Modeling of charge transport in polymers with embedded crystallites

Modern organic semiconductors frequently adopt a semicrystalline morphology which makes the analysis of charge transport challenging. In this paper, we employ Monte Carlo (MC) simulation to study the charge-carrier mobility in an amorphous layer---typically 100 nm thick---that contains well-ordered nanosized domains (crystallites). The case of a low carrier concentration, typical for applications in photovoltaics and LEDs, is considered. We study the dependence of mobility on temperature, on the amount $(V)$ of the crystallites, on the energy offset between crystalline and amorphous regions $({E}_{t})$, and we compare the results with those for a system with solitary traps. We find that, in a system with solitary traps, the mobility can exceed that of the neat phase if the trap depth is comparable with the standard deviation of the density of states (DOS) of the amorphous phase. Controlled by the electronic overlap in the amorphous phase and the crystallites, the mobility in a system with crystallites can increase by up to two orders of magnitude upon increasing V. It features a pronounced maximum when ${E}_{t}$ is close to the standard deviation of the DOS of the amorphous phase, while for large values of ${E}_{t}$, the mobility is practically independent of ${E}_{t}$. The results can be rationalized in terms of an interplay between percolation at large ${E}_{t}$ and hopping transport within the cumulative densities near the transport of the system. We developed a generalized analytic multiple-trapping-release model to rationalize the results of the MC simulations.