Wide-bandgap CuGaSe₂ thin film solar cell fabrication using ITO back contacts

In the tandem Cu(In_1-xGa_x)Se₂ (CIGS) thin film solar cell fabrication, Indium tin oxide thin film (ITO) is a promising material as back contacts of the top cell. ITO thin films were deposited by radio frequency (rf) magnetron sputtering in pure argon atmosphere at a working pressure of 9 × 10^-4 Torr with substrate temperature (T_sub) of 300 °C. The sheet resistance of as-deposited 200 nm-thick ITO thin films was about 1.3 × 10^-4 Ω cm. The ITO thin films were subsequently annealed by rapid thermal annealing (RTA) at a temperature range between 400 and 550 °C for application of the high-temperature CuGaSe₂ (CGS) deposition process. After the annealing processes of ITO thin films, we have examined the optical and electrical properties as transparent conducting oxide (TCO) back contacts. CGS thin films were prepared for the top cell in a tandem solar cell structure with wide bandgap of above 1.6 eV as high open circuit voltage photovoltaic devices. The optical and electrical properties of CGS thin film solar cells with ITO back contacts were investigated as compared with that of metallic Mo back contacts. Also, the CGS thin film solar cells were fabricated using ITO and MO back contacts with a conversion efficiency of 5 and 8.2%, respectively.