Correlation between giant surface potential and enthalpy relaxation in vacuum-deposited organic films

Abstract

It is well known that vacuum deposition of organic molecules possessing electric dipoles leads to spontaneous molecular orientation, resulting in the formation of a giant surface potential (GSP). The GSP is expected to be useful for energy-harvesting devices, and improving carrier injection in organic light-emitting diodes; therefore, maximizing the GSP is crucial for device performance. Here, we systematically investigate the factors governing GSP formation by examining the roles of glass transition temperature (Tg), substrate temperature (Tsub), and deposition rate using a series of organic materials, including adamantane derivatives, diarylethenes, and spiropyrans. The molecular orientation parameter ⟨cosθ⟩ exhibits a clear dependence on Tg, indicating that surface molecular dynamics during deposition play a dominant role. We demonstrate that the GSP slope is maximized when Tsub is maintained at approximately 0.8–0.85 Tg. This condition coincides with the maximum enthalpy relaxation of vapor-deposited organic glasses. Based on these results, we propose a three-regime model describing GSP generation as a function of surface molecular mobility (Tsub/Tg scaling), providing practical guidelines for maximizing GSP in vapor-deposited organic thin films.