Steric Confinement and Dynamic Coordination Synergy for Crystallization Control in 27.9%‐Efficient All‐Perovskite Tandem Solar Cell

Abstract The inhomogeneous Pb/Sn distribution in Sn–Pb narrow‐bandgap (NBG) perovskites (PVK), arising from divergent crystallization dynamics and Sn 2+ segregation, leads to deleterious bandgap fluctuations and interfacial recombination. Herein, a multifunctional cyclodextrin derivative (MCD) additive engineered with 6–8 thiol (‐SH) groups is reported that synchronously addresses these challenges through in situ crystallization modulation and ion‐homogenization, which concurrently suppress Sn 2+ oxidation through their reductive capacity and modulate crystallization kinetics via dual chelation of Pb 2+ /Sn 2+ ions through dynamic sulfur–metal coordination bonds. Theoretical and experimental analyses reveal that this dual‐function mechanism decelerates ion migration during nucleation, enabling the growth of high‐quality NBG PVK films with suppressed defect densities and homogenized Pb/Sn distribution. This homogenization stems from MCD's multidentate thiol coordination, which forms a dynamic chelation network with Pb 2+ /Sn 2+ , regulating ion release rates and suppressing localized supersaturation. The cyclic oligosaccharide backbone further imposes steric confinement, inhibiting ion clustering and disordered nucleation. Consequently, optimized NBG PVK solar cells achieve a 22.3% power conversion efficiency with suppressed non‐radiative losses, enabling 27.9%‐efficient all‐PVK tandem solar cell with extremely high long‐term stability for 1,320 h. This work establishes molecular crowding and dynamic coordination as dual levers to control crystallization thermodynamics in multinary perovskites.