Synthesis and Characterization of Cadmium Zinc Sulphide (Cd1-xZnxS) Thin Films Deposited from Acidic Chemical Baths


Cadmium zinc sulphide thin films with the required composition were successfully deposited from chemical baths containing zinc chloride, cadmium chloride, urea and thioacetamide at modestly acid pH values.  The Cd1-xZnxS films obtained by this method were smooth, uniform, adherent, pinhole free, bright yellow orange in colour and changed to pale yellow with increasing zinc content. ZnS films were white in colour and transparent.


Mr AmpongThe films were annealed in air at 400oC for two hours. Both the as-deposited and annealed samples were characterized using a variety of techniques. Powder X-ray diffraction analysis of the thin films and precipitates of Cd1-xZnxS, over the entire composition range, that is, 0 ≤ x ≤ 1,  showed that, all the compositions up to 75% Zn had the wurtzite structure, with preferred orientation along the (002) plane. Pure ZnS had the sphalerite structure with preferred orientation along the (111) plane. SEM micrographs of the as-deposited thin films showed the presence of uniform and crack-free surface morphologies characterized by well-interconnected globular crystallites. EDX analysis confirmed the film to be consistent with the formation of the ternary compound on silica glass slide. The crystal size measured from the XRD peak width varied from 12 nm (CdS) to 9 nm (ZnS). The variation of lattice parameters with increasing zinc ion content, showed a very good agreement with Vergard’s law.


The fundamental absorption edge of the as-deposited Cd1-xZnxS thin films, showed a blue shift with an increase in zinc ion content. The band gap, determined from optical absorption spectroscopy, varied almost linearly with composition between that of CdS (2.38 eV) and ZnS (3.70 eV) for the as-deposited samples while the annealed samples varied from  CdS (2.40 eV) to ZnS (3.73 eV). This almost linear change in the band gap of CdS by addition of Zn shows formation of a continuous series of solid solutions. The as-deposited thin films possessed poor crystallinity compared to the annealed films.


There is a growing need for energy in the world and since the traditional energy sources based on fossil fuels are limited and will be exhausted in future, PV solar energy is considered a promising energy source candidate. Large-scale application of PV solar energy will also contribute to the diversification of energy sources resulting in more equal distribution of energy sources in the world (NREL Report, 2001 - 2002). The most widely used commercial solar cells are made from single crystalline silicon and efficiencies up to 26.5 % are reported for commercial products.  However, such single crystalline solar cells are relatively expensive with the silicon itself making up 20 to 40 % of the final cost (Afzaal and O’Brien, 2006). Amorphous silicon solar cells can be produced at lower temperatures and deposited on low-cost flexible substrates such as plastics or metal foils. Ultimately, the amorphous cells tend to degrade when exposed to sunlight and their efficiency decreases by 10 to 20 % (Afzaal and O’Brien, 2006). Therefore, a large-scale application of renewable energy sources as electricity power sources is not yet economically attractive in the industrialized countries.


One of the most promising strategies for lowering PV costs is the use of thin film technologies in which the PV materials are deposited onto inexpensive large area substrates such as window glass and flexible substrates. Benefiting from the inherent advantages to thin film PV will require breakthroughs in reducing manufacturing costs, primarily by improving yields and increasing throughput (NREL Report, 2001 - 2002). Copper indium gallium diselenide (CIGS) based thin-film solar cell modules currently represent the highest-efficiency alternative for large-scale, commercial thin-film solar cells. Several companies have confirmed module efficiencies exceeding 13 %. CIGS thin film solar cells were typically fabricated using a high-resistivity cadmium sulphide (CdS) buffer layer deposited by (CBD) in order to avoid the formation of undesirable shunt paths (Liu and Mao, 2009).


The reported efficiencies of CIGS and cadmium telluride (CdTe) based solar cells (using CdS buffer layer) have reached 19.9 and 16.5 %, respectively (Repins et al., 2008; Wu et al., 2001). However, CdS window layer absorbs the blue portion of the solar spectrum for its relatively low band gap and it has lattice mismatch problems associated with a quaternary solar absorber layer, which inhibit the higher performance of the solar cell devices (Meng et al., 2010). These issues have prompted much research in developing a better buffer layer. Cadmium Zinc Sulphide ternary material with wurtzite structure fits this application (AbuShama et al., 2005). Cd1-xZnxS has a larger band gap than CdS and it is suitable to circumvent the problems mentioned above (Borse et al., 2007).


Keeping these aspects in view, more attention is being given in producing good quality cadmium zinc sulphide thin films for comprehensive optical studies and their various applications (Sanap and Pawar, 2011).


Thus, the focus of this research is to develop a novel approach for the deposition of quality cadmium zinc sulphide thin film buffer layers, to improve the performance of the CIGS solar cell.


Francis Kofi Ampong
Senior Lecturer, Department of Physics, KNUST

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