The efficient exciton dissociation and subsequent charge separation occurring in optimized polymer:fullerene devices has been difficult to understand and simulate. Both exciton delocalization and hot charge separation are proposed to facilitate charge separation. To test these theories, both conceptual models were implemented into a dynamic Monte Carlo (DMC) simulation and tested using a simple bilayer device architecture. Using experimentally determined parameters, exciton delocalization accounted for a significant reduction in geminate recombination when compared to the traditional, bound polaron pair model. In addition, the hot charge separation process was able to further reduce the geminate recombination, but only when the hot charge mobility was several orders of magnitude larger than the standard charge mobility.
Given the apparent importance of exciton delocalization, the magnitude of exciton delocalization in regioregular P3HT was investigated further. By fitting exciton delocalization models to previously published experimental data, two separate estimates for the magnitude of exciton delocalization were extracted. A simple lattice model was fit to exciton-exciton annihilation behavior in P3HT films and a detailed DMC model was fit to exciton dissociation dynamics of P3HT:PCBM blend films, leading to exciton delocalization radius estimates of 1.6±0.25 nm and 1.9±0.6 nm, respectively, which were significantly smaller than previously published values.
With such a low magnitude of exciton delocalization, the first exciton dissociation model proposed predicted a large magnitude of geminate recombination. By including charge delocalization, it was hypothesized that formation of a bound polaron pair would be prohibited, resulting in a significant reduction in geminate recombination. This concept was implemented using a simple spherical bead model for both exciton and charge delocalization. To test this model in a more experimentally relevant device, a model bulk heterojunction device was used. In addition, test conditions were expanded to investigate a range of electric fields. A massive reduction in geminate recombination was observed simply by increasing the delocalization radius from 0.5 to 1.5 nm. As a result, it was concluded that both exciton and charge delocalization are dominant factors controlling the exciton dissociation dynamics and the geminate recombination behavior in P3HT:PCBM bulk heterojunction solar cells.