Effect of Annealing Temperature on Structural, Optical and Electrical Properties of Pure CdS Thin Films Deposited by Spray Pyrolysis Technique

A. Hasnat, J. Podder

Department of Physics, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh.
Department of Physics, Jagannath University, Dhaka, Bangladesh.

DOI: 10.4236/ampc.2012.24034                

Abstract

Effect of annealing temperature on the properties of CdS thin films are carried out in this work. Nanocrystalline cadmium sulphide (CdS) thin films were prepared using spray pyrolysis deposition (SPD) technique and the structural, optical and electrical properties were investigated for different annealing temperature (as deposited, 300, 400 & 500 C). The surface morphology and compositional properties studied by SEM and EDX respectively. The crystal structure of CdS thin film was studied by X-ray diffraction. The crystallite size and lattice constant of SPD CdS thin films were investigated. The optical parameters such as transmittance, absorption coefficient and energy band gap of the films with thermal annealing temperature was investigated by UV/VIS spectrophotometer. The variation of band gap values of CdS thin film samples were found to be in the range of 2.51 to 2.8 eV. Electrical resistivity measurements were carried out in fourprobe Vander Pauw method at different temperature. So CdS films may be a good candidate for suitable application in various optoelectronic devices.

Keywords

Spray Pyrolysis; CdS; Annealing Temperature; Structural; Optical Band Gap

1. Introduction

The structural and optical properties of SPD CdS thin films depend on the parameters of relative concentration of the reactants for chemical reaction, thickness of the films, pH value of the aqueous solution and annealing temperature. In the SPD process, film growth occurs either 1) by an ion-by-ion condensation or 2) by adsorption of the colloidal particles of CdS on the substrate [1,2]. The chemically deposited CdS film was found to consist of a continuous film relating to the ion-by-ion deposition of CdS [3,4]. The structural information’s of CdS thin films such as crystal structure, inter-planar distance and lattice constant are determined by the X-ray diffraction (XRD). The optical parameters of optical absorption coefficient and optical band gap are important to design the optoelectronic devices. In this study, nanocrystalline CdS thin films were deposited on glass substrates by SPD. The structural, optical and electrical properties of CdS thin films were investigated. Solar cells are the building blocks of photovoltaic systems. Solar cells are widely used in space program as power source for satellites as well as a major power source to meet earth’s energy needs. Single crystalline CdS has been used extensively for space applications. Cadmium sulphide (CdS) is one of the attractive semiconductor materials and has been investigated for electronic and optoelectronic devices. The solar cells of CdS [5,6] and Cu(In,Ga)Se₂ (CIGS) [7,8] have been researched to increase the efficiency of cells. CdS thin films are prepared by vacuum evaporation [9,10], sputtering [11,12], screen printing [13,14], and CBD [1,3-5,15]. Among the various techniques, SPD is a simple and inexpensive solution deposition technique, in which thin film is prepared by chemical reaction. Also, it is possible to obtain uniform films with good adherence [16,17]. SPD CdS thin films are prepared by the decomposition of thiourea (SC(NH₂)₂) in an alkaline solution containing a cadmium salt, such as cadmium acetate (Cd(CH₃COO)₂).

Other schemes for power generation via solar cells include thin film solar cells [18] and multi-junction solar cells. Multi-junction solar cells although more efficient have higher fabrication cost. Polycrystalline thin film CdS solar cells are one of the important candidates for large scale photovoltaic applications because of their low cost, high efficiency and stable performance and have been used for large area terrestrial applications [19-20].

Cadmium sulphide is an important material for the development of low cost photovoltaic devices for space as well as terrestrial applications [21]. Most of the reported work on CdS solar cells is focused on opto-electronic properties. There are only a few studies on the electrical properties of CdS solar cells [22]. In this paper, the electrical characteristics of CdS solar cells have been investigated and the results are presented.

2. Experimental Details

Spray pyrolysis is basically a chemical process involves spraying aqueous solution onto a substrate held at high temperature. A simple glass nozzle was fabricated to give a fine and very small droplets of precursor solution which is driven by air from the compressor. In the present work, In order to prepare CdS thin films the aqueous solution of Cadmium acetate [Cd(CH₃COO)₂·2H₂O] and thiourea [NH₂CSNH₂] were used as a source of Cd and S respectively. The deposition set up consists of four sections, which include the precursor solution and carrier gas (air) assembly connected to the spray nozzle, the reaction chamber in which the substrate is heated, the pumping and exhausting gas scrubbing systems, and temperature controller with a Copper-Constantan thermocouple to control the substrate temperature. In this study, precursor solutions of 0.1 M concentration was used as raw material to deposit CdS thin films. The glass substrates were cleaned ultrasonically in acetone and methanol respectively for 10 minutes in each case. The solution was sprayed onto pre-cleaned glass substrate. The substrate temperature was maintained constant at 573 K. The normalized distance between the spray nozzle and the substrate was fixed at 29 cm. The pressure of the carrier gas (air) was kept constant at 1 bar. The solution flow rate was maintained 0.5 ml·min^–1 throughout the experiment. The possible chemical reaction that takes place on the heated substrate to produce CdS (please see the formula below).

Characterization

The surface properties of the films were examined by using HITACHI S-3400N model Scanning Electron Microscope (SEM) attached with an EDX to measure quantitatively the sample stoichiometry. CuKα (λ = 1.54178 Å) source was used in XRD. The Optical transmission measurements were carried out within the wavelength range 300 to 1100 nm using UV-1601 PC SHIMADZU scanning double beam spectrophotometer. The experimental accuracy of the transmittance is (±0.005%)

and wavelength is (±0.005%). The observed transmittance data were corrected relative to optically identical uncoated glass substrate. The thicknesses of the films were determined by using Fizeau-fringes method. Fourprobe Vander Pauw method was used for electrical properties measurements.

3. Results and Discussion

3.1. Surface Morphology

Figure 1 shows SEM micrograph samples annealed at different temperature. It shows that the surface is uniform and deposition covers the substrate well. After annealing, the surface roughness is increased. It means that sprayed particles (atoms) are adsorbed onto the substrate to form clusters as the primary stage of nucleation. Clusters have a higher energy than the individual atoms, so at higher annealing temperature growing nuclei come into contact to form island stage and appears as spheroid shape.

Conflicts of Interest

The authors declare no conflicts of interest.

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