Effect of Molarity of Precursor Solution on Nanocrystalline Zinc Oxide Thin Films

Abstract

During the last two decades, the use of transparent conducting films of non-stoichiometric and doped metallic oxides for the conversion of solar energy into electrical energy has assumed great significance. A variety of materials, using various deposition techniques, has been tried for this purpose [1-3]. Among these various materials, zinc oxide (ZnO) is one of the prominent oxide semiconductors suitable for photovoltaic applications because of its high electrical conductivity and optical transmittance in the visible region of the solar spectrum [4]. Furthermore, thin films of ZnO have shown good chemical stability against hydrogen plasma, which is of prime importance in a-Si:H-based solar-cell fabrication. Thus, zinc oxide can serve as a good candidate for replacing SnO2 and indium tin oxide (ITO) films in Si:H-based solar cells. One of the outstanding features of ZnO is its large excitonic binding energy, i.e. 60meV, leading to the existence of excitons at room temperature and even at higher temperatures [5-8]. These unique characteristics have generated a wide range of applications of ZnO. For example, gas sensors [9], surface acoustic devices [10], transparent electrodes and solar cells. Many techniques are used for preparing the transparent conducting ZnO films, such as RF sputtering [11], evaporation [12], chemical vapour deposition [13], ion beam sputtering [14] and spray pyrolysis [15–18]. Among these, the spray pyrolysis technique has attracted considerable attention due to its simplicity and large-scale production combined with low-cost fabrication. By using this technique, one can produce large-area coatings without any need for ultra-high vacuum. Thus, the capital cost and the production cost of high-quality zinc oxide semiconductor thin films are lowest among all other techniques. In the present work, we have synthesized ZnO films by using the spray pyrolysis technique. A number of films have been prepared by changing the molarity of the precursor solution. The prepared films have been characterized with regard to their structural, morphological and electrical properties.