1.1 Introduction
Lead sulphide is a unique direct band gap material which has developed a lot research interest especially in the fabrication of infrared detection and solar cell applications. It is a narrow energy band gap semiconductor of approximately 0.4eV at 300K and Bohr radius of 18nm (Pentia et al., 2001). Lead sulphide (PbS) thin film have been projected as a material of interest to study quantum size effect in recent years (Nanda et al., 2002). In recent years, there has been increasing interest in IV–VI semiconductor materials for their application in optoelectronic and photovoltaic industries. One of the most promising alternative materials is lead sulfide (PbS) thin films owing to their optical and electrical properties being different from those of their bulk counterparts, due to the quantum confinement effect (Gode, et al., 2015).
As a prelude to other parts of this study, this chapter will discuss the background upon which this study was initiated, the statement of problems that led to this study, the Aim and Objectives of the study. Others are Significance of the study, Scope of work, Limitations of the Study and Definition of technical terms.
1.2 Background of Study
The chemical bath deposition (CBD) method is attracting considerable attention, as it does not require sophisticated instrumentation. It is relatively cheap, simple to handle, convenient for large area deposition and capable of yielding good quality thin films (Uhuegbu, 2011)
Thin films of PbS have been prepared with various physical and chemical thin film deposition techniques, such as chemical bath deposition (CBD), electro-deposition (ED), chemical spray deposition (CSP), successive Ionic layer adsorption and reaction (SILAR), electrochemical atomic layer epitaxy (EC-ALE), atomic layer decomposition (ALD) and thermal evaporation technique. Among these different techniques, bath technique is advantageous on account of suitable method in the country. It also allow a uniform wide area deposition (Ahuome and Onimsi, 2016).
In view of the rapid depletion of existing conventional energy resources, due to the high rate of consumption the world over, it’s obvious that mankind is heading towards an energy crisis. Today energy consumption per capita is synonymous with the standard of living of a nation and raise world population, standard of living of developed and developing nation and the trend towards higher technologies on the increases. It is evident that our oil wells, the source of the world well-known major energy resources petroleum will sooner or later dry up. To avert this impending energy crisis and further meet up with energy needs of nations, renewable energy technological revolution is the only answer (Cosmas, 1989).
Renewable energy source that can be developed include non-exhaustible energy source such as sunlight, ocean currents, waves, falling water, wind natural energy stocks whose replenishment is far greater than projected human use. Of this wide range of energy choice, sunlight, or solar energy is certainly one of the most attractive. Since the solar energy is more abundant in the poorer and less developed countries of the world, it is hoped that if this particular source of energy is researched into a developed the bridging of the technological gap between the third world countries and most of the developing countries is then feasible.
Semiconductor materials are always the focus in material science due to their outstanding electronic and structural properties and have potential application in various devices such as light emitting diodes, single electron transistors, and field effect thin film transistors. In principle, the electronic and structural properties of semiconductor materials are tunable by varying their shapes and sizes. So it is one of the desired goals in material science to have precise control of the morphology of semiconductor materials. As an important IV-VI group semiconductor, Lead sulfide (PbS) has attracted considerable attention due to its small direct band-gap (0.41eV at 300K) and a large excitation Bohr radius of 18nm. Lead sulfide (PbS) is an important direct narrow gap semiconductor material with a band gap of 0.4eV and has a cubic structure. Due to their suitable band gaps, PbS thin films are extensively used in IR detectors. Thin film of lead sulfide was establish to have very significant application in the manufacture of photoconductive infrared detectors, transistors, contact rectifiers, prisons, lenses, windows and other components of optical system. This material has also been used in many fields such as humidity, photography, solar absorption photo-resistance, diode lasers, and temperature sensors, decorative and solar control coatings.
The availability of raw materials is an important factor in fabricating solar cells for solar energy conversion. Thin film devices will typically be about 5µm to 50µm thick, in contrast to bulk devices which are about 150µm to 250µm thick, It shall be pointed out that the ultimate lower limit of the cost of bulk devices is defined by the cost of the wafer itself and thus cost lowering below the price of wafers is thus not possible. Hence, even for Si, a thin film technology needs to be developed to meet the cost goals (Cosmas, 1989).
