Lead-Free Solid State Mechanochemical Synthesis of Cs₂NaBi_1-xFe_xCl₆ Double Perovskite: Reduces Band Gap and Enhances Optical Properties

Efficient and stable lead-free halide double perovskites (DPs) have attracted great attention for the future generation of electronic devices. Herein, we have developed a doping approach to incorporate Fe³⁺ ions into the Cs₂NaBiCl₆ crystal unit and reveal a crystallographic and optoelectronic study of the Cs₂NaBi_1-xFe_xCl₆ double perovskite. We report a simple solid-state mechanochemical method that has a solvent-free, one-step, green chemistry approach for the synthesis of Cs₂NaBi_1-xFe_xCl₆ phosphor. The analysis of powder X-ray diffraction (XRD) data determines the contraction of the lattice due to the incorporation of Fe³⁺ cations, and this effect is well supported by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and solid-state nuclear magnetic resonance spectroscopy (ss-NMR). The band gap is reduced with increasing Fe content owing to the strong overlap of the Fe-3d orbitals with Cl-3p orbitals and shift of the valence band maxima (VBM) toward higher energies, as confirmed by ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) analyses. Photoluminescence (PL) studies of Cs₂NaBi_1-xFe_xCl₆ phosphors exhibit a large Stokes shift, broadband emission, and increased PL intensity more than ten times for 15% of Fe content phosphor with enhancement in the average decay lifetimes (up to 38 ns) compared to pristine Cs₂NaBiCl₆ DP. These results indicate that the transition of dark self-trapping of excitons (STEs) into bright STEs enhances yellow emission. XRD, UV, and thermo-gravimetric analysis (TGA) confirmed that the Cs₂NaB_1-xFe_xCl₆ DPs have good structural and thermal stabilities. Our findings indicate that the doping of Fe³⁺ cations into the Cs₂NaBiCl₆ lattice is a constructive strategy to enhance significantly the optoelectronic properties of these phosphors.