Organic solar cells have gained considerable interest over the last few decades as a low-cost, environmentally friendly, lightweight, and flexible alternative to traditional photovoltaics. Developing low-cost, high-performance non-fullerene acceptor materials is crucial for the future development of organic solar cells. In this work, we design and synthesise new acceptor molecules based on the structure of the well-known non-fullerene molecules called Y6 and BTP-eC9. The use of Y6 acceptors and similar molecules has involved a step change in the development of organic solar cells. Significantly higher solar cell efficiencies have been achieved with these acceptors compared to the previously used fullerene derivatives. However, the synthetic complexity and cost of these acceptors limit the future commercialisation of organic solar cells.
By modifying and removing some of the side chains, we substantially reduced the synthetic complexity and cost of the Y6-type acceptor molecules. Single-crystal X-ray analysis reveals an unusual and compact tetrameric packing, possibly originating from the removal of some of the side chains in the structure. Importantly, this packing unlocks multiple intermolecular interactions between the molecular cores, resulting in molecular conformations that create efficient layer-by-layer three-dimensional charge transport pathways. As a result, remarkably high short-circuit current densities of up to 28 mA cm-2 are obtained, among the highest reported for Y-series acceptors when combined with the polymer called PM6. The devices exhibit power conversion efficiencies up to 18.0%, showing comparable device performance to PM6:Y6 and PM6:BTP-eC9 cells, despite the reduced synthetic complexity and estimated acceptor costs. Our result demonstrates that the new acceptor molecules are among the most cost-effective, high-performing, fused-ring acceptors reported to date.