Thermal stability of vapor-deposited stable glasses of an organic semiconductor

TitleThermal stability of vapor-deposited stable glasses of an organic semiconductor
Publication TypeJournal Article
Year of Publication2015
AuthorsWalters, Diane M., R. Richert, and M. D. Ediger
JournalJournal of Chemical Physics
Start Page134504
Date Published04/2015
Vapor-deposited organic glasses can show enhanced kinetic stability relative to liquid-cooledglasses. When such stable glasses of model glassformers are annealed above the glass transition temperature Tg, they lose their thermal stability and transform into the supercooledliquid via constant velocity propagating fronts. In this work, we show that vapor-depositedglasses of an organic semiconductor, N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), also transform via propagating fronts. Using spectroscopic ellipsometry and a new high-throughput annealing protocol, we measure transformation front velocities for TPD glassesprepared with substrate temperatures (TSubstrate) from 0.63 to 0.96 Tg, at many different annealing temperatures. We observe that the front velocity varies by over an order of magnitude with TSubstrate, while the activation energy remains constant. Using dielectric spectroscopy, wemeasure the structural relaxation time of supercooled TPD. We find that the mobility of the liquidand the structure of the glass are independent factors in controlling the thermal stability of TPDfilms. In comparison to model glassformers, the transformation fronts of TPD have similarvelocities and a similar dependence on TSubstrate, suggesting universal behavior. These results may aid in designing active layers in organic electronic devices with improved thermal stability.