Solar Cell
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants.
When research into electricity began and simple batteries were being made and studied, research into solar electricity followed amazingly quickly. As early as 1839, Antoine-Cesar Becquerel exposed a chemical battery to the sun to see it produce voltage. This first conversion of sunlight to electricity was one percent efficient. That is, one percent of the incoming sunlight was converted into electricity. Willoughby Smith in 1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that selenium, when exposed to light, produced an electrical current. Charles Fritts, in the 1880s, also used gold-coated selenium to make the first solar cell, again only one percent efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free solar energy to be a means of decentralization, predicting that solar cells would replace power plants with individually powered residences.
With Albert Einstein’s explanation in 1905 of the photoelectric effect—metal absorbs energy from light and will retain that energy until too much light hits it—hope soared anew that solar electricity at higher efficiencies would become feasible. Little progress was made, however, until research into diodes and transistors yielded the knowledge necessary for Bell scientists Gordon Pearson, Darryl Chapin, and Cal Fuller to produce a silicon solar cell of four percent efficiency in 1954.
Further work brought the cell’s efficiency up to 15 percent. Solar cells were first used in the rural and isolated city of Americus, Georgia as a power source for a telephone relay system, where it was used successfully for many years.
A type of solar cell to fully meet domestic energy needs has not as yet been developed, but solar cells have become successful in providing energy for artificial satellites. Fuel systems and regular batteries were too heavy in a program where every ounce mattered. Solar cells provide more energy per ounce of weight than all other conventional energy sources, and they are cost-effective.
Only a few large scale photovoltaic power systems have been set up. Most efforts lean toward providing solar cell technology to remote places that have no other means of sophisticated power. About 50 megawatts are installed each year, yet solar cells provide only about. 1 percent of all electricity now being produced. Supporters of solar energy claim that the amount of solar radiation reaching the Earth’s surface each year could easily provide all our energy needs several times over, yet solar cells have a long way to go before they fulfill Charles Fritts’s dream of free, fully accessible solar electricity.
Raw Materials
The basic component of a solar cell is pure silicon, which is not pure in its natural state.
To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. At this point, the silicon is still not pure enough to be used for solor cells and requires further purification.
Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz. The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.
The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.
The Manufacturing Process
Purifying the silicon
- 1 The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
- 2 The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure drags the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.
Making single crystal silicon
- 3 Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or boule of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.
Making silicon wafers
- 4 From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer- 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
After the initial purification, the silicon is further refined in a floating zone process. In this process, a silicon rod is passed through a heated zone several times, which serves to ‘drag the impurities toward one end of the rod. The impure end can then be removed. Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is dipped into melted polycrystalline silicon. The seed crystal rotates as it is withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced out of the ingot.
Doping
- 6 The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms burrow into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth. A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.
Placing electrical contacts
- 7 Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated
This illustration shows the makeup of a typical solar cell. The cells are encapsulated in ethylene vinyl acetate and placed in a metal frame that has a mylar backsheet and glass cover.
The anti-reflective coating
- 9 Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer. The most commonly used coatings are titanium dioxide and silicon oxide, though others are used. The material used for coating is either heated until its molecules boil off and travel to the silicon and condense, or the material undergoes sputtering. In this process, a high voltage knocks molecules off the material and deposits them onto the silicon at the opposite electrode. Yet another method is to allow the silicon itself to react with oxygen- or nitrogen-containing gases to form silicon dioxide or silicon nitride. Commercial solar cell manufacturers use silicon nitride.
Encapsulating the cell
- 10 The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.
Quality Control
Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely affect the overall efficiency of the cells. The primary research goal is to find ways to improve the efficiency of each solar cell over a longer lifetime. The Low Cost Solar Array Project (initiated by the United States Department of Energy in the late 1970s) sponsored private research that aimed to lower the cost of solar cells. The silicon itself is tested for purity, crystal orientation, and resistivity. Manufacturers also test for the presence of oxygen (which affects its strength and resistance to warp) and carbon (which causes defects). Finished silicon disks are inspected for any damage, flaking, or bending that might have occurred during sawing, polishing, and etching.
During the entire silicon disk manufacturing process, the temperature, pressure, speed, and quantities of dopants are continuously monitored. Steps are also taken to ensure that impurities in the air and on working surfaces are kept to a minimum.
The completed semiconductors must then undergo electrical tests to see that the current, voltage, and resistance for each meet appropriate standards. An earlier problem with solar cells was a tendency to stop working when partially shaded. This problem has been alleviated by providing shunt diodes that reduce dangerously high voltages to the cell. Shunt resistance must then be tested using partially shaded junctions.
An important test of solar modules involves providing test cells with conditions and intensity of light that they will encounter under normal conditions and then checking to see that they perform well. The cells are also exposed to heat and cold and tested against vibration, twisting, and hail.
The final test for solar modules is field site testing, in which finished modules are placed where they will actually be used. This provides the researcher with the best data for determining the efficiency of a solar cell under ambient conditions and the solar cell’s effective lifetime, the most important factors of all.
The Future
Considering the present state of relatively expensive, inefficient solar cells, the future can only improve. Some experts predict it will be a billion-dollar industry by the year 2000. This prediction is supported by evidence of more rooftop photovoltaic systems being developed in such countries as Japan, Germany, and Italy. Plans to begin the manufacture of solar cells have been established in Mexico and China. Likewise, Egypt, Botswana, and the Philippines (all three assisted by American companies) are building plants that will manufacture solar cells.
