BiOBr Surface-Functionalized Halide Double-Perovskite Films for Slow Ion Migration and Improved Stability.

Surface-tailored lead-free halide double-perovskite (Cs2AgBiX6) thin films are utilized for ion migration studies. A thin surface layer of BiOBr/Cl is grown via intentional annealing of the halide films in ambient conditions. Herein, we physically stacked the two films, viz., Cs2AgBiBr6 and Cs2AgBiCl6, to thermally activate the halide ion migration at different temperatures (room temperature (RT)-150 °C). While annealing, the films' color changes from orange to pale yellow and transparent brown to yellow as a result of the migration of Br- ions from Cs2AgBiBr6 to Cs2AgBiCl6 and Cl- ions from Cs2AgBiCl6 to Cs2AgBiBr6, respectively. Annealing helps in homogenizing the halide ions throughout the films, consequently leading to a mixed phase, i.e., Cs2AgBiClxBr6-x/Cs2AgBiBrxCl6-x (x = 0 to 6) formation. The movement of ions is understood by absorption studies performed at regular time intervals. These investigations reveal a redshift (from 366 to 386 nm) and a blueshift (from 435 to 386 nm) in absorption spectra, indicating the migration of Br- and Cl- toward Cs2AgBiCl6 and Cs2AgBiBr6, respectively. The films characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveal the presence of a peak at 2θ = 10.90° and binding energy of 158.1 eV, respectively, corresponding to the formation of Bi-O bonds at the film surface. Also, XRD studies show a lower 2θ shift of the diffraction peak in the case of Cs2AgBiCl6 films and a higher 2θ shift in the case of Cs2AgBiB6 films, which further confirms the migration of Cl- and Br- from one film to the other. XPS investigations confirm the compositional change with a gradual increment in the concentration of Br-/Cl- with an increase in heating time for Cs2AgBiCl6/Cs2AgBiBr6 films. All these studies confirm thermal diffusion of halide ions in double-perovskite films. Further, from the exponential decay of the absorption spectra, the rate constant for halide (Br) ion diffusion is calculated, which shows an increment from 1.7 × 10-6 s-1 at RT to 12.1 × 10-3 s-1 at 150 °C. The temperature-dependent rate constant follows Arrhenius behavior and renders an activation energy of 0.42 eV (0.35 eV) for bromide (chloride) ion mobility. A larger estimated value as compared to the reported values for Cs2AgBiBr6 wafers (∼0.20 eV) reveals a slow mobility of halide ions in thin films of Cs2AgBiBr6/Cl6. The formation of a BiOBr passivation layer at the surface of Cs2AgBiBr6 thin film might be one of the plausible causes of the slow anion diffusion in the present work. Slow ion migration is an indication that the films are stable and of high-quality.