ABSTRACT
Thin film of CdCoS2 was deposited on glass slide from aqueous solutions of CoCl2.6H2O, CdCl2.2½H2O and thiourea in which ammonium solution and TEA were employed as complexing agents. The film was studied for its optical properties using spectrophotometers. The optical characterization show that the band gap range between 2.50 and 2.70 eV which is between the band gap of CoS and CdS but very close to that of CdS which shows that the film is rich in cadmium. The film was found to have an average transmittance of greater than 55% in the VIS-NIR regions and exhibit average reflectance of greater than 11% in the same regions. Other optical parameters such as refractive index, extinction coefficient, real and imaginary dielectric constants and optical conductivities were also estimated. The film exhibits poor transmittance in the UV regions, hence, they could be effective as coatings for poultry houses and as well good materials for solar cell fabrication.
PDF Abstract XML References Citation
How to cite this article
DOI: 10.3923/jas.2006.1827.1832
URL: https://scialert.net/abstract/?doi=jas.2006.1827.1832
INTRODUCTION
The Chemical Bath Deposition (CBD) method has become very popular in recent decades, especially for thin-film deposition, due to its low cost since no expensive and sophisticated vacuum equipments are required, ease of handling and ease of application to many compounds such as sulfides and selenides which include ZnS, CdS, PbS, CdSe, PbSe, CuS2 Bi2S3 and CoS (Pentia et al., 2004; Eze and Okeke, 1997).
The CBD method, being less expensive than other thin-film deposition methods, allows for the manufacture of relatively low cost devices, especially light detectors and light energy conversion cells. Thin alloy film in the non-isoelectronic system has been prepared by reacting thiourea with a mixture of different complexed ions in chemical baths. For two non-interfering, independent complexing agents used for complexing the two cations, the ions dissociates in an aqueous solution to give metal ions according the reactions (Chopra and Das, 1983).
(1) |
and
(2) |
When one complexing agent is used, only one equation above correctly applies however in this report two non-interfering complexing agents were used.
Since thiourea has a higher dissociation constant, the fraction of S2¯ ions in the solution is expected to be more than the fraction of the remaining thiourea in the solution. As is the case in an atom-by-atom deposition process, the solubility conditions of multi-components in an ion-by-ion condensation process are relaxed (Chopra and Das, 1983).
Some attempts were made to obtain ternary compounds materials using the CBD method (Pentia et al., 2004; Ezema, 2004; Chopra and Das, 1983; Padam and Rao, 1986; Ver ′ onica Estrella et al., 2003). The search for new semiconductor materials for efficient solar energy conversion through photo-electrochemical solar cells, has led to the increasing deposition of metal-metal chalcogenides (Chopra and Das, 1983; Padam and Rao, 1986; Ver ′ onica Estrella et al., 2003). These materials have been known to be potential candidates for photo-electrochemical solar cells (Pawar et al., 1986; Lee et al., 2003).
This study reports on the preparation and optical properties of CdCoS2 films, prepared on glass substrates by CBD. The purpose of this study was to try to get ternary compounds with variable gap by the CBD method and investigation of the optical properties of chemical bath deposited cadmium cobalt sulphide thin film, with potential application in areas of solar energy.
MATERIALS AND METHODS
The preparation of CdCoS2 thin films on glass slide was carried out using chemical bath deposition technique, the glass substrates were previously degreased in HNO3 for 48 h (Eze and Okeke, 1997), cleaned in cold water with detergent, rinsed with distilled water and dried in air. The nitric acid treatment caused the oxidation of the halide ions in glass slides (halide glass) used as substrates, thereby introducing functional groups called nucleation and/or epitaxial centers on which the CdCoS2 thin film is grafted. The degreased cleaned surface has the advantage of providing nucleation centers for the growth of the films, hence yielding highly adhesive and uniformly deposited films.
