Band-gap-engineered mixed-halide perovskite films through vacuum-based deposition approach

Mixed-halide perovskites offer the important characteristic of band-gap tunability, which is essential for tandem solar cells. Herein, the single-source vapor deposition method is reported for MAPbBr₂Cl₁ (MA (methyl ammonium) = CH₃NH₃), MAPbBr_1.5Cl_1.5 and MAPbBr₁Cl₂ perovskite films utilizing the corresponding mixed-halide perovskite nanoparticles as a source for thermal evaporation. Further, band-gap tuning in vacuum-deposited MAPbBr₃ thin films is performed through MACl treatment with successful band-gap tuning from 2.37 to 2.48 eV. The limitation of chlorine (Cl) incorporation into the perovskite structure at room temperature is tackled by MACl treatment of vapor-deposited MAPbBr₂Cl₁, MAPbBr_1.5Cl_1.5 and MAPbBr₁Cl₂ films. The structural properties of MAPbBr_3−xCl_x films show a continuous shift in the X-ray diffraction peak toward a higher 2θ value, confirming chlorine incorporation into the perovskite structure. Band-gap tuning in MAPbBr_3−xCl_x films, from 2.37 to 2.86 eV, is confirmed by absorption spectroscopy. The mixed-halide perovskite films show an improved microstructure with MACl treatment, as observed from the increased grain size of the films. Further, emission studies show good stability of MAPbBr_3−xCl_x films against phase segregation under continuous exposure to 325 nm wavelength illumination of 100 mW/cm² for more than 60 min. The diminished phase segregation might help in advancing the application of mixed-halide perovskites for reliable optoelectronic devices.