The band alignment of CdTe/MoOx hetero-interface contact at back electronic electrode in CdTe thin film solar cell was quantitatively characterized by photon electron emission spectroscopy. The experimental results showed that depending on the CdTe surface chemical state and the stoichiometric value of x in MoOx, energy barrier with a value as low as 0.23-0.39 eV was developed at the CdTe/MoOx contact interface for a MoOx buffer layer with an x value of 2.86-3. CdTe solar cell with relatively high efficiency can be fabricated by using a MoOx as a back contact buffer, which is allowed to have a relatively large stoichiometric x range of 2.86-3. The Te-rich CdTe surface obtained by chemical etching induced a reduction reaction upon the deposition of a MoOx thin layer, leading to the formation of reduced Mo states and other defect states below the Fermi level, which assisted hole carrier transport.