Most current research aims for reducing solar cell cost or increasing efficiency. Innovations in solar cell technology include developing and manufacturing cheaper alternatives to the expensive crystalline silicon cells. These alternatives include solar Windows that mimic photosynthesis, and smaller cells made from tiny, amorphous silicon balls. Already, amorphous silicon and polycrystalline silicon are gaining popularity at the expense of single crystal silicon. Additional innovations including minimizing shade and focusing sunlight through prismatic lenses. This involves layers of different materials (notably, gallium arsenide and silicon) that absorb light at different frequencies, thereby increasing the amount of sunlight effectively used for electricity production.
A few experts foresee the adaptation of hybrid houses; that is, houses that utilize solar water heaters, passive solar heating, and solar cells for reduced energy needs. Another view concerns the space shuttle placing more and more solar arrays into orbit, a solar power satellite that beams power to Earth solar array farms, and even a space colony that will manufacture solar arrays to be used on Earth.
Where To Learn
Books
Bullock, Charles E. and Peter H. Grambs. Solar Electricity: Making the Sun Work for You. Monegon, Ltd., 1981.
Komp, Richard J. Practical Photovoltaics. Aatec Publications, 1984.
Making and Using Electricity from the Sun. Tab Books, 1979.
Periodicals
Crawford, Mark. DOE’s Born-Again Solar Energy Plan, Science. March 23, 1990, pp. 1403-1404.
Waiting for the Sunrise, Economist. May 19, 1990, pp. 95.
Edelson, Edward. Solar Cell Update, Popular Science. June, 1992, p. 95.
Murray, Charles J. Solar Power’s Bright Hope, Design News. March 11, 1991, p. 30.
Solar cells
by Chris Woodford. Last updated: January 22, 2022.
W hy do we waste time drilling for oil and shoveling coal when there’s a gigantic power station in the sky up above us, sending out clean, non-stop energy for free? The Sun, a seething ball of nuclear power, has enough fuel onboard to drive our Solar System for another five billion years—and solar panels can turn this energy into an endless, convenient supply of electricity.
Solar power might seem strange or futuristic, but it’s already quite commonplace. You might have a solar-powered quartz watch on your wrist or a solar-powered calculator. Many people have solar-powered lights in their garden. Spaceships and satellites usually have solar panels on them too. The American space agency NASA has even developed a solar-powered plane! As global warming continues to threaten our environment, there seems little doubt that solar power will become an even more important form of renewable energy in future. But how exactly does it work?
Photo: NASA’s solar-powered Pathfinder airplane. The upper wing surface is covered with lightweight solar panels that power the plane’s propellers. Picture courtesy of NASA Armstrong Flight Research Center.
Contents
- How much energy can we get from the Sun?
- What are solar cells?
- How are solar cells made?
- How do solar cells work?
- How efficient are solar cells?
- Types of photovoltaic solar cells
- How much power can we make with solar cells?
- Power to the people
- Why hasn’t solar power caught on yet?
- A brief history of solar cells
- Find out more
How much energy can we get from the Sun?
Solar power is amazing. On average, every square meter of Earth’s surface receives 163 watts of solar energy (a figure we’ll explain in more detail in a moment). [1] In other words, you could stand a really powerful (150 watt) table lamp on every square meter of Earth’s surface and light up the whole planet with the Sun’s energy! Or, to put it another way, if we covered just one percent of the Sahara desert with solar panels, we could generate enough electricity to power the whole world. [2] That’s the good thing about solar power: there’s an awful lot of it—much more than we could ever use.
Photo: The amount of energy we can capture from sunlight is at a minimum at sunrise and sunset and a maximum at midday, when the Sun is directly overhead.
But there’s a downside too. The energy the Sun sends out arrives on Earth as a mixture of light and heat. Both of these are incredibly important—the light makes plants grow, providing us with food, while the heat keeps us warm enough to survive—but we can’t use either the Sun’s light or heat directly to run a television or a car. We have to find some way of converting solar energy into other forms of energy we can use more easily, such as electricity. And that’s exactly what solar cells do.
What are solar cells?
A solar cell is an electronic device that catches sunlight and turns it directly into electricity. It’s about the size of an adult’s palm, octagonal in shape, and colored bluish black. Solar cells are often bundled together to make larger units called solar modules. themselves coupled into even bigger units known as solar panels (the black- or blue-tinted slabs you see on people’s homes—typically with several hundred individual solar cells per roof) or chopped into chips (to provide power for small gadgets like calculators and digital watches).
Photo: The roof of this house is covered with 16 solar panels, each made up of a grid of 10×6 = 60 small solar cells. On a good day, it probably generates about 4 kilowatts of electricity.
Just like the cells in a battery, the cells in a solar panel are designed to generate electricity; but where a battery’s cells make electricity from chemicals, a solar panel’s cells generate power by capturing sunlight instead. They are sometimes called photovoltaic (PV) cells because they use sunlight (photo comes from the Greek word for light) to make electricity (the word voltaic is a reference to Italian electricity pioneer Alessandro Volta, 1745–1827).
We can think of light as being made of tiny particles called photons, so a beam of sunlight is like a bright yellow fire hose shooting trillions upon trillions of photons our way. Stick a solar cell in its path and it catches these energetic photons and converts them into a flow of electrons—an electric current. Each cell generates a few volts of electricity, so a solar panel’s job is to combine the energy produced by many cells to make a useful amount of electric current and voltage. Virtually all of today’s solar cells are made from slices of silicon (one of the most common chemical elements on Earth, found in sand), although as we’ll see shortly, a variety of other materials can be used as well (or instead). When sunlight shines on a solar cell, the energy it carries blasts electrons out of the silicon. These can be forced to flow around an electric circuit and power anything that runs on electricity. That’s a pretty simplified explanation! Now let’s take a closer look.
How are solar cells made?
