In this work, we report that NiO thin film can be used as a back contact buffer layer in CdTe thin film solar cells. The NiO layer is prepared by electron beam evaporation. To optimize the thickness of the NiO thin film, we fabricate some CdTe solar cells with different NiO thickness values. A NiO/Au back contact CdTe solar cell with an efficiency of 12.17% and an open-circuit voltage Voc of 789 mV is obtained, which are comparable to those of a standard Cu/Au back contact solar cell. The X-ray photoelectron spectroscopy (XPS) is used to quantitatively characterize the band alignment at the CdTe/NiO interface. It can be seen from the band alignment that the valence band offset (EVBO) is 0.52 eV and the conduction band offset (ECBO) is 2.68 eV. The EVBO presents no energy barrier for hole to transport from CdTe to NiO. The value of ECBO indicates that NiO can act as a back surface field layer (BSF) to dramatically reduce carrier recombination in the contact region of a CdTe cell, leading to an improved Voc. The band alignment obtained from XPS measurement shows that the band alignments of NiO and CdTe are perfectly matched. However, the conductivity of NiO film is poor. The insertion of a NiO buffer layer in the back contact increases the series resistance and reduces the fill factor (FF). We propose to use Cu/NiO composite structure as a bi-layer contact to improve the conductivity of the NiO buffer layer, which at the same time can be used to dope the CdTe film surface by Cu to obtain a low resistive contact. We fabricate a cell with a contact structure of 3-nm-Cu/20-nm-NiO/Au and the cell has a Voc of 796 mV, a Jsc (short-circuit currrent) of 24.2 mA/cm2, an FF of 70.2% and an efficiency of 13.5%. In order to study the stability of the solar cell with a Cu/NiO/Au back contact, a thermal stressing test is carried out at a temperature of 80 ℃ in the air atmosphere. For the Cu/NiO/Au back contact structure solar cell, the efficiency decreases from 13.1% to 12.9% after the cell is stressed for 80 h, showing that the stability of the Cu/NiO/Au back contact cell is significantly improved compared with that of the standard Cu/Au contact cell. In summary, the experimental results obtained in this study demonstrate that NiO thin film is a promising buffer layer for manufacturing stable and high efficiency CdTe thin film solar cells.