The reaction bath for the deposition of CdCoS2 contained 0.2M 5 mL cobalt chloride, 0.2M 5 mL cadmium chloride, 14M 2 mL ammonia, 0.01M 2 mL triethanolamine (TEA), 0.1M 10 mL thiourea and 21 mL distilled water, which were added in that order and allowed for 20 h deposition time. The pH after the mixtures were thoroughly stirred with glass stirring rod came to 9. During deposition cations and anions, which are both present in the deposition solution, react with each other and become neutral atoms, which either precipitate spontaneously or very slowly in the bath. Fast precipitation implies that a thin film cannot form on the substrate immersed in the solution. However, if the reaction is slow, which the additives like NH3 and TEA could achieve, then thin solid films of neutral atoms could form on the substrate. The complexing agents used slow down the precipitation action and enables the formation of CdCoS2. The step wise reactions involved in the complex ion formation and film deposition processes for CdCoS2 here are:
Sulphide ions are released by the hydrolysis of thiourea but Co2+ and Cd2+ ions form cobalt-triethanolamine complex and tetra amine cadmium complex ions by combining with TEA and NH3 respectively in the pH range of 9 and 10. The [Co(TEA)]2+ and [Cd(NH3)4]2+ complexes adsorb on the glass, then a heterogeneous nucleation and growth takes place by ionic exchange of reaction S2¯ ions. This process is referred to as ion-by-ion process and in this way brownish (reddish) yellow CdCoS2 was deposited on glass slide in form of transparent, uniform and adherent thin film.
After the films were deposited they were characterized using UNICAM SP8-100 double beam UV spectrophotometer and Fourier transform single beam infrared spectrometer.
The A-T-R spectra of the films were obtained in UV-VIS-NIR regions by means of PYE UNICAM SP8-100 double beam spectrophotometer with uncoated glass slide as reference. The optical properties investigated include the Absorbance (A), Transmittance (T) and Reflectance (R), which were used to calculate the other properties such as refractive index (n), extinction coefficient (k), dielectric constant (ε) and optical conductivity (σ). These optical properties and the band gap of the films were deduced from equations given in literature (Pankove, 1971; Ezema and Okeke, 2002; 2003) while the film thicknesses were obtained by optical methods (Theye, 1985).
RESULTS AND DISCUSSION
Figure 1 shows the combined effect of film - glass system on transmittance of infrared for CdCoS2 when compared with uncoated glass.
This was carried out using a single beam Fourier transform spectrometer. Uncoated glass reduced transmittance from 50.64% at 3527 cm-1 then to 48.62% at 2900 cm-1 and then finally dropped to only about 2% transmittance at 1896 to 2000 cm-1. At about 1999 cm-1, no radiation is transmitted through the glass. Coated glass reduces transmittance from 5.43% at 3744 to 2.72% at 3448 cm-1, it raised the transmittance slightly from 2.75% at 3331 to 2.79% at 2854 cm-1 and then reduced to 0.09% at 2129 cm-1 before finally it drops to only about 0.02% transmittance at 1999-2010 cm-1. By about 1999 cm-1, no radiation is transmitted through the film-glass system.
It is observed that the film-glass system transmit less 1R than the plain glass.
These films are capable of allowing solar radiation (0.3-3.0 μm) to be transmitted into a building but prevent thermal re-radiation out of the building through the glassing system. It is observed that the film-glass system suppresses transmission of IR when compared with the plain glass.
The spectral Absorbance of cadmium cobalt sulphide thin films prepared at 300K is displayed in Fig. 2. Thin film sample absorbs heavily throughout UV-VIS regions but moderately in the NIR regions.
Fig. 1: | Spectral infrared transmittance of plain glass/commbined film-glass for CdCoS2 sample |
Fig. 2: | Spectral absorbance of CdCoS2 thin films |
The transmittance and reflectance spectra (Fig. 3) deduced from absorbance spectra showed that all the films show poor transmittance between 12 and 50% in the UV regions and some portions of the VIS regions but moderately (> 50%) in the remaining portions VIS and NIR regions.
Fig. 3: | Spectral transmittance-reflectance for CdCoS2 thin films |
The film shows high reflectance (>12%) in the VIS-NIR regions.
The properties of poor transmittance in the UV regions and some portions of the VIS regions but moderately high transmittance in some portions of the VIS regions and NIR regions make the film good materials for screening off UV portion of electromagnetic spectrum which is dangerous to human health and as well harmful to domestic animals. The films can be used for coating eye glasses for protection from sunburn caused by UV radiations. Since they show moderately high VIS-NIR transmittance it can be used for coating of poultry roofs and walls. This will ensure that young chicks which have not developed protective thick feather are protected from UV radiation while the heating of the poultry house is maintained by the heating portion of the electromagnetic spectrum and as well there is admittance of VIS light in the house.
The variations of n with hv for sample of CdCoS2 are shown in Fig. 4. The average values of n ranged between 1.97 and 2.27 with maximum values that ranged between 2.63 and 2.64.