Silicon is the stuff from which the transistors (tiny switches) in microchips are made—and solar cells work in a similar way. Silicon is a type of material called a semiconductor. Some materials, notably metals, allow electricity to flow through them very easily; they are called conductors. Other materials, such as plastics and wood, don’t really let electricity flow through them at all; they are called insulators. Semiconductors like silicon are neither conductors nor insulators: they don’t normally conduct electricity, but under certain circumstances we can make them do so.
When we place a layer of n-type silicon on a layer of p-type silicon, a barrier is created at the junction of the two materials (the all-important border where the two kinds of silicon meet up). No electrons can cross the barrier so, even if we connect this silicon sandwich to a flashlight, no current will flow: the bulb will not light up. But if we shine light onto the sandwich, something remarkable happens. We can think of the light as a stream of energetic light particles called photons. 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. The more light that shines, the more electrons jump up and the more current flows.
This is what we mean by photovoltaic—light making voltage—and it’s one kind of what scientists call the photoelectric effect.
How do solar cells work?
Artwork: How a simple, single-junction solar cell works.
A solar cell is a sandwich of n-type silicon (blue) and p-type silicon (red). It generates electricity by using sunlight to make electrons hop across the junction between the different flavors of silicon:
- When sunlight shines on the cell, photons (light particles) bombard the upper surface.
- The photons (yellow blobs) carry their energy down through the cell.
- The photons give up their energy to electrons (green blobs) in the lower, p-type layer.
- The electrons use this energy to jump across the barrier into the upper, n-type layer and escape out into the circuit.
- Flowing around the circuit, the electrons make the lamp light up.
Now for more detail.
That’s a basic introduction to solar cells—and if that’s all you wanted, you can stop here. The rest of this article goes into more detail about different types of solar cells, how people are putting solar power to practical use, and why solar energy is taking such a long time to catch on.
How efficient are solar cells?
A basic rule of physics called the law of conservation of energy says that we can’t magically create energy or make it vanish into thin air; all we can do is convert it from one form to another. That means a solar cell can’t produce any more electrical energy than it receives each second as light. In practice, as we’ll see shortly, most cells convert about 10–20 percent of the energy they receive into electricity. A typical, single-junction silicon solar cell has a theoretical maximum efficiency of about 30 percent, known as the Shockley-Queisser limit. That’s essentially because sunlight contains a broad mixture of photons of different wavelengths and energies and any single-junction solar cell will be optimized to catch photons only within a certain frequency Band, wasting the rest. Some of the photons striking a solar cell don’t have enough energy to knock out electrons, so they’re effectively wasted, while some have too much energy, and the excess is also wasted. The very best, cutting-edge laboratory cells can manage just under 50 percent efficiency in absolutely perfect conditions using multiple junctions to catch photons of different energies.
Chart: Efficiencies of solar cells compared: The very first solar cell scraped in at a mere 6 percent efficiency; the most efficient one that’s been produced to date managed 47.1 percent in laboratory conditions. Most cells are first-generation types that can manage about 15 percent in theory and probably 8 percent in practice.
Real-world domestic solar panels might achieve an efficiency of about 15 percent, give a percentage point here or there, and that’s unlikely to get much better. First-generation, single-junction solar cells aren’t going to approach the 30 percent efficiency of the Shockley-Queisser limit, never mind the lab record of 47.1 percent. All kinds of pesky real-world factors will eat into the nominal efficiency, including the construction of the panels, how they are positioned and angled, whether they’re ever in shadow, how clean you keep them, how hot they get (increasing temperatures tend to lower their efficiency), and whether they’re ventilated (allowing air to circulate underneath) to keep them cool.
Types of photovoltaic solar cells
Most of the solar cells you’ll see on people’s roofs today are essentially just silicon sandwiches, specially treated (doped) to make them better electrical conductors. Scientists refer to these classic solar cells as first-generation, largely to differentiate them from two different, more modern technologies known as second- and third-generation. So what’s the difference?
First-generation
Photo: A colorful collection of first-generation solar cells. Picture courtesy of NASA Glenn Research Center (NASA-GRC).
Over 90 percent of the world’s solar cells are made from wafers of crystalline silicon (abbreviated c-Si), sliced from large ingots, which are grown in super-clean laboratories in a process that can take up to a month to complete. [3] The ingots either take the form of single crystals (monocrystalline or mono-Si) or contain multiple crystals (polycrystalline, multi-Si or poly c-Si).
First-generation solar cells work like we’ve shown in the box up above: they use a single, simple junction between n-type and p-type silicon layers, which are sliced from separate ingots. So an n-type ingot would be made by heating chunks of silicon with small amounts of phosphorus, antimony, or arsenic as the dopant, while a p-type ingot would use boron as the dopant. Slices of n-type and p-type silicon are then fused to make the junction. A few more bells and whistles are added (like an antireflective coating, which improves light absorption and gives photovoltaic cells their characteristic blue color, protective glass on front and a plastic backing, and metal connections so the cell can be wired into a circuit), but a simple p-n junction is the essence of most solar cells. It’s pretty much how all photovoltaic silicon solar cells have worked since 1954, which was when scientists at Bell Labs pioneered the technology: shining sunlight on silicon extracted from sand, they generated electricity.
Second-generation
Photo: A thin-film, second-generation solar panel. The power-generating film is made from amorphous silicon, fastened to a thin, flexible, and relatively inexpensive plastic backing (the substrate). Photo by Warren Gretz courtesy of NREL (image ID #6321083).
