Organic photovoltaics: determining the impact of the 1-chloronaphthalene additive in the PM6:Y6 blends from multi-scale simulations

Abstract

The development of PM6:Y6 active layer has contributed to significant advances in the field of organic photovoltaics. During its fabrication, using solvent additive such as 1-chloronaphthalene (1-CN) at a specific concentration has often been found to play a critical, positive role. There is increasing consensus that 1-CN molecules can tune the global morphology of PM6:Y6 active layer and thus device efficiency by slowing down the solvent evaporation process. However, it remains poorly understood how these 1-CN molecules impact the nano-scale molecular packing and electronic properties of PM6:Y6:1-CN blends (aspects that are challenging to characterize experimentally) as a function of 1-CN concentration in solution. Changes in “global” morphology correspond to modifications in size, connectivity, and crystallinity of PM6 and Y6 domains while changes in “local” morphology correspond to modifications in intermolecular interactions among PM6, Y6, and 1-CN moieties. The latter in turn impact the electronic properties related to hole/electron transfer rates between PM6/Y6 and 1-CN, electron/hole transfer rates between adjacent Y6/PM6 molecules/chains, energetic distributions of interfacial charge-transfer (CT) electronic states, and non-radiative recombination rates and corresponding voltage losses from the CT states to the ground state. By thoroughly investigating these aspects in PM6:Y6 blends with different 1-CN concentrations, via combining density functional theory calculations, all-atom molecular dynamics (MD) simulations, and coarse-grained MD simulations, we are able to rationalize how 1-CN improves the photovoltaic parameters and thus efficiencies of PM6:Y6-based solar cells, and to point out the requirements in terms of electronic properties of solvent additives for further improvement in device efficiency.