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Designing of optical and solar devices based on cesium-based lead halide perovskites have revolutionized the research on direct bandgap semiconducting materials. These perovskites exhibit unique and interesting semiconducting behavior with excellent photophysical properties which suggests their promising utility for solar and optical devices. Nowadays the tailoring of the photophysical, structural and electronic behavior has been continuously done using fully inorganic cesium based mixed halide perovskites using low-cost precursors. Herein to get the physical insight behind the existence of unique and fascinating behavior of CsPb(Cl/Br)3, density functional theory (DFT) have been exploited with experimental validation. Structural behavior has been examined by optimizing the crystal structure for the CsPb(Cl/Br)3 nanoparticles (NPs). Geometry optimization of the CsPb(Cl/Br)3 crystal gives the optimized lattice parameter as 5.812 Å. The cubic nature has been observed in the CsPb(Cl/Br)3 crystal after geometry relaxation. The stability analysis of the CsPb(Cl/Br)3, NPs has been done by calculating the total energy and the formation energy of the system. The negative value of the formation energy suggests the stability of the CsPb(Cl/Br)3 NPs. To investigate the charge neutrality in the system Bader charge analysis has been performed on the optimized crystal geometry of the CsPb(Cl/Br)3. Further to understand the electronic behavior of the CsPb(Cl/Br)3 NPs, band structure has been computed and results show the existence of the occupied state and unoccupied state at the same symmetry point of the Brillouin zone. This clearly indicates the direct bandgap nature of the CsPb(Cl/Br)3 NPs. The calculated value of the bandgap for CsPb(Cl/Br)3 NPs from the electronic band structure is found as 2.672 eV. To support the calculated theoretical results based on DFT we have also synthesized the monodisperse colloidal CsPb(Cl/Br)3 NPs by hot injection method. The structural properties of the as-synthesized CsPb(Cl/Br)3 NPs have been evaluated using X-ray diffraction (XRD) technique. The observed sharp peaks with high intensity in the XRD patterns show the cubic crystalline nature of CsPb(Cl/Br)3 NPs. The evaluated lattice parameters from the XRD pattern is found as 5.781 Å. This calculated value shows very good consistency with the theoretically calculated lattice parameters for the optimized crystal structure of CsPb(Cl/Br)3 NPs. Further, the electronic and photoluminescent nature of the synthesized sample has been studied by Photoluminescence (PL) spectroscopy. The evaluated value of fullwidth half maxima (FWHM) from the PL plot is 16 nm which suggests the presence of homogeneity with orderliness in the cubic lattice of the synthesized NPs. PL plot with a very high-intensity peak around 466 nm shows a good luminescent property. The observed PL peak around 466 nm gives a bandgap of 2.660 eV which is consistent with the DFT calculated results. The convincing combination of electronic and structural robustness make these mixed halide NPs appealing for solar cells and light-emitting diodes (LEDs).
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