Tin-lead halide perovskites, with a bandgap near 1.2 electron-volts, hold significant promise for thin-film photovoltaics. However, the film quality of solution-processed Sn-Pb perovskites is compromised by the asynchronous crystallization behavior between Sn and Pb components, where Sn crystallizes faster than Pb. This study reveals that the rapid crystallization of Sn is due to its stereochemically active lone pair, which impairs coordination between the metal ion and Lewis base ligands in the perovskite precursor. To address this issue, a noncovalent binding agent targeting the open metal site of coordinatively unsaturated Sn(II) solvates is introduced, synchronizing crystallization kinetics and homogenizing Sn-Pb alloying. The resulting single-junction Sn-Pb perovskite solar cells achieve a certified power conversion efficiency of 24.13%, with improved operational stability, retaining 90% of the initial efficiency after 795 hours of maximum power point operation under simulated one-sun illumination. The study provides insights into the underlying chemistry and demonstrates a practical approach to enhancing the performance of Sn-Pb perovskite solar cells.Tin-lead halide perovskites, with a bandgap near 1.2 electron-volts, hold significant promise for thin-film photovoltaics. However, the film quality of solution-processed Sn-Pb perovskites is compromised by the asynchronous crystallization behavior between Sn and Pb components, where Sn crystallizes faster than Pb. This study reveals that the rapid crystallization of Sn is due to its stereochemically active lone pair, which impairs coordination between the metal ion and Lewis base ligands in the perovskite precursor. To address this issue, a noncovalent binding agent targeting the open metal site of coordinatively unsaturated Sn(II) solvates is introduced, synchronizing crystallization kinetics and homogenizing Sn-Pb alloying. The resulting single-junction Sn-Pb perovskite solar cells achieve a certified power conversion efficiency of 24.13%, with improved operational stability, retaining 90% of the initial efficiency after 795 hours of maximum power point operation under simulated one-sun illumination. The study provides insights into the underlying chemistry and demonstrates a practical approach to enhancing the performance of Sn-Pb perovskite solar cells.