Surface equilibrium mechanism controls the stability of co-deposited organic semiconductor glasses

Highly stable glasses of organic semiconductors prepared by physical vapor deposition (PVD) are important materials as they inhibit chemical and physical degradation that leads to a loss of device performance. The highly enhanced stability of single-component PVD organic glasses can be understood with surface equilibrium mechanism. In this work, we investigate the effects of substrate temperature (T_substrate) on co-deposited glasses of m-MTDATA and TPD. Both spectroscopic ellipsometry and differential scanning calorimetry indicate that the stability is controlled by T_substrate/T_{g, mixture} (where T_{g, mixture} is the glass transition temperature of the mixture). An additional series of binary organic glasses have been prepared near T_substrate=0.85T_{g, mixture}. Interestingly, all co-deposited glasses show highly enhanced kinetic and thermal stability, comparable to the most stable single-component PVD glasses. These results indicate that the surface equilibrium mechanism can be extended to understand the stability of co-deposited organic glasses.