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

Abstract

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,N′-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 and 0.98 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.

Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses

“…23 To carry out this study, we have chosen the organic semiconductor TPD (N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine), and the pharmaceutical molecule IMC (1-(4-Chlorobenzoyl)-5methoxy-2-methyl-3-indoleacetic acid), as they form glasses with enhanced stability and have been widely characterised as a function of deposition temperature. 2,3,15,16,22,[24][25][26][27][28][29][30] IMC shows a maximum of stability when deposited around 266 K (0.85T g ) 26 and a transition into the supercooled liquid governed in the first stages by a transformation front whose velocity depends on the deposition conditions of the IMC glass. 15,26 Spectroscopic ellipsometry measurements of thin film TPD glasses have shown that TPD also transforms via a propagating front under isothermal conditions.…”

Orientational anisotropy in simulated vapor-deposited molecular glasses

“…47 Grazing-incidence x-ray scattering measurements on N,N ′ -bis(3-methylphenyl)-N,N ′ -diphenylbenzidine (TPD) indicate anisotropy in nearest-neighbor packing that depends upon substrate temperature during deposition. 48 Based on these last two studies, it has been suggested that anisotropic packing is not a necessary condition for high kinetic stability, but rather that these two properties are independent features of the vapor deposition process. A recent analysis of model PVD polymer glasses reinforces that view.…”

Control of Molecular Orientation in Organic Semiconductor Films using Weak Hydrogen Bonds

“…To quantify the structure–molecular orientation relationships, we analyzed GIWAXD images to obtain the order parameter ( S ) according to the method reported by Hammond et al and Gujral et al (Figures S25–S28, Supporting Information) . This order parameter can be calculated based on the diffraction near q ≈ 1.65 Å −1 and is defined as follows…”