Solar cell use SWCNTs and MoS2 Essay Example
Overview (World consumption of energy and fossil fuel)
The primary sources of energy in the world are coal, hydroelectricity, fossil fuels, geothermal, nuclear, natural gas, and wind, marine, solar and bio-energy. There has been an upward trend in the world’s energy consumption as a result of increasing populations, modern technologies and improved living standards in developing countries. The International Energy Outlook 2016 projects that there will be a significant growth in world demand of energy over a 28-year period, 2012-2040. It is estimated that the world consumption of energy will expand from 549 quadrillion Btu (British thermal units) in 2012, to about 629 quadrillion Btu by 2020, and up to 815 quadrillion Btu by 2040. This makes a 48% increase in the world consumption of marketed energy from 2012 to 2040 (see figure 1) .
Figure 1: Projected world energy consumption from 2012 to 2040 (Source: 12).
In terms of world energy by consumption by source, fossil fuels continue to account for most of the world’s energy demand, while coal remains to be the slowest growing energy source. As the world demand for energy continues to increase, there is more pressure on fossil fuels, a situation that may see the resource beginning to diminish.
A solar cell is an electrical device that converts light energy into electricity by a physical and chemical phenomenon called the photovoltaic effect. Solar cells form the building blocks of solar panels that are used to provide renewable solar energy for both domestic and commercial purposes. Irrespective of whether the source is an artificial light or sunlight, solar cells are described as voltaic. They are used as photodetectors of electromagnetic radiation near the visible light. For a photovoltaic cell to operate, there are three basic attributes required: absorption of light, separation of charge carriers, and separate extraction of the charge carriers to an external circuit.
Single wall carbon nanotube
History and Overview
Carbon nanotubes are allotropes of carbon. They have a cylindrical nanostructure with unusual properties for applications in areas such as nanotechnology, optics, electronics and other areas of material science and technology. Carbon nanotubes are either single-walled or multi-walled, depending on the number of rolled layers of grapheme . The discovery of CNTs is credited to L. V. Radushkevich and V. M. Lukyanovich, who first published images of carbon nanometer diameter tubes in the Soviet Journal of Physical Chemistry in 1952.
Structure and Properties
Most SWNTs have a diameter of approximately 1 nanometer and can be several millions of times longer. Their structure can be conceptualized by wrapping a layer of one-atom-thick grapheme into a seamless cylinder. A pair of indices (n, m) is used to represent the manner in which the grapheme sheet is wrapped. The indices n, m represent the number of unit vectors in the honeycomb crystal lattice of graphine. If m=n, the nanotubes are known as armchair, if m=0, the nanotubes are known as zigzag. Otherwise, they are referred to as chiral.
SWCNTs have good mechanical properties – they are strong and stiff materials with good tensile strength and elastic modulus . Unlike grapheme, CNTs are either metallic or semiconducting along the tubular axis. SWCNT is metallic and have an electric current density as high as 4 × 109 A/cm2. These materials also have useful photoluminescence, Raman spectroscopy and absorption properties .
There are various methods used to produce SWCNTs and nanotubes in general. These include: arc discharge, chemical vapor deposition (CVD), high-pressure carbon monoxide absorption, and laser ablation. Most of these production processes are done with process gases or in a vacuum. The most popular production method is CVD as it produces high purity CNTs with high degree of control over length, diameter and morphology.
Applications of SWCNTs
Bulk CNTs are used as composite fibers in polymers to enhance the electrical, thermal and mechanical properties of the bulk material. Other current applications include: AFM probing tips and scaffold for bone growth in tissue engineering. Potential applications include the field of nano-science in the control of nano-scale structures, production of fiber, electrochemical water treatment, and energy applications.