Classic solar cells are relatively thin wafers—usually a fraction of a millimeter deep (about 200 micrometers, 200μm, or so). But they’re absolute slabs compared to second-generation cells, popularly known as thin-film solar cells (TPSC) or thin-film photovoltaics (TFPV), which are about 100 times thinner again (several micrometers or millionths of a meter deep). Although most are still made from silicon (a different form known as amorphous silicon, a-Si, in which atoms are arranged randomly instead of precisely ordered in a regular crystalline structure), some are made from other materials, notably cadmium-telluride (Cd-Te) and copper indium gallium diselenide (CIGS). [4]
Because they’re extremely thin, light, and flexible, second-generation solar cells can be laminated onto Windows, skylights, roof tiles, and all kinds of substrates (backing materials) including metals, glass, and polymers (plastics). What second-generation cells gain in flexibility, they sacrifice in efficiency: classic, first-generation solar cells still outperform them. So while a top-notch first-generation cell might achieve an efficiency of 15–20 percent, amorphous silicon struggles to get above 7 percent, the best thin-film Cd-Te cells only manage about 11 percent, and CIGS cells do no better than 7–12 percent. [5] That’s one reason why, despite their practical advantages, second-generation cells have so far made relatively little impact on the solar market.
Third-generation
Photo: Third-generation plastic solar cells produced by researchers at the National Renewable Energy Laboratory. Photo by Jack Dempsey courtesy of NREL (image ID #6322357).
The latest technologies combine the best features of first and second generation cells. Like first-generation cells, they promise relatively high efficiencies (30 percent or more). Like second-generation cells, they’re more likely to be made from materials other than simple silicon, such as amorphous silicon, organic polymers (making organic photovoltaics, OPVs), perovskite crystals, and feature multiple junctions (made from multiple layers of different semiconducting materials). Ideally, that would make them cheaper, more efficient, and more practical than either first- or second-generation cells. [6] Currently, the world record efficiency for third-generation solar is 28 percent, achieved by a perovskite-silicon tandem solar cell in December 2018.
How much power can we make with solar cells?
The total solar energy that reaches the Earth’s surface could meet existing global energy needs 10,000 times over.
European Photovoltaic Industry Association/Greenpeace, 2011.
In theory, a huge amount. Let’s forget solar cells for the moment and just consider pure sunlight. Up to 1000 watts of raw solar power hits each square meter of Earth pointing directly at the Sun (that’s the theoretical power of direct midday sunlight on a cloudless day—with the solar rays firing perpendicular to Earth’s surface and giving maximum illumination or insolation, as it’s technically known).
In practice, after we’ve corrected for the tilt of the planet and the time of day, the best we’re likely to get is maybe 100–250 watts per square meter in typical northern latitudes (even on a cloudless day). That translates into about 2–6 kWh per day (depending on whether you’re in a northern region like Canada or Scotland or somewhere more obliging such as Arizona or Mexico). [11] Multiplying up for a whole year’s production gives us somewhere between 700 and 2500 kWh per square meter (700–2500 units of electricity). Hotter regions clearly have much greater solar potential: the Middle East, for example, receives around 50–100 percent more useful solar energy each year than Europe.
Unfortunately, typical solar cells are only about 15 percent efficient, so we can only capture a fraction of this theoretical energy: perhaps 4–10 watts per square meter. [7] That’s why solar panels need to be so big: the amount of power you can make is obviously directly related to how much area you can afford to cover with cells. A single solar cell (roughly the size of a compact disc) can generate about 3–4.5 watts; a typical solar module made from an array of about 40 cells (5 rows of 8 cells) could make about 100–300 watts; several solar panels, each made from about 3–4 modules, could therefore generate an absolute maximum of several kilowatts (probably just enough to meet a home’s peak power needs).
What about solar farms?
But suppose we want to make really large amounts of solar power. To generate as much electricity as a hefty wind turbine (with a peak power output of maybe two or three megawatts), you need about 500–1000 solar roofs. And to compete with a large coal or nuclear power plant (rated in the gigawatts, which means thousand megawatts or billions of watts), you’d need 1000 times as many again—the equivalent of about 2000 wind turbines or perhaps a million solar roofs. (Those comparsions assume our solar and wind are producing maximum output.) Even if solar cells are clean and efficient sources of power, one thing they can’t really claim to be at the moment is efficient uses of land. Even those huge solar farms now springing up all over the place produce only modest amounts of power (typically about 20 megawatts, or about 1 percent as much as a large, 2 gigawatt coal or nuclear plant). The UK renewable company Ecotricity has estimated that it takes about 22,000 panels laid across a 12-hectare (30-acre) site to generate 4.2 megawatts of power, roughly as much as two large wind turbines and enough to power 1,200 homes. [8]
Photo: The vast 91-hectare (225-acre) Alamosa Solar Generating Project in Colorado generates up to 30 megawatts of solar power using three cunning tricks. First, there are huge numbers of photovoltaic panels (500 of them, each capable of making 60kW). Each panel is mounted on a separate, rotating assembly so it can track the Sun through the sky. And each has multiple Fresnel lenses mounted on top to concentrate the Sun’s rays onto its solar cells. Photo by Dennis Schroeder courtesy of NREL (image ID #10895528).
Power to the people
Photo: A micro-wind turbine and a solar panel work together to power a bank of batteries that keep this highway construction warning sign lit up day and night. The solar panel is mounted, facing up to the sky, on the flat yellow lid you can see just on top of the display.
Some people are concerned that solar farms will gobble up land we need for real farming and food production. Worrying about land-take misses a crucial point if we’re talking about putting solar panels on domestic roofs. Environmentalists would argue that the real point of solar power is not to create large, centralized solar power stations (so powerful utilities can go on selling electricity to powerless people at a high profit), but to displace dirty, inefficient, centralized power plants by allowing people to make power themselves at the very place where they use it. That eliminates the inefficiency of fossil fuel power generation, the air pollution and carbon dioxide emissions they make, and also does away with the inefficiency of transmitting power from the point of generation to the point of use through overhead or underground power lines. Even if you have to cover your entire roof with solar panels (or laminate thin-film solar cells on all your Windows), if you could meet your entire electricity needs (or even a large fraction of them), it wouldn’t matter: your roof is just wasted space anyway. According to a 2011 report [PDF] by the European Photovoltaic Industry Association and Greenpeace, there’s no real need to cover valuable farmland with solar panels: around 40 percent of all roofs and 15 percent of building facades in EU countries would be suitable for PV panels, which would amount to roughly 40 percent of the total electricity demand by 2020.
