Solvent engineering enables tin-lead perovskite films with long carrier diffusion lengths and reduced tin segregation.

All-perovskite tandem solar cells offer great promise for achieving low levelized cost of electricity, but their performance remains limited by insufficient near-infrared photon absorption in narrow bandgap tin-lead (Sn-Pb) subcells. Micron-thick Sn-Pb layers are essential for maximizing absorption, yet high-concentration precursor solutions often cause non-uniform crystallization, stoichiometric imbalance and limited carrier diffusion lengths. Here we identify the root cause of these limitations as the insufficient coordination of tin(II) iodide (SnI) in conventional dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) binary solvent system at high precursor concentrations, resulting in Sn-rich colloids that nucleate detrimental Sn-rich phases in final films. To address this, we develop a ternary solvent system that fully coordinates with SnI, suppressing Sn-rich phases and enabling stoichiometric, micron-thick Sn-Pb films with carrier diffusion lengths of ~11 μm. The enhanced Sn-Pb absorber achieves efficiencies of 24.2% in single-junction cells and 29.3% in tandem devices, along with significantly improved long-term operational stability.