Balancing driving force, charge transport, and non-radiative recombination in organic solar cells with non-fused ring acceptors

Abstract

The design of low-cost, stable, and high-efficiency non-fullerene acceptors requires a deeper understanding of the impact of the molecular structure on photovoltaic performance. In this study, we investigate the influence of gradual side-chain modifications of non-fused ring acceptors. The transition from non- (L0) to partially (L2) and fully chlorinated (L4) side chains enhances the molecular selfassembly, condenses the intermolecular packing, and balances the electron and hole mobility. Additionally, we observe lower bimolecular recombination coefficients and field-independent exciton dissociation upon gradual chlorination of the side chains, which improves the fill factor of the devices. However, the accompanying higher non-radiative voltage loss restricts the performance of the fully chlorinated L4 systems. Thus, the blend PM6:L2 balances efficient exciton dissociation with reduced non-radiative recombination, yielding the highest efficiency. This study emphasizes the pivotal role of side-chain halogenation in fine-tuning molecular packing and charge dynamics, offering guidelines for the next generation high efficiency photovoltaic materials.