A photovoltaic cell, which specifically converts sunlight into electrical energy, is one of the most promising technologies towards achieving global renewable energy goals. Crystalline silicon solar cells have become mainstream because of their higher conversion efficiency, yet they are restricted to their massive size and higher cost of production. However, in the last few years, there has been new innovations in this sector giving rise to silicon thin-film materials which is extremely encouraging because of their accessibility, light weight, lower cost, and less demanding manufacture technique.
Amorphous silicon (a-Si) is the non-crystalline form of silicon. It is primarily used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells where it is deposited in thin films onto a variety of substrates, such as glass, metal or plastic. Though this technology has been present for many years now, with wide usage in small electronic items such as pocket calculators, etc., it was mainly used for very small power requirements. The low efficiency of the cells made them unviable for larger commercial usage. However, the recent developments in thin film solar cell technology with increase in efficiency, yield and durability all point towards a promising future for this technology.
Amorphous silicon panels are formed by depositing a thin layer of silicon material on a substrate material such as glass or metal. Amorphous silicon can be deposited at very low temperatures, as low as 75 degrees Celsius, which allows for deposition on plastic as well. In its simplest form, the cell structure has a single layers. However, single layer cells suffer from significant degradation in their power output when exposed to the sun. Better stability requires the use of a thinner layers in order to increase the electric field strength across the material. This has led the industry to develop double and even triple layer devices that contain cells stacked one on top of the other.
While crystalline silicon achieves a yield of about 18 percent, amorphous solar cells’ yield remains at around 7 percent. The low efficiency rate is partly due to the Staebler-Wronski effect, which manifests itself in the first hours when the panels are exposed to sunlight, and results in a decrease in the energy yield of an amorphous silicon panel from 10 percent to around 7 percent. However, new adaptations in the production processes of amorphous silicon modules using Silane Gas has resulted in increase of output without any additional costs. Banking on improvements in light-trapping, high-rate deposition, and a HybridNano technology, the efficiency figures will substantially increase with each passing year.
The principal advantage of amorphous silicon solar cells is their lower manufacturing costs, which makes these cells very cost competitive. One of the main advantages of a-Si over crystalline silicon is that it is much more uniform over large areas. Since amorphous silicon is full of defects naturally, any other defects, such as impurities, do not affect the overall characteristics of the material too drastically. Amporphous silicon can be produced in a variety of shapes and sizes (e.g., round, square, hexagonal, or any other complex shape. This makes it an ideal technology to use in a variety of applications such as BIPV applications, powering garden lights, car accessories, powering electronic devices, etc. Unlike crystalline solar cells in which cells are cut apart and the recombined, amorphous silicon cells can be connected in series at the same time the cells are formed, making it is easy to create panels in a variety of voltages. The development process of a-Si solar panels also renders them much less susceptible to breakage during transport or installation. This can help reduce the risk of damaging your significant investment in a photovoltaic system.
As mentioned previously, these panels have a lower efficiency than mono-crystalline solar cells, or even poly-crystalline solar cells. Attempts to increase the efficiency, such as building multi-layer cells or alloying with germanium to reduce its band gap and further improve light absorption all have an added complexity. Namely, the processes are more complex and process yields are likely to be lower and costs are likely to be higher as a result – thus reducing the cost advantage of this type of solar cell. Another disadvantage is the expected lifetime of amorphous cells is shorter than the lifetime of crystalline cells, although how much shorter is difficult to determine, especially as the technology continues to evolve.
Despite the many advantages of thin-film silicon (Si) solar cells, their low efficiencies remain a challenge that must be overcome. Efficient light utilization across the solar spectrum is required to achieve efficiencies over 15%, allowing them to be competitive with other solar cell technologies.