First-principles approach to the electronic structure in the pentacene thin film polymorph

Density functional theory ( DFT) calculations have been performed to investigate the electronic structure of pentacene in the so- called 'thin film' phase, focusing on the effects on the relevant electronic properties of the relative orientation of the molecules within the crystalline unit cell, which are so far experimentally unknown. Our results show that the energy bandwidth and bandgap are crucially affected by the molecular stacking. Furthermore, by comparing our theoretical spectra with scanning tunnelling spectroscopy measurements, we propose a molecular arrangement that gives good agreement with experiments as far as the relevant orbitals are concerned. For this polymorph, we find highest occupied molecular orbital ( HOMO) and lowest unoccupied molecular orbital ( LUMO) bandwidths of similar to 0.7 and similar to 0.8 eV, respectively, which are significatively larger than those obtained for the pentacene bulk phase and are consistent with the larger conductivity observed experimentally in pentacene thin films.