Vapor pressures of organic semiconductors:

The sublimation vapor pressures of the thin film transistor materials, α-hexathiophene (α-6T) and α-quaterthiophene (α-4T) were measured using the Knudsen effusion method and enthalpies of sublimation were calculated. The criteria for Knudsen cell design to obtain accurate vapor pressures of large organic molecules were examined. A recording microbalance with an accuracy of ±0.1 μg was used in conjunction with a specially designed Knudsen cell. Vapor pressures from 10−6 to 10−2 Torr were measured. At low pressures, the mean free path was large even for oligomers with hard sphere diameters of 22 Å, so that Knudsen criteria were fulfilled. The vapor pressure results are used to discuss the observations from previously published physical vapor crystal growth experiments.