Nanotechnology for Photovoltaic Energy: Challenges and Potentials

Nanotechnology for Photovoltaic Energy: Challenges and Potentials

Salahuddin Qazi (State University of New York Institute of Technology, USA) and Farhan A. Qazi (Syracuse University, USA)
Copyright: © 2013 |Pages: 29
DOI: 10.4018/978-1-4666-1996-8.ch006
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Abstract

Solar radiation is plentiful and a clean source of power. However, despite the first practical use of silicon based solar cell more than 50 years ago, it has not been exploited to its full potential due to the high cost of electrical conversion on a per Watt basis. Many new kinds of photovoltaic cells such as multi-junction solar cells dye –sensitized solar cells and organic solar cell incorporating element of nanotechnology have been proposed to increase the efficiency and reduce the cost. Nanotechnology, in the form of quantum dots, nanorods, nanotubes, and grapheme, has been shown to enhance absorption of sunlight, makes low cost flexible solar panels and increases the efficiency of photovoltaic cells. The chapter reviews the state of current photovoltaic cells and challenges it presents. It also discusses the use of nanotechnology in the application of photovoltaic cells and future research directions to improve the efficiency of solar cells and reduce the cost.
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Introduction

Photovoltaic is derived from Photo meaning “light” and voltaic meaning “electric” and is defined as the conversion of sunlight to electricity through a photovoltaic cell (PV). It was first discovered by French physicist Alexandre-Edmond Becquere (1839). The first Solar cell was built by Fritts (1883), who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions (1% efficient). The modern age of solar power technology started when Chapin, Fuller, & Pearson (1954) from Bell Laboratories, discovered that silicon doped with certain impurities was able to generate electricity for satellites. This device originally known as the solar battery is currently called Solar cell, exploited the principle of P-N junction. Initially the energy conversion of the cell, was 6% and reached 11% by the year 1957 and 14% by the year 1960 (Pearson, 1957; Rappaport, 1961). Photovoltaic cell is a non –mechanical device usually made from silicon which creates an electron imbalance across the cell and produces direct current (DC) as a result of incident sunlight. To convert DC current into workable alternating current (AC) electricity a device known as power converter is used. Solar energy or solar radiation can also be used for thermal energy source.

The world current energy consumption is 4.1 x 1020 Joules/year which is equivalent to a continuous power consumption of 13 trillion watts or 13 terra watts (TW). The earth surface receives an average of 120,000 TW from the sun, ignoring the energy being scattered by the atmosphere and clouds. Thus with Solar cells as low as 10% conversion efficiency, the world’s energy needs can be satisfied with solar panels covering 0.16% of earth surface which would supply 20 TW of power. In US it will take 1.6% of the land area to meet the country domestic needs (“Basic Research Needs for Solar Energy Utilization,” 2005). According to Pike Research forecasts, the worldwide demand for solar energy will nearly double between 2010 and 2013, reaching 19.3 Giga watts by the end of that period, caused by the shift of solar industry from supply-constrained to demand-driven over the past two years. This shift is driven by a new abundance of polysilicon, as well as the effects of the worldwide financial crisis, and the plunging price of solar modules. This market realignment will set the stage for a new era of solar growth over the next several years (“Global Solar Energy Outlook,” 2010).

Photovoltaic energy as an alternative renewable energy source is emerging as a viable solution to energy problems worldwide. Despite its immense potential it is still a long way from becoming the world’s major energy source because of many challenges it presents in its implementation. This includes higher cost of making large silicon solar panels and inefficient Solar cells due to inadequate absorption of sunlight and low efficiency of 31% set by Shockley and Queisser (1961) for a single semiconductor junction. The high price for Solar cells is largely due to the use of expensive substrate materials and costly microfabrication processing. The efficiency of thin film photovoltaic cell increased from 10% in 1970 to over 24% in the most recent years by the use of new photovoltaic materials. Ongoing research in photovoltaic energy and new developments in nanotechnology is overcoming some of these challenges. Nanotechnology in the form of nanoparticles, nanowires and nanostrucures has been shown to enhance absorption of sunlight, make low cost flexible solar panels and increase the efficiency of photovoltaic cells beyond Shockly and Queissar (1961) limit by using multiple exciton generation in nanostructures. The advantages of nanotechnology based Solar cells also reduce manufacturing costs as a result of using a low temperature manufacturing process instead of high temperature vacuum deposition process typically used to produce conventional crystalline semiconductor based Solar cells.

