BAND STRUCTURES OF ORGANIC THIN-FILM-TRANSISTOR MATERIALS

We report extended Huckel theory (EHT) band structure calculations for the high-temperature polymorph of alpha-6T (alpha-sexithiophene) and C-60, two organic materials that have shown promising device characteristics, as the active element in thin-film transistors (TFTs). We also report calculations for alpha-3T and kappa-(ET)(2)Cu(NCS)(2) [where ET = bis(ethylenedithio)tetrathiafulvalene] for comparison purposes. Both alpha-6T/HT and C-60 show well developed band dispersions. Whereas C-60 is an isotropic three-dimensional (3D) molecular solid, alpha-6T/HT shows a strongly 2D electronic structure, and the band structure is quite similar to that of the ET organic superconductors such as kappa-(ET)(2)Cu(NCS)(2). Factors important in determining the mobilities of organic semiconductors are reviewed.