ENHANCING THE CRYSTALLIZATION OF LEAD-FREE CS3BI2I9 MATERIAL USING CONTROLLED THERMAL ANNEALING AND ITS ROLE IN PHOTOVOLTAIC PERFORMANCE: A COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION

Lead-free Cs3Bi2I9 thin films were synthesized and annealed at 80 to 110 °C to investigate structural and optical
properties for photovoltaic applications. X-ray diffraction confirms phase-pure hexagonal Cs3Bi2I9 (P63/mmc), with
crystallite size increases as annealing temperature rises, indicating improved crystallinity and reduced defects. The
density-functional-theory calculations reveal a semiconducting bandgap of 2.07 eV. The material shows a high
refractive 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 for
perovskite solar cells.