In this work we have carried out a theoretical study of structural, electronic, elastic and optical properties for the three ternary compounds KGaS$_2$, KGaSe$_2$ and KGaTe$_2$, using the pseudo-potential plane-wave scheme in the framework of the density functional theory. The obtained structural parameters are very close to the available experimental data. The analysis of band structure indicates semiconducting behaviour of these compounds with a direct bandgap. The mechanical properties of the considered KGaQ$_2$ are investigated by predicting the single-crystal elastic constants and the macroscopic elastic parameters, such as the bulk modulus and shear modulus. Our results indicate that all the compounds are mechanically stable and both KGaSe$_2$ and KGaTe$_2$ exhibit a significant elastic anisotropy compared to KGaS$_2$ compound. A set of frequency dependent optical parameters were calculated for incident radiation polarized in the [100], [010] and [001] crystallographic directions, including the complex dielectric function, the refractive index and the absorption coefficient. The studied materials show anisotropic in the optical properties, especially in the real and imaginary part of the dielectric function and in the refraction index. In addition, the obtained results show that our compounds absorb in the UV region.

Recently, photovoltaic solar cells have been revolutionized by adopting ABX$_3$ halide perovskite materials as photoabsorbers. In the recent past, lead halide perovskites have attracted significant research interest. However, owing to its toxicity, alternative lead-free materials are currently being actively sought. In this work, we report the computational results of the structural, electronic, optical and elastic properties of the unexplored lead-free halide fluoroperovskite material ASiF$_3$ (A = Li, Na, K and Rb) by means of first-principles calculations. The computed energetic and elastic properties assert that the formation of the ASiF$_3$ cubic systems is energetically favourable and mechanically stable. According to the results, LiSiF$_3$ and NaSiF$_3$ are indirect bandgap semiconductors, which are not appropriate for solar cell applications. The calculated elastic, electronic and optical properties indicate that KSiF$_3$ and RbSiF$_3$ alloys are isotropic and exhibit high ductility. They have reasonable direct bandgaps, a small effective mass and good light absorption, making them suitable for single-junction photovoltaic cells and other optoelectronic applications in the visible and UV regions of the electromagnetic spectrum.