Abstract
Organic photovoltaics (OPVs) have recently exceeded 20% power conversion efficiency (PCE), reinforcing their potential for commercialisation. However, large-scale adoption also depends on environmental impact, toxicity, cost, stability, and scalability. Conventional wet-processing routes typically rely on hazardous organic solvents, whereas aqueous nanoparticle (NP) dispersions offer a greener alternative and have already enabled device efficiencies above 10%. Yet strategies to tailor conjugated materials specifically for NP-based processing remain unexplored. This work examines how the alkyl side-chain length influences the performance of nanoparticle bulk heterojunctions. We investigated three conjugated polymers, FO4-T, FO6-T, and FO8-T, which were derived from a benzo[c][1,2,5]thiadiazole-based backbone and bear 2-butyl-1-octoxy, 2-hexyl-1-decoxy and 2-octyl-1-dodecoxy side chains, respectively. Donor-acceptor blend NPs were prepared via the miniemulsion method using Y6 as the non-fullerene acceptor. We correlated optical properties, NP characteristics, film microstructures, thermal annealing behaviour, and device performances to the macromolecular structure. Our findings show that the side-chain length strongly influences nanoparticle-based thin film morphology, its behaviour upon thermal treatment and the resulting photovoltaic efficiency. Among the series, FO8-T : Y6 exhibited the most favourable microstructure with a thermal treatment lower than its FO4-T and FO6-T counterparts and delivered power conversion efficiencies up to 10.64%. This study establishes structure-property relationships for water-processed organic solar cells and highlights side-chain engineering as a key lever for advancing eco-friendly, high-performance active layers.