---

In this research, we deal with the detailed simulation of CIGS solar cells using band-gap grading. Our main goal is to improve the electrical and optical properties of these cells. In order to check the performance, we also evaluate the change in the thickness of the absorbent layer as a key factor. In particular, we analyze the effect of these changes on the efficiency and filling factor of the cells. In conventional CIGS solar cells, the efficiency is about 20.8% that we have achieved so far. But by taking advantage of the bandgap grading on the absorber layer and by using engineering techniques in the absorber and buffer layers, we have achieved the most impressive efficiency ever of 33.7%. This increase is the result of successful achievements in optimizing the structure and properties of cells. In summary, the values of important output parameters for these advanced cells include 33.7% efficiency, 79% filling factor, 1mV open circuit voltage and 40.96mA/cm2 short circuit current.