Studies of rf sputtered Sb2Te3 thin films for back contacts to CdS/CdTe solar cells

Sb₂Te₃ films were magnetron sputtered using a 99.999% pure Sb₂Te₃ target from Cerac to study their growth morphology and transport properties. Optical emission spectroscopy was used to monitor the plasma conditions during growth. The films were grown on aluminosilicate glass substrates at temperatures from ~30 °C to 270 °C and characterized by x-ray diffraction, four point probe, and Hall-effect studies. All the films had a metallic luster, were opaque in the visible, and increased in grain size as substrate temperature increased. The best films were obtained when the sputtering was done at 240 °C substrate temperature. The grain size reached 80 nm, hole carrier concentration 2 × 10²⁰ cm^-3, mobility 146 cm²/V-s and the resistivity 4.1 Ω-μm. Based on these results, two-micron-thick sputtered CdS/CdTe solar cells were completed with Sb₂Te₃ as part of the back contact and yielded efficiency >;12%, with open-circuit voltage ~780 mV, fill-factor ~72% and short-circuit current ~22 mA/cm² when 3 nm Cu and 20 nm Au was evaporated on top of the Sb₂Te₃ layer. Cells finished with Sb₂Te₃/Mo back contacts (without copper) yielded up to 10% efficiency. The lower efficiency when Cu is not used suggests that the Cu is still needed to modify the CdTe/Sb₂Te₃ interface when Sb₂Te₃ is used.