Dynamic Effects on Hole Transport in Amorphous Organic Semiconductors: a Combined QM/MM and kMC Study.

Abstract

Small-molecule-based amorphous organic semiconductors (OSCs) are essential components of organic photovoltaics and organic light-emitting diodes. The charge carrier mobility of these materials is an integral and limiting factor in regard to their performance. Integrated computational models for the hole mobility, taking into account structural disorder in systems of several thousand molecules, have been the object of research in the past. Due to static and dynamic contributions to the total structural disorder, efficient strategies to sample the charge transfer parameters become necessary. In this paper, we investigate the impact of structural disorder in amorphous OSCs on the transfer parameters and charge mobilities in different materials. We present a sampling strategy for incorporating static and dynamic structural disorder which are based on QM/MM methods using semiempirical Hamiltonians and extensive MD sampling. We show how the disorder affects the distributions of HOMO energies and intermolecular couplings and validate the results using kinetic Monte Carlo simulations of the mobility. We find that dynamic disorder causes an order of magnitude difference in the calculated mobility between morphologies of the same material. Our method allows the sampling of disorder in HOMO energies and couplings, and the statistical analysis enables us to characterize the relevant time scales on which charge transfer occurs in these complex materials. The findings presented here shed light on the interplay of the fluctuating amorphous matrix with charge carrier transport and aid in the development of a better understanding of these complex processes.