Lead-free Cs3Bi2I9 thin films were synthesized and annealed at 80 to 110 °C to investigate structural and opticalproperties for photovoltaic applications. X-ray diffraction confirms phase-pure hexagonal Cs3Bi2I9 (P63/mmc), withcrystallite size increases as annealing temperature rises, indicating improved crystallinity and reduced defects. Thedensity-functional-theory calculations reveal a semiconducting bandgap of 2.07 eV. The material shows a highrefractive index (3.03), low reflectivity (<0.4), and large absorption coefficient, enabling efficient light harvesting.Enhanced dielectric response and optical conductivity further support strong photo-induced carrier generation.Improved morphology and favorable optoelectronic properties establish Cs3Bi2I9 as a stable, non-toxic absorber forperovskite solar cells.