Composition-Dependent Structural & Optical Properties of Pb-Based Mixed Halides (Br/I)

Metal halide perovskites have emerged as promising candidates for next-generation photovoltaics due to their low-cost fabrication, tunable bandgaps, and strong light-harvesting ability. However, their deployment is limited by instability and halide segregation under illumination. A rational strategy to address this is composition engineering, where partial substitution of bromide (Br⁻) for iodide (I⁻) tunes the bandgap and stabilizes the lattice. Here, we investigate the composition-dependent structural and optical properties of Pb-based mixed halides (Br/I). Thin films with varying Br/I ratios were prepared by spin coating precursor solutions in DMF: DMSO (4:1). Samples were characterized using UV-Vis, XRD, and AFM. UV-Vis revealed systematic bandgap tuning with halide substitution. XRD showed lattice expansion with increasing I content, while AFM indicated reduced surface roughness for mixed compositions of 75% PbBr₂ and 25% PbI₂. This study demonstrates how controlled Br/I substitution tunes the bandgap, stabilizes the lattice, and improves surface morphology—providing critical guidance for developing efficient and stable perovskite solar cell technologies.