Enhancing Sn-Pb perovskite homogeneity via thioether coordination for efficient and stable all-perovskite tandem...

All-perovskite tandem solar cells (TSCs) hold a substantial promise for achieving ultrahigh-efficiency photovoltaics beyond the Shockley-Queisser limit. However, their development has been hampered by challenges associated with a narrow-bandgap tin-lead (Sn-Pb) perovskite subcell. A key issue is inhomogeneous Sn/Pb distribution during crystallization, which generates trap states and accelerates degradation. Here, we introduce a molecular stabilization strategy by using S-allyl-l-cysteine (SALC) as a ligand that preferentially coordinates with tin(II) iodide (SnI₂), thereby modulating crystallization kinetics. The strong thioether coordination leads to spatially uniform Sn/Pb distribution, and 3.5-fold reduction in Sn(IV) content due to the reduction capability of functional groups in SALC. Consequently, the resulting Sn-Pb perovskite solar cells achieve a champion power conversion efficiency (PCE) of 22.99% with an exceptional open-circuit voltage of 0.892 V. When integrated into all-perovskite TSCs, a certified PCE of 28.84% (29.44% laboratory-measured) is achieved along with a great improvement in operational stability compared to control devices, retaining nearly 90% of initial PCE after 420 hours of the maximum power point tracking under 1 sun illumination in ambient air.