Preparation and characterization of zinc sulfide thin film by electrostatic spray deposition of nano-colloid

Highlights

• Polyvinylpyrrolidone-capped ZnS nanoparticles were synthesized by thermolysis.

• The nanocolloid of zns-capped nanoparticles was stabilized in ethanol.

• ZnS particle thin films deposited by electrostatic spray deposition.

• ZnS films realized using a low-temperature and non-vacuum method.

Abstract

In this investigation, condensed and adhesive zinc sulfide (ZnS) layers were productively sprayed onto Fluorine doped Tin Oxide glass substrate at room temperatures using a facile, non-vacuum and economical electrostatic spray deposition (ESD) method. Used ZnS colloidal in the ESD process prepared by a one-pot thermolysis synthesis method. The effects of applied voltage, liquid flow rate, and distance of the nozzle to substrate on the morphology of ZnS thin films, were investigated. It revealed optimized conditions at the voltage of 24 kV, the flow rate of 1 μl/min, and nozzle to substrate distance of 8 cm. The ultraviolet-visible absorption spectroscopy revealed direct optical band gap of the deposited layer is around 3.44 eV. At a wavelength larger than 350 nm, the visible region (transparency area) of ZnS film was observed. Energy dispersion X-ray analysis and scanning electron microscope showed the ZnS film has a thickness of more than 300 nm with Zn_1-xS (x ≈ 0.3–0.4) chemical composition.

Introduction

The nontoxic and wide band-gap zinc sulfide (ZnS) is a promising material which can effectively replace the cadmium sulfide (CdS) as the buffer layer in thin film solar cells. In these solar cells, a buffer layer protects the absorber layer from the atmosphere and deposition of other layers of a solar cell device [1]. CdS compound also is similar to ZnS but is a toxic semiconductor and should be avoided in the final solar modules. One method to enhance the efficiency of some thin film solar cells, is to replace the CdS (e.g. = 2.4 eV) buffer layer with an alternative buffer layer having a higher energy gap. ZnS (e.g. = 3.8 eV) improves the device efficiency by eliminating absorption loss [2], [3], [4]. ZnS is such an environment-friendly substance. Its use has decreased the concern of toxic issues of cadmium elements considerably [2,[5], [6], [7], [8]]. One trend in optical device fabrication is reducing the production cost by introducing a facile and vacuum-free deposition process. Numerous deposition methods have been used in ZnS buffer layers, such as chemical bath deposition [8], [9], [10], [11], [12], RF Sputtering [13], [14], [15], and spray pyrolysis [15], [16], [17], [18]. Chemical bath deposition (CBD) is a frequently used method for depositing buffer layers, but the waste of raw materials is high while using this method. Also, in the CBD method, the thickness control is difficult because the layers should be thin in the range of 50–80 nm to keep minimal resistance [5,19,20]. Atomic layer deposition is another technique for deposition of buffer layers which has good control over the composition of the layer during the process and the thickness of the layer is controllable but it is a high-cost deposition technique. Spray pyrolysis is also a widely used method for deposition of buffer layers, which reduces material loss, but high deposition temperature in this method and the penetration of buffer elements in the absorbent layer are some major problems [21]. Electrostatic spray deposition (ESD) has been developed recently to facile, non-vacuum, and inexpensive deposition method of optical device thin layers. In many researches, the ESD process has been used as a synthesis method to prepare nano-layer of oxides [22,23], sulfides [24,25], phosphates [26,27], by depositing the precursor solution.

This investigation reports the use of the ZnS nanocolloids that have been synthesized by one-pot polyol thermolysis with green and polar solvents and printing them by the ESD process without a hot plate. Electrostatic spray deposition is a simple and efficient method used in nanoparticle deposition and thin films formation and emerged as an alternative and more economical method to the conventional deposition methods, such as chemical vapor deposition, physical vapor deposition and sputtering. It is a process in which droplets of a solution or suspension of any material are sprayed on a substrate to form a smooth surface layer. It is a highly effective method in the production of crack-free thin films. Its usage is favorable as its setup is easy with minimal wastage of material. ESD can be used effectively for creating multiple layers from a common solvent as well as higher rate of solvent extraction during droplets’ flight towards the substrate [28], [29], [30], [31], [32]. In order to achieve better results by ESD, physico-chemical properties of the solution such as electric conductivity, surface tension, vapor pressure, viscosity and dielectric constant should be suitable because these factors play an important role in the formation of Taylor cone [29]. Based on this condition ethanol is a desirable base fluid to prepare colloids. K.L. Choy et al. [24], also used ethanol to deposit CdS by the electrostatic spray process. The advantages of this method include simplicity, cost-friendly and the ability to carry out at low temperature.