Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (PV) cell, commonly called a solar cell. A PV cell is a non-mechanical device usually made from silicon alloys.
Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum.
When photons strike a PV cell, they may be reflected, pass-through or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight (energy) is absorbed by the material (a semiconductor), electrons are dislodged from the material’s atoms.
Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons migrate naturally to the surface.
When the electrons leave their position, holes are formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell’s front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. When the two surfaces are connected through an external load, electricity flows.
The PV cell is the basic building block of a PV system. Individual cells can vary in size from about 1cm to about 10cm across. However, one cell produces only one or two watts, which is not sufficient for most applications. To increase power output, cells are electrically connected into a packaged module. Modules can be further connected to form an array.
The term array refers to the entire generating plant, whether it is made up of one or several thousand modules. Many modules can be connected to form the required array size (power output). Photovoltaic technology generally allows electricity to be generated without waste, noise or pollution.
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.
1. The Photoelectric Effect
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 thep/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.
1.1 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 2×10-7to 4×10-6 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.
1.2 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.
1.3 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.
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 websites:
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.
2. 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.
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.
3. PV Devices
Photovoltaic devices can be made from various types of semiconductor materials, deposited or arranged in various structures, to produce solar cells that have optimal performance.
In this section, we first review the three main types of materials used for solar cells. The first type is silicon, which can be used in various forms, including single-crystalline, multicrystalline, and amorphous. The second type is polycrystalline thin films, with specific discussion of copper indium diselenide (CIS) cadmium telluride (CdTe), and thin-film silicon. Finally, the third type of material is single-crystalline thin film, focusing especially on cells made with gallium arsenide.
We then discuss the various ways that these materials are arranged to make complete solar devices. The four basic structures we describe include homojunction, heterojunction, p-i-n and n-i-p, and multijunction devices.
4. PV Systems
A photovoltaic (PV) or solar cell is the basic building block of a PV (or solar electric) system. An individual PV cell is usually quite small, typically producing about 1 or 2 watts of power. To boost the power output of PV cells, we connect them together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, we can build PV systems able to meet almost any electric power need, whether small or large.
PV systems can be classified into two general categories: flat-plate systems or concentrator systems. We will talk about the differences between these two types of systems later on.
By themselves, modules or arrays do not represent an entire PV system. We also need structures to put them on that point them toward the sun, and components that take the direct-current electricity produced by modules and “condition” that electricity, usually by converting it to alternate-current electricity. We might also want to store some electricity, usually in batteries, for later use. All these items are referred to as the “balance of system” (BOS) components.
Combining modules with the BOS components creates an entire PV system. This system is usually everything we need to meet a particular energy demand, such as powering a water pump, or the appliances and lights in a home, or, if the PV system is large enough, all the electrical requirements of a whole community.
The basic photovoltaic or solar cell typically produces only a small amount of power. To produce more power, cells can be interconnected to form modules, which can in turn be connected into arrays to produce yet more power. Because of this modularity, PV systems can be designed to meet any electrical requirement, no matter how large or how small.