Modeling of charge transport in polycrystalline sexithiophene from quantum charge transfer rate theory beyond the first-order perturbation

Charge mobility in polycrystalline organic semiconductors is often thermally activated, so a semiclassical Marcus charge transfer rate theory has long been used to investigate the charge transport properties of organic semiconductors. However, the classical treatment for the nuclear degrees of freedom and the first-order perturbative nature of electronic coupling in the semiclassical Marcus charge transfer rate theory is often invalid in organic semiconductors. Furthermore, traps in polycrystalline organic semiconductors are not considered during the simulations with the semiclassical Marcus charge transfer rate theory. In the present work, we propose a model to study charge transport properties in polycrystalline organic semiconductors which consist of trap-free crystallitic grains separated by boundaries between them. The charge transfer rate in grains is evaluated with a quantum charge transfer rate theory without weak electronic coupling approximation while the charge transport at grain boundaries is limited by energy barriers there. We find that a thermally activated mobility can be obtained from the quantum charge transfer rate theory when traps at grain boundaries are considered. Meanwhile, a roughly linear dependence of mobility on grain size is shown for large grain size while a rapid variation of mobility with grain size is observed when the grain size is small, which reconciles the discrepancy of the mobility versus grain size in experiments. In addition, the different mobilities for sexithiophene crystal structures in high- and low-temperature phases show that the mobility in polycrystalline organic semiconductors not only depends on the boundary property between grains but also the molecular packing in grains.