Crystallographic and electronic structures of three different polymorphs of pentacene

Abstract

Calculations are presented for the energy bands of three polymorphs of pentacene: the thin-film, bulk, and single-crystal phases. In the calculation of the thin-film phase, the structural data from our recent results on the x-ray diffraction reciprocal space mapping were applied. The band structures are essentially two dimensional, i.e., only small dispersions are found along the ${c}^{*}$ direction. The energy dispersion of the thin-film phase is found to be larger and more isotropic than those of the other phases. The energy dispersions of the bands derived from highest occupied molecular orbital (HOMO), HOMO-1, lowest unoccupied molecular orbital (LUMO), and $\mathrm{LUMO}+1$ levels are analyzed by comparing with the corresponding results on the basis of the tight-binding approximation; the dispersions are well described by transfer integrals involving only the nearest neighbor molecules. In accordance with this finding, a simple model is presented to explain the relation between the crystal structure and the energy dispersion. From the calculated bands, the effective masses are derived to discuss the transport properties of the hole and electrons. Angle-integrated photoemission spectra were also measured for the thin-film and bulk phases. Finally, spectral features of the HOMO-derived bands are interpreted by the calculated density of states.