Effect of thermal treatment and deposition conditions of ZnO by radio frequency sputtering on the photovoltaic response of CdTe solar cells

ZnO thin films grown by radio frequency sputtering were studied as a function of the experimental parameters: source power, chamber deposition pressure, substrate temperature and oxygen partial pressure; in addition to post-deposition thermal treatment. The ZnO thin films were characterized by stylus profilometry, optical measurements, electrical resistivity, X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. Current-voltage and external quantum efficiency measurements were performed on the CdS/CdTe-based solar cell. In the case of ZnO/CdS thin films with a high-temperature thermal treatment, an increment in transmittance was observed in the ZnO wavelength region. With the incorporation of the ZnO thin film in the front contact of the solar cell, we obtained a photovoltaic efficiency of 7.4%, with the following optimal deposition conditions: 230 W source power, 2.67 Pa of deposition pressure and 300°C for the substrate temperature with a thermal treated of 450 °C for ZnO/CdS thin films. While a photovoltaic efficiency of 6.0% was obtained, when the ZnO/CdS was thermal treated at 500°C with the ZnO deposited at 140 W source power, 1.33 Pa deposition pressure and 250 °C for the substrate temperature. Finally, a photovoltaic efficiency of 10% was achieved for CdTe solar cells with ZnO thin film by combining the experimental conditions and the thermal treatment described above.