Ab initio method was used to calculate the structural phase transitions and physical properties of semiconductors. At the beginning, the complex compounds which have been widely used in solar cell industry such as CuInxGa₁₋xSe₂ were studied. For high pressure phases and electronic properties of CuInSe₂, it was found that the CIS transform in the following way; I42d → Fm3m → Cmcm at 12 GPa and 42 GPa respectively. The sX-LDA band gap in the structure increases at the rate of 39.6 meV/GPa, in fair agreement with 30 meV/GPa obtained from the photoabsorption experiment. The band gap is closed in the Fm3m and Cmcm structures. Therefore, the metallic phase has been assumed for both structures. The path of transformation from to Cmcm was derived. The barrier between these two phases was estimated. The upper bound of the potential barrier is 17 meV which is equivalent to thermal energy at 198 K. For studying the effects of Na on high pressure phases of CuIn₀.₅Ga₀.₅Se₂, it was found that the high pressure phase transitions of CuIn₀.₅Ga₀.₅Se₂ appear to be remarkable similar to that of the CuInSe₂ phase transitions. The Na concentrations were chosen at 0.1, 1.0 and 6.25 %. The positive mixing energy of Na[subscript InGa] is higher than that of Na[subscript Cu]. Thus, Na substitutes on Cu sites are more energetically favorable than on Ga or In sites which is in good agreement with the previous reports. The Na substitutes reduce the hardness of CIGS and they can be easily added into the CuIn₀.₅Ga₀.₅Se₂ under high pressure comparing to the addition under ambient pressure. The most significant effects occur at Na concentration of 6.25%. It was found that the EDOS near the VBM is increased noticeably in the chalcopyrite phase. This should lead to the increase of the hole density.