(151j) Surface Equilibration Mechanism Controls the Stability of a Model Co-Deposited Glass Mixture of Organic Semiconductors

Organic glasses are important materials in organic electronics, such as organic light-emitting diodes (OLEDs). The active components in OLEDs are glassy layers (10-30 nm thick) of physical vapor-deposited (PVD) organic semiconductors. A typical OLED architecture would have 3-7 layers of PVD glasses. Some layers might be pure components (e.g., an electron transport layer) and some of them might be glassy mixtures (e.g. the emitter layer). The kinetic and thermodynamic stability of these layers are directly relevant to the lifetime of these electronic devices. Previous work has shown that ultrastable single-component organic semiconductor glasses can be prepared via PVD, however, little is known about the stability of vapor-deposited glass 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 (T_sub). 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 T_sub=0.78-0.90T_g (where T_g is the conventional glass transition temperature). When deposited at 0.94T_g, the enthalpy of m-MTDATA/TPD glass equals that expected for the equilibrium liquid at that temperature. These results provide an avenue for designing high-performance organic electronic devices.