Abstract
Cd1-xZnxS thin films have been studied extensively as window layers for solar cell applications. However, a mismatch between the Cd1-xZnxS and copper-indium-gallium-selenide absorber layers increases with Zn film concentration, which reduces the device efficiency. In this work, Cd1-xZnxS thin films with low Zn concentrations were analyzed. The effect of the addition of different molar Zn concentrations to the reaction mixture on the growth mechanism of Cd1-xZnxS thin films and the influence of these mechanisms on structural, optical and morphological properties of the films has been studied. Cd1-xZnxS thin films were synthesized by chemical bath deposition using an ammonia-free alkaline solution. Microstructural analysis by X-ray diffraction showed that all deposited films grew with hexagonal structure and crystallite sizes decreased as the Zn concentration in the film increased. Optical measurements indicated a high optical transmission between 75% and 90% for wavelengths above the absorption edge. Band gap value increased from 2.48 eV to 2.62 eV, and the refractive index values for Cd1-xZnxS thin films decreased as the Zn increased. These changes in films and properties are related to a modification in growth mechanism of the Cd1-xZnxS thin films, with the influence of Zn(OH)2 formation being more important as Zn in solution increases.
Highlights
Cd1–xZnxS thin films have attracted attention for a long time, since their energy band gap can be tuned and their lattice parameters can be modified with variation in Zn concentration1-3
Cd1–xZnxS thin films were deposited by chemical bath deposition (CBD), which is a very simple, inexpensive and effective technique for the deposition of various semiconductor materials
It is well known that the initial reaction solution conditions in CBD determine the properties of the obtained films; few studies have focused on the growth mechanisms and subsequent film properties
Summary
Cd1–xZnxS thin films have attracted attention for a long time, since their energy band gap can be tuned and their lattice parameters can be modified with variation in Zn concentration. Rodríguez et al demonstrated that the growth mechanism affects the morphological properties and chemical composition of ZnS thin films grown by CBD from a nontoxic alkaline solution, which can affect optical properties. Another important factor in the synthesis of Cd1–xZnxS thin films by CBD is the reaction solution composition. Most reports found in literature use ammonia as complexing agent in the aqueous synthesis of semiconductor thin films1,2,6,7 This is a highly volatile, toxic and harmful material, and industrial production would produce serious environmental and health problems. The effects of very low Zn concentration (less than 5 at.%) on chemical, structural, optical and morphological properties are analyzed and interpreted in terms of growth mechanisms
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