The formation of molybdenum diselenide (MoSe.sub.2) is widely observed at the back-contact interface for copper zinc tin selenide (CZTSe) thin-film solar cells. Depending on individual selenium (Se) supply and thermal conditions for forming CZTSe absorbers on molybdenum (Mo) substrates, the thickness of MoSe.sub.2 can vary from a few hundreds of nanometers up to [almost equal to] 1 [mu]m, which is comparable to the commonly adopted thickness of 1 ~ 1.5 [mu]m for CZTSe absorbers. In this study, for controlling the thickness of interfacial MoSe.sub.2, thin diffusion barrier layers of silicon oxynitride (SiO.sub.x N.sub.y) are deposited onto Mo layers prior to the growth of CZTSe absorbers in the fabrication process. As a result, a reduction in the thicknesses of MoSe.sub.2 layers is achieved. In terms of energy conversion efficiency ([eta]), CZTSe solar cells grown on Mo/SiO.sub.x N.sub.y back contacts suffer a deterioration as the SiO.sub.x N.sub.y layers get thicker. CZTSe solar cells grown on Mo/SiO.sub.x N.sub.y /Mo back contacts preserve their efficiencies at [almost equal to] 11% with thin 10 nm SiO.sub.x N.sub.y layers.