Damping the phase segregation in mixed halide perovskites: Influence of X-site anion

It has been observed that presence of chlorine in perovskite nanocrystals (NCs), incorporates stability whereas bromine content enhances radiative lifetime that makes mixed halides competitive candidates for both higher stabilities and longer radiative lifetimes as compared to pristine cesium lead halides. In this study, we demonstrate an important, yet least explored phenomenon observed in mixed halides perovskites of photo-induced phase segregation, which unfortunately limits the practical utility of mixed halide perovskites. This study reveals that phase segregation becomes more prominent on decreasing bromine content in mixed halide CsPbBr_3-xCl_x perovskites, when bromine content is 50% or less, it shows no phase segregation. Triple halide perovskite ABX_3-y-zY_yZ_z NCs have been synthesized to mitigate the phase segregation. The grain size of the synthesized NCs exists in the range of 20–25 nm. The selected area electron diffraction exhibits the faint rings of the CsPbBr_1.5Cl_1.5 indicating the presence of defects in NCs and the X-rays diffraction analysis confirms the NCs exist in monoclinic crystal structure. Mixed halides perovskites have same crystal structure but exhibit a slight shift in 2θ values. The trend in stability follows the order as, CsPbBr_3-xCl_x > CsPbCl₃ > CsPbBr₃ that is estimated by the values of octahedral factor (μ) and tolerance factor (t). The UV–Vis and steady-state photoluminescence (PL) analysis depict a blue-shift in the spectra by substituting bromine with chlorine in CsPbBr₃. The PL kinetics analysis predicts the CsPbBr_2.5Cl_0.5 exhibits the longest average PL lifetime amongst all other CsPbBr_3-xCl_x perovskites NCs. Our study provides insights in designing stable mixed halides perovskites by suppressing the phase segregation.