Analysis of the charge generation and recombination processes in the PM6:Y6 organic solar cell†

Abstract

Closing the efficiency gap between organic solar cells and their inorganic and perovskite counterparts requires a detailed understanding of the exciton dissociation and charge separation processes, energy loss mechanisms, and influence of disorder effects. In addition, the roles played by excitations delocalized over two or more (macro)molecules and by localized triplet states remain to be well-defined. To address these issues, we have combined molecular dynamics simulations with density functional theory calculations to provide a comprehensive analysis of charge generation and charge recombination in the representative PM6:Y6 blend, describe loss mechanisms, and assess the influence of disorder on the electronic processes. The results allowed the identification of Y6 excimer-like states that can efficiently dissociate into states with hole–electron separation distances larger than those in conventional donor:acceptor interfacial charge-transfer states. They also point to the appearance of low-energy defect states upon formation of Y6 twisted conformations, which can negatively impact the Y6 chemical stability and device performance. Importantly, it is found that the local triplet states formed via non-geminate recombination can efficiently transfer back to triplet CT states, opening the way to eventual dissociation into free charges. Overall, our work provides valuable insight into the charge dynamics within PM6:Y6 active layers.