Growth and Characterization of Ternary Chalcogenide Thin Films for Efficient Solar Cells and Possible Industrial Applications
Chapter One
Aim and Objectives of the Study
Aim
The aim of this work is to grow cheaper and more efficient thin films which to the best of our knowledge, have not been grown else where. The decorative applications of thin films will also be considered in addition to their applications in agriculture, electronics and opto-electronic devices.
Objectives
- To grow thin films using solution growth techniques (SGT)
- To characterize the thin films grown by measuring their optical and solid state properties which include the following:
(i) Absorbance (A),
(ii) Transmittance(T),
(iii) Reflectance(R)
(iv) Absorption coefficient(α),
(v) Band gap energy (Eg),
(vi) Refractive index (n),
(vii) Extinction coefficient (k)
(viii) Dielectric constant (ε),
(ix) Optical conductivity and film morphology.
To test the thin films in order to decide which thin films are suitable for solar cells, anti-reflecting coatings, electronics, optoelectronic and other applications.
CHAPTER TWO
Background Knowledge and Literature review on the Optical Properties of Thin Film.
Optical and Solid State Properties of Thin Film
The optical and solid state properties studied in this work include: Absorbance (A), Transmittance (T), Reflectance ( R ), Absorption coefficient (α), Optical density (O.D). Others are the band gap, optical constants, refractive index (n) ,extinction coefficient (k), the dielectric constants- real (εr) and imaginary (ε i), Optical conductivity (σ o), dispersion and Electrical conductivity (σ e).
Transmittance
The transmittance (T) of a specimen is defined ( Wooten, 1972, Pankove, 1971, Lothian, 1958) as the ratio of the transmitted flux
(I t) to the incident flux (I o) that is,
T = It/Io 2.1
Reflection at surfaces are usually taken into consideration, hence transmission is corrected for reflection and for scattering as well. With the corrections, the transmittance is called internal transmittance. If a specimen has a thickness d, an absorption coefficient, α and a reflectivity, R, the radiation reaching the first interface is (1 – R )Io, the radiation reaching the second interface is (1 – R) Io exp (-α d) and only a fraction, (1 – R)(1 – R)Io exp(-α d) emerges. The portion internally reflected eventually comes out considerably attenuated. The end result is that overall transmission is given by
( Wooten,1972, Pankove,1971, Lothian,1958 ) as:
T = It/Io = (1 –R)2 exp(-α d)/ (1 –R2) exp (-2αd) 2.2
Equation 2.2 accounts for the effect of multiple reflections in the film. When the product α d is large, the second term in the denominator becomes negligible and the transmittance is expressed as ( Lothian,1958)
T = It/Io = (1 – R)2 exp (α d) 2.3
If R and d are known, equation 2.3 is used to solve for α. The measurements of the transmittance of two samples having different thickness d1 and d2 can also be used to solve for α using equation 2.4
T1/T2 = exp [α(d2 – d1 ) ] 2.4
Absorbance
The absorbance (A) is the fraction of radiation absorbed from the radiation that strikes the surface of the material. Alternatively, A is the logarithm to base 10 of the transmittance, i.e,
A = log 10 It /Io = log 10 T 2.5
It follows from equation 2.5 that the transmittance and absorbance are related by
T = 10 A 2.6
Hence knowing one, the other can be calculated. The absorbance (A) is determined directly from absorbance spectra measurements and the instrument scales are often calibrated in this unit ( Lothian, 1958).
During the optical characterization of thin films, it is the spectral absorbance of the films that are obtained directly from the spectrophotometer equation 2.6 is used for calculating transmittance. The other properties are obtained from calculations based on the above quantities (transmittance and absorbance).
Reflectance
This is the fraction of the incident radiation of a given wavelength that is reflected when it strikes a surface. A relation between transmittance (T), spectral absorbance (A) and spectral reflectance (R), according to the law of conservation of energy is given by
A + T + R = 1 2.7
Equation 2.7 is used for calculating reflectance.
