Abstract: Based on the experimental results, the effect of the temperature on the optical and structural characters of the FeS2 thin films produced using Chemical Bath Deposition (CBD), has been symmetrically studied. The deposited thin films are characterized with X-ray Diffraction (XRD) and UV-Vis spectrometer. The XRD pattern indicates that the numbers of peaks are increased from 2 to 6 as temperature increased from 50-80°C. The diffraction peaks deposited at 80°C are very sharp with the high intensity indicating the significance in grain size and Polycrystalline orthorhombic nature. The variation of the direct band gap of the films with temperature is estimated and correlated with theoretical values. All the films show a gradually increasing absorbance throughout the visible region which makes it possible for this material to be used in photo electrochemical cell.
INTRODUCTION
The metal chalcogenides are used for the solar energy conversion through photo electrochemical cells. As the energy band gap is between 1.15 and 2.5 eV, the materials acquire importance in solar cells. Among the metal chalcogenides, Iron Sulfide thin films are most powerful candidates for photovoltaic and photo electrochemical applications (Anuar et al., 2009a). The thin films of FeS2 consist of abundant and non-toxic elements. FeS2 is an element with a minority carrier diffusion length much larger than its absorption length and higher carrier mobility even in the thin film form. In the visible range this material has a high absorption. With such a high absorption coefficient, this material is projected as the most suitable material for sensitization type solar cells. These films are prepared using a variety of methods like flash evaporation method, CVD, thermal sulfuration etc. The present method CBD is simple, yet economically viable and greatly suitable for large area deposition. In this method, it is possible to deposit the films of large area and well adherent. The thin films deposited by this method include PbS, MnS, CdS, ZnS, SnSc XnS which are available in literature (Bessergenev et al., 2007). However, there are no reports in the literature on the aqueous solution deposition of FeS2 thin films in the presence of TMA and thiourea for sulfur ion. This method is based on the reaction between dissolved iron and sulfur ions in acidic water. In this study, films of iron sulfide are deposited using CBD method from aqueous medium at different temperatures (Anuar et al., 2009b). The TEA is used as complexing agent. This is a resultant component from slow precipitation of the compound reducing the releasing rate of the metallic ions. Using UV-Vis λ-20 spectrophotometer, the optical properties of the films are studied. For structure determination; the films have been studied by using X-ray diffractometer.
MATERIALS AND METHODS
In the present study, the chemicals we used are of analytical grade. They include ferric chloride (FeCl3), thiourea ((H2NCSNH2) and triethanolamine (TEA) [(HOC2H4)3N]. Distilled water is used as the solvent for the preparation of the solutions. During the deposition, the TEA is used as a complexing agent while ferric chloride is the source of Iron ion and Thiourea is the source of sulfide ion. In the process, iron sulfide thin film is prepared using chemical bath deposition technique. To know the effect of temperature, the FeS2 films are deposited at various temperatures between 50 and 80°C.
RESULTS AND DISCUSSION
Optical properties: The optical properties of FeS2 films have been studied using UV/Vis λ-20 spectrophotometer in the wavelength ranging from 300-700 nm. At different temperatures, the relation between the absorbance and wavelength of the films is shown in Fig. 1.
| Fig. 1: | A graph of wavelength vs. absorbance at different temperatures |
All the films prepared at 50-80°C show a gradually increasing absorbance throughout the visible region which makes it possible to use in the photo electrochemical cells. The films prepared at higher bath temperature, 80°C have higher absorption. This is because of the most homogeneity and higher crystallinity at higher temperatures. To calculate the energy gap of the films, the equation, of Stern is used,
| (1) |
where, υ is the frequency, h is the Planck’s constant; k is a constant while n has values 1 or 4. n = 1 for a direct band gap material and n=4 for the indirect band gap material. The plot of (Ahυ)2 versus hυ is shown in Fig. 2.
The direct band gap energy is calculated by extrapolating the curve (Ahυ)2 = 0. Based on the optical spectrum analysis, the band gap energy of the Iron Sulfide thin films are deposited at 80°C is 1.82 eV.
Structural properties: The change in the structure and identification of phases is studied with the help of Seifert XRD PW3710 X-ray diffractometer in which CuKα (λ = 1.5418 Å) is used as radiation. The X-ray diffraction patterns at temperatures 50, 60 and 80°C of the FeS2 thin films, respectively are shown in Fig. 3.
The XRD patterns of the deposited films at different temperatures reveal that the samples show a polycrystalline and orthorhombic nature. The color of the sample is yellowish brown. . The diffraction angle 2θ value is 33.6° at lower bath temperature 50°C and has two peaks and a diffraction peak corresponding to Silicon Oxide at 2θ = 42.3° is also observed.
| Fig. 2: | A graph between the optical absorbance {(Ahυ) 2} in cm2 eV-1 and photon energy. (The arrow indicates the extrapolation of the linear part of the Fig. 2) |
| Fig. 3: | XRD pattern of FeS2 thin films deposited at various bath temperatures; a: 50°C, b: 60°C and c: 80°C |
When the temperature is increased, the number of peaks and intensity of the peaks is also increased. The diffracted peaks of silicon oxide observed at 50°C is disappeared and the number of peaks are increased to six at 80°C. The corresponding peaks referred to (011), (101), (111), (121), (210), (121) and (211) planes of FeS2 and no elemental peaks for Fe and S were observed. The observed peaks were identified by comparing the d-spacing values obtained from XRD patterns with Joint Committee on Powder Diffraction Standards (JCPDS) data for FeS2 (JCPDS reference no. 00-024-0074) (Prabahar et al., 2009). The crystallinity also increased which can observe from the sharpness of the peaks.
CONCLUSION
The iron sulfide thin films are deposited using low cost chemical bath deposition method at different bath temperatures. The XRD patterns reveal that all the films have orthorhombic structure. The band gap energy of the films varied from 2.25-1.8 eV as the temperature ranges from 50-80°C. These films produced the highest absorption characteristics with band gap. Therefore, deposition at 80°C is suitable to produce good quality of FeS2 thin films.