Structural Characterization of Vapor-Deposited Glasses of an Organic Hole Transport Material with X-ray Scattering

Vapor-deposited organic glasses can be produced with enhanced thermal stability and tunable molecular orientation by controlling the substrate temperature during deposition. Recent work has also Shown improved charge carrier mobility associated with anisotropic molecular packing in organic electronics. Here grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to characterize the structural anisotropy in glasses of a hole transport material, N,Ni-bis(3-methylphenyl)-N,N'-diphenylbenzidine, commonly referred to as TPD. The TPD glasses were prepared by physical vapor deposition at substrate temperatures between 0.79 T-g, where T-g where T-g is the glass transition temperature. A GIWAXS-derived orientation order parameter is used to quantify the anisotropy observed in the scattering patterns of the glasses. The GIWAXS-order parameter exhibits both positive and negative values as a function of substrate temperature, indicating either face-on or edge-on packing, and correlates well with a spectroscopic ellipsometry-derived order parameter that is sensitive to molecular orientation. We propose molecular packing arrangements based on the combination of the two order parameters and explore the relationship between kinetic stability and glass structure.