In this work, we studied the impact of using Zn1-xMg xO to replace the intrinsic ZnO in CIGS solar cells. The effect of Mg content and layer thickness of the Zn1-xMgxO on device formation was investigated. We found that the amount of Mg in the Zn 1-xMgxO layer and the layer thickness significantly alter the cell properties. Device characterization indicated that the impact of the Mg content is not limited to the front-window layer, but also extends to underneath the CIGS layer. The numerical simulation nicely explains the relationship between various front-window layers and device performance. Our observation indicates that properties of the front-window layer may be important to junction formation in CIGS devices. Any changes to this part of the device could seriously alter device performance.