Absorption Coefficient
Absorption coefficient is the decrease in the intensity of a beam of photons or particles in its passage through a particular substance or medium. This is true when applied to electromagnetic radiation, atomic and subatomic particles. When radiation of intensity Io is incident on material of thickness d (µm) the transmitted intensity It is given by
( Pankove,1971, Lothian, 1958) as:
It = Io exp (-α d) 2.8
For pure absorption, the constant (α) is the absorption coefficient. For scattering, obeying Bouguer-Beer’s law, α is the scattering coefficient. And for the total attenuation including both is the extinction coefficient given by the sum of the absorption and scattering coefficient.
T = It/Io = exp (-α d ) 2.9
and
α = – [ ln T ]/d 2.10
For a unit distance transversed, we have
CHAPTER THREE
Methods of Thin Film Growth
There are several ways of preparing halide and chalcogenide films. Preparation techniques and method of producing these films range from very simple and cheap to complex and very expensive ones depending on the substrate coating materials and on the required performance of the films ( Quijada, et al, 1998, Chopra, 1969). The methods for depositing thin films may be broadly classified under two headings: physical and chemical techniques. The physical techniques include the broad categories of physical vapour deposition (PVD), sputtering techniques, plasma techniques etc. The chemical techniques include chemical vapour deposition (CVD), spray pyrolysis, electrochemical deposition (ECD), anodization, spin coating, dip coating, solution growth technique (SGT) etc.
Chemical techniques are simple, cost effective and offer large area of uniform and controlled deposition for the preparation of halide and chalcogenide films. The growth technique is the simplest and the cheapest chemical technique. In all these techniques, three basic steps are followed in the formation of films. These steps are: creation of the species required for film formation, the transport of these species through a medium, and finally the condensation of the species on the substrate and subsequent coalescence to form the film. We now discuss some methods of preparing thin films with emphasis on the solution growth technique employed in this work.
CHAPTER FOUR
THE MEASUREMENT TECHNIQUES OF THIN FILM CHARACTERISTICS AND MATERIALS
Measurement Techniques of Thin Film Characteristics.
The thickness, composition, solid state and optical characteristics of thin films are tested and measured for the possible applications of thin films and its effectiveness. Methods for determining film properties range from very simple, chemical and mechanical to the very complex electronic and spectroscopic techniques. Spectroscopic method is used to study many film properties which include thickness, film composition, nature of film, crystallographic orientations, lattice parameters, crystallite size and preferred orientations.
CHAPTER FIVE
Results and Observations
Optical properties of Iron Copper Sulphide (FeCuS)
The optical properties of the films studied are presented below:
Absorbance (A)
The absorbances of the specimens plotted against wavelength are presented in fig. 5.1.
The spectral absorbance of the specimens vary with wavelengths in the same manner, increasing rapidly from a value of about 0.24 at 260nm to maximum values of 0.42 at 280nm for FeCuS1, 0.72 at 300nm for FeCuS2 and FeCuS3. From the maxima, it decreases sharply to about 0.2 at 400nm for FeCuS1, 0.2 at 440nm for FeCuS2 and 0.2 at 420nm for FeCuS3 and thereafter decreases gently with wavelength.
CHAPTER SIX
ANALYSIS AND DISCUSSION
The Spectral Analysis
The five categories of thin films grown ( i.e. FeCuS, FeZnS, PbAgS, CuAgS and CuZnS), absorbance was high in UV and low in VIS-NIR region, while the transmittances were low in UV-region and high in VIS-NIR regions. The reflectances were high in UV-region and low in the VIS-NIR regions. As a result of this property, UV radiation is screened off and the infrared and visible radiation is admitted into the building by the films. The films are also suitable for eye glass coating for protection. These findings are in agreement with that of Ezema (2004) for the films FeCdS Ezema et al (2006) for the film MgCdS.
