The effect of asymmetric external reorganization energy on electron and hole transport in organic semiconductors

Abstract

Understanding the relationship between charge mobilities and the molecular stacking structures of π-conjugated organic semiconducting materials is essential for their development. In this study, a quantum mechanics (QM)-derived state-specific polarizable force field (SS-PFF) is applied to explicitly estimate the external reorganization energies (λ_ext) during electron transfer (ET) or hole transfer (HT) processes using our recently proposed two-point model (J. Phys. Chem. A, 2018, 122, 8957–8964). Different from the Marcus two-sphere model, the application of the explicit two-point model produces a notably asymmetric λ_ext for ET and HT processes in oligoacene crystals. For the same charge transfer channels, the λ_ext of ET is 7–10 times higher than that of HT, which results in a larger intrinsic hole mobility. This successfully rationalizes why acenes are prone to be p-type conducting materials. Perfluorination can change the polarity of the molecular surface electrostatic potential (ESP). Thus, perfluorination is a possible approach to reduce the external reorganization energies for ET reactions, which can be used to tune the order of λ_ext of ET and HT processes. The two-point model with the SS-PFF, therefore, opens the door to revisit the intrinsic mobilities of electrons and holes in organic semiconductors.