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Electriciansparadise -- Electricians' Guide to Photovoltaic Power


PV Physics

What do we mean by photovoltaics? First used in about 1890, the word has two parts: photo, derived from the Greek word for light, and volt, relating to electricity pioneer Alessandro Volta. So, photovoltaics could literally be translated as light-electricity. And that's what photovoltaic (PV) materials and devices do „ they convert light energy into electrical energy (Photoelectric Effect), as French physicist Edmond Becquerel discovered as early as 1839. Commonly known as solar cells, individual PV cells are electricity-producing devices made of semiconductor materials. PV cells come in many sizes and shapes „ from smaller than a postage stamp to several inches across. They are often connected together to form PV modules that may be up to several feet long and a few feet wide. Modules, in turn, can be combined and connected to form PV arrays of different sizes and power output.

The size of an array depends on several factors, such as the amount of sunlight available in a particular location and the needs of the consumer. The modules of the array make up the major part of a PV system, which can also include electrical connections, mounting hardware, power-conditioning equipment, and batteries that store solar energy for use when the sun isn't shining.

Did you know that PV systems are already an important part of our lives? Simple PV systems provide power for many small consumer items, such as calculators and wristwatches. More complicated systems provide power for communications satellites, water pumps, and the lights, appliances, and machines in some people's homes and workplaces. Many road and traffic signs along highways are now powered by PV. In many cases, PV power is the least expensive form of electricity for performing these tasks.


The Photoelectric Effect

To broaden their scientific perspective on a new generation of silicon PV devices, the U.S. Department of Energy's National Renewable Energy Laboratory has pioneered a new class of materials „ microcrystalline silicon alloys „ which may have application in the photovoltaic and microelectronics industries. To date, the scientists have deposited and characterized 50 microcrystalline silicon films.

In 1839, Edmond Becquerel discovered the process of using sunlight to produce an electric current in a solid material. But it took more than another century to truly understand this process. Scientists eventually learned that the photoelectric or photovoltaic (PV) effect caused certain materials to convert light energy into electrical energy at the atomic level.

The photoelectric effect is the basic physical process by which a PV cell converts sunlight into electricity. 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. With their newfound energy, these electrons escape from their normal positions in the atoms of the semiconductor PV material and become part of the electrical flow, or current, in an electrical circuit. A special electrical property of the PV cell„what we call a "built-in electric field"„provides the force, or voltage, needed to drive the current through an external "load," such as a light bulb.

To induce the built-in electric field within a PV cell, two layers of somewhat differing semiconductor materials are placed in contact with one another. One layer is an "n-type" semiconductor with an abundance of electrons, which have a negative electrical charge. The other layer is a "p-type" semiconductor with an abundance of "holes," which have a positive electrical charge.

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. When n- and p-type silicon come into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along 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 we call the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, where they become available to the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons. How do we make the p-type ("positive") and n-type ("negative") silicon materials that will eventually become the photovoltaic (PV) cells that produce solar electricity? Most commonly, we add an element to the silicon that either has an extra electron or lacks an electron. This process of adding another element is called doping.


Light and the PV Cell

We've looked at how to construct a solar cell using crystalline silicon. And we've used this basic type of cell to explain the photoelectric effect, which is the phenomenon operating at the heart of a solar cell. Here, we want to take a look at sunlight, the energy source actually used by solar cells. A brief discussion of several terms will help us better understand aspects of light's interaction with solar cells.

Wavelength, Frequency, and Energy

The energy from the sun is vital to life on Earth. It determines the Earth's surface temperature and supplies virtually all the energy that drives natural global systems and cycles. Some other stars are enormous sources of energy in the form of X-rays and radio signals, but our sun releases the majority of its energy as visible light. However, visible light represents only a fraction of the total spectrum of radiation. Specifically, infrared and ultraviolet rays are also significant parts of the solar spectrum.

The sun emits almost all of its energy in a range of wavelengths from about 2x10-7 to 4x10-6 meters. Most of this energy is in the visible light region. Each wavelength corresponds to a frequency and an energy: the shorter the wavelength, the higher the frequency and the greater the energy (which is expressed in electron-volts, or eV). Red light is at the low-energy end of the visible spectrum and violet light is at the high-energy end, where it has half again as much energy as red light. In the invisible portions of the spectrum, radiation in the ultraviolet region, which causes the skin to tan, has more energy than that in the visible region. Likewise, radiation in the infrared region, which we feel as heat, has less energy than the radiation in the visible region.

