Solar cell is optimized to convert solar radiation to electrical current with conversion efficiency as high as possible. Due to their superior performance compared to conventional silicon devices we used the Si xGe 1-x strained layer for increasing the efficiency of solar cell device. By using simulation's tools, i.e. pcld version 5.6, we investigate and analysis the performance of Si 0.2G 0.8/Si solar cell, especially open circuit voltage, short circuit current, fill factor which will afTect the efficiency of the device. We also compare it with conventional silicon solar cell in order to examine their performances and the thickness of both device structures. The Si xGe 1-x strained layer we applied contents 80% germanium. Results show that by inserting Si 0.2Ge 0.8 strained layer in device structure open circuit voltage and short circuit current has been optimal and the thickness of the device compared to conventional silicon solar cell is 1/17 times.