Experimental investigation of structure, morphology, and optical properties of lead-free halide double perovskites K₂XBiBr₆ (X=Na, Ag, and Cu) for photovoltaic applications

In the present study, K₂XBiBr₆ (X = Na, Ag, and Cu) are synthesized by the antisolvent recrystallization technique. The potential of studied compounds K₂XBiBr₆ (X = Na, Ag, and Cu) as viable substitutes for lead-based perovskites, in light of the environmental issues related to lead toxicity, along with material instability. X-ray diffraction (XRD) confirmed the synthesis and high structural stability of the materials, and this finding is further proved by the calculated Goldschmidt tolerance factors. The scanning electron microscopy (SEM) surface morphology showed multi-directional crystals and clear grains with clear boundaries with average grain sizes of 0.30 μm, 0.35 μm, and 0.25 μm of K₂NaBiBr₆, K₂AgBiBr₆ and K₂CuBiBr₆ respectively. Energy-dispersive X-ray spectroscopy (EDX) confirmed the elemental composition of K, X (Na/Ag/Cu), Bi, and Br, while FTIR analysis identified characteristic vibrational bands in the 450-4000 cm⁻¹ range. The materials have good optical absorption throughout the visible and UV–visible ranges, indicating the appropriateness of the materials in light-harvesting applications. In general, the present work will contribute to the understanding of environmentally friendly double perovskites as potential applications of photoelectrochemical and solar-energy-conversion technologies.