Impact of Active Layer Morphology on Bimolecular Recombination Dynamics in Organic Solar Cells

Using kinetic Monte Carlo simulations, we present a reaction-diffusion model to describe the impact of the morphology of the active layer and charge-transfer lifetime on the bimolecular recombination kinetics in organic solar cells. The morphologies we consider range from bilayers to bulk heterojunctions with coarse and fine intercalated domains. We find that within the morphologies simulated by the potential model, it is the density of states that affects the order of bimolecular recombination kinetics. The results show that the morphology of the active layer, modeled by the potential model, only influences the average delay time between the exciton dissociation and the onset of bimolecular recombination. The results also indicate that the donor or acceptor domain size and the degree of Gaussian disorder have very similar effects on the charge recombination dynamics. Our findings suggest one possible way to explain (i) why bimolecular recombination deviates from second-order (Langevin) kinetics and (ii) why Langevin theory overestimates the bimolecular rate constant.