VAPOR-PRESSURES OF ORGANIC SEMICONDUCTORS - ALPHA-HEXATHIOPHENE AND ALPHA-QUATERTHIOPHENE

The sublimation vapor pressures of the thin him transistor materials, alpha-hexathiophene (alpha-6T) and alpha-quaterthiophene (alpha-4T) we re measured using the Knudsen effusion method and enthalpies of sublim ation were calculated The criteria for Knudsen cell design to obtain a ccurate vapor pressures of large organic molecules were examined. A re cording microbalance with an accuracy of +/-0.1 mu g was used in conju nction with a specially designed Knudsen cell. Vapor pressures from 10 (-6) to 10(-2) Torr were measured. At low pressures, the mean free pat h was large even for oligomers with hard sphere diameters of 22 Angstr om, so that Knudsen criteria were fulfilled. The vapor pressure result s are used to discuss the observations from previously published physi cal vapor crystal growth experiments. (C) 1998 Elsevier Science B.V. A ll rights reserved.