Optical and Electrical Characterization of Cdxni1-Xs and Sb2S3 Thin Films for Photovoltaic Application
Shikambe, Reuben Tsisambo
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The demand for silicon in microelectronics has put a lot of strain on the availability and eventually high prices for silicon type of solar cells. In this work, nickel doped cadmium sulphide (CdxNi1−x S) was investigated. This material was used as a window layer, combined with antimony sulphide (Sb2S3) as an absorber layer in fabricating glass/ SnO2: F/CdxNi1−x S /Sb2S3 /C-Ag p-n junction solar cell. Thin films of CdX Ni1−XS (CdS: Ni) and Sb2S3 were prepared by CBD method on ordinary glass substrates and the cell fabricated on SnO2: F glass. Characterization was done using the following techniques: EDX-Spectrometer applied in the determination of the elements present in the samples. Carrier concentration and mobility measurements were obtained using Hall Effect measurement while transmittance and reflectance data was measured using UV-VIS-NIR Spectrophotometer in the 300-1100 nm spectral range. The thickness for Sb2S3 thin films increased with deposition time and varied from 199-294 nm. The carrier density for Sb2S3 thin films was found to be between 9.7 x 1015 and 1.07 x 1017 cm-3 while their carrier mobility was between 5.11 x 10 -4 and 9.60 x 10 -3 cm2 V-1S-1. The carrier mobility was observed to decrease with the deposition time. Transmittance below 21% and absorbance above 66% was observed for these films across UV-VIS region. Their Optical conductivity was within the range 4.09 x 1011 - 8.36 x 1012 S-1. Band gap for these samples varied with deposition time and annealing. The band gap for as-grown and annealed samples ranged between 1.56 to 2.24 eV. The resistivity for these thin films decreased with the rise in the deposition duration causing an increment in the electrical conductivity. Samples sheet resistivity was within the range 5.09 x 104 and 1.50 x 105 Ω-cm while the electrical conductivity was between 6.66 x 10-6 and 1.96 x 10-5 (Ω-cm)-1. In the elemental composition analysis for Sb2S3 samples, S ions was at 51.23%, Sb ions at 48.740% with the other impurities being less than 0.01%. The hall voltages hence ratio of hall voltage to current and hall coefficients for Sb2S3 thin films were positive proving that these thin films were p-type semiconductor materials. On the other hand the film thickness for CdS: Ni samples decreased with the increase in the volume of NiCl2 solution and the concentration of the reaction bath and ranged between 101 and 234 nm. In the elemental composition analysis for CdS sample; Cd ions was at 44.64% and S ions at 55.22% with other impurities contributing percentages below 0.1 %. For CdS: Ni samples, Ni ions was at 4.006%, Cd ions at 74.35% and S ions at 21.64% with other impurities being less than 0.01%. The hall voltages hence ratio of hall voltage to current and hall coefficients for CdXNi1−XS thin films were negative proving that they were n-type semiconductor materials. Their carrier density ranged between 7.23 x 1015 and 2.35 x 1016 cm-3 and the carrier mobility ranged between 6.16 x 10-2 and 7.51 x 10-1 cm2 V-1S-1. The carrier mobility reduced as the volume of NiCl2 solution in the reaction bath increased. Carrier mobility for CdS: Ni thin films was higher compared to that of Sb2S3 thin films. Spectral transmittance above 68% and absorbance below 20% in the VIS-NIR region was observed for CdX Ni1−XS thin films. Their optical conductivity was within the range 3.78 x 1011 - 2.40 x 1012 S-1. Band gap for as-grown and annealed samples varied with doping and ranged between 2.55 to 3.50 eV. The resistivity increased with the increase in the volume of NiCl2 causing a decrease in the electrical conductivity. The resistivity ranged between 4.53 x 102 and 6.98 x 103 Ω-cm while the electrical conductivity was between 1.43 x 10-4 and 2.21 x 10-3 (Ω-cm)-1. After optimizing the properties for n-type and p-type materials for the operation of a solar cell, a glass/ SnO2: F / CdX Ni1−XS /Sb2S3 /C-Ag p-n junction solar cell was fabricated on SnO2: F glass substrate. I-V characteristics graph gave the following solar cell parameters; value of Voc was 0.384 V, value of Isc was 0.0323 A, value of FF was 0.62 and the solar cell conversion efficiency was 0.63 %. The maximum voltage of 0.280 V and maximum current of 0.0273 A were found leading to a maximum power output of 0.00764 W. Research involving varying the concentration of the complexing agents used in the preparation of Sb2S3 and Cdx Ni1−xS thin films is hereby recommended to study its effect on optical and electrical properties.