Solar cells respond differently to the different wavelengths, or colors, of light. For example, crystalline silicon can use the entire visible spectrum, plus some part of the infrared spectrum. But energy in part of the infrared spectrum, as well as longer-wavelength radiation, is too low to produce current flow. Higher-energy radiation can produce current flow, but much of this energy is likewise not usable. In summary, light that is too high or low in energy is not usable by a cell to produce electricity. Rather, it is transformed into heat.

Air Mass

The sun is continually releasing an enormous amount of radiant energy into the solar system. The Earth receives a tiny fraction of this energy; yet, an average of 1367 watts (W) reaches each square meter (m2) of the outer edge of the Earth's atmosphere. The atmosphere absorbs and reflects some of this radiation, including most X-rays and ultraviolet rays. Still, the amount of the sun's energy that reaches the surface of the Earth every hour is greater than the total amount of energy that the world's human population uses in a year.

How much energy does light lose in traveling from the edge of the atmosphere to the surface of the Earth? This energy loss depends on the thickness of the atmosphere that the sun's energy must pass through. The radiation that reaches sea level at high noon in a clear sky is 1000 W/m2 and is described as "air mass 1" (or AM1) radiation. As the sun moves lower in the sky, the light passes through a greater thickness (or longer path) of air, losing more energy. Because the sun is overhead for only a short time, the air mass is normally greater than one„that is, the available energy is less than 1000 W/m2.

Scientists have given a name to the standard spectrum of sunlight at the Earth's surface: AM1.5G (where G stands for "global" and includes both direct and diffuse radiation, described next) or AM1.5D (which includes direct radiation only). The number "1.5" indicates that the length of the path of light through the atmosphere is 1.5 times that of the shorter path when the sun is directly overhead.

The standard spectrum outside the Earth's atmosphere is called AM0, with no light passing through the atmosphere. AM0 is typically used to predict the expected performance of PV cells in space. The intensity of AM1.5D radiation is approximated by reducing the AM0 spectrum by 28%, where 18% is absorbed and 10% is scattered. The global spectrum is 10% greater than the direct spectrum. These calculations give about 970 W/m2 for AM1.5G. However, the standard AM1.5G spectrum is "normalized" to give 1000 W/m2, because of inherent variations in incident solar radiation.

Direct and Diffuse Light

As we have noted, the Earth's atmosphere and cloud cover absorb, reflect, and scatter some of the solar radiation entering the atmosphere. Nonetheless, an enormous amount of the sun's energy reaches the Earth's surface and can therefore be used to produce PV electricity. Some of this radiation is direct and some is diffuse, and the distinction is important because some PV systems (flat-plate systems) can use both forms of light, but concentrator systems can only use direct light.

Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.

* Direct light consists of radiation that comes straight from the sun, without reflecting off of clouds, dust, the ground, or other objects. Scientists also talk about direct-normal radiation, referring to the portion of sunlight that comes directly from the sun and strikes the plane of a PV module at a 90-degree angle.

* Diffuse light is sunlight that is reflected off of clouds, the ground, or other objects. It obviously takes a longer path than a direct light ray to reach a module. Diffuse light cannot be focused by the optics of a concentrator PV system.

* Global radiation refers to the total radiation that strikes a horizontal surface. Global sunlight is composed of direct-normal and diffuse components of sunlight. Additionally, diffuse and direct-normal sunlight generally have different energy spectra or distributions of color.

Insolation

The actual amount of sunlight falling on a specific geographical location is known as insolation„or "incident solar radiation." Insolation values for a specific site are sometimes difficult to obtain. Weather stations that measure solar radiation components are located far apart and may not carry specific insolation data for a given site. Furthermore, the information most generally available is the average daily total„or global„radiation on a horizontal surface. To learn more about solar and other resource data, please visit the following Web sites:

Renewable Resource Data Center (RReDC)

The RReDC provides information on several types of renewable energy resources in the United States, in the form of publications, data, and maps.

NASA's Surface Meteorology and Solar Energy Data This is a renewable energy resource web site sponsored by NASA's Earth Science Enterprise Program that contains over 200 satellite-derived meteorological and solar energy parameters, monthly averaged from 10 years of data, and data tables for a particular location.

