Enhancing the photophysical properties of absorption and luminescence in lightly doped halide double perovskite nanomaterials (Cs2Ag1-aBi1-bPbxBr6): Theory and experiment

Lead-free halide double perovskites, such as Cs2AgBiX6, have emerged as less toxic alternatives to the lead-based solar cell absorbers, APbX3 (where A is a monovalent cation and X is halide). Despite their potential application in photovoltaics and x-ray detectors, halide double perovskites exhibit limited optical properties due to their relatively high band gap and the indirect nature of the band gap. We propose doping by introducing small amounts of Pb2+ into the Cs2AgBiBr6 perovskite structure at the Ag1+ and Bi3+ sites through a controlled solution synthesis. We explored the crystal structure, surface morphology, optical properties, and electronic structure of as-synthesized Pb-doped double perovskite crystals. Using X-ray diffraction and X-ray photoelectron spectroscopy analysis, the formation of Pb-doped Cs2AgBiBr6 double perovskite was confirmed. The crystal structure parameters and band gap values of doped Cs2AgBiBr6 double perovskites, as predicted by density functional theory (DFT) calculations, were validated through XRD and UV/Visible spectroscopy analysis. The strategic Pbdoping assisted in altering the band topology, which resulted in the decrease of disparity between indirect and direct band gaps in Pb-doped perovskite Cs2AgBiBr6. Pb-doping also induced a notable reduction in the energy bandgap of the perovskite compound, making it spectroscopically more relevant, thereby enhancing its applicability in photovoltaics.