substrate temperatures. Consistent with previous work, the substrate temperature during deposition controls the anisotropic structure of the glass. Crystal growth at the free surface of CBP was very fast and was eliminated by covering the free surface with a thin overlayer of vapor-deposited tris(8-hydroxyquinoline) aluminum (Alq3). It was observed that vapor-deposited glasses of CBP with an overlayer crystallized into either the α or γ polymorph, depending on the glass structure. To the best of our knowledge, this is the first demonstration that two glasses of the same organic compound, subjected to the same annealing conditions, grow crystals of different polymorphs. The α polymorph exhibited a fine-grained morphology, reminiscent of previous studies on glass-crystal (GC) growth, while the γ polymorph exhibited dendritic growth. Crystals of both polymorphs grew rapidly below the glass transition temperature Tg in comparison to previously studied GC growth systems such as o-terphenyl. Surprisingly, Tg for CBP was found to be about 385 K, well above previously reported values.