CHAPTER SEVEN
CONCLUSION AND RECOMMENDATION
Conclusion
New ternary thin films of iron copper sulphide, iron zinc sulphide, lead silver sulphide, copper silver sulphide and copper zinc sulphide using solution growth technique (SGT) have been grown and characterized. The deposited films were characterized using UNICO-UV-2102 PC Spectrophotometer and Olympus PMG which showed that the films have crystal structure.
The following properties were studied: absorbance, transmittance, reflectance, absorption coefficient, refractive index, extinction coefficient, optical conductivity, thickness, band gap energy, and dielectric constant.
- The spectral analysis revealed that FeCuS, FeZnS, PbAgS, CuAgS and CuZnS films have high absorbance in the UV-region and low absorbance in the VIS-NIR-regions. They have low transmittance in the UV-region and high transmittance in the VIS-NIR-regions. FeCuS, FeZnS, CuAgS and CuZnS films have high reflectance in the UV-region and low reflectance in the VIS-NIR-regions, while PbAgS film has low reflectance in UV-region and high reflectance in the VIS-NIR-regions.
- The other optical and solid state properties revealed that (1) FeCuS films have absorption coefficient (α ) ranging from 0.1×106to 1.6×106m-1, while the refractive index (n) ranged from 1.2 to 2.3. The optical conductivity (σo ) ranged from 0.03×1014s-1 to 0.6×1014s-1, while the extinction coefficient (k) ranged from 0.005 to 0.038. The direct band gap (Eg) ranged from 2.4eV to 2.8eV and the indirect band gap (Eg) ranged from 0.6eV to 1.0eV, while the real and imaginary parts of the dielectric constant (ε ) ranged from 1.4 to 5.2 and 0.008 to 0.136 respectively. The thickness (t) ranged from 0.0103µm to 0.873µm.
- FeZnS films have absorption coefficient (α ) ranging from 0.2×106m-1, while the refractive index ranged from 0.72 to 2.3. The optical conductivity (σo) ranged from 0.07×1014s-1to 0.6×1014s-1 while the extinction coefficient (k) ranged from 0.004 to 0.056. The direct and indirect band gap energy (Eg) were 2.9eV and 1.9eV respectively, while the real and imaginary part of the dielectric constant (ε ) ranged from 0.7 to 5.2 and 0.008 to 0.164 respectively. The thickness (t) ranged from 0.0899µm to 1.25µm.
- PbAgS films have absorption coefficient (α) ranging from 0.5×106m-1to 0.9×106m-1, while the refractive index ranged from 0.1 to 2.3. The optical conductivity (σo ) ranged from 0.06×1014s-1 to 0.6×1014s-1, while the extinction coefficient (k) ranged from 0.010 to 0.140. The direct and indirect band gap energy (Eg) ranged from 1.5eV to 2.1eV and 0.3eV to 0.8eV respectively, while the real and imaginary parts of the dielectric constant (ε ) ranged from 0.4 to 5.2 and 0.010 to 0.390 respectively. The thickness (t) ranged from 0.0103µm to 0.873µm.
- CuAgS films have absorption coefficient (α) ranging from 0.5×106m-1to 1.28×106m-1, while the refractive index (n) ranged from 1.94 to 2.28. The optical conductivity ((σo ) ranged from 0.24×1014s-1 to 0.6×1014s-1,while the extinction coefficient (k) ranged from 0.025 to 0.064. The direct and indirect band gap energy (Eg) were 2.3eV and 1.1eV respectively, while the real and imaginary parts of the dielectric constant (ε) ranged from 3.8 to 5.2 and 0.100 to 0.290 respectively. The thickness (t) ranged from 0.0103µm to 0.592µm.
- CuZnS films have absorption coefficient ((σo) ranging from 0.24×106m-1to 1.6×106m-1, while the refractive index (n) ranged from 1.6 to 2.3. The optical conductivity (σo) ranged from 0.12×1014s-1 to 0.6×1014s-1, while the extinction coefficient (k) ranged from 0.008 to 0.082. The direct and indirect band gap energy (Eg) ranged from 2.2eV to 2.4eV and 0.4eV to 0.9eV respectively. The thickness ranged from 0.048µm to 0.627µm.