It’s important not to forget that solar shifts power generation to the point of power consumption —and that has big practical advantages. Solar-powered wristwatches and calculators theoretically need no batteries (in practice, they do have battery backups) and many of us would relish solar-powered smartphones that never needed charging. Road and railroad signs are now sometimes solar powered; flashing emergency maintenance signs often have solar panels fitted so they can be deployed in even the remotest of locations. In developing countries, rich in sunlight but poor in electrical infrastructure, solar panels are powering water pumps, phone boxes, and fridges in hospitals and health clinics.
Why hasn’t solar power caught on yet?
The answer to that is a mixture of economic, political, and technological factors. From the economic viewpoint, in most countries, electricity generated by solar panels is still more expensive than electricity made by burning dirty, polluting fossil fuels. The world has a huge investment in fossil fuel infrastructure and, though powerful oil companies have dabbled in solar power offshoots, they seem much more interested in prolonging the lifespan of existing oil and gas reserves with technologies such as fracking (hydraulic fracturing). Politically, oil, gas, and coal companies are enormously powerful and influential and resist the kind of environmental regulations that favor renewable technologies like solar and wind power. Technologically, as we’ve already seen, solar cells are a permanent work in progress and much of the world’s solar investment is still based on first-generation technology. Who knows, perhaps it will take several more decades before recent scientific advances make the business case for solar really compelling?
One problem with arguments of this kind is that they weigh up only basic economic and technological factors and fail to consider the hidden environmental costs of things like oil spills, air pollution, land destruction from coal mining, or climate change—and especially the future costs, which are difficult or impossible to predict. It’s perfectly possible that growing awareness of those problems will hasten the switch away from fossil fuels, even if there are no further technological advances; in other words, the time may come when we can no longer afford to postpone universal adoption of renewable energy. Ultimately, all these factors are interrelated. With compelling political leadership, the world could commit itself to a solar revolution tomorrow: politics could force technological improvements that change the economics of solar power.
And economics alone could be enough. The pace of technology, innovations in manufacturing, and economies of scale continue to drive down the cost of solar cells and panels. Look what’s happened over the last decade or so. Between 2008 and 2009 alone, according to the BBC’s environment analyst Roger Harrabin, fell by about 30 percent, and China’s increasing dominance of solar manufacturing has continued to drive them down ever since. Between 2010 and 2016, the cost of large-scale photovoltaics fell by about 10–15 percent per year, according to the US Energy Information Administration; overall, the price of switching to solar has plummeted by around 90 percent in the last decade, further cementing China’s grip on the market. Six of the world’s top ten solar manufacturers are now Chinese; in 2016, around two thirds of new US solar capacity came from China, Malaysia, and South Korea.
Photo: Solar cells aren’t the only way to make power from sunlight—or even, necessarily, the best way. We can also use solar thermal power (absorbing heat from sunlight to heat the water in your home), passive solar (designing a building to absorb sunlight), and solar collectors (shown here). In this version, 16 mirrors collect sunlight and concentrate it onto a Stirling engine (the gray box on the right), which is an extremely efficient power producer. Photo by Warren Gretz courtesy of NREL (image ID #6323238).
Catching up fast?
The tipping point for solar is expected to arrive when it can achieve something called grid parity, which means that solar-generated electricity you make yourself becomes as cheap as power you buy from the grid. Many European countries expected to achieve that milestone by 2020. Solar has certainly posted very impressive rates of growth in recent years, but it’s important to remember that it still represents only a fraction of total world energy. In the UK, for example, the solar industry boasted of a milestone achievement in 2014 when it almost doubled the total installed capacity of solar panels from roughly 2.8 GW to 5 GW. But that still represents only a couple of large power stations and, at maximum output, a mere 8 percent of the UK’s total electricity demand of roughly 60 GW (factoring in things like cloudiness would reduce it to some fraction of 8 percent).
According to the US Energy Information Administration, in the United States, where photovoltaic technology was invented, in 2020, solar represented only 3 percent of the country’s total electricity generation. That’s about 2.3 times more than in 2017 (when solar was 1.3 percent), 3.3 times more than in 2016 (when the figure was 0.9 percent) and about 7.5 times as much as in 2014 (when solar stood at just 0.4 percent). [9] Even so, it’s still less than a third as much as coal and 26 times less than all fossil fuels. [10] Even a doubling in US solar would see it producing not much more than half as much electricity as coal does today (10 × 3 = 6 percent, compared to 10 percent for coal in 2020). It’s telling to note that two of the world’s major annual energy reviews, the BP Statistical Review of World Energy and the International Energy Agency’s Key World Energy Statistics, barely mention solar power at all, except as a footnote.
Chart: Solar power is making more of our electricity every year, but still nowhere near as much as coal (which is in steep decline). This chart compares the percentage of electricity generated in the United States by solar power (green line) and coal (red line). The position is better than this in some countries and worse in others. Drawn by explainthatstuff.com using historic and current data from US Energy Information Administration (historic data from that page is available from the Wayback Machine).
Will that change anytime soon? It just might. According to a 2016 paper by researchers from Oxford University, the cost of solar is now falling so fast that it’s on course to provide 20 percent of the world’s energy needs by 2027, which would be a step change from where we are today, and a far faster rate of growth than anyone has previously forecast. modestly, the US EIA predicts that solar will be providing 20 percent of all US electricity by 2050. Can the pace of growth possibly continue? Could solar really make a difference to climate change before it’s too late? Watch this space!
