Characterization of CuxNy AND CdO:Al thin films for solar cell applications
Mosoti, Obare B.
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A solar cell is a device that converts light energy into electricity by photovoltaic effect. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge careers for an electric current. Materials presently used for solar cell fabrication include silicon, gallium arsenide (GaAs) among others. However, these materials are expensive and not readily available. As a result, there is need to search for alternative materials which are readily available and give a high conversion coefficient. The purpose of this research was to prepare and study the optical and electrical properties of CuxNy and CdO:Al thin films so as to optimize them for solar cell applications. CuxNy and CdO:Al thin films were deposited by dc magnetron sputtering and reactive evaporation techniques respectively using Edwards Auto 306 vacuum coater system. CuxNy films were deposited at Nitrogen flow rates between 2 sccm and 10 sccm. CdO:Al films were deposited with Al doping concentrations of 0%, 1%, 3%, 5%, 7% and 9% atomic weight. The optical measurements were done using spectrophotometer Solid Spec 3700 DUV in the range (300-2000nm) while electrical characterization was done by four point probe method. Transmittance of CuxNy films was found to increase with increase in wavelength and was below 20% in the 300-2000 nm range. For CdO:Al thin films, integrated transmittance was above 78% in the 400-800 nm range for all samples prepared. Reflectance of CuxNy thin films was found to be below 30% in the 300-2000 nm range while that of CdO:Al thin films was below 15% in 400-800 nm range. From scout software, optical properties such as absorption coefficient, refractive index and extinction coefficient of the thin films were also obtained. For both CuxNy and CdO:Al thin films, absorption coefficient decreased with increase in wavelength of incident radiation. Optical band gap of CuxNy thin films increased with increase in Nitrogen flow rate to a maximum of 1.72 ± 0.02 eV at 10 sccm. For CdO:Al thin films, optical band gap increased with increase in Al doping to a maximum of 3.10 ± 0.06 eV at 5% Al doping, after which it started to decrease with increase in Al doping. Electrical resistivity of CuxNy ranged from 31.70 Ωcm to 87.46 Ωcm while that of CdO:Al thin films ranged from 5.526 x 10-2 Ωcm to 11.33 x 10-2 Ωcm. Optimized deposition parameters such as nitrogen flow rate and Al doping concentration were used to fabricate a thin film solar cell on transparent glass slide. I-V characteristics of the solar cell were measured using a Keithley 2400 source meter interfaced with a computer running Labview program. An open circuit voltage of 476 mV, short circuit current density of 0.48 mA/cm2, fill factor of 0.54 and conversion efficiency of 0.21 % were obtained. From these results, CuxNy and CdO:Al thin films serve as potential alternative materials to the widely used silicon solar cells.