Apart from saving cost as given above conversion efficiencies of these cells have been improved upon. In recent years, efficiencies of 10 to 17% or more in crystallized and/or epitaxial thin Si and GaAs solar cells respectively, have been made possible.
n comparison to thin film CuS/CdS cells for example, the production of bulk Si (primarily single crystal) solar cells is in the range of 5MW internationally for large scale applications, Si production capability does not exist and the trend today is toward ribbon technology. However, single crystal ribbon technology is energy intensive and costly and requires a large material input. Although the scope for immediate application exists, the long range potential is low. On the other hand, thin film a Silicon solar cells offer a very promising future, at present Sanyo, Fuji, and Sharp of Japan are producing 2 to 3 MW/year of such cell for low power electronic applications.
The simple production involved and the simple module/panel fabrication make thin film solar cells the only viable system. To further stress the importance, I quote Ehrenrich, “the potential payoff would be immense if truly inexpensive technology based on thin film system were to be developed” (Cosmas, 1989). Finally, the Pbs thin film is a p-type semiconductor with a crystal structure of centered cubic structure, looking at the uses, benefits and applications of the thin films in different areas of technology to be given shortly, one does not have any other alternative for technological breakthrough than the „thin film technology'.
Therefore, in Nigeria where the research was carried out, the activities that was conducted is to study the Structural and Electrical Properties of Lead Sulphide (PBS) Thin Film Deposited through Chemical Bath Deposition.
1.3 Statement of Problems
Investigation revealed that there is a problem of electric power crisis. Non uniformity and poor adhesion were the common problems occurred when the thin films were deposited on smooth substrates by chemical bath deposition method. Hence, much attention was paid on the cleaning and activating the substrate surface. The selection of the substrate and methods used for the cleaning of the surface of the substrate are very important in the formation of thin films with reproducible process. The contaminants present in the surface of the substrate can be broadly categorized into two types, namely organic and inorganic. Organic contaminants can be easily removed by emulsifying the surface of the substrate with washing solutions. However, for the removal of inorganic contaminants a direct mechanical approach is needed when they are in particle form.
1.4 Aim and Objectives of Study
The aim of the study is to Study the Structural and Electrical Properties of Lead Sulphide (PBS) Thin Film Deposited through Chemical Bath Deposition. In achieving this aim, the following specific objectives were laid out as follows:
- To study the components of the deposited layer through X-ray diffraction meter.
- To deposit lead sulphide thin film through chemical bath decomposition (CBD).
- To study its thickness of deposited layer.
- To investigate its charge carrier mobility.
- To deposit lead sulphide (PbS) thin film and study its electrical and structural properties, as a solar cell material.
1.5 Significance of Study
This study will be of immense benefit to researchers who intend to know more on this study and can also be used by non-researchers to build more on their research work. This study contributes to knowledge and could serve as a guide for other study.
1.6 Scope of Study
The scope of the research is focused on the Study of Structural and Electrical Properties of Lead Sulphide (PBS) Thin Film Deposited through Chemical Bath Deposition.
This work only deals with the internal parameter of lead sulphide thin film such as thickness of the deposited layer analysis, charge carrier mobility and X-ray diffraction analysis in dark. This work does not involve investigation of above properties of PbS thin films under illumination meaning that it does not discuss the external parameter.
1.7 Limitations of the Study
During the course of this study, many things militated against its completion, some of which are:
- Time Constraint: The time frame given to accomplish this project was very short due to school academic calendar and it was carried out under pressure which made the researcher not to implement some necessary features.
- Research material: availability of research material is a major setback to the scope of the study.
- Frequent power failure: This made the researcher append more money on fuel to ensure sustainable power.
- Financial Constraint: Insufficient fund tends to impede the efficiency of the researcher in sourcing for the relevant materials, literature or information and in the process of data collection (internet, questionnaire and interview).
1.8 Definition of Terms
Lead Sulphide: Is an inorganic compound and semi conducting material with niche uses, with the formula (PbS).
Thin Film: Is defined as a thin material created animation by an atom, molecule, ion or cluster of species as condensation process. Film materials may also be formed from a liquid or a paste, in which case it is called a thick film. It is net the thickness that is important in defining a film but rather the way it is created with the consequential effects on its micro structure and properties.
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