Find out more
On this website
- Climate change and global warming
- Electronics
- Energy
- Passive-solar energy
- Photoelectric cells
- Pyranometers (devices that measure sunlight)
- Renewable energy
- Wind turbines
Books for older readers
Books for younger readers
- Solar Power: Capturing the Sun’s Energy by Laurie Brearley. Scholastic, 2018. A 48-page introduction billed as suitable for grades 3–5, ages 8–10.
- Eyewitness Energy by Jack Challoner. New York/London, England: Dorling Kindersley, 2012: Explains the basic concepts of energy and the history of how people have harnessed it. Grades 4–6; ages 9–12.
- Energy by Chris Woodford. New York/London, England: Dorling Kindersley, 2007: My own colorful little book about energy in the modern world. Ages 9–12.
- Power and Energy by Chris Woodford. New York: Facts on File, 2004. Another of my books, this is a 100-page introduction to humankind’s efforts to harness energy. Suitable for grades 4–6; ages 9–12.
Articles
On other websites
- Solar Research: Lots of information on the latest from the US DOE’s National Renewable Energy Laboratory—home of cutting-edge research into sustainable power.
- Measuring solar insolation: Simple maps from NASA compare the amount of sunlight received by different regions of our planet.
Videos
- Energy 101: Solar PV: The US Department of Energy’s quick introduction explains how solar panels work and summarizes their advantages.
- How solar farms could work: The CSEM company of Switzerland have animated the idea of a solar farm that could work in the oceans or the desert.
- Suncatchers and Sterling Engines: Tessera Solar explain how solar power plants can use Stirling engines to convert thermal energy into electricity.
References
Please do NOT copy our articles onto blogs and other websites
Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.
Text copyright © Chris Woodford 2007, 2022. All rights reserved. Full copyright notice and terms of use.
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Solar Electric Photovoltaic Modules
Photovoltaic (PV) Power
PV is emerging as a major power resource, steadily becoming more affordable and proving to be more reliable than utilities. Photovoltaic power promises a brighter, cleaner future for our children.
Using the technology we have today we could equal the entire electric production of the United States with photovoltaic power plants using only about 12,000 square miles.
In 1839, Edmund Becquerel discovered the process of using sunlight to produce an electric current in a solid material, but it wasn’t until a century later that scientists eventually learned that the photovoltaic effect caused certain materials to convert light energy into electrical energy.
The photovoltaic effect is the basic principal 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. The absorbed light generates electricity.
In the early 1950s, photovoltaic (PV) cells were developed as a spin-off of transistor technology. Very thin layers of pure silicon are impregnated with tiny amounts of other elements. When exposed to sunlight, small amounts of electricity are produced. Originally this technology was a costly source of power for satellites but it has steadily come down in price making it affordable to power homes and businesses.
| Semiconductor device that converts sunlight into direct current (DC) electricity |
| PV modules consist of PV cell circuits sealed in an environmentally protective laminate and are the fundamental building block of PV systems |
| PV panels include one or more PV modules assembled as a pre-wired, field-installable unit |
| A PV array is the complete power-generating unit, consisting of any number of PV modules and panels |
Photovoltaic Cell
A single PV cell is a thin semiconductor wafer made of two layers generally made of highly purified silicon (PV cells can be made of many different semiconductors but crystalline silicon is the most widely used). The layers have been doped with boron on one side and phosphorous on the other side, producing surplus of electrons on one side and a deficit of electrons on the other side.
When the wafer is bombarded by sunlight, photons in the sunlight knock off some of excess electrons, this makes a voltage difference between the two sides as the excess electrons try to move to the deficit side. In silicon this voltage is.5 volt
Metallic contacts are made to both sides of the semiconductor. With an external circuit attached to the contacts, the electrons can get back to where they came from and a current flows through the circuit. This PV cell has no storage capacity, it simply acts as an electron pump.
The amount of current is determined by the number of electrons that the solar photons knock off. Bigger cells, more efficient cells, or cells exposed to more intense sunlight will deliver more electrons.
Photovoltaic Modules
A PV module consists of many PV cells wired in parallel to increase current and in series to produce a higher voltage. 36 cell modules are the industry standard for large power production.
The module is encapsulated with tempered glass (or some other transparent material) on the front surface, and with a protective and waterproof material on the back surface. The edges are sealed for weatherproofing, and there is often an aluminum frame holding everything together in a mountable unit. In the back of the module there is a junction box, or wire leads, providing electrical connections.
There are currently four commercial production technologies for PV Modules:
Single Crystalline This is the oldest and more expensive production technique, but it’s also the most efficient sunlight conversion technology available. Module efficiency averages about 10% to 12%
Polycrystalline or Multicrystalline This has a slightly lower conversion efficiency compared to single crystalline but manufacturing costs are also lower. Module efficiency averages about 10% to 11%
String Ribbon This is a refinement of polycrystalline production, there is less work in production so costs are even lower. Module efficiency averages 7% to 8%
Amorphous or Thin Film Silicon material is vaporized and deposited on glass or stainless steel. The cost is lower than any other method. Module efficiency averages 5% to 7%
Check with manufacturer for module’s accurate conversion efficiency.
Photovoltaic Panels
PV panels include one or more PV modules assembled as a pre-wired, field-installable unit. The modular design of PV panels allows systems to grow as needs change. Modules of different manufacture can be intermixed without any problem, as long as all the modules have rated voltage output within 1.0 volt difference.
Photovoltaic Array
A PV Array consists of a number of individual PV modules or panels that have been wired together in a series and/or parallel to deliver the voltage and amperage a particular system requires. An array can be as small as a single pair of modules, or large enough to cover acres.
