Crystalline silicon or c-Si encompasses the crystalline forms of silicon. It is used as umbrella term to describe polycrystalline silicon (multi-Si) and monocrystalline silicon (mono-Si), the two dominant semiconducting materials used in photovoltaic technology for the production of solar cells.
However, in the electronics industry, the term crystalline silicon refers exclusively to monocrystalline silicon, as only this material is used in the production of microchips. Moreover, the purity of c-Si in electronics is required to be higher than for solar cells.
Solar cells made of crystalline silicon are often called conventional or traditional solar cells as they account for 90 percent of current worldwide PV production and were first developed more than half a century ago at Bell Labs in 1954. They are also called wafer-based solar cells, as c-Si solar cells are made of about 160 µm thick solar wafers — slices from bulks of solar grade silicon. Solar cells made from c-Si are always single-junction cells and are more efficient than their rival technologies.:29
The rival solar cell technology to crystalline silicon is called thin-film. Thin-film itself is an umbrella term that includes several solar cell materials, such as CdTe and CIGS, as well as amorphous silicon (a-Si). Amorphous silicon is an allotropic variant of silicon, and amorphous means "without shape" to describe its non-crystalline form. 1
- 1 Overview
- 2 Mono-silicon
- 3 Multi-silicon
- 4 Not classified as crystalline silicon
- 5 Transformation of amorphous into crystalline silicon
- 6 References
The allotropic forms of silicon range from a single crystalline structure to a completely unordered amorphous structure with several intermediate steps in between. In addition, each of these different forms can possess several names and even more abbreviations, and often cause confusion to non-experts, especially as some materials and their application as a PV technology are of minor significance, while other materials are of outstanding importance.
Classification by the PV industry
The photovoltaic industry, however, groups them into two distinct categories:
- Crystalline silicon (c-Si), used in wafer-based solar cells:
- Not classified as crystalline silicon, used in thin-film solar cells:
As of 2013, crystalline silicon, also know as silicon wafer-based technology used in conventional solar cells accounted for about 90 percent of worldwide PV production. For the last ten years, multi-Si always lead the market ahead of mono-Si, and currently account for 54 percent and 36 percent, respectevly. Thin-film techonologies never reached more than 20 percent of worldwide market-share and currently stand at 9 percent. In that segment of the market, CdTe leads with an annual production of 2 GWp or 5 percent, followed by a-Si and CIGS, both around 2 percent.:4,18
The conversion efficiency of PV devices decribes the energy-ratio of the outgoing electrical power compared to the incoming radiated light. A single solar cells has generally a better, or higer efficiency than a entire solar module. Also lab efficiency is always significantly ahead of commercially available products in the market.
In 2013, record lab cell efficiency was highest for crystalline silicon. However, multi-silicon is followed closely by Cadmium Telluride and Copper indium gallium selenide solar cells
- 25.0% – mono-Si cell
- 20.4% – mulit-Si cell
- 19.8% – CIGS cell
- 19.6% – CdTe cell
These are all single-junction solar cells. For high concentration, multi-junction cells the record currently stands at 44.7 percent.:6
The average commercial crystalline silicon module increased its efficiency from about 12 to 16 percent over the last ten years. In the same period CdTe-modules impromed ther efficiency from 9 to 13 percent. Currently the best-performing modules under lab conditions are made of monocrystalline silicon. They are 7 percentage points above the efficeincy of currently produced modules (23% vs 16%) and indicate that the conventional silicon technology still has potential to improve and therefor maintain its leading position.:6
Best lab module efficiencies for multi-junction modules with concentrator technology reached 36.7 percent.:6
Energy payback time
The energy payback time (EPBT) describes the time span a PV system needs to operate in order to generate the same amount of energy that was used for its manufacture and installation (energy amortization). The EPBT depends vastly on the location where the PV system is installed and on the efficiency of the system, namely the type of used PV technology and the system's components.
|Crystaline Silicon||Thin-film||Global Solar Potential|
Manufacturing improvements for c-Si
The EPBT has always been longer for PV systems using crystalline silicon than thin-film technology. This is due to the fact, that silicon is produced by the reduction of high-grade quartz sand in electric furnaces. This carbo-thermic smelting process occurs at high temperatures of more than 1000 °C and is very energy intensive, using about 11 kilowatt-hours (kWh) per produced kilogram of silicon. However, the energy payback time has shortened significantly over the last years, as crystalline silicon cells became ever more efficient in converting sunlight, while the thickness of the wafer material was constantly reduced and therefore required less silicon for its manufacture. Within the last ten years, the amount of silicon used for solar cells declined from 16 to 6 grams per watt-peak. In the same period, the thickness of a c-Si wafer was reduced from 300 µm, or microns, to about 160 µm. Crystalline silicon wafers are nowadays only 40 percent as thick as they used to be in 1990, when they were around 400 µm.:29 The sawing techniques that slice crystalline silicon ingots into wafers have also improved by reducing the kerf loss and making it easier to recycle the silicon sawdust. 
