Influence of Vapor Deposition on Structural and Charge Transport Properties of Ethylbenzene Films

Abstract

Organic glass films formed by physical vapor deposition exhibit enhanced stability relative to those formed by conventional liquid cooling and aging techniques. Recently, experimental and computational evidence has emerged indicating that the average molecular orientation can be tuned by controlling the substrate temperature at which these “stable glasses” are grown. In this work, we present a comprehensive all-atom simulation study of ethylbenzene, a canonical stable-glass former, using a computational film formation procedure that closely mimics the vapor deposition process. Atomistic studies of experimentally formed vapor-deposited glasses have not been performed before, and this study therefore begins by verifying that the model and method utilized here reproduces key structural features observed experimentally. Having established agreement between several simulated and experimental macroscopic observables, simulations are used to examine the substrate temperature dependence of molecular orientation. The results indicate that ethylbenzene glasses are anisotropic, depending upon substrate temperature, and that this dependence can be understood from the orientation present at the surface of the equilibrium liquid. By treating ethylbenzene as a simple model for molecular semiconducting materials, a quantum-chemical analysis is then used to show that the vapor-deposited glasses exhibit decreased energetic disorder and increased magnitude of the mean-squared transfer integral relative to isotropic, liquid-cooled films, an effect that is attributed to the anisotropic ordering of the molecular film. These results suggest a novel structure–function simulation strategy capable of tuning the electronic properties of organic semiconducting glasses prior to experimental deposition, which could have considerable potential for organic electronic materials design.

Emergence of a substrate-temperature-dependent dielectric process in a prototypical vapor deposited hole-transport glass

“…demonstrated for BSB-Cz that the orientation anisotropy of VD glasses, which depends on deposition conditions, can improve their charge carrier transport characteristics with respect to the ordinary isotropic glass 20 . The same effect was also recently observed in computational simulations of ethylbenzene 54 . The site energy of the hole state of a molecule depends strongly on the relative orientation of the adjacent dipoles 55 .…”

Bidimensional H‐Bond Network Promotes Structural Order and Electron Transport in BPyMPMs Molecular Semiconductor

“…The understanding of the driving forces regulating molecular orientation is mandatory to optimize material properties, such as charge transport, and to design processes and/or compounds to improve device efficiency. In this matter, the molecular dynamics (MD) at atomic resolution is a powerful computational method able to rationalize the microscopic origin of the molecular preferred orientation, [ 17–26 ] and several studies have been reported to successfully study interfacial properties and molecular arrangement of OLED materials. [ 7,18,20,27 ] For a general description of the challenges and the strategies currently used in the computational modeling of OLED materials, we refer the interested reader to a recent perspective [ 19 ] and the references therein.…”

The interplay of interfaces, supramolecular assembly, and electronics in organic semiconductors

“…Greater orientational anisotropy around T g was also found for longer molecules [14,17,55]. Additionally, allatom MD simulations of ethylbenzene, a model glassy system which can be considered to have similar structural properties to common organic semiconductors showed the same dependence on T sub [90]. Furthermore, coarse-grained (CG) MD simulations of rod-like or disc-like molecules showed the same trends as found experimentally despite the models only being parameterized to reproduce the molecular shape and not the specific interactions of the experimentally studied molecules [15,17,91].…”

“…Controlling emitter orientation has thus been the subject of extensive study in recent years. A number of factors have been shown to influence the alignment of emitter molecules with respect to their substrate, including the choice of host [18,86], the shape of the emitter molecule [14,18], strength of interactions with the substrate [14,82,83], emitter chemical structure [87], the presence of permanent dipole moments [88], processing technique [87,89], and temperature [14,15,17,56,[90][91][92][93].…”