Optical and electrical analysis of Cd1-Xfexs/CuS p-n junction for solar cell application
Kang’ethe, Michael Ngahu
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The rapid population growth in Kenya and the world in general has led to very high energy demands; this has necessitated urgent exploration of other alternative energy sources. Solar energy is one such alternative that has not been fully utilised hence many researchers have developed much interest in them. Solar cells absorb light from the sun and convert it in to useful electrical energy however the challenge has been how to harness it as well as the very low efficiencies recorded by the fabricated cells.Thin films of metal chalcogenides have also gained much popularity in the production of solar cells due to their flexibility for optimization towards higher efficiency hence this study investigated the optical and electrical properties of Cd1-xFexS/CuS p-n junction for solar cell application. Chemical bath deposition (CBD) technique was used to prepare Cadmium iron sulfide (Cd1-xFexS) and copper sulfide (CuS) thin films and to fabricate the solar cell. The thin films were prepared using thiourea and sulfates of cadmium, iron and copper. Deposition of Cd1-xFexS was done at 750C while that of CuS was done at 80oC both at normal atmospheric pressure utilizing aqueous conditions. This study concentrated on optimizing the optical properties of the films. Optical properties were determined using UV- VIS –NIR 3700 DUV spectrophotometer for the range 300nm to 1100nm and the electrical properties were investigated using four point probe connected to a Keithley 2400 source meter. The optical band gap of the as-deposited Cd1-xFexS films was found to vary from 2.5eV to 3.41 eV with transmittance of above 80% in the VIS - NIR region for the concentration range of x = 0.0 to 0.5. This means that the band gap decreased with increasing Fe2+ concentration in the alloy and Cd0.7Fe0.3S sample showed the widest band gap. It was observed that the presence of Fe2+ decreased the optical band gap. The average extinction coefficients for the as deposited Cd1-xFexS samples were very low (k = 0.00784 to 0.0899) revealing that they absorb very little radiation hence a good window layer material. Electrical resistivity decreased with increase in Fe2+ in the bath in Cd1-xFexS and resistivities of order 103 Ω-cm were obtained. These properties are appropriate for window layers used for photovoltaic cell applications. CuS thin films showed a band gap of 2.31 to 2.33eV, a transmittance of below 55% and 50% in the VIS and NIR regions respectively, an absorbance of above 40% was also recorded in the VIS and NIR regions with an average extinction coefficient of the range (k = 0.191to 2.785) and a resistivity range of 0.34 to 23.28 x103 Ω-cm. The solar cell was then fabricated using the optimum conditions, which were Cd0.7Fe0.3S and CuS at 60% Cu concentration. The fabricated solar cell registered a short circuit current, Isc = 0.057 A, open voltage, Voc = 0.57V, efficiency, η = 3.69% and a fill factor, FF = 0.6814. These values imply that these two materials are appropriate for solar cell applications especially in the VIS and NIR light spectrum and should be adopted.