Origin of Anisotropic Molecular Packing in Vapor-Deposited Alq3 Glasses

Abstract

Anisotropic molecular packing is a key feature that makes glasses prepared by physical vapor deposition (PVD) unique materials, warranting a mechanistic understanding of how a PVD glass attains its structure. To this end, we use X-ray scattering and ellipsometry to characterize the structure of PVD glasses of tris(8-hydroxyquinoline) aluminum ( Alq3), a molecule used in organic electronics, and compare our results to simulations of its supercooled liquid. X-ray scattering reveals a tendency for molecular layering in Alq3 glasses that depends upon the substrate temperature during deposition and the deposition rate. Simulations reveal that the Alq3 supercooled liquid, like liquid metals, exhibits surface layering. We propose that the layering in Alq3 glasses observed here as well as the previously reported bulk dipole orientation are inherited from the surface structure of the supercooled liquid. This work significantly advances our understanding of the mechanism governing the formation of anisotropic structure in PVD glasses.

Influence of intermolecular interactions on the formation of spontaneous orientation polarization in organic semiconducting films

“…The anisotropy has often been observed in the amorphous films of vacuum‐deposited organic semiconductors 1,2,19 . Ediger et al reported a surface equilibrium mechanism, which shows that anisotropy is inherited from the surface structure 14,19 . When the molecules arrive at the film surface during the vacuum deposition, they can move to find their stable positions even below the glass transition temperature because mobility at the surface could be much higher than that in the bulk.…”

“…Finally, anisotropic amorphous films are formed. This surface equilibration mechanism has been recently applied to explain the formation mechanism of SOP in Alq 3 film 14 …”

“…Although the influence of SOP on the device performance of OLED has been gradually revealed, our knowledge of the formation mechanism of SOP is still limited. Only recently, PDM and van der Waals (vdW) interactions were suggested as key factors, 6,10–13 and surface equilibration mechanism was proposed for describing SOP formation process 14 . In this study, we investigated the dominant factors of SOP formation for the understanding and control of SOP.…”

Molecular Orientation Effects in Organic Light‐Emitting Diodes

“…[110,[117][118][119] More research efforts are required to develop faster and more universal methodologies so that molecular simulations may become an important tool for the experimentalists, in silico screening the OLED emitter-host pairs for optimizing the light outcoupling efficiencies. As pointed out by Bagchi et al, [129] the demonstrated correlation between the molecular orientations in vacuum-deposited films and at the surface of a supercooled liquid may open up a possible route towards a far less computationally expensive algorithm. We envision that more research efforts in this field would shorten the development of new OLED molecules.…”

Extreme Elasticity Anisotropy in Molecular Glasses

“…Even molecules with a nearly spherical shape can form anisotropic glasses, exhibiting a tendency toward molecular layering and polar alignment of the molecular dipoles. [ 10 ]…”