Long Minority-Carrier Diffusion Length and Low Surface-Recombination Velocity in Inorganic Lead-Free CsSnI₃ Perovskite Crystal for Solar Cells

Sn-based perovskites are promising Pb-free photovoltaic materials with an ideal 1.3 eV bandgap. However, to date, Sn-based thin film perovskite solar cells have yielded relatively low power conversion efficiencies (PCEs). This is traced to their poor photophysical properties (i.e., short diffusion lengths (<30 nm) and two orders of magnitude higher defect densities) than Pb-based systems. Herein, it is revealed that melt-synthesized cesium tin iodide (CsSnI₃) ingots containing high-quality large single crystal (SC) grains transcend these fundamental limitations. Through detailed optical spectroscopy, their inherently superior properties are uncovered, with bulk carrier lifetimes reaching 6.6 ns, doping concentrations of around 4.5 × 10¹⁷ cm⁻³, and minority-carrier diffusion lengths approaching 1 µm, as compared to their polycrystalline counterparts having ≈54 ps, ≈9.2 × 10¹⁸ cm⁻³, and ≈16 nm, respectively. CsSnI₃ SCs also exhibit very low surface recombination velocity of ≈2 × 10³ cm s⁻¹, similar to Pb-based perovskites. Importantly, these key parameters are comparable to high-performance p-type photovoltaic materials (e.g., InP crystals). The findings predict a PCE of ≈23% for optimized CsSnI₃ SCs solar cells, highlighting their great potential.