Effect of Cu and Cl on EVT-CdTe solar cells

The stoichiometry of CdTe plays an important role in the formation of point defects and can therefore impact solar cell performance. In this study, the effect of Cu and Cl on CdTe films is investigated as a function of stoichiometry. The stoichiometry of CdTe films was altered by varying the gas phase composition (amount of Cd and Te vapors) during the Elemental Vapor Transport (EVT) deposition process. Superstrate CdTe/CdS solar cells were fabricated using EVT-CdTe. Controlled amounts of Cu were incorporated into the CdTe cells during the back contact formation process. Solar cell performance improved for cells fabricated with EVT-CdTe deposited under Te-rich conditions. The dual benefits of Cu viz. improving doping concentration and eliminating the Schottky barrier at the back contact were observed. Capacitance-voltage (CV) measurements revealed an increase in net doping with a decrease in the Cd/Te vapor ratio, which can be attributed to an increase in the native acceptor defects, cadmium vacancies (VCd's). Open circuit voltages up to 850 mV with FF up to 70 % were measured for EVT-CdTe solar cells with Cd/Te vapor ratio 0.7. 1-photon Time Resolved Photoluminescence (TRPL) measurements revealed higher lifetimes for solar cells fabricated with Cd/Te ratio 0.7. The incorporation of Cu reduced lifetimes.