CNT-Si solar cell
SWCNT-Si p-n junction hybrid solar cells are a new generation of photovoltaic devices with superior opto-electronic properties of CNTs integrated with well-established Si photovoltaic technology . SWNTs have a direct band gap energy matching with a wider range of solar spectrum and superior charge carrier transport properties, which makes them suitable in CNT-Si solar cell applications than double or multi-walled CNTs. CNT films function as transparent conducting front electrodes, a feature that offers high performance of Si solar cells by reducing optical shading . A typical CNT-Si heterojunction has a similar structure as that of a conventional Si-based solar cell – with the expensive p-type Si layer replaced with a conductive transparent CNT film electrode .
Molybdenum disulfide (MoS2)
History and overview
Molybdenum disulphide (MoS2) has become of interest as an alternative semiconductor material for nanoelectronic applications in the next generation of electrical devices due to large band gap and electron mobility, excellent stability and lack of dangling bonds
. The material has been widely studied and finds applications in areas like energy harvesting, field-effect transistors, co-catalysts, optoelectronics, and counter electrodes
. MoS2 has been reported to have an on/off ratio of about 103 and a carrier mobility of about 80 cm2/Vs, indicating that MoS2 is a promising candidate for fabrication of photovoltaic solar cells
Structure and properties
Photoluminescence evolution and band structure:
According to , MoS2 has two excitation peaks associated with the split of energy from valence band spin-orbital coupling. With the MoS2 layers varying from single to multiple layers, there is quantitaive change in its band structure, explaining the prominent resonance effect 
Single layer MOS2 has a wider direct bandgap of 1.8 eV, making it suitable applications in switching nanodevices. In general, MoS2 transistors exhibit the n-type behavour . The switching behavour of an MoS2 – based photoresistor has an outstanding switching character, with photocurrent and annihilation taking place within 50 ms . The photoresponsivity of the MoS2 – based switching devices is much higher compared to graphene based devices. Thus, the MoS2 has bets advantage for applications in transistors, memory devices, and photodetectors .
There are a number of methods used in the production of MoS2. They include: exfoliation method, CVD synthesis method, sulfurization of Mo-based compound, sulfurization of Mo and Mo-based oxides, thermal decomposition of (NH4)2MoS4, vapor-solid growth from MoS2 powder, and direct synthesis of graphene/MoS2 composites .
Application of MoS2
According to , the photovoltaic properties of graphene/MoS2/n-Si solar cells are significantly enhanced when a thin film of MoS2 is inserted between graphene and SiO2 layer (see figure 2). The thin MoS2 film functions as an electron-blocking/hole-transproting layer.The graphene/MoS2/n-Si solar cell exhibits a hihger efficiency of photovoltaic conversion . Therefore, the MoS2 thin film layer between n-Si and graphene contributes to the improvement in photovoltaic performance as a blocking/carrier transport layer. For this reason, MoS2 is used in the fabrication of graphene/MoS2/n-Si solar cells. MoS2-based structures are used in FET, photodetectors, memory devices, and electrodes in Li-ion batteries and in hydrogen evolution reaction .
Figure 2: Schematic diagram of the grapheme/MoS2/n-Si solar cell (Source: ).
Aim of the Project
After preparing the MoS2 monolayer films integrated with SWCNTs, the main objective will be to fabricate thin film solar cells with n-type silicon as the n-type semiconductor, while the SWCNTs as the p-type semiconductor. The MoS2 will be added to to the CNTs-Si based solar cells as an n-type semiconductor. This will result in a new device – the design MoS2/SWCNTs-n-type Si solar cells, which will be of high efficiency at a lower fabrication cost. Thus, hybridizing of CNTs with MoS2 intends to give a novel photovoltaic device. In this research, we intend to fabricate three high performing solar cells; the SWCNT n-type Si solar cells, the hybrid MoS2/SWCNTs-n-type Si solar cells (prepared by
filtering different volumes of MoS2 (100-2000μL) and CNTs
with 300μL as a control volume of SWCNTs), and the layered structure of MoS2/SWCNTs-n-type Si solar cells (prepared by filtering different volumes of MoS2 (100-2000μL) and CNTs with 300μL as a control volume of SWCNTs via vacuum filtration method. After fabricating the cells, the second type and third type were compared in terms of their SWCNTs-n-type Si based solar cell efficiency.
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