Copper has been introduced in CdTe solar cell manufacturing for a long time and is critical for achieving high performance devices. In this work, CuCl thin films were prepared by vacuum thermal deposition and then used as a buffer layer with a thermal annealing treatment for activation in the CdTe solar cells. The results indicate that Cu and Cl diffused into the CdTe absorber which was beneficial for doping and copassivation. The carrier density for CdTe solar cells in the vicinity of the pn junction was significantly improved from 6.1 x 10(13) to 4.3 x 10(14) cm(-3), and V-oc was increased by > 70 mV. The cell with the CuCl thin films was more uniform and revealed higher photocurrent response than the reference cell. The results also indicate that the formation of ohmic back contacts to CdTe solar cells due to the presence of the CuxTe (Cu7Te4) thin films. An optimal device with open circuit voltage of similar to 820 mV, and fill factor of similar to 72% and efficiency reaching 16.69% was obtained by the control of the copper concentration and the thermal activation conditions of the CuCl buffer layer.