Influence of alkyl side-chain length on the carrier mobility in organic semiconductors: herringbone vs. pi–pi stacking

Abstract

The influence of the alkyl side-chains on the electronic structures, molecular packing and charge transport for five naphthalene diimide (NDI) derivatives, four [1]benzoselenopheno[3,2-b][1]benzoselenophene (BSBS) derivatives and three [1]benzothieno[3,2-b]benzothiophene (BTBT) derivatives is systematically investigated by density functional theory calculations coupled with a tunneling enabled hopping model. The results show that the introduction of long alkyl chains does not affect the energy levels of frontier molecular orbitals and reorganization energies, but can modify the molecular packing in organic semiconductors. It is found that for the π–π stacking systems such as NDI derivatives and BSBS-C10/C14, the charge mobility decreases with alkyl chain length, which is due to the displaced stacking along the short axis of the molecular plane, while for the herringbone stacking systems such as BTBT derivatives and BSBS-C8/C12, the charge mobility increases with alkyl chain length, due to enhanced π⋯π interaction along the stacking direction and the displaced stacking along the short axis of the molecular plane, respectively.