• Replacing NMP with 2-pyrrolidinone (PDI) suppresses crystallization of mixed-halide intermediates, enabling high-quality wide-bandgap perovskite films.
• The strategy mitigates phase segregation and reduces defect densities, leading to improved open-circuit voltage and fill factor in perovskite/CIGS tandem solar cells.
• The approach is compatible with scalable blade-coating in ambient air, addressing a key bottleneck for commercial deployment.
• The work demonstrates a significant efficiency improvement in flexible all-thin-film tandem solar cells, advancing the potential for lightweight photovoltaics.