Introduction
Purpose
This report investigates Photovoltaic Cells(PC). PC are materials and devices that generate a flow of electrons directly from sunlight through an electronic process that occurs in semiconductors of electricity such as silicon (citation).
Background
The energy crisis of the 1970s because of over-consumption and the declination of oil production in the US, main source of energy then, resulted in a major interest for a different source of electricity(citation). Photovoltaic cells were first discovered in 1839 by the French Physicist Alexandre Edmond Becquerel who noticed that some metals are photoelectric which means they produce electric when exposed to light. However, the first practical photovoltaic cell came to life in 1954 by Bell Labs researchers Daryl Chapin, Calvin Fuller, and Gerald Pearson in the United States of America who accidentally discovered that a doped junction of semiconductors produced a usable amount of electricity when exposed to light than just some simple metals. By 1958 they made photovoltaic silicon solar cells for scientific and commercial applications which was about 6 percent efficient (a later version manages 11 percent (citation). The prices were impractically high. Industry developments and scientific research in the following years made photovoltaic cells more accessible with decreased productions costs (citation).
The functionality of photovoltaic cells is based on two principals, namely the law of conservation of energy and the photo electric effect.
The Photoelectric Effect(PE) was first discovered by German physicist Heinrich Hertz in in 1887 but improved by Albert Einstein who gave an almost complete explanation of the phenomenon in 1905(citation). PE is the production of electrons or other free carriers when light is shone onto a material. Electrons emitted in this manner can be called photoelectrons(citation).
Equation: hf=∅+Ek
h = the Plank constant 6.63 x 10-34 J s
f = the frequency of the incident light in hertz (Hz)
phi = the work function in joules (J)
Ek = the maximum kinetic energy of the emitted electrons in joules (J)
Law of Conservation of Energy was discovered by Julius Robert Mayer in 1842. However, to establish the more general law of conservation of energy, it required nineteenth century physicists to recognize electrical, chemical and other forms of energy in addition to heat (Physics Book). Now it is called the First Law of Thermodynamics which states: The total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one object to another, but the total amount remains constant. (Physics Book).
Ein – Eout + (change in all other forms of energy)= Constant
Uin + Kin – Uout + Kout = constant
E-Mechanical Energy
E= kinetic energy + potential energy
U-Potential Energy
K-Kinetic Energy
Scope
This report discusses the principles, functionality, application and efficiency of Photovoltaic cells as well as the advantages of its use.
Discussion
Composition
Photovoltaic cells or solar cells are two layers of silicon sandwiched in order to create an electric field. Silicon does not conduct electricity but under certain circumstances it is possible to do so. The sunlight shines on the silicon and may free electrons within the crystal lattice but for these electrons to do useful work they must be directed into an electrical circuit made of positive and negative charges. (citation)
Fig 1: Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a: p/n junction at their interface, thereby creating an electric field.
Therefore, the silicon is doped with Phosphorus and Boron in order to usefully conduct electricity. In solar panel manufacture, doping is introduces an atom of another element into silicon crystal to alter its electrical properties(). The dopant, which is the introduced element, has either: three or five valence electrons—which is: one less or one more than silicon’s four (citation)
Fig 2
Phosphorus atoms are replaced with five valence electrons for a silicon atom in a silicon crystal leaving an extra, unbonded electron that is relatively free to move around the crystal.
Phosphorus atoms, which have: five valence electrons, are used to dope n-type silicon because phosphorus provides its fifth free electron. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free, having no bonding responsibilities. When phosphorus atoms are substituted for silicon in a crystal, many free electrons become available.
But the: n-type silicon cannot form an electric field by itself. It also needs: p-type silicon. Boron, which has only three valence electrons, is used for doping: p-type silicon. Boron is introduced during silicon processing when the silicon is purified for use in photovoltaic devices. When a boron atom takes a position in the crystal lattice formerly occupied by a silicon atom, a bond will be missing an electron. In other words, there is an extra positively charged hole.
To create an electric field within a silicon photovoltaic (PV) cell, two different layers of silicon that have been specially treated are sandwiched so they will let electricity flow through them in a particular way. The lower layer is doped so it has slightly few electrons. It’s called p-type or positive-type silicon (because electrons are negatively charged and this layer has too few of them). The upper layer is doped the opposite way to give it slightly too many electrons. It’s called n-type or negative-type silicon.
