Highly stable all-perovskite tandem solar cells with targeted conversion of tin–lead surfaces

All-perovskite tandem solar cells (APTSCs) have rapidly improved in both power conversion efficiency (PCE) and room-temperature stability. However, achieving device stability under combined light–heat stresses (ISOS-L-3 conditions) remains challenging. The critical limitation stems from the highly reactive tin–lead surface which, even with molecular passivation strategies, remains susceptible to severe photothermal degradation. Here we develop a targeted conversion strategy to transform the metastable surface into a solid protection layer. Our method relies on treatment with alkaline caesium hydroxide, which releases OH⁻ to mediate the dual transformation of SnI₄ and the defective surface into solid metal oxides, as well as replacing volatile organic cations with Cs⁺. This strategy leads to improved stability under ISOS-L-3 testing conditions and overall optoelectronic performance. The resulting tin–lead cells achieve a champion PCE of 23.65%, enabling the corresponding APTSCs to reach a PCE of 29.52% (certified, 28.56%). The APTSCs retain 90.3% of their initial PCE after 500 h under ISOS-L-3 conditions, outperforming traditional amine-treated counterparts. Our findings demonstrate a promising pathway towards photothermally stable and efficient APTSCs.