When sunlight reaches the Earth, it is distributed unevenly in different regions. Not surprisingly, the areas near the Equator receive more solar radiation than anywhere else on the Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen and shorten days with the changing seasons. For example, the amount of solar energy falling per square meter on Yuma, Arizona, in June is typically about nine times greater than that falling on Caribou, Maine, in December. The quantity of sunlight reaching any region is also affected by the time of day, the climate (especially the cloud cover, which scatters the sun's rays), and the air pollution in that region. Likewise, these climatic factors all affect the amount of solar energy that is available to PV systems.

Although the quantity of solar radiation striking the Earth varies by region, season, time of day, climate, and air pollution, the yearly amount of energy striking almost any part of the Earth is vast. Shown is the average radiation received on a horizontal surface across the continental United States in the month of June. Units are in kWh/m2/day.


The Crystalline Silicon Solar Cell

PV cells can be made of many different semiconductors. But we'll use crystalline silicon as an example, for three reasons. First, crystalline silicon was the material used in the earliest successful PV devices. Second, and more important, it's still the most widely used PV material. And third, although other PV materials and designs exploit the photoelectric effect in slightly different ways, if you know how the effect works in crystalline silicon, then you'll have a basic understanding of how it works in all PV devices.


An Atomic Description of Silicon

All matter is composed of atoms, which are made up of positively charged protons, negatively charged electrons, and neutral neutrons. Protons and neutrons, which are about the same size, are in the close-packed, central nucleus of the atom. The much lighter electrons orbit the nucleus. Although atoms are built of oppositely charged particles, their overall charge is neutral because they contain an equal number of positive protons and negative electrons whose charges offset one another.

As depicted in this simplified diagram, silicon has 14 electrons. The four electrons that orbit the nucleus in the outermost "valence" energy level are given to, accepted from, or shared with other atoms.

Electrons orbit at different distances from the nucleus, depending on their energy level. For example, an electron with less energy orbits close to the nucleus, whereas one with greater energy orbits farther away. The higher energy electrons farthest from the nucleus are the ones that interact with neighboring atoms to form solid structures.

In the basic unit of a crystalline silicon solid, a silicon atom shares each of its four valence electrons with each of four neighboring atoms.

A silicon atom has 14 electrons, but their natural orbital arrangement allows only the outermost four electrons to be given to, accepted from, or shared with other atoms. These outermost four electrons, called valence electrons, play a very important role in the photoelectric effect.

Large numbers of silicon atoms bond with each other by means of their valence electrons to form a crystal. In a crystalline solid, each silicon atom normally shares one of its four valence electrons in a covalent bond with each of four neighboring silicon atoms. The solid thus consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares valence electrons.

The solid silicon crystal is thus made up of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the crystal lattice.


Bandgap Energies of Semiconductors and Light

When light shines on crystalline silicon, electrons within the crystal lattice may be freed. But not all photons „ as packets of light energy are called „ are created equal. Only photons with a certain level of energy can free electrons in the semiconductor material from their atomic bonds to produce an electric current.

This level of energy, known as the "bandgap energy," is the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit. To free an electron, the energy of a photon must be at least as great as the bandgap energy. However, photons with more energy than the bandgap energy will expend that extra amount as heat when freeing electrons. So, it's important for a PV cell to be "tuned"„through slight modifications to the silicon's molecular structure„to optimize the photon energy. A key to obtaining an efficient PV cell is to convert as much sunlight as possible into electricity.

Crystalline silicon has a bandgap energy of 1.1 electron-volts (eV). (An electron-volt is equal to the energy gained by an electron when it passes through a potential of 1 volt in a vacuum.) The bandgap energies of other effective PV semiconductors range from 1.0 to 1.6 eV. In this range, electrons can be freed without creating extra heat.

The photon energy of light varies according to the different wavelengths of the light. The entire spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. For example, red light has an energy of about 1.7 eV, and blue light has an energy of about 2.7 eV. Most PV cells cannot use about 55% of the energy of sunlight, because this energy is either below the bandgap of the material or carries excess energy.

Different PV materials have different energy band gaps. Photons with energy equal to the band gap energy are absorbed to create free electrons. Photons with less energy than the band gap energy pass through the material.


Built-In Electric Field

Light shining on crystalline silicon may free electrons within the crystal lattice. But for these electrons to do useful work„as in providing electricity to light a light bulb„they must be separated and directed into an electrical circuit. To separate the electrical charges, the silicon solar cell must have a built-in electric field.