Key Terms in this Chapter

Efficiency of Photovoltaic Cell: It is defined as the ratio of electrical power produced by a photovoltaic cell at any instant to the power of the solar input which sunlight is striking the cell measured in Watts /m2. A major factor limiting the conversion efficiency is due to large bandgaps for low energy to be absorbed, while much of the energy from electrons freed up by high-energy is lost as the electron is extracted.

Quantum Dot: Quantum dots (QDs) are semiconductor nanocrystals of nanometers dimensions whose electrons-holes (excitons) are confined in all three spatial dimensions. Quantum dots are used to improve efficiency of photovoltaic cells beyond standard thermodynamic limit. They have unique quantum optical properties that are not found in the bulk material due to the property of quantum confinement exhibited by the nanoscale structures.

Shockley and Queisser Limit: Refers to the maximum theoretical efficiency of a Solar cell of around 33.7% assuming a p-n junction band gap of 1.1 eV (for silicon). In other words only 33.7% of all the power contained in sunlight falling on a silicon Solar cell, could ever be turned into electricity. In this model excitation energy above the bandgap is lost to heating and excitation energy below the band gap is not absorbed.

Multiple Exciton Generation (MEG): Multiple exciton generation involves the generation of more than one exciton from the absorption of a single photon. This a method is used to increase the efficiency of Solar cells beyond the Shockley-Queisser limit which has recently been demonstrated in quantum dots.

Nanorods: Nanorods are one dimensional structure which provides a directed path for electrical transport and are used to control the bandgap by varying the radius of rods and using the quantum size effect. The efficiency of quantum dot conjugated polymer Solar cell can be enhanced by using quantum confinement effect which will affect the length and width of nanorods leading to thinner devices for optimal absorption of incident light

Single Wall Nanotubes (SWNTs): have a nanometer-scale diameter and exhibit ballistic electrical conductivity and are very efficient for transporting electrons. In addition SWCNT offers unique properties of offering a wide range of bandgaps to match the solar spectrum, enhanced optical absorption and reduced carrier scattering for hot carrier transport.

Photovolatic (Solar) Cell: A photovoltaic cell is a Solar cell essentially made of a large area p-n junction diode. Energy from the incident photons creates excitation of the electron to the conduction band leaving behind a hole in the valence band resulting into electron-hole pairs or excitons in the case of organic semiconductors.

Solar Radiation: The annual energy input of solar irradiation on Earth exceeds the world’s yearly energy consumption by several thousand times. Most of the energy coming from sun is in the visible and infrared part of the electromagnetic spectrum, with less than 1% emitted in the radio, UV and X-ray spectral bands.

Semiconductor Based Solar Cell: The conventional Solar cells are made of semiconductor material, usually crystalline silicon (c-Si) as light absorbing semiconductor. The first type of crystalline Solar cells is monocrystalline which are wafers, of about 0.3 mm thick, sawn from Si ingot of single crystal silicon. The second type of crystalline Solar cells are Polycrystalline (multicrystaline) made by sawing a cast square ingot block of silicon first into bars and then wafers.

Graphene Based Solar Cell: The epitaxial graphene is basically a single layer of graphite, consisting of a hexagonal array of carbon atoms, just like those found in bulk graphite. Recent work shows that graphene is highly conductive and highly transparent form of carbon which is a critical requirement for organic photovoltaic cell. It also shows outstanding thermal and chemical stability.

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