12 volt module is the industry standard for battery charging. Systems processing up to about 2000 watt-hours should be fine at 12 volts. Systems processing 2000. 7000 watt-hours will function better at 24 volt. Systems running more than 7000 watt-hours should probably be running at 48 volts.
Follow the link below to see samples of complete photovoltaic-based electrical systems: Configured Solar Electric Systems
Photovoltaic Module Performance
The performance of PV modules and arrays are generally rated according to their maximum DC power output (watts) under Standard Test Conditions (STC). Standard Test Conditions are defined by a module (cell) operating temperature of 25o C (77 F), and incident solar irradiant level of 1000 W/m2 and under Air Mass 1.5 spectral distribution. Since these conditions are not always typical of how PV modules and arrays operate in the field, actual performance is usually 85 to 90 percent of the STC rating.
Today’s photovoltaic modules are extremely safe and reliable products, with minimal failure rates and projected service lifetimes of 20 to 30 years. Most major manufacturers offer warranties of twenty or more years for maintaining a high percentage of initial rated power output. When selecting PV modules, look for the product listing (UL), qualification testing and warranty information in the module manufacturer’s specifications.
Photovoltaic Applications
PV has been routinely used for roadside emergency phones and many temporary construction signs, where the cost and trouble of bringing in utility power outweighs the higher initial expense of PV, and where mobile generator sets present more fueling and maintenance trouble.
than 100,000 homes in the United States, largely in rural sites, now depend on PVs as a primary power source, and this figure is growing rapidly as people begin to understand how clean and reliable this power source is, and how deeply our current energy practices are borrowing from our children.
PV costs are now down to a level that makes them the clear choice not just for remote applications, but for those seeking environmentally safer solutions and independence from the ever-increasing utility power costs.
Photovoltaic Benefits
- Solar power provided by photovoltaic systems lower your utility bills and insulate you from utility rate hikes and price volatility due to fluctuating energy prices
- Installing a solar system increases property value and home resale opportunities
- Purchase of a solar power system allows you to take advantage of available tax and financial incentives
- Because they don’t rely on miles of exposed wires, residential PV systems are more reliable than utilities, particularly when the weather gets nasty.
- PV modules have no moving parts, degrade very, very slowly, and boast a life span that isn’t fully known yet, but will be measured in decades.
- Solar electric systems are quiet, reliable, fossil-fuel free
- Unlike mobile power generators, avoids greenhouse gas emissions
View all PV solar panels available on our shopping cart
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The Ultimate Guide to Solar Cell with Jackery
Whether the source is sunshine or artificial light, solar cells are referred to as photovoltaic. They serve as photodetector devices, such as infrared detectors, that can detect light or other electromagnetic radiation close to the visible range or measure light intensity. Solar cells are frequently grouped to create solar modules, which are then connected to solar panels, which are even larger units.
Some solar-powered gadgets don’t even have an off button and never require batteries. They operate indefinitely as long as there is sufficient sunlight. But how does solar energy use? Of course, the solar cell plays a part. You will discover what a solar cell is, how it functions, how it was developed, and how to choose the best solar panel with Jackery in this post.
What is A Solar Cell
The solar cell is a crucial component of photovoltaic energy conversion, which transforms light energy into electrical energy. Semiconductors are typically utilized as the material for solar cells. Converting energy entails charge carrier separation and the production of electron-hole pairs in a semiconductor from light’s photon energy absorption.
French scientist Edmond Becquerel initially showed the photovoltaic phenomenon in 1839. Based on fabrication methods, solar or photovoltaic technology can be divided into three generations. First-generation solar cells are constructed using silicon, which is both efficient and profitable. The original generation of silicon-based solar cells still accounts for 80% of today’s solar cell production. The first generation consists of monocrystalline silicon solar cells, polycrystalline, amorphous, and hybrid.
Source: Anthony Fernandez
What is Solar Photovoltaic Cell?
A photovoltaic (PV) cell, also known as a solar cell, can either reflect, absorb, or pass through a light that strikes it. The semiconductor material that makes up the PVPV cell can conduct electricity more effectively than an insulator but is less effective than a good conductor like a metal. In PVPV cells, a variety of semiconductor materials are employed.
When a semiconductor is exposed to light, the light’s energy is absorbed and transferred to the semiconductor’s negatively charged electrons. The additional energy enables the electrons to conduct an electrical current through the material. This current can be used to power your home and the rest of the electric grid by extracting it through conductive metal contacts, which are the grid-like lines on solar cells.
How is The Solar Cell Market Today?
The efficiency of solar cells has increased dramatically in recent years, going from reports of roughly 3% in 2009 to over 25% presently. Although solar cells have rapidly increased their efficiency, several obstacles must be overcome before they can be considered viable commercial technology.
Wafer-based PV and thin-film cell PVsPVs are the two primary divisions of photovoltaic technologies. The wafer-based PVsPVs include conventional crystalline silicon cells and gallium arsenide cells, with c-Si cells currently controlling the PVPV market with a market share of roughly 90% and GaAs having the highest efficiency.
What Are Solar Cells Made of
A layer of p-type silicon is sandwiched between a layer of n-type silicon to form a solar cell. There are too many electrons in the n-type layer and too many positively charged holes in the p-type layer. The electrons on one side of the junction (n-type layer) migrate into the holes on the opposite side, which is close to the intersection of the two layers (p-type layer). As a result, a region known as the depletion zone is formed surrounding the connection, where the electrons fill the holes.
The p-type side of the depletion zone now contains negatively charged ions, and the n-type side of the depletion zone now includes positively charged ions when all the holes in the depletion zone have been filled with electrons. These ions’ opposite charges provide an internal electric field that inhibits the n-type layer’s electrons from filling the p-type layer’s holes.
