Energy from the Sun is not only freely available but also the most abundant form of energy on the planet. The Earth receives twenty-five times more power from the Sun than we use in the world every day. To harness some of that energy, we use solar panels.
Solar energy’s contribution to the total energy supply of humanity has grown significantly in the last couple of decades. In this article, we’ll deal with how solar panels are made as well as how their photovoltaic cells produce electricity.
How are Solar Panels Made?
To utilize the plentiful energy provided by our Sun, we require the help of one of the most abundant materials on earth – sand. Sand has to be converted to 99.999% pure silicon crystals to use in solar cells and has to go through a complex purification process to achieve this. The process consists of mixing it with carbon and applying temperatures of up to 2000°C.
The result is a gaseous silicon compound form that the raw silicon gets converted to, which is then mixed with hydrogen to obtain highly purified polycrystalline silicon. Scientists then reshape these silicon ingots and turn them into silicon wafers – very thin slices of silicon. The silicon wafer is the heart of a photovoltaic cell.
If you were to look into the structure of the silicon molecule, you would see that the atoms bond together, which means the electrons in the structure lose their freedom of movement. A sufficient amount of energy needs to be added to the system to allow the particles to move freely. This is where the Sun comes in.
How do Solar Cells Work?
When sunlight strikes the silicon, the electrons will gain photon energy and will be free to move once again. However, at this moment, the movement of the particles is random, and it does not result in any current through the load.
A driving force is needed to ensure the unidirectional flow of electrons. An easy and practical way to produce the driving force is a P-N junction. Boron, with three valence electrons, is injected into pure silicon to create ‘holes’ for the particles to move towards. When these two materials are connected, some electrons will migrate into this positively-charged region and fill the holes available there. This creates a depletion region with no free electrons and holes.
Due to the electron migration, the boundary of the negative side becomes slightly positively charged, while the positive side becomes slightly negatively charged. Fundamental physics tells us that an electric field will form between these charges. This is the electric field that produces the necessary driving force to move the electrons in our desired direction.
When light strikes the solar panel, it hits the negatively-charged region of the PV-cell, called the N-region, and it penetrates it, reaching down to the depletion region. The energy inserted into the system in this way is enough to generate electron-hole pairs in the depletion region.
Holes and electrons are driven out of the depletion region by this electric field, increasing the concentration of both the electrons in the N-region as well as holes in the P-region. This creates a difference between them, which allows the electrons from the N-region to flow through the load we connect to the panel and complete their path, recombining with the holes in the P-region. This is how a solar cell continuously gives a direct current.
In a practical solar cell, the top N-Layer is very thin and abundant with electrons, whereas the P-layer is a lot thicker. Solar cells are constructed in this way to increase their performance. In this type of arrangement, the electron-hole pairs are generated in a broader area, resulting in more current generation by the PV cell. The other advantage is that due to the thin top layer, more light energy can reach the depletion region.
Solar Panel Materials
A solar panel has several layers. One of them is a layer of cells, connected in series through copper strips. Another is a layer of EVA sheeting on each side of the cells to protect them from dirt, humidity, vibrations, and shocks.
For ground-mounted or rooftop installations, two main types of solar panels exist– thin-film and crystalline silicon.
Crystalline silicon solar modules are the most common. They look like black or blue rectangular grids consisting of smaller square-shaped cells. Those are silicon solar cells, and they link together in a series to form a circuit—the more interconnected cells in a series, the more electricity produced by the system.
When you connect these series-connected solar cells parallel to another cell series, you get the solar panel. Only around 0.5V is produced by a single photovoltaic cell. The combination of parallel and series connection of the cells increases the current and voltage values to a usable range.
Monocrystalline vs. Polycrystalline
Two types of crystalline silicon designs exist – monocrystalline and polycrystalline. The difference between the internal crystalline lattice structure leads to different appearances between these two types of panels. Even though the principles of operation do not differ, monocrystalline cells are capable of producing higher electrical conductivity. On the other hand, their more complex production process makes monocrystalline cells costlier, which contributes to their more limited use.
Molten silicon is poured into a cast to create polycrystalline solar cells. Because of this method of construction, however, the crystal structure forms imperfectly, with randomly oriented crystals showing clear boundaries where the crystal formation was broken. This gives the polycrystalline silicon solar panel its distinctive grainy appearance, as the gemstone-type pattern highlights the boundaries in the crystal. The lower efficiency and lower price of polycrystalline modules are due to these impurities.
Monocrystalline silicon solar is created by growing a single crystal. In a monocrystalline solar cell, the crystal framework produces a uniform blue color without any grain marks, giving the highest efficiency levels and best purity.
Less Common Variants
The other type of modules – thin film, are more traditionally used in large utility-scale installations. A thin semiconductor adheres to the metal foil, plastic, or glass substrates. As its name suggests, the thin film can be fragile and sometimes flexible. Its lightweight and flexibility have led to its use on automobiles, curved roofs, and other unique installations.
Most solar panels consist of solar cells with a glass layer on the front and a protective back sheet on the rear. With new bifacial designs, where solar cells are exposed on both the front and back sides of the panel, back sheets are no longer needed. Some manufacturers are also deciding to go frameless, choosing to either use sturdier back sheets that don’t warrant a frame or sandwich solar cells between two pieces of glass.
It all sounds relatively simple. All you need is a semi-conductor for the photovoltaic effect, and a conductor to allow electricity to flow, but don’t think you can build your own solar panels just yet.
There are a lot of details at play when it comes to any decent level of efficiency, and that is a real engineering challenge. The basic idea is simple, but the details are incredibly tricky. Complexity aside, solar energy provides a sustainable way to power our lives, and all that is thanks to silicon, the Sun, and some intelligent engineering.