Determining the Ideal Lead-Free Perovskite CsSnI ₃ Absorber Phase for Photovoltaic Solar Cells: A Deep Density Functional Theory Study

The structural, electronic, optical, and transport properties of the four crystallographic phases of the lead-free perovskite CsSnI 3, namely α (cubic), β (tetragonal), γ (black orthorhombic), and δ (yellow orthorhombic), are systematically investigated using first-principles calculations based on DFT for photovoltaic solar cell applications. Their structural parameters are well reproduced using the GGA-PBE functional, while their electronic and optical properties are accurately obtained using the screened Coulomb hybrid HSE06 functional. The α-, β-, and γ-phases exhibit direct narrow band gaps (1.0–1.2 eV), whereas the δ-phase displays a wide indirect gap (2.7 eV) and negligible visible-light absorption. DOS analysis reveals that the VBM is dominated by I-5 p states, while the CBM originates primarily from Sn-5 s /Sn-5 p orbitals. Among the four investigated polymorphs, the γ-phase exhibits the most favorable optoelectronic characteristics, combining near-optimal band gap (1.2 eV), strong visible-light absorption (>10 5 cm –1 ), high dielectric constant (14.2), efficient charge carries transport with averaged electron and hole effective masses of m e * = 0.38 m 0 and m h * = 0.14 m 0, respectively, together with a low exciton binding energy of 6.7 meV. Remarkably, direction-resolved analysis further reveals pronounced anisotropy, with the electron effective mass reduced to m e * = 0.06 m 0 and the exciton binding energy to 2.5 meV along the [001] direction. These findings identify γ-CsSnI 3 as a highly promising lead-free absorber and highlight the critical role of crystallographic orientation in enhancing charge transport and exciton dissociation, providing fundamental design guidelines for optimizing lead-free perovskite absorbers in next-generation photovoltaic devices.