Why waste our time digging for oil or shoveling coal when there is a huge power station high above us that sends out free, clean energy? The Sun, a smoldering ball of nuclear energy, has enough fuel to power our Solar System for five billion more years. Solar panels can convert this energy into an inexhaustible supply of electricity.
Although solar power may seem futuristic or strange, it is already very common. A solar-powered watch or calculator for your pocket might be on your wrist. Many gardeners have solar-powered lights. Solar panels are often found on satellites and spaceships. NASA, the American space agency, has even created a solar-powered plane. Global warming is threatening our environment and it seems certain that solar energy will be an increasingly important source of renewable energy. How does it work?
What is the maximum amount of solar power we can get from the Sun?
It is incredible how solar power works. Each square meter on Earth receives an average 163 watts solar energy. We’ll discuss this figure in detail in a moment. This means that you could place a 150 watt table lamp on every square meter of Earth and use the Sun’s electrical energy to light up the entire planet. Another way to put it, if we covered only 1% of the Sahara desert in solar panels, we could produce enough electricity to solar power the entire world. The great thing about solar energy is that there’s a lot of it, much more than we could ever need.
There is a downside. The Sun’s energy arrives as a mixture of light and heat. Both are vital. The light is what makes plants grow and provides food for us. Heat keeps us warm enough to live. However, we cannot use the Sun’s heat or light directly to solar power a TV or car. It is necessary to convert solar energy into another form of energy that we can use more easily such as electricity. That’s precisely what solar cells do.
- The cell’s surface is illuminated by sunlight
- Photons carry energy through the cells’ layers.
- Photons transfer their energy to electrons in lower layers
- This energy is used by electrons to escape from the circuit and jump back into the upper layers.
- The power for a device is provided by the electrons that flow around the circuit.
What are solar cells?
A solar cell is an electronic device which captures sunlight and converts it into electricity. It is about the same size as an adult’s hand, octagonal in form, and colored bluish-black. Many solar cells can be bundled together to create larger units called modules. These are then connected into bigger units known by solar panels. (The black- or blue-tinted tiles you see on homes – usually with hundreds of individual solar cells per roof) Or chopped into chips (to power small gadgets such as digital watches and pocket calculators).
The cells of a solar panel work in the same way as a battery. However, unlike a battery’s cells that produce electricity from chemicals, solar panels’ cells capture sunlight to create electricity. Photovoltaic cell (PV), as they make electricity from sunlight (photo comes from the Greek word meaning light). The term “voltaic”, however, is a reference to Alessandro Volta (1745-1827), an Italian electricity pioneer.
Light can be thought of as tiny particles called photons. A beam of sunlight is like an enormous yellow firehose that shoots trillions upon trillions. A solar cell can be placed in the path of these photons to capture them and then convert them into an electric current. Each cell produces a few volts, so the job of a solar panel is to combine energy from many cells to produce a useful amount of electric current and voltage. Today’s solar cells are almost all made of slices of silicon (one the most common chemical elements found on Earth, found within sand). However, as we’ll soon see, other materials may also be possible. The sun’s energy blasts electrons from a solar cell when it is exposed to sunlight. They can then be used to power any electrical device that is powered by electricity.
How are solar cells made?
Silicon is the material from which microchips’ transistors (tiny switches), are made. Solar cells also work in a similar manner. A semiconductor is a type of material. Conductors are materials that allow electricity to flow easily through them, such as metals.
Others, like plastics and wood, don’t allow electricity to flow through them; they are called insulation. Semiconductors such as silicon are not conductors or insulators. However, we can make them conduct electricity under certain conditions.
A solar cell is made up of two layers of silicon, each one having been doped or treated to allow electricity to flow through it in a specific way. The lower layer has slightly less electrons because it is doped. This layer is called p-type, or positive-type silicon. It has too many electrons and therefore is negatively charged. To give the layer an excess of electrons, it is doped in the opposite direction. This is referred to as n-type and negative-type silicon. (Read more about doping and semiconductors in our articles on integrated circuits and transistors.
A barrier is formed at the junction between two layers of n-type and p-type silica. This barrier is the crucial border where both types of silicon meet. The barrier is inaccessible to electrons so even if the silicon sandwich is connected to a flashlight, the current won’t flow and the bulb will not turn on. However, if you shine light on the sandwich, it will produce something amazing. The light can be thought of as a stream or “light particles”, which are energetic, called photons. Photons that enter the sandwich give up their energy to the silicon atoms as they pass through. The incoming energy knocks electrons from the lower, p type layer. They then jump across the barrier to reach the n-type above and flow around the circuit. The more light there is, the more electrons will jump up and more current will flow.
