| High thermal stability and anisotropic molecular orientation enhances the performance of vapor-deposited organic semiconductors, but controlling these properties is a challenge in amorphous materials. Here, I present the thermal stability and anisotropic molecular orientation in vapor-deposited glasses of six molecules used as organic semiconductors and emitters. Using a high-throughput preparation protocol, I simultaneously deposit many glasses with different substrate temperatures. I characterize them using spectroscopic ellipsometry. For all the systems studied, I find that a wide range of substrate temperatures can prepare glasses with enhanced thermal stability. For one organic semiconductor, I study the transformation behavior when it is annealed above Tg and observe that it transforms via a front mechanism. By developing a high throughput annealing protocol, I establish that the front velocity is independently determined by the mobility of the transformed liquid and the structure of the underlying glass.;Additionally, all the systems studied show anisotropic molecular orientation when vapor-deposited with a wide range of substrate temperatures. Molecules with similar molecular shapes show similar trends in molecular orientation when the substrate temperature is normalized by Tg. However, rod- and disk-shaped molecules have different trends in molecular orientation.;Simulations of the equilibrium liquid show anisotropic molecular orientation near the surface, and rod- and disk-shaped molecules have different trends in this anisotropic structure. Simulated liquids of coarse-grained models of the rod- and disk-shaped molecules show similar anisotropic surface structure. Furthermore, simulations of the vapor-deposition process reproduce the trends in the experiments and suggest an orientation mechanism.;We propose that enhanced thermal stability and anisotropic molecular orientation in vapor-deposited glasses are due to enhanced equilibration at the glass surface during the deposition. Molecules on the surface equilibrate to more stable structures and obtain molecular orientations favorable near the free surface of the equilibrium liquid, resulting in a bulk glass that is stable and has anisotropic molecular orientation otherwise only seen at the free surface. I show this mechanism is general for different molecular shapes. Establishing a general mechanism for stability and molecular orientation could inform the choice of materials and deposition conditions for preparing active layers in organic electronics. |
