This study investigates the synchronized crystallization of tin-lead perovskite solar cells (Sn-Pb PSCs) by addressing the asynchronous crystallization behavior between Sn and Pb components. The rapid crystallization of Sn-based perovskites is attributed to the stereochemically active lone pair on Sn(II), which hinders coordination between the metal ion and Lewis base ligands in the perovskite precursor. To synchronize crystallization kinetics and homogenize Sn-Pb alloying, a noncovalent binding agent, p-phenylenediamine (PPD), is introduced. PPD selectively binds to Sn(II) over Pb(II) in the precursor solution, circumventing the lone pair repulsion and enabling high-quality Sn-Pb perovskite films with homogenized composition and enhanced optoelectronic properties. The resulting single-junction Sn-Pb PSCs achieve a certified power conversion efficiency of 24.13%. The encapsulated device retains 90% of the initial efficiency after 795 h of maximum power point operation under simulated one-sun illumination. The study reveals that the sterically active Sn(II) lone pair repels Lewis base ligands and destabilizes the high-coordination solvate intermediates, leading to faster crystallization of Sn perovskites. The introduction of PPD, which selectively binds to Sn(II) over Pb(II), suppresses the formation of Sn-I-Pb bridge structures and enhances the crystallinity and stability of the perovskite films. The PPD binding also reduces surface Sn-enrichment and enhances the film's optical and electronic properties, leading to improved photovoltaic performance and operational stability. The study demonstrates that the noncovalent binding agent PPD effectively addresses the asynchronous crystallization issue in Sn-Pb perovskites, enabling the fabrication of high-performance solar cells with enhanced efficiency and stability. The results highlight the importance of understanding the underlying chemistry of Sn and Pb in perovskite precursors to achieve optimal device performance.This study investigates the synchronized crystallization of tin-lead perovskite solar cells (Sn-Pb PSCs) by addressing the asynchronous crystallization behavior between Sn and Pb components. The rapid crystallization of Sn-based perovskites is attributed to the stereochemically active lone pair on Sn(II), which hinders coordination between the metal ion and Lewis base ligands in the perovskite precursor. To synchronize crystallization kinetics and homogenize Sn-Pb alloying, a noncovalent binding agent, p-phenylenediamine (PPD), is introduced. PPD selectively binds to Sn(II) over Pb(II) in the precursor solution, circumventing the lone pair repulsion and enabling high-quality Sn-Pb perovskite films with homogenized composition and enhanced optoelectronic properties. The resulting single-junction Sn-Pb PSCs achieve a certified power conversion efficiency of 24.13%. The encapsulated device retains 90% of the initial efficiency after 795 h of maximum power point operation under simulated one-sun illumination. The study reveals that the sterically active Sn(II) lone pair repels Lewis base ligands and destabilizes the high-coordination solvate intermediates, leading to faster crystallization of Sn perovskites. The introduction of PPD, which selectively binds to Sn(II) over Pb(II), suppresses the formation of Sn-I-Pb bridge structures and enhances the crystallinity and stability of the perovskite films. The PPD binding also reduces surface Sn-enrichment and enhances the film's optical and electronic properties, leading to improved photovoltaic performance and operational stability. The study demonstrates that the noncovalent binding agent PPD effectively addresses the asynchronous crystallization issue in Sn-Pb perovskites, enabling the fabrication of high-performance solar cells with enhanced efficiency and stability. The results highlight the importance of understanding the underlying chemistry of Sn and Pb in perovskite precursors to achieve optimal device performance.