How efficient are Solar Panels?
The law of conservation energy, a fundamental rule of physics, states that energy cannot be created or made to disappear into thin air. We can only convert it from one form of energy to another. A solar cell cannot produce more electricity than it gets in light each second. As we will see, most solar cells can convert between 10-20% of the energy they receive to electricity. The theoretical maximum efficiency of a typical single-junction silicon solar panel is about 30%. This limit is known as the Shockley Queisser limit. Because sunlight is a wide variety of wavelengths and energies, any single-junction silicon solar cell will only capture photons within a narrow frequency range. The rest of the photons will be wasted. Some photons that strike a solar cell are too weak to produce enough electrons. Others have too much energy and are wasted. In the most ideal conditions, laboratory cells with cutting-edge technology can achieve just below 50 percent efficiency. They use multiple junctions to capture photons of different energies.
A real-world domestic panel might have an efficiency of around 15 percent. Single-junction, first-generation solar cells won’t achieve the 30 percent efficiency limit set by Shockley-Queisser, or the laboratory record of 47.1 percent. There are many factors that can affect the nominal efficiency of solar cells, such as how they are constructed, angled and positioned, whether they are ever in shadow, how clean they are, and how cool they are.
Different types of Photovoltaic Cell
The majority of solar cells that you will see today on roofs are simply silicon sandwiches. They have been “doped” to improve their electrical conductivity. These classic solar cells are called first-generation by scientists to distinguish them from the two more advanced technologies, second- and third generation. What is the difference?
First-generation Solar Cells
More than 90 percent of the world’s solar cell production is made from wafers containing crystalline silicon (abbreviated “c-Si”), which are sliced from large ingots. This process can take as long as a month and takes place in super-clean laboratories. Ingots can be single crystals (monocrystalline solar panels) or multi-crystalline (polycrystalline solar panels), depending on whether they contain multiple crystals.
First-generation solar cells function as we have shown them in the box above. They use one, simple junction between n and p-type layers of silicon, which is cut from separate ingots. An n-type ingot is made by heating small pieces of silicon with small amounts (or antimony or phosphorus) as the dopant. A p-type one would use boron. The junction is made by fusing slices of p-type and n-type silicon. There are a few extra bells and whistles that can be added to photovoltaic cells (like an antireflective layer, which increases light absorption and gives them their blue color), and metal connections so they can be wired into circuits. But a simple p–n junction is what most solar cells rely on. This is how photovoltaic solar cells have been working since 1954 when Bell Labs scientists pioneered it: by shining sunlight onto silicon sand they produced electricity.
Second-generation Solar Cells
The classic solar cells are thin film solar cell wafers. They’re usually only a fraction of millimeter thick (around 200 micrometers or 200mm). They aren’t as thin as second-generation solar cells (TPSC), or thin film solar cells, which are 100 times thinner (several millimeters or millionths of a meter deep). While most of them are still made of silicon (a form called amorphous siliu, a-Si), in which atoms are arranged in random crystalline structures, some are made out of other materials such as cadmium-telluride, Cd-Te, and copper indium gallium diselenide, (CIGS).
Second-generation cells are extremely thin and light and can be laminated to windows, skylights and roof tiles. They also work well with all types of “substrates”, which are backers such as metals and plastics. Second-generation cells have less flexibility than first-generation ones, but they still perform better than them. A top-quality first-generation cell may achieve efficiency of 15-20%, but amorphous silicon struggles to get above 7%) while the best thin-film CdTe cells manage only about 11 percent and CIGS cells no better than 7-12%. This is one of the reasons why second-generation solar cells have not had much success in the market despite their many practical benefits.
Third-generation Solar cells
These new technologies combine the best characteristics of both first- and 2nd generation cells. They promise high efficiency (up to 30 percent) just like first-generation cells. They are more likely to be made of materials other than silicon (making second-generation photovoltaics, OPVs), and perovskite crystals. Additionally, they may feature multiple junctions (made up of multiple layers from different semiconducting material). They would be more affordable, more efficient, and practical than first- or second generation cells. The current world record for efficiency of third-generation solar cells is currently 28 percent. This was achieved in December 2018 by a tandem perovskite-silicon solar cell.
How are they made?
As you can see, there are seven steps to making solar cells.
