Cadmium sulfide (CdS) is an important II–VI compound semiconductor material with applications in several heterojunction photovoltaic systems including CdTe and CIGS solar cells. There are several physical and chemical deposition techniques used for deposition of CdS thin films including RF sputtering , evaporation, close space sublimation (CSS), closed space vapor transport (CSVT), metal organic chemical vapor deposition (MOCVD), chemical bath deposition (CBD), spray pyrolysis, successive ionic layer adsorption and reaction (SILAR), etc.
Among these, CBD is one of the simple stand inexpensive large-area deposition techniques. CBD is most widely used since it is not only available at low temperatures and easy to control but also it helps produce uniform, compact and adherent films.
Kim Hyon Chol, a researcher at the Faculty of Electronics, has investigated the influence of deposition parameters such as reactant concentration, bath temperature and deposition time on the CdS thin film properties when a CdS thin film, a window layer of a CdTe solar cell, was deposited by the chemical bath deposition (CBD) method without stirring.
Influence of solution concentration: When the reactant concentration is too high, the reaction rate becomes fast and the color of solution turns yellow, resulting in films with brittle nature and poor uniformity due to the adherence of a great amount of CdS precipitates onto the surface, which is undesirable for solar cell fabrication. On the other hand, low concentration leads to a decrease in deposition rate and non-uniformity of thin films. When the reactant concentration is controlled suitably, the residual solution after the reaction can be colorless to obtain uniform, compact and transparent films.
Influence of bath temperature: The bath temperature affects the surface morphology, microstructure and adhesion of CdS thin films. At lower temperatures, an abundance of CdS particles are directly deposited because of lower solubility products of CdS (Ksp). Meanwhile, higher bath temperature yields thin films of poor quality due to the increase in Ksp and the decrease in the precipitation rate. The increase in the bath temperature decreases the grain size and promotes the formation of CdS thin films.
Influence of deposition time: Deposition time influences the thickness of films with constant reactant concentration. At the initial stage of the formation of a thin film, a large number of voids made by CdS colloid particles form discontinuous films. For CdS thin films with these voids, the performance of solar cell is strongly affected by the decrease in shunt resistance. As the deposition time increases, the surface of a film becomes compact, uniform and continuous.
The measurements of transmittance were carried out for all deposition parameters. The performances of solar cells with different S/Cd ratios, the variation of the crystal phase with NH4Cl concentration and the surface morphology with the bath temperature were studied. Finally, he determined the optimum deposition parameters through the systematic analysis.
You can find more information in his paper “Effect of the reactant concentration, bath temperature and deposition time on the properties of CdS thin film prepared by the chemical bath deposition method” presented to the SCI Journal “optical materials”.
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