In this study, Cul was employed as a back contact buffer layer which not only assisted hole transport, but also acted as an electron reflector at back contact. Quantitative analysis of the band alignment at the CdTe/CuI interface was carried out by using X-ray photoelectron spectroscopy (XPS). The results showed that the valence band offset and conduction band offset between CuI and CdTe were 0.08 and 1.63 eV, respectively. By optimizing the thickness of the CuI layer, a CdS/CdTe solar cell with a maximum efficiency of 14.5% has been fabricated. Capacitance-voltage (C-V) and impedance spectroscopy measurements were carried out and the results demonstrated that the CuI buffer layer between CdTe and metal electrode eliminated contact barrier and reduced carrier recombination at the back contact. The current-voltage (J-V) characteristics measured under different spectrum illumination conditions provided convincing evidences that Cul buffer layer acted as an electron reflector and increased the open-circuit voltage (V-oc) of the CdTe solar cells. Long time device stressing test results demonstrated that CdTe solar cells with a CuI buffer layer have much better device stability compared to the cells with a Cu/Au bi-layer metal contact.