Stage 1: Purify Silicon
The silicon dioxide is heated in an electric furnace. To release the oxygen, a carbon arc can be applied. The result is carbon dioxide and molten silica, which can be used to make solar systems. However, even though this yields silicon with only 1% impurity it is still not good enough. The floating zone technique is a method that allows the 99% pure silicon rods to be passed through a heated zone several times in the same direction. This process removes all impurities from one end of the rod and allows it to be removed.
Stage 2: Making Single Crystal Silicon
Czochralski Method is the most popular method for creating single-crystalline silicon. This involves placing a seed crystal made of silicon in melted silicon. This creates a boule or cylindrical ingot by rotating the seed crystal while it is being removed from the melted silicon.
Stage Three: Cut the Silicon Wafers
The second stage boule is used to cut silicon wafers with a circular saw. This job is best done with diamond, which produces silicon slices that can be further cut to make squares or hexagons. Although the saw marks are removed from the sliced wafers, some manufacturers leave them because they believe that more light may be absorbed by rougher solar cell efficiency.
Stage 4: Doping
After purifying the silicon at an earlier stage, it is possible to add impurities back into the material. Doping is the use of a particle accelerator to ignite phosphorus ions in the ingot. You can control the penetration depth by controlling the speed of the electrons. You can skip this step by using the traditional method of inserting boron during cutting the wafers.
Stage Five: Add electrical contacts
The electrical contacts are used to connect the solar system and act as receivers for the generated current. These contacts, made of metals like palladium and copper, are thin to allow sunlight to enter the solar cell efficiently. The metal is either deposited on the exposed cells or vacuum evaporated using a photoresist. Thin strips of copper coated with tin are usually placed between the cells after the contacts have been installed.
Stage Six: Apply the Anti-Reflective Coating
Because silicon is shiny, it can reflect up to 35% of sunlight. To reduce reflections, a silicon coating is applied to it. This is done by heating the material until the molecules boil off. The molecules then travel onto the silicon and condense. A high voltage can also be used to remove the molecules and deposit them onto the silicon at the opposite electrode. This is called “sputtering”.
Stage Seven: Encapsulate and Seal the Cell
The solar cells are then encapsulated with silicon rubber or ethylene vinyl Acetate. Finally, they are placed in an aluminum frame with a back sheet and glass cover.
What amount of electrical energy can solar cells produce?
Theoretically, it is a lot. For the moment, let’s forget about solar cells and focus on pure sunlight. Each square meter of Earth can receive up to 1000 watts of solar power. This is the theoretical power of direct sunlight on a clear day. The solar rays are firing perpendicularly to the Earth’s surface, giving maximum illumination.
After we adjust for the tilt of our planet and the time, we can expect to get 100-250 watts per sq. meter in northern latitudes, even on cloudless days. This translates to about 2-6 kWh per daily. Multiplying the entire year’s production yields 700 to 2500 kWh per sq. m (700-2500 units) of electricity. The sun’s energy potential in hotter regions is clearly greater than Europe. For example, the Middle East receives between 50 and 100 percent more solar energy per year than Europe.
Unfortunately, solar cells are only around 15 percent efficient so we can only capture 4-10 watts per square foot. This is why panels with solar power must be large: how big you are able to cover with cells will directly affect the power you can generate. A typical solar panel made up of 40 cells (each row of 8 cells) will produce about 3-4.5 watts. However, a solar panel made up of 3-4 modules could generate several kilowatts, which is enough to power a home’s peak energy needs.
How about Solar Panel Farms?
However, what if we need to generate large amounts of solar energy? You will need between 500 and 1000 solar roofs to generate the same amount of electricity as a large wind turbine with a peak output of about two or three megawatts. To compete with large nuclear or coal power plants (rated in the gigawatts), you would need about 1000 solar roofs. This is equivalent to approximately 2000 wind turbines and perhaps a million of them. These comparisons assume that our solar and wind produce maximum output. Even though solar cells can produce clean, efficient power, they cannot claim to be efficient land uses. Even the huge solar farms that are popping up all over the country produce modest amounts of power, typically around 20 megawatts or 1 percent less than a large 2 gigawatt nuclear or coal plant. Nevada Solar Group, a renewable company, estimates that it takes approximately 22,000 panels to cover a 12-hectare (30-acres) area to produce 4.2 megawatts. This is roughly the same amount as two large wind turbines. It also generates enough power to power 1,200 homes.
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