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.

To create this electric field within a photovoltaic (PV) cell, two separate semiconductors are sandwiched together. P-type (or "positive") semiconductors have an abundance of positively charged holes, whereas n-type (or "negative") semiconductors have an abundance of negatively charged electrons.

When n- and p-type silicon come into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along 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 we call the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.


Doping Silicon to Create n-Type and p-Type Silicon

In a crystalline silicon cell, we need to contact p-type silicon with n-type silicon to create the built-in electrical field. The process of doping, which creates these materials, introduces an atom of another element into the 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 that silicon's four.

Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal.

Phosphorus atoms, which have five valence electrons, are used in doping n-type silicon, because phosphorus provides its fifth free electron. A phosphorus atom occupies the same place in the crystal lattice formerly occupied by the silicon atom it replaced. 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.

Substituting a boron atom (with three valence electrons) for a silicon atom in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal.

The most common method of doping is to coat a layer of silicon material with phosphorus and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. The temperature is then reduced so the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray-on process, and a technique where phosphorus ions are precisely driven into the surface of the silicon.

The n-type silicon doped with phosphorus cannot form an electric field by itself. We also need 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 PV 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.


Absorption and Conduction

In a PV cell, photons are absorbed in the p-layer. And it's very important to "tune" this layer to the properties of incoming photons to absorb as many as possible, and thus, to free up as many electrons as possible. Another challenge is to keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. To do all this, we design the material to free the 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 all these characteristics, we improve the PV cell's conversion efficiency, which is how much of the light energy is converted into electrical energy by the cell.

To make an efficient solar cell, we try to maximize absorption, minimize reflection and recombination, and thus maximize conduction.


Electrical Contacts

Grid contacts on the top surface of a typical cell are designed to have many thin, conductive fingers spreading to every part of the cell's surface.

Electrical contacts are essential to a photovoltaic (PV) cell because they bridge the connection between the semiconductor material and the external electrical load, such as a light bulb. The back contact of a cell „ on the side away from the incoming sunlight „ is relatively simple. It usually consists of a layer of aluminum or molybdenum metal. But the front contact „ on the side facing the sun „ is more complicated. When sunlight shines on the PV cell, a current of electrons flows all over its surface. If we attach contacts only at the edges of the cell, it will not work well because of the great electrical resistance of the top semiconductor layer. Only a small number of electrons would make it to the contact.

To collect the most current, we must place contacts across the entire surface of a PV cell. This is normally done with a "grid" of metal strips or "fingers." However, placing a large grid, which is opaque, on the top of the cell shades active parts of the cell from the sun. The cell's conversion efficiency is thus significantly reduced. To improve the conversion efficiency, we must minimize these shading effects.

Another challenge in cell design is to minimize the electrical resistance losses when applying grid contacts to the solar cell material. These losses are related to the solar cell material's property of opposing the flow of an electric current, which results in heating the material.

Therefore, in designing grid contacts, we must balance shading effects against electrical resistance losses. The usual approach is to design grids with many thin, conductive fingers spreading to every part of the cell's surface. The fingers of the grid must be thick enough to conduct well (with low resistance), but thin enough not to block much of the incoming light. This kind of grid keeps resistance losses low while shading only about 3% to 5% of the cell's surface. Grids can be expensive to make and can affect the cell's reliability. To make top-surface grids, we can either deposit metallic vapors on a cell through a mask or paint them on via a screen-printing method. Photolithography is the preferred method for the highest quality, but has the greatest cost. This process involves transferring an image via photography, as in modern printing.

An alternative to metallic grid contacts is a transparent conducting oxide (TCO) layer such as tin oxide (SnO2). The advantage of TCOs is that they are nearly invisible to incoming light, and they form a good bridge from the semiconductor material to the external electrical circuit.

TCOs are very useful in manufacturing processes involving a glass superstrate, which is the covering on the sun-facing side of a PV module. Some thin-film PV cells, such as amorphous silicon and cadmium telluride, use superstrates. In this process, the TCO is generally deposited as a thin film on the glass superstrate before the semiconducting layers are deposited. The semiconducting layers are then followed by a metallic contact that will actually be the bottom of the cell.

-- The above information courtesy of the U.S. Dept. of Energy.


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