This work focuses on determining theoreticaly quite accurately the optimal parameters of the CIGS cell with structure ZnO:Al/CdS/CIGS/Mo/Glass such as: thickness of CdS and the thickness of antireflection layer. The calculations of optical losses are carried out due to reflections from all interfaces and absorption in the transparent conducting oxide (TCO) and n-type layers. The effect of thickness of antireflection coating above TCO surface has been taken into account. It is shown that, the amount of the optical losses is about 23%. The 100 nm thickness of the antireflection layer is sufficient to reduce these losses up to 16 %. At certain parameters of the used materials and with taking into account the reflectivity from back contact, the efficiency of CIGS has been improved and records a value of 21.27%.