1. Chemistry of Photovoltaic Cells
Photovoltaics
Exercise 2 1 1
Objectives
Chemistry of Photovoltaic Cells
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Objectives
define electron shells, valence electrons
define crystal lattice structure of silicon
define free electrons, dopants, pn junction
describe how current is generated
describe the contacts, base, antireflective cover
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2. Chemistry of Photovoltaic Cells
Photovoltaics
Exercise 2 2 2
Chemistry of Photovoltaic Cells
Chemistry of Photovoltaic Cells
All matter is composed of atoms. An atom is an indivisible particle of matter which defines a chemical element; it is the smallest part of an element which has all of the properties of that element. In 1913, Niels Bohr described a simple representation of an atom as a central nucleus which contains positively-charged protons and uncharged neutrons, which are surrounded by negatively charged electrons. The electrons are attracted by the protons and thus encircle the nucleus, similar to planets orbiting the sun. The number of electrons surrounding the nucleus is equal to the number of protons; the amount of neutrons may vary. The electrons are arranged in specific leveled bands about the nucleus, called electron shells. An electron shell is the orbital area where an electron is located around an atom’s nucleus. All of the electrons in one shell are the same distance from the nucleus; however, they do not always orbit in the same pattern.
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3. Structure of Electron Shells
Photovoltaics
Exercise 2 3
Structure of Electron Shells 3
Structure of Electron Shells
This figure illustrates a nucleus with levels of surrounding electron shells. The number of electrons in each electron shell follows the same pattern regardless of the number of electrons in a specific atom. The first shell is labeled K. The K shell may hold up to 2 electrons; an atom with more electrons will have electrons in the second shell which is labeled L. The L shell has two levels. Electrons will fill up each level, and then move to the next shell.
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Basic Photovoltaic Principles and Methods (Zwiebel 1984)
Photovoltaics (Seippel 1983)
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4. Continued Photovoltaics Exercise 24 4
Structure of electron shells
Continued
For example, an atom of copper has 29 electrons, 29 protons, and 35 neutrons. The copper atom’s K shell has 2 electrons, the L shell has 8 electrons, the M shell has 18 electrons, and the N shell has only 1 electron. Most conductor materials have a similar atomic structure with 1 electron in the outermost shell. The electrons in the outer shell can be moved, which is the electron flow that produces electricity.
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5. Continued Photovoltaics Exercise 25 5
Structure of electron shells
Continued
The electron shells represent different energy levels. When an atom is exposed to light or heat energy, their electrons can be moved to a shell further from the nucleus, which means that they can be elevated to a higher energy level. Likewise, an electron can be reduced to a lower energy level and return to a lower shell.
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EnergyLevels01
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6. Continued Photovoltaics Exercise 26 6
Structure of electron shells
Continued
When an electron returns to a lower shell, light or heat energy is released. The energy released is in the form of energy packets as described by Planck and Einstein, called quanta. The quantum of energy needed to move an electron to a higher shell is equal to the quantum of energy released when the electron falls back to the lower shell. Although the energy is the same to move back and forth, the quantum required to move an electron from lower shells (such as, from the L shell to the K shell) is less than the quantum required to move an electron from higher shells (such as, from the M shell to the N shell).
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7. Crystal Lattice Structure of Silicon
Photovoltaics
Exercise 2 7 7
Crystal Lattice Structure of Silicon
Crystal Lattice Structure of Silicon
An electron in the outermost shell is called a valence electron. Valence electrons interact with electrons from other atoms and combine to make larger structures. Most semiconductor materials have 4 valence electrons. Silicon is the most common semiconductor material used in PV cells. An atom of silicon has 14 electrons of which 4 valence electrons can be shared, given, or accepted by other atoms. In pure silicon, multiple silicon atoms bond together to form a solid. Each silicon atom shares one of its valence electrons with 4 other silicon atoms. Pure silicon shares each valence electron at an exact angle which forms an arrangement that is similar to a cube. This arrangement repeats throughout the silicon solid; it is called a crystal lattice.
