Lewis Base–Zwitterion Dual Additives Enable Efficient Blade‐Coated Tin‐lead Perovskite Solar Cells and Tandems

High‐efficiency all‐perovskite tandem solar cells are severely constrained by narrow‐bandgap Sn‐Pb subcells, which confront an inherent contradictory bottleneck: Sn 2+ ‐rich precursors undergo fast, unregulated crystallization during blade coating, yet Sn 2+ cations are easily oxidized to harmful Sn 4+ under ambient conditions. Conventional single additive materials can only address one of the two critical drawbacks instead of balancing both. Herein, we develop a dual‐additive strategy combining Lewis‐base amidinothiourea (FATU) and zwitterionic glycine hydrochloride (GlyHCl) with two core synergistic functions. FATU utilizes C═S and −NH 2 groups to preferentially chelate Sn 2+ , inhibiting its oxidation and pre‐nucleation aggregation; GlyHCl simultaneously coordinates Pb 2+ and Sn 2+ to slow global crystallization kinetics and suppress heterogeneous nucleation. This dual synergy fabricates high‐quality Cs 0.1 FA 0.5 MA 0.4 Pb 0.5 Sn 0.5 I 3 films with over 800 nm large grains and minimized trap density. The blade‐coated single‐junction device reaches 21.02% PCE with V OC boosted from 0.797 to 0.850 V. When integrated with a 1.76 eV wide‐bandgap top cell, the tandem achieves 27.84% peak efficiency (stabilized at 27.33%) and retains more than 75% initial efficiency after 680 h continuous MPP tracking. This work offers a practical pathway for scalable blade‐coating production of high‐performance all‐perovskite tandem photovoltaics.