Surface enthalpy recovery/relaxation of organic films determined by Mg deposition property

Abstract

Estimating the surface glass transition temperature (T g) of materials is important for making organic devices by vacuum evaporation because the surface T g affects the interface of layered devices and their performance. The enthalpy state of organic films is related to T g. Bulk T g is ordinarily measured by differential scanning calorimetry (DSC), although there is no established method for measuring the surface T g. The Mg deposition property reflects the surface T g; no Mg deposition denotes a low surface T g. In this study, we investigated how annealing affects the bulk and surface T g of N,N′-Bis-(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) using DSC and the Mg deposition properties. For bulk NPB, enthalpy relaxation occurred after annealing at a temperature below T g. On the other hand, for vacuum-deposited NPB film, the enthalpy was in a relaxed state just after preparation, although the surface’s enthalpy recovered by annealing at same condition, as identified by the Mg deposition property.