Suppressing Sn-Rich Phase Precipitation Enables Controlled Crystallization in MA-free Sn–Pb Perovskite Solar Cells

Methylammonium-free (MA-free) tin–lead perovskites are essential for durable all-perovskite tandem solar cells (APTSCs), yet the lower activation energy of Sn-based phases triggers a premature nucleation driving force, leading to detrimental Sn-rich phase precipitation and significant lattice strain. Herein, we introduce rubidium thiocyanate (RbSCN) as a multifunctional regulator to orchestrate the crystallization dynamics and strain evolution. SCN– selectively coordinates with Sn2+, retarding premature Sn-rich phase precipitation and synchronizing the growth rates of Sn/Pb-mixed phases. Simultaneously, Rb+ mitigates local lattice distortion, yielding structurally homogeneous films with minimized nonradiative recombination. Consequently, the optimized MA-free Sn–Pb device achieves a 22.92% power conversion efficiency (PCE) and exhibits enhanced operational stability. Integrating this into monolithic APTSCs realizes a PCE of 28.77% (TÜV-certified 28.53%). Notably, the VOC × FF product represents the highest value reported for MA-free APTSCs, highlighting the efficacy of synchronized crystallization in unleashing the potential of narrow-bandgap perovskites.