1.1 General Introduction

The sun releases an estimated 1017 Joules of energy, which it delivers to earth in one second [1]. Earth’s ultimate recoverable resource of crude oil, estimated at 3 trillion barrels, contains 1.7×1022 joules of energy, which the sun supplies to earth in 1.5 days [1]. The amount of energy humans use annually, about 4.6×1020 Joules, is delivered to earth by the sun in one hour [2]. The enormous power that the sun continuously delivers to earth is 1.2×105 terawatts (TW), dwarfs every other energy source [2], renewable or non renewable. It dramatically exceeds the rate at which human civilization produces and uses energy, currently about 13TW [3]. The impressive supply of solar energy is complemented by its versatility. Sunlight can be converted into electricity by exciting electrons in a solar cell. It can yield chemical fuel via natural photosynthesis in green plants or artificial photosynthesis in human-engineered system’s [4] Concentrated or unconcentrated sunlight can produce heat for direct use or further conversion to electricity. Despite the abundance and versatility of solar energy, we use very little of it to directly power human activities. Solar electricity accounts for a minuscule 0.015% of world electricity production, and solar heat for 0.3% of global heating of space and water [5]. Biomass produced by natural photosynthesis is by far the largest use of solar energy; its combustion or gasification accounts for about 11% of human energy need [6].  However, more than two-third of that is gathered unsustainably, that is, with no replacement plan and burned in small, inefficient stores where combustion is and the resulting pollutants are uncontrolled.

               Between 80% and 85% of world energy comes from fossil fuels, a product of ancient biomass stored beneath earth’s surface for up to 200 million years [7]. Fossil fuel resources are of finite extent and are distributed unevenly beneath earth’s surface. When fossil fuels are converted into useful energy through combustion they produce green house gases and other harmful environmental pollutants. In contrast, solar photons are inexhaustible and unrestricted by geographical boundaries. Their direct use for energy production does not threaten heat or climate. The solar resources, magnitude, wide availability, versatility and benign effect on the environment and climate make it an appealing energy source. It is anticipated that by the year 2030 the world demand for electricity will double and the demands for fuel and heat will increase by 60% [7]. The utilization gap between solar energy’s potential and our use of it can be overcome by raising the efficiency of the conversion processes.

              The use of photovoltaic (PV) modules for electricity generation has come under intensive research in recent times, with a view to increasing the efficiency and reducing investment capital [8]. Generating significant fraction of future energy requirement from PVs is a major challenge to solar energy physicists [9], particularly because present PV production is almost insignificant relative to fossil fuel-based generation [10].

        1.2   Purpose of the Study

    The purpose of the study is to:

  •  deposit cadmium oxide (CdO) thin film on glass substrate using the Successive Ionic Layer Adsorption and Reaction techniques (SILAR), for two complexing agents,NH3 and NaOH.
  • characterize the deposited CdO films using the UV/VIS/NIR spectroscopy.
  • investigate the effects of the number of cycles on the band gaps of the deposited CdO thin film.
  • study the variation between the optical conductivity and the band gap energy of the film.
  • determine the relationship between the dielectric function of the material and the band gap energy and number of cycles.
  • investigate the effect of complexing agents on deposition of CdO films.
  • to study the structural properties of the deposited films.

         1.3 Scope of the Study

               This research work is limited to the growth and characterization of CdO thin films grown on glass substrate using SILAR technique. The deposition of CdO will be carried out using ammonia (NH3) and sodium hydroxide (NaOH) as  complexing agents.

                                                               CHAPTER TWO

                                                            LITERATURE REVIEW

During the last few years, CdO has revealed itself as a very promising material for use in the photovoltaic industry. Because of its high optical transparency in the spectral region of the solar radiation, electrical conductivity (in the absence of doping), refractive index between those of air and CdTe and a good match for CdTe lattice, it may also be used as a substitute for CdS and SnO2 in the SnO2/CdS/CdTe photovoltaic heterostructures [11]. In the last ten years, the optical and electrical properties of CdO thin films prepared by various techniques such as spray pyrolysis [12], oxidation of cadmium films [13], chemical vapour deposition [14] and many others have been studied [15]. It was experimentally proven that these properties are very sensitive to the film structure and deposition conditions [16].

               R,S  Rusu and G.I Rusu  worked on the electrical and optical characteristics of CdO thin films. In their work, CdO thin film of thickness range of 0.15mm to 0.70mm were deposited by thermal evaporation under vacuum onto glass substrates kept at 300K and 473K respectively. Depending on the substrate temperature, films with polycrystalline or amorphous structured were obtained, the influence of temperature and post deposition heat treatment on the electrical conductivity (s) and optical transmittance (T) in visible region was investigated. An irreversible temperature dependence of s during heating/cooling cycles of as grown samples were observed and the optical band gap was determined to be   Eg = 2.4eV [17].

                 Hann, Tachen ,lupan ,Dutta and Heinrich deposited CdO using chemical bath deposition (CBD) technique and three different complexing agents; ammonia (NH3), ethanolamine(C2H7NO) and methylamine (C3H9NO). CdSO4 was used as the Cd precursor and H2SO4 as the oxidizing agent. Grown films were cubic CdO2 with some Cd(OH)2 as well as CdO phases being detected. Annealing at 400oC in air for 1hr transformed films into cubic CdO. The calculated optical band gap of as grown film ranges from 3.37eV to 4.64eV. Annealed films have a band gap of about 2.53eV, a carrier density as high as 1.89×1020cm-3 and a resistivity as low as 1.04×10-2Ωcm-1 were obtained [18].

               Mohaboob deposited CdO using the SILAR approach, Cadmium acetate {Cd(CH3COO)2}was used as a source of CdO and ammonium hydroxide (NaOH) was the complexing agent. The study determined the effect of molarity of solution on the structural, optical and morphological properties of deposited films.x-ray diffraction  (XRD) and scanning electron microscopy (SEM) revealed that the crystallite size is increased with molarity of the precursor solution. UV/VIS spectrum of the films showed that the optical band gap energy increases with concentration of cadmium acetate in the precursor solution [19].      

               Balu, Nagarathinam, Suganya, Arunkumar and Selvan also, deposited CdO using the SILAR technique; the effect of solution concentration on the structural, optical, electrical, and morphological properties of the deposited sample was analyzed. The structural study revealed that the films were polycrystalline with preferred orientation along the 200 plane. The lattice parameter was found to be equal to 4.690A, grain size increased from 15.96nm to 21.00nm as the solution concentration increased. Optical absorption measurements showed that the films coated with 0.05M had a maximum transmittance of 84%, band gap energy of the coated films decreased with the increase in solution concentration. It was recorded that the sheet resistance increased from 14×102Ω/m2 to 17.5×102Ω/m2 as the concentration increased from 0.05M to 0.2M. Film coated with 0.1M had low temperature coefficient of resistance -1.75×10-3/K [20].

The use of thin film semiconductors has attracted much interest in an expanding variety of applications in various electronic and opto-electronic devices due to their low production costs. It is also common belief that lowering PV costs to a level competitive with conventional power sources will require significant reduction in manufacturing cost, which may be realized in thin films, and or new architectures that lead to dramatic improvement in efficiency. Cadmium(Cd) is one of the most promising Group 11B with the following characteristics: symbol Cd, atomic number 48, atomic weight 112.41,  this metallic element with ionization potential of 8.994eV, electron configuration {kr}4d105S2; valency state +2, standard electrode potential, E0-0.404 [21].