Fig. 4: | Plot of n against hv for CdCoS2 thin films |
Fig. 5: | Plot of k against hv for CdCoS2 thin films |
Fig. 6: | Plot of σo against hv for CdCoS2 thin films |
Fig. 7: | Plot of εr against hv for CdCoS2 thin films |
The variations of k with hv for samples of CdCoS2 are shown in Fig. 5. The average values of k ranged between 1.59x10-2 and 2.47x10-2 with maximum values that ranged between 4.02x10-2 and 5.54x10-2.
Plots of optical conductivity σo against hv are shown in Fig. 6. It has maximum values that ranged between 0.72x1014S-1 and 0.76x1014S-1 with average values that ranged between 0.20x1014S-1 and 0.37x1014S-1.
Table 1 below shows the optical properties and thickness, while Table 2 shows the solid state properties and thickness of CdCoS2 thin films prepared under varying conditions at 300 K.
Fig. 8: | Plot of εi against hv for CdCoS2 thin films |
The plots of εr against hv are displayed in Fig. 7. εr has maximum values that ranged between 6.91 and 6.99 with average values that ranged between 3.97 and 5.23.
The plots of εi against hv are displayed in Fig. 8. εi has maximum values that ranged between 1.80x10-1 and 2.17x10-1 with average values that ranged between 6.15x10-2 and 1.11x10-1.
A plots of (ahv)2 against hv for CdCoS2 films are shown in Fig. 9. It shows band gaps that ranged between 2.50 and 2.70 eV.
Fig. 9: | Plot of (αhv)2 against hv for CdCoS2 thin films |
Table 1: | Optical properties and thickness of CdCoS2 thin films prepared at 300 K |
Table 2: | Solid state properties and thickness of CdCoS2 Thin Films at 300 K |
The band gap for CoS has been reported to be 3.20 eV (Ndukwe, 1992) while the band gap for CdS was reported to be 2.37 eV (Choi et al., 1998; Osuji, 1994) and 2.40 eV (Pawar et al., 1986). When the band gaps 2.37 and 2.40 eV for CdS and 3.20 eV for CoS were compared with that of CdCoS2 thin films, the band gap of CdCoS2 thin films are observed to be between that of CdS and CoS. This band gap lies between the band gap of CoS of 3.20 eV (Ndukwe, 1992) and the band gap of 2.37 eV of CdS (Choi et al., 1998; Osuji, 1994) and 2.40 eV (Pawar et al., 1986). The difference between the band gap of CdCoS2 and CoS and that of CdS shows the deposited films are rich in cadmium. This band gap of the CdCoS2 film makes it a good material for solar cell fabrication.
CONCLUSIONS
CdCoS2 thin films with thickness of 0.461 μm and energy band gaps between 2.50 and 2.70 eV which are band gaps between that of CoS and CdS has been successfully deposited using chemical bath deposition technique. The FTIR spectroscopy showed the percentage transmittance that ranged between 7 and 46% in the far infrared regions. The films were found to have average transmittance of greater than 50% in the VIS-NIR regions and exhibit high reflectance of greater than 12% in the same regions. The film exhibits poor transmittance in the UV regions, hence, they could be effective as coatings for poultry houses and as well good materials for solar cell fabrication.
REFERENCES
- Eze, F.C. and C.E. Okeke, 1997. Chemical bath deposited cobalt sulphide films: Preparation effects. Mater. Chem. Phys., 47: 31-36.
Direct Link - Lee, J.H., W.C. Song, J.S. Yi and Y.S. Yoo, 2003. Characteristics of the CdZnS thin film doped by thermal diffusion of vacuum evaporated indium films. Solar Energy Materials Solar Cells, 75: 227-234.
Direct Link - Padam, G.K. and S.U.M. Rao, 1986. Preparation and characterization of chemically deposited cuins2 thin films. Sol. Ener. Mater., 13: 297-305.
Direct Link - Pawar, S.H., S.P. Tamhankar and C.D. Lokhande, 1986. Studies on electrochemical photovoltaic cells formed with thin film Bi2CdS4 Photoelectrodes. Sol. Ener. Mater., 14: 71-77.
Direct Link - Pentia, E., V. Draghici, G. Sarau, B. Mereu, L. Pintilie, F. Sava and M. Popescu, 2004. Structural, electrical and photoelectrical properties of CdxPb1-xS thin films prepared by chemical bath deposition. J. Electrochem. Soc., 151: G729-G733.
Direct Link - Estrella, V., M.T.S. Nair and P.K. Nair, 2003. Semiconducting Cu3BiS3 thin films formed by the solid-state reaction of CuS and bismuth thin films, Semicond. Sci. Technol., 18: 190-194.
CrossRefDirect Link