CdTe has become a leading contributor in the thin-film photovoltaic market. A suitable back contact is still one of the most crucial issues to realize efficient CdTe thin film solar cells. Herein, we intensively studied the mechanisms of interfacial modification and device performance enhancement for the representative ZnTe:Cu back contact structure. It's found that, in spite the as-deposited ZnTe:Cu buffer could reduce the contact barrier, the device performance is still limited by the increased defect-related recombination. A controlled heat treatment process is proved to be effective in alleviating the recombination loss at the back contact. Detailed characterizations demonstrate that the CdTe/ZnTe:Cu interface reaction happens and the interfacial composition is modified during the heat treatment process, which optimize the interfacial chemical states and band alignment. The improved interfacial properties decrease the defect-related recombination and promote the holes transport, and consequently improve the device efficiency greatly.