Abstract: Four critical issues related to high efficiency CdTe thin film solar cell are discussed and studied: the quality of CdS films, back contact, CdS/CdTe interface formation, and CdCl2 heat treatment. Through control of the copper doping in the back contact we have eliminated the roll-over phenomenon in the I-V curve. The back contact barrier was reduced by formation of a Cu1.4Te compound which has a good contact with CdTe. Systematic investigations showed that relatively strong interdiffusion at the CdS/CdTe interface began to occur at around 350°C. This temperature coincides with the CdS phase transformation from cubic to hexagonal. Both S- and Te-rich CdSxTe1-x alloy formed at the interface, with x as much as 11%. Through optimization of the cell fabrication process, a CdTe solar cell efficiency as high as 14.6% has been obtained.