When light reaches the surface of a silicon crystal, it may reflect, pass through, or be absorbed. Light that is absorbed by silicon effects the energy level of the electrons within the crystal lattice.
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Silicon01; Silicon02
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Image 01 shows valence electrons of silicon connecting with other silicon atoms. Image 02 is a graphical representation of the lattice structure of silicon a 3-D view.
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8. Electron Bound States Photovoltaics Exercise 28 8
Electron Bound States
Electron Bound States
Electrons in their natural shell configuration are said to be in the ground state. When low energy light, in the form of photons, interacts with atoms, the energy levels raise but the electrons remain bound to the atom. These electrons are said to be in an excited state. Low energy light interacting with a silicon crystal may cause the electrons to vibrate or move to a higher shell but they do not leave their position. The physical properties of atoms in the excited state change from the ground state. For example, an excited atom may be larger than the same atom in the ground state.
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9. Continued Photovoltaics Exercise 28 8
Electron Bound States
Continued
When photons strike an atom, those with high enough energy can bump an electron to a higher shell or move the electron out of the valence shell to create a free electron. The area of the outermost shell containing valence electrons is called the valence band. The area beyond the outermost shell is called the conduction band. The conduction band is the area where free electrons move. The area between the two bands is an energy barrier called the band gap. Electrons must gain enough energy to pass through the band gap to move along the conduction band. An electron can gain energy to move through the band gap from the application of an electric field, heat, or light. The quantum of energy required to move an electron through the band gap varies with the atomic weight of the specific atom. The size of the band gap decreases as temperature increases, which permits greater movement of free electrons.
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10. Continued Photovoltaics Exercise 210 10
Electron Bound States
Continued
When electrons move across the band gap, the remaining spaces (called holes) cause a positive charge in the atom. The holes allow for other electrons to move, thereby creating more spaces, and thus the holes appear to move. Electrons from nearby atoms move into the empty spaces causing more holes throughout the crystal.
The formation of free electrons and holes created by light energy is the central process in the photovoltaic effect, but this process itself does not produce current. For these free electrons to be effective and create electricity, the silicon material must form an electric field, which is not possible with pure silicon. However, it is possible to influence the movement of electrons and holes in a silicon crystal lattice by adding atoms from other materials called dopants. A dopant is an element, which is added in small amounts, to change the conductive properties of silicon or other semiconductor material.
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Image01 shows the space left when an electron moves to the conduction band. Image 02 shows the electron moving through the crystal lattice structure.
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11. Dopants Photovoltaics Exercise 211 11
Dopants
Dopants
Atoms that are used as dopants have either one more (unbound) valence electron than silicon or one less valence electron. A dopant with one unbound valence electron is called a donor, because when it is bound to silicon it can give an electron to another atom. Donors have a negative charge and are used to create n-type semiconductor material. A dopant with one less valence electron is called an acceptor, because when it is bound to silicon it creates a hole that can receive an electron from another atom. Acceptors have a positive charge and are used to create p-type semiconductor material.
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12. Continued Photovoltaics Exercise 212 12
Structure of electron shells
Continued
Two common dopants are phosphorus and boron, they are used to create n-type and p-type materials.
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Dopants02a; Dopants02b
Basic Photovoltaic Principles and Methods (Zwiebel 1984)
Photovoltaics (Seippel 1983)
Image 01 is a two-dimensional illustration of silicon lattice doped with a boron atom. Image 02 is a two-dimensional illustration of silicon lattice doped with a phosphorus atom.
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13. Continued Photovoltaics Exercise 213
Dopants 13
Continued
When these materials are joined, free electrons move from the n-type material to the holes in the p-type material. The area where these materials meet is called the pn junction, which is an the essential component of a photovoltaic cell. The pn junction creates a barrier which separates free electrons and holes, and thereby hinders them from rejoining. The strength of the barrier is dependent on the amount of dopant in the silicon.
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14. Continued Photovoltaics Exercise 214 14
Dopants
Continued
This barrier creates a difference in electrical charge (an electric field) at the surface on each side of the semiconductor material which is used to produce an electric current within an external circuit. The region with an electric field is called the depletion region because it quickly carries off free electrons. The electric field causes the electrons to move out from the surface of the semiconductor materials where they can join the external electric circuit.
