Characterization of CuᵪOᵧ - ZnO:Sn P-N Junction for solar cell applications
Omayio, Enock Osoro
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The Cu.O, - ZnO:Sn p-n junction solar cell has been fabricated in steps. Cu.O, and ZnO:Sn thin films were first deposited by reactive de magnetron sputtering and reactive evaporation techniques respectively using Edwards Auto 306 vacuum coater system .Cu.O, films were deposited at oxygen flow rates between 5-30 seem, ZnO:Sn films were deposited with Sn doping concentrations reaching 6%. Transmittance of ZnO:Sn films was above 70% for Sn doping less than 5% whereas reflectance was below 40% within the visible range. For Cu.O, films, transmittance increased with increase in wavelength and reflectance was below 35 % within the visible range. From simulation of optical spectra of films using scout software absorption coefficient, refractive index and optical band gap of thin films were obtained. For both Cu.O, and ZnO:Sn films, absorption coefficient decreases with increase in wavelength of incident radiation. Optical band gap of CuxOy films increased with increase in oxygen flow rate to a maximum of 2.64 eV at 10 seem of O2 flow after which it started to decrease. For ZnO:Sn films, optical band gap decreased with increase in Sn doping to a minimum of 2.48 eV at 4% Sn doping after which it started to increase with increase in Sn doping. Electrical resistivity of Cu.O, films increased slightly with increase in 02 flow whereas for ZnO:Sn film's resistivity decreased with increase in Sn doping concentration with minimum resistivity of 21.1 ncm. Solar cell was fabricated on transparent glass slide from 250nm thick p-type Cu.O, film deposited at oxygen flow of 5 seem and 140 nm thick n-type ZnO:Sn film deposited with 2% Sn doping concentration. I-V characteristics of the solar cell were measured by Keithley 2400 sourcemeter interfaced with computer running labview program. It had Voc of 480 mV and fill factor of 0.63 which is higher than those of other cells in literature. The fabricated solar cell had conversion efficiency of 0.232%.