First-Principles Investigation into Structural and Optoelectronic Properties of Mixed Halide Perovskites CsPbBr3– y X y (X = I, Cl, F; y = 0, 1, 2, 3) Using GGA, mBJ, mBJ + SOC, and HSE Functionals

Abstract This study presents a comprehensive density functional theory investigation into the structural, electronic, and optical properties of inorganic halide perovskites with the general formula CsPbBr3–yXy (where X = I, Cl, F; and y = 0, 1, 2, 3). Using the Vienna Ab initio Simulation Package, electronic band structures were calculated employing a hierarchy of functionals: the generalized gradient approximation, Tran-Blaha-modified Becke-Johnson (TB-mBJ), TB-mBJ combined with spin–orbit coupling, and the hybrid HSE06 functional. Structural analysis confirms that substituting bromine with smaller halides such as fluorine (F) and chlorine (Cl) contracts the lattice, whereas iodine (I) substitution leads to lattice expansion, a trend consistent with the relative ionic radii. The results reveal systematic trends in bandgap tuning; incorporation of F and Cl widens the band gap, whereas I substitution narrows it. These changes are attributed to shifts in orbital interactions and varying electronegativities. Optical calculations demonstrate that F and Cl substitution decreases the low-energy electronic polarizability, yielding lower static real dielectric constants, indicating reduced electronic polarizability favorable for wide bandgap and UV-reflective coating components, whereas increasing I concentration raises the refractive index and stretches optical absorption deep into the visible range, matching solar illumination requirements. Spectroscopic limited maximum efficiency analysis of ten distinct compositions identifies CsPbI3 as the premier candidate, yielding a maximum theoretical spectroscopic efficiency of approximately 31% under radiative limits. These findings underscore halide substitution as an effective strategy for engineering the optoelectronic behavior of CsPbBr3-based perovskites for next-generation energy applications.