CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

Conventional energy resources are being depleted quickly because of high consumption worldwide, and the world is heading toward an energy crisis. Energy consumption per capita is now closely tied to a nation's standard of living, and demand keeps rising with population growth, with rising living standards in developed and developing countries, and with the move toward more advanced technologies. Oil, the best-known conventional energy resource, will eventually run out. Meeting the world's energy needs will therefore depend heavily on renewable energy technologies. Chanchangi et al. (2023) point out that access to stable and reliable energy is crucial to economic development and living standards, and that in Nigeria rapid population growth has pushed energy demand beyond supply. They also argue that the economic and environmental merits of photovoltaic devices make them a suitable clean energy option for developing countries such as Nigeria.

Renewable energy sources are those whose natural replenishment far exceeds projected human use, such as sunlight, ocean currents, waves, falling water and wind. Of these, solar energy is among the most attractive. It is also more abundant in poorer and less developed countries, so if it is researched and developed, it can help to narrow the technological gap between those countries and the developed world.

Semiconductor materials are a constant focus of materials science because of their electronic and structural properties and their use in devices such as light-emitting diodes, single-electron transistors and thin-film field-effect transistors. In principle these properties can be tuned by changing the shape and size of the material, so precise control of morphology is an important goal. Lead sulphide (PbS) is a IV–VI semiconductor that has received considerable attention. Elete et al. (2023) note that it has excellent optoelectronic properties and finds use in light-emitting diodes, photovoltaic cells, sensors and lasers, and that it crystallises in the cubic (halite) structure with a lattice constant of about 5.936 Å. As a narrow direct band-gap material, bulk PbS has a band gap of about 0.41 eV, and Altiokka and Onal (2023) report that the band gap of PbS thin films can range from 0.41 to 2.3 eV depending on crystallite size. Because of these suitable band gaps, PbS thin films are widely used in infrared detectors, and Altiokka and Onal (2023) add that they have also been preferred for solar energy converters, optical switches and chemical, temperature and flame sensors.

PbS thin films can be prepared by many physical and chemical techniques, including electrodeposition, spray pyrolysis, successive ionic layer adsorption and reaction (SILAR), chemical vapour deposition, thermal evaporation and chemical bath deposition (CBD) (Altiokka & Onal, 2023). Among these, CBD attracts particular interest because it needs no sophisticated instruments. Elete et al. (2023) describe it as a cost-effective method that is well suited to large-scale production, and Karabulut et al. (2014) note that it can grow PbS films at low temperatures and is relatively simple. It is also convenient for large-area deposition and capable of yielding good-quality films. These features matter for a country like Nigeria, where the availability of raw materials and affordable equipment is an important factor in fabricating solar cells. Thin-film devices are also far thinner than bulk devices, which reduces the quantity of material required.

Understanding the structure and electrical behaviour of CBD-grown PbS films is essential for their use in devices. In an investigation of PbS films grown on glass and GaSe crystal substrates, Karabulut et al. (2014) examined structural, morphological, optical and electrical properties and found a cubic structure with a lattice constant of about 5.93 Å on glass. Elete et al. (2023) used X-ray diffraction and four-point probe measurements to study copper-doped PbS films grown by CBD in Nigeria, and found a polycrystalline structure and a resistivity that changed with dopant concentration. This study builds on such work by examining the structural and electrical properties of PbS thin films deposited by chemical bath deposition.

1.2 Statement of the Problem

Compound semiconductors offer a wide range of materials with different band gaps for solid-state devices. These devices are usually made as thin films, and the crystalline quality and stoichiometry of the films strongly influence the stability and performance of the device. Vacuum deposition is a popular and reliable way of preparing thin films, but it has a well-known drawback: compound semiconductors tend to dissociate thermally into their constituent elements during evaporation. Elements with different vapour pressures, such as sulphur and metals, are difficult to deposit in the correct proportions, and vacuum-evaporated sulphide films are often non-stoichiometric and deficient in sulphur. Vacuum methods also require expensive and sophisticated equipment that many laboratories in developing countries cannot afford.

Chemical bath deposition offers a simple and inexpensive alternative. However, the properties of chemically deposited PbS films depend strongly on the growth conditions, and their electrical behaviour is that of polycrystalline material built from grains and grain boundaries. There is therefore a need to deposit PbS films by CBD under controlled conditions and to establish their crystal structure and electrical properties. This study addresses that need by depositing PbS thin films by chemical bath deposition and characterising their structural and electrical properties.

1.3 Objectives of the Study

The general objective is to study the structural and electrical properties of lead sulphide (PbS) thin films deposited by chemical bath deposition. The specific objectives are to:

1. synthesise PbS thin films by the chemical bath deposition technique and understand their growth mechanism;
2. investigate the effect of the deposition conditions on the growth and properties of the PbS thin films;
3. determine the structural properties and crystallite size of the films from X-ray diffraction (XRD) patterns; and
4. study the electrical properties of the films from their current–voltage (I–V) characteristics.

1.4 Significance of the Study

Lead sulphide is an important IV–VI chalcogenide semiconductor that has attracted considerable attention because of its optical and optoelectronic properties and its applications in solar cells, optoelectronic devices, photoconductors, sensors and infrared detectors (Altiokka & Onal, 2023; Elete et al., 2023). Its properties depend on how it is prepared, so careful characterisation of films made by a particular route is of practical value.

Chemical bath deposition is a very simple, cost-effective method that is convenient for large-area scaling and can produce good-quality films whose properties compare well with those from other methods (Elete et al., 2023; Karabulut et al., 2014). Since CBD allows crystallite characteristics to be varied by controlling the deposition parameters, a study of the structural and electrical properties of CBD-grown PbS films will help to guide the choice of deposition conditions for devices. The findings will also benefit researchers in materials science and solar energy, who can use them as a reference, and will support efforts to develop low-cost thin-film technology that can contribute to meeting Nigeria's energy needs.

References

Altiokka, B., & Onal, M. (2023). Production of PbS by the method of chemical bath deposition at relatively low solution temperatures. Journal of Optoelectronics and Advanced Materials, 25(1–2), 42–48.

Chanchangi, Y. N., Adu, F., Ghosh, A., Sundaram, S., & Mallick, T. K. (2023). Nigeria's energy review: Focusing on solar energy potential and penetration. Environment, Development and Sustainability, 25, 5755–5796. https://doi.org/10.1007/s10668-022-02308-4

Elete, D. E., Emegha, J. O., Nenuwe, N. O., & Omagbemi, O. W. (2023). Synthesis and characterization of chemical bath deposited copper doped lead sulfide thin films. Bulletin of the Chemical Society of Ethiopia, 37(5), 1237–1251. https://doi.org/10.4314/bcse.v37i5.15

Karabulut, M., Ertap, H., Mammadov, H., Uğurlu, G., & Öztürk, M. K. (2014). Properties of PbS thin films grown on glass and layered GaSe crystal substrates by chemical bath deposition. Turkish Journal of Physics, 38(1).