Abstarct: Abstract The third generation of solar cells is baxsed on organic-inorganic hybrid perovskite materials. Research has shown that finding stable non-toxic lead-free perovskites is very important for the further development of perovskite-baxsed optoelectronic technology. In this thesis, the structural, electronic and optical properties of the optimal structures of Sn-Ge and Bi-Cu baxsed perovskites have been investigated using the approaches of basic principles baxsed on quantum mechanics (density functional theory (DFT), GGA-BLYP approximation and SIESTA computational code). The electronic, optical and structural properties of the 3D double halide perovskites 〖Cs〗_2 CuBi〖Cl〗_6 and 〖Cs〗_2 CuBi〖Br〗_6 show that the 3D double halide perovskites have an indirect band gap. The bandgap increased by decreasing the ionic radius of halogen in 〖Cs〗_2 CuBi〖Cl〗_6 structure, which is a suitable candidate for optical and photovoltaic applications. In this research, lead-free three-dimensional halide perovskites MA 〖Sn〗_(0.5) 〖Ge〗_(0.5) I_3 and Cs 〖Sn〗_(0.5) 〖Ge〗_(0.5) I_3 have been investigated baxsed on basic principles calculations. As a result, MA cation has shown better results in electronic and optical properties, and Cs cation is not a suitable option for solar cell due to plasma fluctuations and showing mextallic properties in the visible region in terms of optical properties. Our results showed that 3D perovskites have a direct band gap. The band gap increases with increasing the ionic radius of A cation in MA 〖Sn〗_(0.5) 〖Ge〗_(0.5) I_3 structure. In addition, we investigated the electronic, optical and structural properties of two-dimensional organic-inorganic perovskites 〖(PEA)〗_2 GeI_4, 〖(PEA)〗_2 SnI_4 and 〖(PEA)〗_2 〖Sn〗_(0.5) 〖Ge〗_(0.5) I_4. The excellent results obtained in the electronic and optical properties of 2D structures are due to the quantum confinement effect, which has led to high stability and increased band gap of 2D perovskites compared to 3D. which is a suitable candidate for optical and photovoltaic applications. According to the obtained results, double hybrid perovskites are considered as semiconductors with tunable band gap characteristics. In addition, high dielectric constant, high absorption, high optical conductivity, and low reflectance indicate that these materials have the potential to be used in a wide range of optoelectronic applications, including solar cells.