The effect of a graphene oxide (GO) intermediate layer on the performance of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells was studied. The formation of dielectric MoSe₂, which can be induced by a reaction between CZTSSe and Mo, was successfully suppressed. The GO layer prevented the decomposition of CZTSSe along with the formation of binary chalcogenides. Furthermore, the interference effect generated between the incident light and the light reflected by Mo was effectively alleviated, thereby reducing photon loss to effectively absorb light. The incorporation of the GO layer resulted in a stronger electric field in the absorber, as determined by the transfer matrix method, and accordingly, a higher absorption spectrum was observed in absorption measurements. Consequently, the series resistance was reduced because MoSe₂ formation was inhibited, and a significant improvement in the J_sc value was confirmed via the mitigation of the interference effect and an improved series resistance. The CZTSSe solar cell exhibited a notable power conversion efficiency of 11.2%, which represented an improvement over a conventional reference cell that was fabricated with a GO layer.