Abstract <p>A comparative theoretical investigation of A₃PI₃ (A = Ca, Ba, Mg) has been performed using full potential linear augmented plane wave method (FP-LAPW) performed by WIEN2K code based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The volume optimization is performed using the Birch–Murnaghan equation of states. The compound belongs to the space group 221. The structural, electronic and optical properties including the band structure and density of states are obtained. The band structure analysis revealed direct bandgaps of 1.465 eV (Ca₃PI₃), 0.842 eV (Ba₃PI₃), and 0.566 eV (Mg₃PI₃). Optical calculations showed high static dielectric constants, notable at 1 eV for Ba₃PI₃, and significant absorption in the visible region, with Ca₃PI₃ exhibiting the most suitable optical profile. Mg₃PI₃ displayed the highest reflectivity (~0.72), indicating potential for photonic applications. The novelty of this work lies in the first comparative study of these A₃PI₃ systems, identifying Ca₃PI₃ as the most promising candidate for lead-free solar cell applications due to its optimal bandgap and balanced optical response. Fortunately, A₃PI₃ compounds are used for photovoltaic purposes because of their optical and thermoelectric properties. It also contributes to the low-cost, nontoxic and earth—abundant materials.</p>