Monovalent Cation Doping in Cesium Bismuth Bromide Perovskites: Insights into Structure–Property Correlation

All-inorganic halide perovskites have emerged as promising candidates for optoelectronic applications with tunable properties. Doping with monovalent alkali metals in perovskites has been proven to be effective in modulating the physical and optical properties. Herein, a lead-free layered cesium bismuth bromide, Cs3Bi2Br9, perovskite is doped with potassium, wherein the lower dopant level transpires as a substitutional and the higher level as an interstitial dopant, and the structural, morphological, optical, and electronic modifications are systemically investigated. The structural characterization through XRD analysis specifically revealed anisotropic lattice distortion on doping with a monovalent cation, leading to a conspicuous shift in the (003) plane. The morphology was tuned from hexagonal to rectangular sheets at the optimal dopant concentration. Incorporation of the potassium dopant also reduced the bandgap and increased the photoluminescence quantum yield by 40%. The conductivity of the doped samples showed a considerable increase by 37%. DFT analysis cohesively provided the impact of potassium doping on the electronic states of Cs3Bi2Br9, effectively raising the Fermi level and increasing the charge carrier concentration, clearly elucidating the experimental observations. The study provides an integrated view on the impact of monovalent cation doping in lead-free bismuth halide perovskites, underscoring piloted property tuning with potential applications in optoelectronic devices.