Academic Open Internet Journal

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Volume 11, 2004

 

OPTICAL PROPERTIES OF CHEMICAL BATH DEPOSITED FeCdS3 THIN FILMS

 

F. I. EZEMA

fiezema@yahoo.com

DEPT OF PHYSICS & ASTRONOMY / SCHOOL OF GENERAL STUDIES, NATURAL SCIENCE UNIT, UNIVERSITY OF NIGERIA, NSUKKA.

 

ABSTRACT

Multi-component thin film of FeCdS3 was deposited on glass slide from aqueous solutions of Fe(NO3)3.9H2O, CdCl2.2½H2O and thiourea in the which ammonium solution and EDTA were employed as complexing agents. The film was studied for its optical properties using spectrophotometers. The optical characterization shows that the band gap of the film is 2.30eV and this is a band gap between the band gap of FeS2 (2.16 -2.18 eV) and CdS (2.37-2.40 eV). This obtained band gap was found to be very close to that of CdS which shows that the film is rich in cadmium. The average transmittance of film is greater than 50% in the VIS-NIR regions and high reflectance of greater than 12% in the same regions, and exhibits poor transmittance in the UV regions. Hence, they could be effective as coatings for poultry houses and as well a good material for solar cell fabrication.

Keywords: Chemical bath deposition technique, FeCdS3 thin films, poultry house coating, Solar cells.

 

 

 

 

INTRODUCTION

In the search for new semiconductor materials for efficient solar energy conversion through photo-electrochemical solar cells, metal-metal chalcogenides are increasingly being studied (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).

FeCdS3 is a group VIII-IIB-VIB material. This alloy film in the non-isoelectronic system has been prepared by reacting thiourea with a mixture of different complexed ions.  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),

M(A)2+n               M2+ + nA  …………………………………………………………  1

and

M(B)2+n               M2+ + nB  ………………………………………………………….  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 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).

This paper reports on an investigation of the optical properties of chemical bath deposited ferrous cadmium sulphide thin film. 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 (e), and optical conductivity (s). These optical properties and the band gap of the films were deduced from equations given in literatures (Pankove 1971, Ezema and Okeke 2002, 2003) while the film thicknesses were obtained by optical methods (Theye 1984).

EXPERIMENTAL DETAILS

The preparation of FeCdS3 thin films on glass slide was carried out using solution growth technique, the glass substrates were previously degreased in HNO3 for 48 hours (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 FeCdS3 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 FeCdS3 contained 0.2M 5ml ferrous nitrate, 0.2M 5ml cadmium chloride, 14M 2ml ammonia, 0.01M 2ml Ethylenediaminetetraacetate (EDTA), 0.1M 10ml thiourea and 21ml distilled water which were added in that order and allowed for 20hours 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 EDTA 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 FeCdS3. The step wise reactions involved in the complex ion formation and film deposition processes for FeCdS3 here are:

  Fe(NO3)3.9H2O + EDTA                               [Fe(EDTA)]2+  +  (NO3)-

                [Fe(EDTA)]2+                                  Fe2+  +  EDTA

        CdCl2.2½H2O + NH3                              [Cd(NH3)4]2+  +  2Cl-

[Cd(NH3)4]2+                                       Cd2+  +  4NH3

(NH2)2CS + OH-                                  CH2N2 + H2O + HS-

HS- + OH-                                            H2O + S2-

             Fe2+ + Cd2+ + 3S2-                              FeCdS3

Sulphide ions are released by the hydrolysis of thiourea but Fe2+ and Cd2+ ions form ferrous-ethylenediaminetetraacetic complex and tetra amine cadmium complex ions by combining with EDTA and NH3 respectively in the pH range of 9 and 10. The [Fe(EDTA)]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 FeCdS3 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.  

RESULTS AND DISCUSSION

Figure 1 shows the combined effect of film – glass system on transmittance of infrared for FeCdS3 when compared with uncoated glass. This was carried out using a single beam Fourier transform spectrometer. Uncoated glass reduced transmittance to

 

 

 

 

 

 

 

 

 

 

 

 

 

50.64% at 3527cm-1 then to 48.62% at 2900cm-1 and then finally dropped to only about 2% transmittance at 1896cm-1 to 2000cm-1. By about 2001 cm-1, no radiation at all is transmitted through the glass.  Coated glass reduced transmittance to 4% at 3525cm-1; 4.01% at 2866cm-1 and finally dropped to only about 0.2% transmittance at 1896 –2000cm-1. By about 2001cm-1, no radiation at all is transmitted through the film-glass system.  These films are capable of allowing solar radiation (0.3 – 3.0mm) 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 ferrous cadmium sulphide films prepared at 300K is displayed in Figure 2.The film Sample absorbs heavily throughout UV-VIS regions but moderately in the NIR regions. The transmittance and reflectance spectra (Figure 3) deduced from absorbance spectra showed that all the films show poor transmittance

