Assessing alkyl side chain effects on electron transport properties of Y6-derived non-fullerene acceptors

Abstract

Recent advances in non-fullerene acceptors (NFAs) have significantly increased the efficiency of organic photovoltaics, reaching approximately 20% in single junction solar cells. These advancements are attributed to the introduction of the promising L8-R series, derived from the high-performance NFA known as Y6. To gain a deeper insight about such efficiency improvement, here we computationally characterise Y6 and its derivatives, focusing on the supramolecular structure of the resulting aggregates and their electron transfer properties. The applied computational protocol indicates an evident relationship between different side chains and the electronic features of the NFA supramolecular architectures, and provides a possible rationale for their impact on photovoltaic device efficiency. Using density functional theory and experimental crystal structures, we show that shorter and branched alkyl side chains on L8-R derivatives improve electron transfer integrals and charge mobilities compared to Y6. This improvement is attributed to differences in crystal packing. The effect of thermal fluctuations on the performance of the NFA aggregate is instead investigated through molecular dynamics simulations with quantum-mechanically derived force fields. Results indicate that, despite a minimal impact on electron transport capabilities due to dynamic disorder, the various substitution patterns significantly influence the supramolecular arrangement of the aromatic cores. The validation of the presented computational protocol, which integrates in a multi-level procedure accurate charge transfer rates computed on reliable morphologies obtained through classical molecular dynamics with refined force fields, paves the way towards a bottom-up modelling of donor/acceptor interfaces with polymeric donors, where the accurate description of structural and electronic disorder is key to reach a computationally-driven identification of higher performing components for organic photovoltaics.