- Purify the Silicon:Silicon dioxide is put in an electric arc furnace, where oxygen is released using a carbon arc. Carbon dioxide and molten silicon are left, but even this is not pure enough to be used in solar cells. This silicon will produce one with just 1% impurities.
- Create Single Crystal Silicon:The Czochralski Method, in which a seed silicon crystal is dipped into molten polycrystalline silicon, is the most used technique for producing single-crystal silicon.
- Cut the Wafers:A circular saw is used to slice the second-stage boule into silicon wafers. The best raw material for this task is diamond, which produces silicon slices that can then be further cut to create squares or hexagons that are simpler to slot together into the surface of a solar cell.
- Doping:This technique, also known as doping, often entails firing phosphorous ions into the ingot using a particle accelerator.
- Add Electrical Contacts:Electrical contacts serve as a conduit for the current generated by solar cells and connect them. These connections, made of metals like palladium or copper, are thin so as not to prevent sunlight from reaching the cell.
- Add Anti-Reflective Coating:To lessen the quantity of sunlight lost through reflection, an anti-reflective coating is put on the silicon.
- Encapsulate the Cell:To complete the process, the solar cells are sealed in silicon rubber or ethylene vinyl acetate and mounted in an aluminum frame with a glass or plastic cover for added protection and a back sheet.
Source: US Energy Information Administration
How Does A Solar Cell Work
The solar cell is a technological innovation that directly converts light energy into electricity through the photovoltaic effect, creating electrical charges free to travel through semiconductors. All solar cells share a similar fundamental design. An optical coating or antireflection layer that reduces the quantity of light lost through reflection allows light to enter the system. As a result, the light is trapped and is more likely to reach the layers below that do energy conversion. Spin-coating or vacuum deposition creates this top antireflection layer, commonly an oxide of silicon, tantalum, or titanium.
Below the top antireflection layer are three energy conversion layers. These are the top junction layer, the absorber layer, and the back junction layer. Two additional electrical contact layers carry the electric current to an external load and then back to the cell to complete the electric circuit. A solar cell is a sandwich of n-type silicon and p-type silicon. It generates electricity by using sunlight to make electrons hop across the junction between the different flavors of silicon:
- Photons (light particles) pelt the cell’s upper surface when sunlight shines.
- The photons (yellow blobs) transport energy through the cell at a downward angle.
- In the lower p-type layer, photons transfer their energy to electrons (green blobs).
- With the help of this energy, the electrons can penetrate the barrier into the top n-type layer and break out into the circuit.
- As the electrons move across the circuit, the lamp begins to glow.
Source: Advanced Renewable Energy Systems
The Types of Solar Cells
The three main categories of solar cells are crystalline silicon-based, thin-film solar cells, and a more recent innovation that combines the other two. P-type and n-type silicon are two types of semiconductors used to make solar cells. Atoms with one fewer electron in their outer energy level than silicon, like boron or gallium, are added to create p-type silicon.
Crystalline Silicon Cells
Crystalline silicon (c-Si) wafers, cut from massive ingots manufactured in laboratories, make about 90% of solar cells. These nuggets can develop into single or numerous crystals and can take up to a month to grow. Monocrystalline solar panels are made from a single crystal, whereas polycrystalline are made from multiple crystals.
Thin Film Solar Cells
While thin-film solar cells, also known as thin-film photovoltaics, are around 100 times thinner than crystalline silicon cells, they are still produced from wafers that are only a tiny fraction of a millimeter deep (about 200 micrometers, or 200m). Amorphous silicon (a-Si), in which the atoms are randomly organized rather than in an ordered crystalline structure, is the material used to create these thin film solar panels and cells. These films can also be produced using organic photovoltaic (PVPV) materials, copper indium gallium diselenide (CIGS), and cadmium-telluride (Cd-Te).
Third Generation of Solar Cells
The most recent solar cell technologies combine the best aspects of thin-film and crystalline silicon solar cells to deliver high efficiency and enhanced usability. They frequently have several junctions made up of various semiconducting materials’ layers. Also, they are typically made of amorphous silicon, organic polymers, or perovskite crystals.
Solar Cell Development
Solar energy already offers consumers many advantages while reducing the harmful environmental effects of fossil fuel power generation. Switching to solar energy has benefits on a more local level and reduces air pollution and carbon dioxide emissions because it locates power generation at the point of use.
Smaller devices like watches and calculators may now operate without batteries, and road and train maintenance signs can now be powered by the sun so that they can be used in even the most remote regions. In some nations, solar energy is used to power telephone booths, water pumps, and even refrigeration systems in medical facilities. As the supply of fossil fuels declines, there will be a greater need to turn to renewable energy sources, such as solar.
Best Portable Solar Panels with Jackery
Jackery portable solar panels can be folded and strapped for easy carrying and use. One of the highest efficiency rates in the industry makes it possible to maximize the sun’s energy and transform it into clean energy. Use solar power and the Jackery rechargeable portable power stations to keep your equipment charged. For better backup power, it is the advanced off-grid solar generating system.
With mono crystalline solar cells and adjustable supports, Jackery portable solar panels have a charging efficiency of up to 25%. Make the most of solar energy’s potential. The solar panel can be easily connected to your power source. Connect the DCDC input of your portable power source to the DCDC interface.
Jackery SolarSaga 200W Solar Panel
The Jackery SolarSaga 200W solar panels are your best option if you want a solution that can power your entire home. Solar panels can generate more power under similar circumstances with a higher conversion rate of 24.3%. A Jackery Portable Power Station Explorer 2000 Pro can be fully charged in 2.5 hours using 6 SolarSaga 200. Additionally compatible with various Jackery power stations is the portable solar panel.
Compatible With
Recharging Time
Conversion Efficiency
Explorer 2000 Pro 6SolarSaga 200