Significant efforts have been made to improve the performance of the Cu(In1-xGax)Se-2 (CIGS) solar cells by tuning the band gap of the CIGS absorber to match it with the solar spectrum. However, the performance of the current record-holding CIGS solar cells is still far from theoretical expectations. Various researchers reported that the open circuit voltage (Voc) and the fill factor (FF) degrade in wide band gap CIGS solar cells. However, the limiting factors on further boosting the efficiency are still a matter of debate. In this study, we focus on tuning the properties of the interfacial layer between the rear contact and the wide-gap CIGS absorber to lower the contact resistance and recombination rate. Based on the numerical simulation using SCAPS (a solar cell capacitance simulator), we find that a MoO3 interfacial layer with high work function is more effective than its MoSe2 counterpart in reducing the back barrier, which in turn increases the V-oc and the FF of the solar cell. We further predict that an overall efficiency of 24% can be achieved by reducing the back surface recombination and Schottky barrier with sub-micrometer a thick CIGS absorber. This work puts forward a strategy to improve the efficiency of wide band gap CIGS solar cells whilst reducing the raw materials consumption.