In this work, a numerical analysis of a CIGS ($CuIn_{1-x}Ga_xSe_2$) solar cell with a rectangular grating on the electrode is presented. The effects of the grating on the energy conversion efficiency are calculated using the RCWA (rigorous coupled wave analysis) method. In conventional CIGS solar cells, the thickness of the light absorption layer (CIGS) is $2;{mu}m$, at which the incident light is almost absorbed. By adopting a grating on the electrode and using a less than $1;{mu}m$ CIGS layer, we obtained a higher efficiency compared to the conventional solar cells.