Manipulating molecular orientation in vapor-deposited organic semiconductor glasses via in situ electric fields: a molecular dynamics study

Abstract

The manipulation of molecular orientation is a well established target in organic electronics and energy-harvesting applications since it may affect relevant parameters such as energy levels, electron transport, exciton recombination or light outcoupling. In vapor-deposited thin film organic glasses, thermal evaporation offers control of the molecular orientation by changing the deposition temperature or the deposition rate. However, the deposition conditions are strongly linked to other properties of the glass such as density and thermal stability. The deposition conditions that yield optimal orientation may result in poorly stable and low-density glasses, detrimental for device performance. Here, we use molecular dynamics simulations to investigate the possibility of manipulating molecular orientation of physicalvapor-deposited (PVD) glasses by applying external electric fields during the vapor deposition process. Our aim is to align the electric dipole moments of these molecules in the direction of the electric field, whether in-plane or out-of-plane. We consider three distinct molecules: TPD and two brominated TPD derivatives with larger intrinsic dipole moments. Our results demonstrate that the electric field effectively modifies the orientation polarization of the glass without modifying its density, opening a possible route to manipulate glass properties and device performance at will.