When n- and p-type silicon layers contact, excess electrons move from the n-type side to the p-type side. Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet what is called the p/n junction
P-Layer Design
In a PV cell, photons are absorbed in the: p-layer. It is therefore important that this layer be “tuned” to the properties of incoming photons so it can absorb as many as possible and, thus, free up as many electrons as possible. The design of the: p-layer must also keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. To accomplish these goals, p-layers are designed to free electrons as close to the junction as possible, so that the electric field can help send the free electrons through the conduction layer (the n-layer) and out into the electrical circuit. By optimizing these characteristics, the PV cell’s conversion efficiency (how much light energy is converted into electrical energy) is improved.
Photovoltaic Electrical Contacts and Cell Coatings
The outermost layers of photovoltaic (PV) cell, or solar cell, are the electrical contacts and anti-reflective coating. These layers provide essential functions to the cell’s operation.
Solar cell materials:
Silicon (Si)—including single-crystalline Si, multicrystalline Si, and amorphous Si
Polycrystalline Thin Films—including copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon Single-Crystalline Thin Films— including high-efficiency material such as gallium arsenide (GaAs).
Functionality
Each cell generates a few volts of electricity, therefore in order to make a useful amount of electric current and voltage the results cells are combined and put into a solar panel. Solar panel is the combination of photovoltaic cells(citation).
When light shines on a PV cell, it may be reflected, absorbed, or pass right through. But only the absorbed light generates electricity.
The energy of the absorbed light is transferred to electrons in the atoms of the PV cell semiconductor material.
With their new energy, these electrons escape from their normal positions in the atoms and become part of the electrical flow, or current, in an electrical circuit. A special electrical property of the PV cell—what is called a: “built-in electric field”—provides the force, or voltage, needed to drive the current through an external load, such as a light bulb.
Fig 2:As photons enter our sandwich, they give up their energy to the atoms in the silicon. The incoming energy knocks electrons out of the lower, p-type layer so they jump across the barrier to the n-type layer above and flow out around the circuit
Performance
Photovoltaic (PV), or solar cells use the energy in sunlight to produce electricity. However, the amount of electricity produced depends on the quality of the light available and the performance of the PV cell. Researchers make measurements of conversion efficiency and quantum efficiency to characterize the performance of PV cells.
Conservation efficiency is Energy efficiency is “using less energy to provide the same service”.(citation)
Quantum effieicncy is The “quantum efficiency” (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell. The quantum efficiency may be given either as a function of wavelength or as energy.(citation)
Based on these results, researchers may redesign aspects of the cell—e.g., material compositions or thicknesses of layers—to improve performance.
Photovoltaic cells
-10-20 % efficient, inefficient use of land-limited applications-dependent on temperature, position, shadow, and cleanliness-size(octagonal in shape, about the size of an adult’s palm) -colorful(black, blueish or orange)-rejects unwanted ultraviolet and infrared light-costs by inexpensive wiring installation-90% visible light transmitted -over 10% efficient-size(1 micrometer thick)-colorless(transparent)
Transparent Photovoltaic Cells
Transparent Photovoltaic Cells
Transparent Photovoltaic Cell is a device that converts infrared and ultraviolet light into a flow of electrons reaching up to 1% efficiency initially. This technology/invention is still under research by Dr. Richard Lunt who came up with the idea, along with Yimu Zhao, Benjamin Levine and Garrett Meek.
Conclusion
Summary: Transparent Photovoltaic Cell is a promising technology that can revolutionize the use of solar cells by diversifying its application and increasing building efficiency at the point of electricity utilization having in mind aesthetics which provides a level of transparency to enjoy natural lighting and a view of surroundings.
Recommendation:
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Works Cited
“How Do Solar Panels Work?” LiveScience. Purch, 16 Dec. 2013. Web. 30 Nov. 2016.
@mitenergy. “Transparent Solar Cells.” MIT Energy Initiative. N.p., 20 June 2013. Web. 28 Nov. 2016.
University, Michigan State. “Solar Energy That Doesn’t Block the View.” MSUToday. N.p., 19 Aug. 2014. Web. 28 Nov. 2016.
Woodford, Chris. “How Do Solar Cells Work?” Explain That Stuff. N.p., 28 Apr. 2016. Web. 28 Nov. 2016.