 

 

 

 

 

 

 

 

 

 

between 12 and 50% in the UV-VIS regions but moderately (> 50%) in the VIS- NIR regions. The film shows high reflectance (>12%) in the VIS-NIR regions. The properties of poor transmittance in the UV-VIS but moderately high transmittance in the VIS-NIR 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 film 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 and k and σo with hν for sample of FeCdS3 are shown in Figures 4 and 5.  The average value of n occurred 1.98 with maximum and minimum values of 2.28 at photon energies of 2.48eV and 1.57 at 3.27eV respectively. The values of k was observed 3.30 x 10-2 and with maximum and minimum values of 4.72 x 10-2 at 3.10eV

 

 

 

 

 

 

 

 

 

 

 

and 2.02 x 10-2 at 1.38eV respectively.    It observed a maximum value of 0.60 x 1014S-1 at 3.65eV and minimum value of 0.11 x 1014S-1 at 1.38eV with an average value of 0.40 x 1014S-1.

The plots of εr and εi against hν are displayed in Figure 6.  εr has a minimum value of 2.47 at 3.27eV and maximum value of 5.21 at 2.48eV with an average value of 4.03. εi has a minimum value of 0.66 x 10-3 at 1.38eVand a maximum value of 1.81 x 10-1 at 2.70eV with an average value of 1.27 x 10-1.

A plots of (αhn)2 against hν for FeCdS3 films are shown in figure 7.  This reveals band gap of 2.30eV.

 

 

 

 

 

 

 

 

This band gap lies between the band gap of FeS2 of 2.16eV and 2.18eV reported by Uhuegbu and Okeke 1995, and the band gap of CdS of 2.37eV reported by Choi et al 1994 and 2.40eV reported by Pawar et al 1996. The difference between the band gap of FeCdS3 and FeS2 that of CdS shows the deposited film is rich in cadmium. This band gap of the FeCdS3 film makes it a good material for solar cell fabrication.   

 

 

CONCLUSION

FeCdS3 thin film with thickness of 0.461μm with energy band gap of 2.30eV which is band gap between that of FeS2 and CdS has been successfully deposited using solution growth technique.  The FTIR spectroscopy showed the percentage transmittance that ranged between 7 and 46% in the far infrared regions. The deductions from the spectrophotometers showed average values n of 1.98, k of 3.30 x 10-2 and σo of 0.40 x 1014S-1.  The film was found to have an average transmittance of >50% in the VIS-NIR regions while exhibiting high reflectance of >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 a good material for solar cell fabrication.

REFERENCES

Choi J.Y., K.J. Kim, J.B. Yoo and D. Kim (1998), Properties of Cadmium Sulphide    Films Deposited by Chemical Bath Deposition with ultrasonication, Solar Energy 64 (1-3), 41-47.

 

Chopra, K.L. and S.R. Das (1983), Thin Film Solar Cells, Plenium Press, New York.

Eze, F.C. and C.E. Okeke (1997), Chemical Bath Deposited Cobalt Sulphide Films; Preparation Effects, Materials Chemistry and Physics 47, 31-36.

 

Ezema F.I. and Okeke C.E., 2002, “Preparation and Characterization of Bismuth Bromide Oxide Film Preparation by Solution Growth Technique”, Nig. Journal of Physics 14 (2) 48 – 54.

 

Ezema F.I. and Okeke C.E., 2003, “Chemical Bath Deposition of Beryllium Sulphide Thin Film and Its Applications” Academic Open Internet Journal www.acadjournal.com vol. 9

 

Ezema F.I. and Okeke C.E, 2003, “Chemical Bath Deposition of Bismuth Oxide (Bi2O3) Thin Film and Its Application”, Greenwich Journal of Science and Technology 3(2) 90 – 109.

 

Jae-Hyeong Lee, Woo-Chang Song, Jun-Sin Yi and Yeong-Sik Yoo, 2003, Characteristics of the CdZnS thin film doped by thermal diffusion of vacuum evaporated indium films, Solar Energy Materials and Solar Cells 75 (1-2), 227-234.

 

Padam, G.K. and S.U.M. Rao (1986), Preparation and Characterization of Chemically        Deposited CuInS2 Thin Films, Sol. Ener. Mater. 13,297-305.

 

Pankove, J.I. (1971), Optical processes in semiconductors Prentice-Hall, New York.

 

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.

 

Theye, M. (1985), In “Optical properties of thin films”, K.L. Chopra and L.K. Malhota, eds, thin film technology and Applications, Tata McGraw-Hill, New Delhi.

 

Uhuegbu C.C. and C.E. Okeke, 1995, Deposition of Iron Disulphide Thin Film by Solution Growth Technique using EDTA as complexing agent, Nig. Journ. of Phys. 7, 24-27.

 

Ver ´ onica Estrella, 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

 

 

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