Dynamic Monte Carlo modeling of exciton dissociation in organic donor-acceptor solar cells

A general dynamic Monte Carlo model for exciton dissociation at a donor-acceptor interface that includes exciton delocalization and hot charge separation is developed to model the experimental behavior observed for the poly(3-hexylthiophene):fullerene system and predict the theoretical performance of future materials systems. The presence of delocalized excitons and the direct formation of separated charge pairs has been recently measured by transient photo-induced absorption experiments and has been proposed to facilitate charge separation. The excess energy of the exciton dissociation process has also been observed to have a strong correlation with the charge separation yield for a series of thiophene based polymer:fullerene systems, suggesting that a hot charge separation process is also occurring. Hot charge separation has been previously theorized as a cause for highly efficient charge separation. However, a detailed model for this process has not been implemented and tested. Here, both conceptual models are implemented into a dynamic Monte Carlo simulation and tested using a simple bilayer donor-acceptor system. We find that exciton delocalization can account for a significant reduction in geminate recombination when compared to the traditional, bound polaron pair model. In addition, the hot charge separation process could further reduce the geminate recombination, but only if the hot charge mobility is several orders of magnitude larger than the standard charge mobility.