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

Cesium tin iodide (CsSnI₃) 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-CsSnI₃ 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 CsSnI₃-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 CsSnI₃. 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-CsSnI₃ phase formation. As a result, the optimized CsSnI₃-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-CsSnI₃ 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.