Most of the solar panels available on the market today can be roughly divided into three types: monocrystalline, polycrystalline and thin film . They differ from each other in production technology, appearance, performance, cost and scope. Poly and monocrystalline panels account for almost 85% of market sales. But thin-film (amorphous) solar panels are considered promising due to their own advantages and a wide range of applications.
They work in very hot climates and overcast/cloudy weather, are especially effective in diffused light, and small-sized amorphous silicon photocells are actively used for domestic needs, integrated into the exterior of buildings, camping chargers for gadgets, pocket calculators.
For a long time, the low power output of amorphous silicon solar cells limited their use to only a narrow range of applications. This problem was partially solved by layering several amorphous solar cells on top of each other, which increases their performance and makes them more compact.
How are amorphous silicon panels made?
Amorphous silicon (a-Si) is a non-crystalline form of silicon. The word “amorphous” literally means formless. Such a silicon material is not structured and crystallized at the molecular level, as in many other types of silicon-based solar cells.
Amorphous silicon panels are a well-established thin film technology that has been on the market for about two decades. Such modules are formed by deposition of silicon vapor in a thin layer of about 1 micrometer (sometimes ranging from several nanometers to tens of micrometers) on a substrate material, such as glass or metal. Amorphous silicon can also settle at very low temperatures, down to 75°C, which also allows it to be applied to plastic or other flexible materials that let the sun’s rays through. It is precisely because of the ability to use a flexible base that thin-film modules have a wide range of applications.
In its simplest form, an amorphous module structure has one layer. But single-layer elements have a low output power. For greater stability, it is necessary to increase the electric field strength on the material. However, this reduces the absorption of light and, consequently, the efficiency of the modules. This has led the industry to develop two- and even three-layer devices that contain contact modules stacked on top of each other.
Efficiency of thin-film amorphous silicon panels
Initially, if we compare the output power of thin-film panels and crystalline silicon panels, the figure does not speak in favor of thin films: 18-22% versus about 7-8%. This low efficiency is partly due to the Stabler-Wronsky effect (photoinduced degradation) that occurs in the first hours the panels are exposed to sunlight. This phenomenon causes the energy yield of the amorphous silicon panel to decrease from 10 percent to about 7 percent.
However, with the development of technology in recent years, these indicators are rapidly improving. Today, scientists and engineers working to improve the panels have gradually brought the efficiency of amorphous panels to 12.5% in the laboratory. The efficiency of amorphous silicon solar cells, which are produced in large volumes, ranges from 6% to 9%.
Now there are three generations of thin-film panels based on amorphous silicon.
1st generation. Here refers to the ancestor of technology – a single-junction solar battery. It is characterized by a short service life (up to 10 years) and a low level of productivity (about 5%).
2nd generation. The same single-junction solar battery with increased efficiency (up to 8%).
3rd generation . Today’s most highly efficient thin-film amorphous panels with a service life of more than 10 years and a productivity level of 12-12.5%.
Most often, thin-film panels are used for solar systems operating on an industrial scale with dependent (slave, grid-tie) inverters and supplying electricity to the general network, since amorphous modules show the highest efficiency when used in powerful systems (from 10 kW).
Advantages of amorphous panels
- The production of elements from amorphous silicon is waste-free, which significantly reduces the cost of thin-film panels. Therefore, low production cost is their main advantage and makes amorphous silicon PV modules very competitive in the solar panel market.
- Substrates/bases of amorphous panels can be made of both metal and inexpensive materials such as glass and plastic.
- Better uniformity over large areas. This is due to the fact that amorphous silicon itself is full of defects. Therefore, any other defects, such as impurities, do not affect the overall characteristics of the material too much.
- Amorphous silicon can be produced in various shapes and sizes (round, square, hexagonal, or any other complex shape). This makes it an ideal technology for use in a variety of applications such as powering electronic calculators, solar wristwatches, garden lights, and automotive accessories.
- The human eye is sensitive to light with wavelengths between 400 and 700 nanometers. Since amorphous silicon solar cells are sensitive to light with almost the same wavelengths, this means that in addition to being used as solar cells, they can also be used as light sensors (for example, outdoor light sensors, etc.).
- Flexibility, light weight, easy installation. The strength and flexibility of amorphous silicon thin film panels are determined by the surfaces or substrates to which the thin film solar cells are attached. The flexible thin film module gives designers and installers more room to be more creative when it comes to panel application. For example, they can be placed on curved surfaces, and among the developments there is even their use in clothing.
- Thin film solar cells perform relatively well in poor lighting conditions and are less affected by shading, generating up to 20% more power in diffuse or weak sunlight compared to crystalline panels. They have less power loss in cloudy weather. They have a better cost per watt of power.
- The manufacturing process and technology of a-Si solar panels makes them much less susceptible to breakage during shipping or installation than crystalline panels. Thin-film amorphous panels are less likely to be defective.
- Another fundamental advantage of this type of technology is greater resistance to heat, heat. For example, according to research by the US National Renewable Energy Laboratory, it was found that when the temperature rises, amorphous silicon PV modules perform better than crystalline silicon panels.
- Much less silicon is required to produce thin-film panels. Because amorphous silicon is a direct bandgap material, amorphous modules require only about 1% of the silicon that would be used to produce crystalline silicon photovoltaic cells.
- Amorphous silicon thin film panels are well suited for large area solar power plants and wall-mounted solar power plants.
- All of the above benefits can help reduce the risk of significant investment in a photovoltaic system using such panels.
Disadvantages of Amorphous Silicon Thin Film Panels
- Amorphous thin film solar cells have lower efficiency than mono- and polycrystalline ones. Attempts to increase it, for example, by creating multilayer modules or doping with germanium to reduce the band gap and further improve light absorption, is a rather complicated process that makes production more expensive. If these processes are implemented, thin-film panels will cost more and thus lose their competitive advantage.
- Thin film modules tend to break down faster than mono- and polycrystalline solar panels. The life expectancy of amorphous batteries is less than that of crystalline ones. It is not always possible to determine how much, especially given the constant development of technologies used in the production of thin-film panels. To date, the service life of such panels, on average, is from 10 to 25 years, and in power they lose from about 10 to 40% in the first few seasons, after which their power is fixed at this indicator and, as a rule, no longer falls. . Many manufacturers promise an output power of about 80% by the end of the service life.
- Large occupied area. It is necessary to cover a much larger area (about 2.5 to 3 times) with amorphous silicon solar panels than with crystalline based panels to get equal power.
Solar panels made of amorphous silicon are constantly being improved and are encouraging with their prospects today. They are lighter in weight, more flexible and potentially cheaper to manufacture. On the other hand, this technology must become more mature in order to compete with mono- and polycrystalline solar cells.