Congruent and Oriented Crystallization of Mixed Sn–Pb Perovskite From the Nano‐ to Centimeter Scale

Abstract Metal halide perovskites have garnered widespread attention for research and applications, thanks to their high adaptability in elemental composition and optoelectronic properties. The mixed Sn–Pb perovskite FASn _0.5 Pb _0.5 I ₃ (FA ⁺ is formamidinium) features a narrow bandgap of 1.25 eV, appropriate for building tandem solar cells, but faces challenges in fabricating uniform, compact films having protected surfaces and scalable dimensions. Herein, a Lewis‐base molecule trimethylthiourea (3T) is applied as a ligand to the FASn _0.5 Pb _0.5 I ₃ system, and find that it favors binding to Sn ²⁺ over Pb ²⁺ . As a result, the tin and lead components crystallize congruently at the unit cell scale. The 3T ligand helps the FASn _0.5 Pb _0.5 I ₃ crystal grains develop into regular shapes and micron sizes, so as to fill in the film thickness and closely contact the substrate. Also, slow evaporation of 3T during annealing inhibits surface defects and renders centimeter‐wide film smoothness. Solar cells made of such FASn _0.5 Pb _0.5 I ₃ films has achieved a power conversion efficiency of 21.5% and a fill factor of 81%. Eliminating methylammonium and a hole transport layer (HTL) from these solar cells substantially boosts their short‐term and storage stabilities. These results will contribute to making streamlined, durable, and large‐area perovskite tandem solar cells.