All-atom molecular dynamics simulations of vapor deposition as a tool to study thermodynamic stability and structure...

Thin films of organic semiconductors obtained by vapor deposition under certain conditions are known to have unique thermodynamic and structural properties, for which they have been named ultrastable. The use of all-atom molecular dynamics simulations for their study is gaining interest due to the possibility of simplifying the process of their deposition and examining their properties at the molecular level. This work provides a systematic and thorough investigation of the key parameters of thin films that characterize their stability and morphology: energy, density, fictive temperature, and orientational order parameter. The results of studying these properties using molecular dynamics for vapor-deposited thin films of three organic semiconductors (TNB, TPB, and TPD) led to a number of valuable insights. First, it was shown that the dependencies of thermodynamic properties on deposition temperature (Tdep) obtained from simulations are consistent with those observed experimentally but shifted toward higher temperatures. Moreover, it was demonstrated for the first time that all glass stability indicators for each of the compounds studied show a maximum at a single deposition temperature. These observations highlight the close relationship between Tdep and the characteristics of the resulting thin films, which can be used to develop a methodology for producing ultrastable glasses with tunable characteristics. The results of this work show that further application of the molecular dynamics method to study vapor-deposited thin films of organic semiconductors may reveal intriguing aspects of their formation and structure.