<p>Recently, two-dimensional (2D) lead-free halide perovskites Cs₃Bi₂ × ₉ have attracted great attention due to their unique quantum confinement and (easy) tunable compositions. In the present work, all-inorganic, 2D lead-free halide perovskite materials Cs₃Bi₂ × ₉ and their mixed halides are investigated using density functional theory (DFT) for solar cell and other optoelectronic applications. The 2D lattice has a trigonal P3̅m1 symmetry, where the 2D layers corner-sharing octahedral [BiBr₆]³⁻ are separated by 1D Cs layers. The calculated structural parameters, such as bond lengths, bond angles, and lattice constants, are consistent with the experimental results. These materials have direct band gaps and the calculated values lie mostly in the visible range (2.20–3.24&#xa0;eV) of electromagnetic spectrum. The values of dielectric constant and refractive index decrease with increasing band gaps from Cs₃Bi₂I₉ to Cs₃Bi₂Cl₉. Cs₃Bi₂I₉ has the highest power conversion efficiency (PCE) of 20%, quantum efficiency (QE) of ~ 85%, and current density (J) of 14&#xa0;mA/cm², because of its smaller band gap. The incorporation of light anions (I to Br) makes the band gap wider and lowers light absorption, current density, voltage, and overall efficiency. The mixed halide Cs₃Bi₂I₆Br₃ gives an excellent balance among all other doped compounds in these properties, but has reduced efficiency than pure iodine-based compound due to a wider band gap and lower light absorption. The computed ground state characteristics of Cs₃Bi₂(X_1−uY_u)₉ (X/Y = I, Br, Cl; u = 0, 1/3, 2/3/ 1) demonstrate the materials’ significant promise for highly efficient optoelectronic applications, such as solar cells, light-emitting diodes, laser diodes, and photo-detectors.</p>