Ab initio calculation of electron effective masses in solid pentacene

We have calculated the band structures of solid pentacene. One of our models makes use of experimental data obtained for a single crystal [R. B. Campbell et al., Acta. Crystallogr. 14, 705 (1961); R. B. Campbell and J. M. Robertson, Acta. Crystallogr. 15, 289 (1962)] and the other one considers a thin film fabricated by the present authors. Our calculations indicate that the electronic properties of a pentacene thin film are markedly different from those of a single crystal, although the densities of pentacene molecules in these solids are not significantly different. The energy band gaps of the single crystal and the thin film have been estimated to be $0.73\mathrm{eV}$ and between 0.26 and $0.72\mathrm{eV}$ , respectively. The bandwidths of the highest valence band and the lowest conduction band of the film are wider than those of the single crystal. These results suggest that the electron or hole carriers in the film have effective masses lighter than those of the single crystal, and in order to estimate the carrier mobility we first calculate the effective mass.