Optical and Structural Properties of Copper Aluminum Diselenide (Cualse2) Compound Thin Films
Chapter One
OBJECTIVES OF THE STUDY
To study the optical and structural properties of Copper Aluminum Diselenide (Cualse2) compound thin films.
CHAPTER TWO
LIITERATURE REVIEW
ELECTRO-CONDUCTIVE MATERIALS
Electrical resistivity (also known as resistivity, specific electrical resistance, or volume resistivity) is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. It is commonly represented by the Greek letter ρ (rho) and its SI unit is the ohm metre (Ω. m).
Electrical conductivity or specific conductance is the reciprocal quantity, and measures a material’s ability to conduct an electric current. It is commonly represented by the Greek letter σ, and its SI unit is Siemens per metre (S·m−1):
σ = 1/ ρ
Resistivity of various materials
The conductivity of a solution of water is highly dependent on its concentration of dissolved salts, and other chemical species that ionize in the solution. Electrical conductivity of water samples is used as an indicator of how salt-free, ion-free, or impurity-free the sample is; the purer the water, the lower the conductivity (the higher the resistivity). Conductivity measurements in water are often reported as specific conductance, relative to the conductivity of pure water at 25 °C.
The effective temperature coefficient varies with temperature and purity level of the material. The 20 °C value is only an approximation when used at other temperatures. For example, the coefficient becomes lower at higher temperatures for copper, and the value 0.00427 is commonly specified at 0 °C.
The extremely low resistivity (high conductivity) of silver is characteristic of metals. George Gamow (1947) tidily summed up the nature of the metals’ dealings with electrons in his science-popularizing book, One, Two, Three…Infinity: “The metallic substances differ from all other materials by the fact that the outer shells of their atoms are bound rather loosely, and often let one of their electrons go free. Thus the interior of a metal is filled up with a large number of unattached electrons that travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric force applied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call an electric current”. Table 2.1 shows the resistivity, conductivity and temperature coefficient of various materials at 20 °C (68 °F).
The cold spray process offers advantages to produce, with minimum thermal exposure, coatings from functional materials such as thermoelectric, magneto-caloric, photo-voltaic, piezo-electric, super-magnetic, and high-temperature superconductive formulations.
CHAPTER THREE
RESEARCH METHODOLOGY
CuAlSe2 thin films were prepared by chemical bath deposition technique and studied the effects of deposition temperature (above room temperature (27°C)) on the properties of this material. We report the chemical bath deposition of CuAlSe2 thin films at variable temperature in an alkaline medium (pH = 8.86) at deposition time of 24 hours. The deposition parameter was optimized to obtain good quality thin films. The properties like absorbance, reflectance and transmittance with wavelength were analyzed under varying bath temperature (50°C – 70°C). The film thicknesses of the samples were analyzed with respect to the various deposition temperatures. The optical conductivities and the refractive index against photon energy were determined. The band gap energy was determined from the influence of the deposition temperature.
CHAPTER FOUR
RESULTS AND DISCUSSION
Fig. 1 shows the optical thickness of CuAlSe2 thin films in the wavelength region from 300 to 700 nm at respective deposition temperature of 50°C, 65°C and 70°C. As seen, the film thicknesses increase as the bath temperature increase.
Fig. 2 shows the optical absorption spectrum of the compound thin films, which shows high absorption in the visible range of 420nm to 680nm indicating that these materials can be used for photovoltaic solar cell applications. The graphs show the increasing order of absorbance of samples as 20, 19 and 16 which has deposition temperature of 70°C, 65°C, and 50°C respectively. This shows that at minimum temperature of 50°C, highest absorbance of visible radiation occurred.
The graph of transmittance versus wavelength in fig. 3 shows that it is only sample 16 has low transmittance in general while others have high transmittance at UV region. Sample 19 has highest transmittance. This shows that it can be used for thermal window glass coating.
Chapter Five
CONCLUSION
The films of CuAlSe2 were successfully deposited using chemical deposition technique onto commercial microscope glass substrates at variation of deposition temperature. The optical properties show that films could be useful in photovoltaic solar cell applications and thermal window coatings. The influence of deposition temperature in this research shows that: absorbance and optical conductivity increase as the deposition temperature decrease, but increase in reflectance and film thicknesses bring about increase in bath temperature. The energy band gap is in the range of 2.00 to 2.14 eV which shows that it is potentially useful for blue light-emitting diodes. The band gap energy decreases as the deposition temperature increases. It does agree with the fact that increase in temperature brings about decrease in difference between the conduction band minimum and valence band maximum which is called the energy band gap.
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