Stability, photovoltaic, and optoelectronic properties of eco-friendly $$CsGeCl_{3-n}F_n$$ C s G e C l 3 - n F n halide perovskite for solar cells applications: insights from DFT calculations

This study explores the characteristics of fluorine-substituted lead-free halide perovskites $$CsGeCl_{3-n}F_n$$ C s G e C l 3 - n F n for clean energy applications, focusing on their structural, stability, and optoelectronic characteristics. Density functional theory (DFT) calculations, using Quantum Espresso code (QE), were performed to assess the material performance of the $$CsGeCl_{3-n}F_n$$ C s G e C l 3 - n F n series ( $$n=0,1,2,3$$ n = 0 , 1 , 2 , 3 ). The results reveal that fluorine substitution significantly enhances both thermodynamically and mechanical stability, which are confirmed by calculated formation energy and elastic constants. The Pugh’s and Poisson ratios are estimated to confirm reduced brittleness and increased ductility in $$CsGeCl_3$$ C s G e C l 3 upon the increase of fluorine content. The electronic analysis reveals a direct semiconducting nature with a tunable band gap, increasing from 1.03 eV to 2.06 eV upon fluorine substitution. Optical analysis indicates anisotropic behavior in mixed halides perovskites, $$CsGeCl_2F$$ C s G e C l 2 F and $$CsGeClF_2$$ C s G e C l F 2 , exhibiting strong absorption coefficients on the order of $$10^5$$ 10 5 cm $$^{-1}$$ - 1 , low reflectivity, and weak electron–hole interactions. These findings underscore the potential of $$CsGeCl_{3-n}F_n$$ C s G e C l 3 - n F n perovskites for improved performance in solar cells and other optoelectronic devices. The results offer valuable insights into optimizing these materials for advanced optoelectronic applications and sustainable energy technologies. Graphic Abstract