From these results, it was observed that the films have the property of screening off UV portion of the electromagnetic radiation by absorbing and reflecting and the admittance of the visible and infrared radiation by transmission. These properties confirm the films good materials for coating poultry buildings, eye glasses coating, solar thermal conversion, solar control, anti-reflection coating and solar cells fabrication. Comparing the maximum absorption coefficient of the five films grown, it was observed that FeZnS has the highest value followed by FeCuS, CuZnS, CuAgS and PbAgS in that order. While the maximum refractive index of the films is of the same value of 2.3 except for CuAgS with value of 2.28. The maximum value of the optical conductivity for the films is the same. PbAgS films has the highest value of extinction coefficient followed by CuZnS, CuAgS, FeZnS and FeCuS in that order. While the maximum value of 5.2 for the real part of the dielectric constant are the same for all the films. The values of band gap energy, imaginary part of dielectric constant and thickness vary from one film to another. We believe that the similarities are due to the fact that the same substrate (glass) was used for the growing of the films.
The films were found to be photo-conducting with voltage ranging from 0.1mV to 0.6mV inside the room and 0.6mV to 15mV outside the room around 9.30 am.
The various films grown in this work could be used in the following areas: (a) In agricultural industry such as green house effect, (b) In car industry such as anti-dazzling on the windscreen, (c) In electronics industry such as in camera lens and coating of eye glasses and (d) in architectural industry such as poultry house and coating of windows and doors for passive heating and cooling.
The method of growing thin films may be similar all over the scientific world but the films grown are different. This work was based on growing new sets of thin films of different substances using solution growth technique (SGT).
Recommendation
1 It is recommended that other methods of deposition such as physical vapour deposition (sputtering) etc should be used. The result obtained will be compared with the results of this work
2 Testing of new combinations of semi-conducting materials and electrolyte-solvent systems
3 Using other types of substrates such as metals etc
4 Assembling of the solar cells into modules
5 Development of assembling technologies
6 Using concentration greater than 0.5mole for the various materials used
7 To look into the possibility of using other group III –VIII elements for thin film growth
8 To look into the possibility of coating the windows and doors for passive cooling.
REFERENCES
- Agoston, G.A (1977), Colour theory and its applications in art and design, Springer-Verlag, Berlin
- Anderson, W. W. (1982), Analysis of dispersion in the measured capacitance of CdS/Cu2S solar Cells, Solar Cell 6 (2), 97
- Arthur, P. and O. M. Smith (1964), Qualitative analysis 2nd ed. McGraw-Hill Book Co. Inc. New York 105-115
- Berman, R. (1976), Thermal conduction in solids, Claredon Press Oxford
- Bertin, E. P (1975), Principles and practice of X-spectrometric analysis 2nd edn, Plenum New York
- Bhattacharya, R. N. and P. Pramanik (1983), Growth kinetics and polymorphism of chemically deposited CdS films, Sol. Ener. Mater. 7, 431
- Bhattacharya, S., P. Pramanick and P. K. Basa (1987), Solution growth technique for deposition of cobalt solenoid thin films, Thin Solid Films Lett. 149, 181
- Bhattacharya, R. N. and P. Pramanik (1982), A photoelectrochemical cell based on chemically deposited Sb2Se3 thin film electrode and dependence, Sol. Ener. Mater. 6, 317
- Bindu, K., J. Campos, M.T.S. Nair, A. Sanchez and P. K. Nair (2005), Semiconducting AgSbSe2 thin film and its application in photovoltaic structure, Semicond. Sci. Technol. 20, 496
- Blatt, F. J. (1968), Physics of electronic conduction in solids, McGraw-Hill Book Company New York, 335-350