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15. Review Glossary Photovoltaics Exercise 2No 15
Review Glossary
Review Glossary
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16. Function of a Photovoltaic Cell
Photovoltaics
Exercise 2 16 16
Function of a Photovoltaic Cell
Function of a Photovoltaic Cell
If wires were connected to a silicon crystal with a pn junction, it by itself would not produce electricity, the free electrons and holes would recombine and therefore cancel each other. However, if there is an imbalance within the flow of electrons, electricity can be produced from the pn junction. This imbalance is created from light energy which produces more free electrons and accelerates electron flow. When light energy is absorbed by the photovoltaic cell, the photons cause free electrons to flow out of the n-type material. The electric current performs work through a load, and then the electrons flow to the p-type material to recombine with holes. The light energy that was absorbed by the electrons is lost to power the external circuit. Photons from sunlight continually create more free electrons and holes, which creates current and uses the energy to perform work, such as powering a calculator or water heater.
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17. Continued Photovoltaics Exercise 217 17
Function of a Photovoltaic Cell
Continued
To further understand how the PV cell functions, consider this simple model: a metal base holds a small piece of n-type silicon covered by a thick piece of p-type silicon, which is covered with a thin wire mesh.The base and wire mesh serve as contacts. A contact is a piece of conductive material (usually metal) that allows electric current to flow through a circuit. When a photon passes through the PV cell, if it has enough energy to free an electron in the n-type or p-type layers of the semiconductor material, the free electron will combine with a hole. However, if the photon frees an electron in the depletion region, the free electron will be pushed by the electric field to produce a current of 1 electron which will move through an attached wire. If the wire was connected from the metal base to the top mesh, the current could be measured. The voltage showing on a measuring device would correspond to the energy in 1 electron.
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18. Continued Photovoltaics Exercise 218 18
Function of a Photovoltaic Cell
Continued
Silicon allows sunlight to pass through its surface. Photons with long wavelengths pass through to the deeper part of the semiconductor material, and photons with short wavelengths are absorbed near the surface. The n-type layer of semiconductor material is usually made very thin because photons are absorbed in the p-type layer. Most semiconductor material is designed with the pn junction positioned near the top surface, which increases the probability of photons creating free electrons at the pn junction rather than deeper in the p-type layer where they could combine with holes. This design helps the electric field to carry free electrons to the external circuit.
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19. Structure of a Photovoltaic Cell
Photovoltaics
Exercise 2 19 19
Structure of a Photovoltaic Cell
Structure of a Photovoltaic Cell
Electric contacts are required for all PV cells because they connect the semiconductor material to the external electric circuit. The contact on the back side, which does not receive sunlight, is usually a simply designed metal base. The contact on the front side, which receives sunlight, is designed to effectively cover the entire surface of the PV cell by using a grid of metal strips. The strips of the grid are made thick enough to conduct electricity, yet thin enough to prevent blocking sunlight or producing a shadow on the material. Grid contacts are usually designed with multiple thin strips which spread through the cell surface. Semiconductor material can reflect one-third of the sunlight reaching a PV cell, therefore an anti-reflective coating or texturized surface is used to reduce the reflection. This figure shows the internal structure of a common PV cell.
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20. Review Glossary Photovoltaics Exercise 2No 20
Review Glossary
Review Glossary
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21. Glossary Photovoltaics Exercise 2 SAMPLE: 21 GlossaryNO 21
Glossary
Glossary
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22. End of Lesson Quiz Photovoltaics Exercise 222 22
End of Lesson Quiz
End of Lesson Quiz
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23. Quiz Photovoltaics Exercise 2NO 23
Quiz
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24. Conclusion Photovoltaics Exercise 224 24
Conclusion
Conclusion
In this lesson you learned about the function and structure of photovoltaic cells. You also learned how dopants are used in semiconductor material. In the next lesson, you will learn how resistance and load effect electrical circuits.
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