Suppressing Nongeminate Charge Recombination Triggers 19.5% Efficiency Bilayer Organic Solar Cells

Bilayer organic solar cells, composed of a donor and acceptor layer, provide independent optimization of each layer to enhance the photovoltaic performance. However, the power conversion efficiency remains lower than that of bulk heterojunction cells. Herein, we focus on suppressing nongeminate charge recombination by tuning the acceptor layer's morphology with fullerene derivatives to improve the performance of bilayer organic solar cells. We use the PM6/Y6 derivatives as model systems and incorporate fullerenes such as PC71BM into the acceptor layer to enhance aggregation, improve crystallinity, increase electron mobility, and reduce trap density. Consequently, bilayer devices based on PM6/BTP-eC9 and PM6/L8-BO achieved efficiencies of 18.0% and 19.5%, respectively, approaching the performance of bulk heterojunction cells. The improved fill factor results from reduced bimolecular recombination and suppressed trap-assisted recombination, offering insights into optimizing the active layer morphology and designing high-mobility acceptor materials for efficient bilayer organic solar cells.