Characterization of chemical bath deposited nickel-doped cadmium sulphide thin films for solar cell applications.

Abstract

CdS films have received a lot of attention attributed to their high transparency, high absorption coefficient, high electron affinity, and outstanding photoconductive property. So far, the best performance of thin film solar cells has been achieved using ultra-thin polycrystalline CdS as a buffer layer. While CdS thin film is the best choice for use as a window/buffer layer, it still experiences optical losses in the low-wavelength region of the solar spectrum due to its low bandgap (2.42 eV). Previous research has shown that nickel widens the bandgap of some semiconductors such as ZnO, CdS, and Sb2S3. Therefore, nickel doping of CdS could be a feasible way of widening its bandgap. The current study is intended to enhance the optical properties of CdS, hence the performance of the solar cells and optoelectronic devices by doping with nickel during film growth to widen the bandgap of CdS. CdS:Ni thin films were synthesized using chemical bath deposition (CBD) with different concentrations of Cd2+ and Ni2+ (15 wt%, 25 wt%, 35 wt%, and 45 wt%). The films were prepared from an aqueous solution of 0.1M cadmium chloride, 1M thiourea, 0.05M nickel (II) chloride, 1M triethanolamine (complexing agent), and 35 wt% ammonia solution (pH regulator). The pH of the reaction bath was ≈11. The samples were annealed in air at varying temperatures (150 – 450 ℃). The influence of nickel concentration and annealing temperatures on structural and optical properties of CdS thin films was studied. The incorporation of nickel into the CdS structure was recognized using X-ray diffraction (XRD). Optical properties, reflectance, and transmittance, in the range 200 nm-1500 nm were determined by UV-VIS-NIR spectrophotometer. The generated data were used to calculate other optical and solid-state properties like extinction coefficient (k), bandgap (Eg), absorption coefficient (α), refractive index (n), and Urbach energy (EU). The films were found to be polycrystalline and exhibited a mixed-phase structure (cubic and hexagonal structures). It was further observed that the diffraction peaks shifted slightly to the lower angle with increasing Ni2+ concentration. This could be as a result of compressional micro-stress in the CdS lattice, due to the difference in ionic radii of Cd2+ ion and Ni2+ ion. The transmittance of the films was found to increase with an increase in both dopant concentration and annealing temperature. This may be as a result of the improvement in the crystallinity of the films with increasing dopant concentration and annealing temperature. The bandgap of as-prepared CdS:Ni was observed to widen as the nickel concentration was increased. This could be due to donor electrons occupying the states at the bottom of the conduction band blocking thus the low energy transitions (Burstein- Moss effect). On annealing the films, their bandgaps decreased when the temperature was raised upto 250 ℃ and then increased with further annealing at 350 and 450 ℃. The decrease in the energy bandgap could be attributed to the increase in the grain size leading to denser films with lower bandgaps. The increase in the energy bandgap of films annealed at 350 and 450 ℃ could be attributed to the phase transition from cubic (zinc-blend) to hexagonal (wurtzite) structure. The absorption coefficients for all CdS:Ni thin films were found to be greater than 104 cm-1 in the visible region (380 nm to 780 nm) and near-infra red (780 and 2500 nm) regions which confirmed that the films have a direct optical energy gap. CdS:Ni thin films with 25 wt % and annealed at 250 ℃ were found to be the most appropriate films for use in solar cell as window/buffer layers as they recorded the highest transmittance with minimum optical properties (of Urbach energy of 0.16 and the lowest extinction coefficient) and had negative charges as the majority charge carriers. To realize higher conversion efficiencies in thin-film solar cells using CdS as window/buffer, we recommend further studies on film thickness and composition of the CdS:Ni thin films by varying the deposition time, pH of the solution, the concentration of the reagents, and temperature of the reaction bath.