The Study Of Structural And Electrical Properties Of Lead Sulphide (PBS) Thin Film Deposited Through Chemical Bath Deposition
Lead sulfide (PbS) thin films were prepared on glass substrates at room temperature by chemical bath deposition (CBD) technique. The influence of dip times on the lead sulphide (PbS) thin films deposited on glass slide substrates via chemical bath deposition (CBD) technique using chemical precursors, nitrate Pb(NO3)2, and thiourea SC(NH2)2, and sodium hydroxide (NaOH) at room temperature was investigated. The thicknesses of the layers were measured as the structural property, the thickness of , and were obtained for the time deposits of 60, 80, and 100 minutes respectively. The charge carrier mobility µ was studied as the electrical properties and the charge carrier mobility (µ) of , and for the time deposits of 60, 80 and 100 minutes respectively.
1.1 Background of the Study
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 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).
As I have earlier mentioned, 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.
In 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’.
1.2 Applications of Thin Films
Metallic films are used as electrical conductors in numerous micro-electronic devices. For most applications of silicon semiconductor devices the conducting films used (usually referred to as metallization) are 1-2µm thick. The films can be as narrow as 1 or 2µm but occasionally they are several millimeters wide.
Thin films can also be used in information recording systems based on light or electron recording. The recorded data appears as a change in electrical or optical properties of the film. Reflective films have been used in photo-plastic recording. This is a method of recording light images in the form of surface deformations in a photoconductive thermoplastic film. Also ferroelectric films are used in display devices like the microwave capacitors, thermistors, bolometer, pyro-electrics, piezoelectric transducers, and optical display devices.
Thin films of silver and lead halides as well as arsenic sulphide are made use of as photographic information storage devices.
It is easy to carry out absorption, reflection, emission, and generally optical studies for visible or near ultraviolet radiation because of availability of good filters, compensators, or transmission polarizers. Studies below the 2000Ȧ wavelength are not possible because of lack of equivalent filters, compensators, and polarizers but the use of thin film in ultraviolet spectroscopy eliminates some of the difficulties encountered in determining the optical constants of materials. With the thin films, the design of good filters, polarizers, and compensators is possible for optical studies under high intensity synchrotron radiation and for the use of the hydrogen laser which operates at 1600Ȧ by solid state physicists and biophysicists i n research centers and laboratories.
A practical application of Yttria (Y203) films as a dielectric for thin film capacitors has been reported from Japan. Dielectric thin films have attracted attention as convenient media for
surface propagation of optical waves, the technique of which is used in two dimensional integrated optical circuits like in the two dimensional processors. Wave guide characteristics of the films (refractive index and attenuation) permit coloration between the optical and electrical properties of thin film to be made.
It is very interesting and exciting to observe that superconducting transition temperatures, of some materials are raised after depositing these materials on substrates. For instance the transition temperatures of Cd and Zn have been raised from 0.3K to 0.9K, and 0.8K to 1.5K respectively. The for Cd condensed on a substrate held at that 0.3K is almost twice the one of
the bulk Cd which is 0.53K. Though the increase in the is small, it can be noted that if efforts by solid state physicists are geared towards this direction, definitely there should be a remarkable breakthrough in superconductivity studies in no distant future. Then the problem of electric power crisis will have been solved and man will therefore live to enjoy.
1.3 Aim and Objectives of the Study
The aim of this work is:
To deposit lead sulphide (PbS) thin film and study its electrical and structural properties, as a solar cell material.
Objectives of the work include:
- 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 study the components of the deposited layer through X-ray diffraction meter.
1.4 Scope and Limitations
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.
1.5 Definition of Terms
i. Lead Sulphide:
Is an inorganic compound and semi conducting material with niche uses, with the formula (PbS).
ii. Solar Cells:
Are semi-conducting device that converts the radiant energy of sunlight in to electrical energy.
Are substance with electrical properties intermediate between a good conductor and a good insulator.
Are materials that transmit or allow electricity, heat, light and sound through it.
Are non-conductive materials that do not transmit sound or electricity.
vi. Band Gap:
Is the energy difference between the valence band and conduction band.
Is the phenomenon associated with electromagnetic radiation. It is produced by the photo excitation of material, resulting in the production of thermal energy.
Deals conversion of light into electricity using semi conducting materials that exhibit the photovoltaic effect.
The production of a thin film of material onto an existing material.
x. Chemical Bath Deposition:
Is a method to deposit thin semiconductor films are deposited on substrates immersed in dilute solutions.
xi. X-Ray Diffraction:
X-ray diffraction is the most widely used characterization technique for the determination of crystallographic structure of the films.
xii. 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.
Summary, Conclusion and Recommendations
CBD, a novel and simple method was used to prepare PbS thin film. The deposition is uncomplicated, environmentally friendly and economical. It increases the utilization efficiency of chemicals, simplifies the experimental process, and reduces cost. The structural property and electrical properties studies revealed that the PbS thin film could be suitably employed in window layer for solar cell.
However, the thickness of the layers were measured as the structural property, the thicknesses of
the layers were measured using gravimetric method and thickness of , and for the time deposits of 60, 80, and 100 minutes respectively. And the charge carrier mobility µ was used to analyzed the electrical properties, the charge carrier mobility µ of , and for the time deposits of 60, 80 and 100 minutes respectively.
Lead sulfide (PbS) thin films were successfully deposited on glass substrates by chemical bath deposition (CBD) method. The structural and electrical studies of the thin films were carried out. The thickness of the layers were measured as the structural property, the thicknesses of the layers
were measured using gravimetric method and thickness of, and for the time deposits of 60, 80, and 100 minutes respectively. And the charge carrier mobility µ were measured as the electrical properties and the charge carrier mobility µ of, and for the time deposits of 60, 80 and 100 minutes respectively.
However, physically, the colour of the deposited specular reflecting thin film is deep dark brown, i can conclusively said that the structural property and electrical properties studies revealed that the PbS thin film could be suitably employed in window layer for solar cell.
- It is interesting to note that since we are blessed with abundant free solar energy, being very close to the equator, all hands must be on deck to research in to various ways of tapping this energy for a possible and effective replacement of the world well known major energy resource petroleum threatened by world population explosion.
- Specific areas of research should include development of films of all chalcogenide and halogens of metals and alkali metals especially the cadmium sulphide for photovoltaic cells.
- Of special interest is the development of halogens of alkali metals known to possess enviable property of super ionic conductivity in thin forms, A possible breakthrough in superconductivity in metals can also be possible if thin films of some metals like zinc are deposited on certain substrates, This is because the transition temperature is usually raised when deposition is achieved.
- Last but not the least, doping profiles or multiple layers tried by structures including hetrojunction of films should be suitably modifying solution composition and concentration.
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