Surface Equilibration Mechanism Controls the Stability of a Model Co-deposited Glass Mixture of Organic Semiconductors

Abstract

While previous work has identified the conditions for preparing ultrastable single-component organic glasses by physical vapor deposition (PVD), little is known about the stability of co-deposited mixtures. Here, we prepared binary PVD glasses of organic semiconductors, TPD (N,N-Bis(3-methylphenyl)-N,N-diphenylbenzidine) and m-MTDATA (4,4,4-Tris[phenyl(m-tolyl)amino]triphenylamine), with 50:50 mass concentration over a wide range of substrate temperatures (Tsub). The enthalpy and kinetic stability are evaluated with differential scanning calorimetry and spectroscopic ellipsometry. Binary organic semiconductor glasses with exceptional thermodynamic and kinetic stability comparable to the most stable single-component organic glasses are obtained when deposited at Tsub=0.78-0.90Tg (where Tg is the conventional glass transition temperature). When deposited at 0.94Tg, the enthalpy of m-MTDATA/TPD glass equals that expected for the equilibrium liquid at that temperature. Thus, the surface equilibration mechanism previously advanced for single-component PVD glasses is also applicable for these co-deposited glasses. These results provide an avenue for designing high-performance organic electronic devices.