Lattice‐Matched 2D Template Enables Efficient Cesium Tin Halide Perovskite Solar Cells

ABSTRACT Cesium tin iodide (CsSnI 3 ) has emerged as a promising inorganic light‐absorber for lead‐free perovskite solar cells (PSCs) due to its ideal bandgap of 1.3 eV and intrinsic thermal stability. However, its application is limited by random crystallization and spontaneous δ‐CsSnI 3 phase transition, causing substantial photovoltaic performance losses. Here, we employ lattice‐matched 2D perovskite templates to induce the oriented growth of 3D perovskites, enabling phase‐pure 2D/3D perovskite heterostructures for efficient and durable CsSnI 3 ‐based PSCs. Compared to their alkyl counterparts, 2D perovskites adopting aromatic spacer cations introduce additional interlayer π – π stacking to inhibit octahedral tilting and minimize their lattice spacing mismatch with 3D CsSnI 3 . This improved crystallographic compatibility promotes oriented growth of 2D/3D heterostructures along the (110) plane, regulating the crystallization kinetics and creating an additional energy barrier that suppresses δ‐CsSnI 3 phase formation. As a result, the optimized CsSnI 3 ‐based PSCs deliver a champion power conversion efficiency (PCE) of 15.27% with a high open‐circuit voltage of 0.90 V. Benefiting from reduced trap states and eliminated δ‐CsSnI 3 phase impurities, the target devices exhibit markedly improved operational stability, retaining over 95% of their initial PCE after 1280 h at maximum power point tracking under continuous one‐sun illumination in nitrogen without encapsulation.