Second highest occupied molecular orbital effects on the valence band structure of organic semiconductors

Abstract

We investigate the electronic states of organic semiconductors with herringbone structures using the multi-orbital tight-binding model for highest and second highest occupied molecular orbitals (HOMOs and SHOMOs) to elucidate the effect of SHOMOs on the valence-band state. We diagonalize the Hamiltonian analytically and derive the valence band width considering hybridization of HOMO and SHOMO. It is shown that the SHOMO effect is enhanced in the condition where the sum of the intermolecular transfer integrals of HOMO-SHOMO in the sublattice and the sum between the sublattices is respectively so large that their product is not negligible compared to the energy difference between HOMO and SHOMO. We also investigate acene, phenacene and thienoacene using density functional theory. It is shown that [1]benzothieno[3,2-b][1]benzothiophene(BTBT) based materials satisfy this condition, and SHOMO contributes significantly. It implies that the origins of the high-mobility of BTBT based materials can be ascribed to the light effective mass due to the contribution of SHOMO.