Tsuyoshi Tsujioka

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.