| Summary: | Abstract Precise control over halide perovskite crystallization is pivotal for realizing efficient solar cells. Here, we introduce a strategy utilizing in-situ-formed oxide-based ABX3-structured seeds to regulate perovskite crystallization and growth. Introducing potassium stannate into perovskite precursors triggers a spontaneous reaction with lead iodide, producing potassium iodide and lead stannate. Potassium iodide effectively passivates defects, while PbSnO3 (ABX3-structured), exhibiting a 98% lattice match, acts as a template and seed. This approach facilitates pre-nucleation cluster formation, preferential grain orientation, and the elimination of intermediate-phase processes in perovskite films. Incorporating potassium stannate into both the perovskite precursors and the buried hole transport layers enables single-junction 1.25 eV-bandgap Sn-Pb perovskite solar cells to achieve a steady-state efficiency of 23.12% and enhanced stability. Furthermore, all-perovskite tandem devices yield efficiencies of 28.12% (two-terminal) and 28.81% (four-terminal). This versatile templating method also boosts the performance of 1.77 eV and 1.54 eV-bandgap cells, underscoring its broad applicability.
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