Monocrystalline silicon (mono c-Si) is a form in which the crystal structure is homogenous throughout the material; the orientation, lattice parameter, and electronic properties are constant throughout the material. Dopant atoms such as phosphorus and boron are often incorporated into the film to make the silicon n-type or p-type respectively. Monocrystalline silicon is fabricated in the form of silicon wafers, usually by the Czochralski Growth method, and can be quite expensive depending on the radial size of the desired single crystal wafer (around $200 for a 300 mm Si wafer). This monocrystalline material, while useful, is one of the chief expenses associated with producing photovoltaics where approximately 40% of the final price of the product is attributable to the cost of the starting silicon wafer used in cell fabrication.
Polycrystalline silicon (poly c-Si) is composed of many smaller silicon grains of varied crystallographic orientation. This material can be synthesized easily by allowing liquid silicon to cool using a seed crystal of the desired crystal structure. Additionally, other methods for crystallizing amorphous silicon to form polysilicon exist such as high temperature chemical vapor deposition (CVD).
Not classified as crystalline silicon
These allotropic forms of silicon are not classified as crystalline silicon. They belong to the group of thin-film solar cells.
Amorphous silicon (a-Si) has no long-range periodic order. The application of amorphous silicon to photovoltaics as a standalone material is somewhat limited by its inferior electronic properties. When paired with microcrystalline silicon in tandem and triple-junction solar cells, however, higher efficiency can be attained than with single-junction solar cells. This tandem assembly of solar cells allows one to obtain a thin-film material with a bandgap of around 1.12 eV (the same as single-crystal silicon) compared to the bandgap of amorphous silicon of 1.7-1.8 eV bandgap. Tandem solar cells are then attractive since they can be fabricated with a bandgap similar to single-crystal silicon but with the ease of amorphous silicon.
Nanocrystalline silicon (nc-Si), sometimes also known as microcrystalline silicon (μc-Si), is a form of porous silicon. It is an allotropic form of silicon with paracrystalline structure—is similar to amorphous silicon (a-Si), in that it has an amorphous phase. Where they differ, however, is that nc-Si has small grains of crystalline silicon within the amorphous phase. This is in contrast to polycrystalline silicon (poly-Si) which consists solely of crystalline silicon grains, separated by grain boundaries. The difference comes solely from the grain size of the crystalline grains. Most materials with grains in the micrometre range are actually fine-grained polysilicon, so nanocrystalline silicon is a better term. The term Nanocrystalline silicon refers to a range of materials around the transition region from amorphous to microcrystalline phase in the silicon thin film.
Protocrystalline silicon has a higher efficiency than amorphous silicon (a-Si) and it has also been shown to improve stability, but not eliminate it. A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form.
Protocrystalline Si also has a relatively low absorption near the band gap owing to its more ordered crystalline structure. Thus, protocrystalline and amorphous silicon can be combined in a tandem solar cell where the top layer of thin protocrystalline silicon absorbs short-wavelength light whereas the longer wavelengths are absorbed by the underlying a-Si substrate.
Transformation of amorphous into crystalline silicon
Amorphous silicon can be transformed to crystalline silicon using well-understood and widely implemented high-temperature annealing processes. The typical method used in industry requires high-temperature compatible materials, such as special high temperature glass that is expensive to produce. However, there are many applications for which this is an inherently unattractive production method.
Low temperature induced crystallization
Flexible solar cells have been a topic of interest for less conspicuous-integrated power generation than solar power farms. These modules may be placed in areas where traditional cells would not be feasible, such as wrapped around a telephone pole or cell phone tower. In this application a photovoltaic material may be applied to a flexible substrate, often a polymer. Such substrates cannot survive the high temperatures experienced during traditional annealing. Instead, novel methods of crystallizing the silicon without disturbing the underlying substrate have been studied extensively. Aluminum-induced crystallization (AIC) and local laser crystallization are common in the literature, however not extensively used in industry.
In both of these methods, amorphous silicon is grown using traditional techniques such as plasma-enhanced chemical vapor deposition (PECVD). The crystallization methods diverge during post-deposition processing.
In aluminum-induced crystallization, a thin layer of aluminum (50 nm or less) is deposited by physical vapor deposition onto the surface of the amorphous silicon. This stack of material is then annealed at a relatively low temperature between 140°C and 200°C in a vacuum. The aluminum that diffuses into the amorphous silicon is believed to weaken the hydrogen bonds present, allowing crystal nucleation and growth. Experiments have shown that polycrystalline silicon with grains on the order of 0.2 – 0.3 μm can be produced at temperatures as low as 150°C. The volume fraction of the film that is crystallized is dependent on the length of the annealing process.
Aluminum-induced crystallization produces polycrystalline silicon with suitable crystallographic and electronic properties that make it a candidate for producing polycrystalline thin films for photovoltaics. AIC can be used to generate crystalline silicon nanowires and other nano-scale structures.
Another method of achieving the same result is the use of a laser to heat the silicon locally without heating the underlying substrate beyond some upper temperature limit. An excimer laser or, alternatively, green lasers such as a frequency-doubled Nd:YAG laser is used to heat the amorphous silicon, supplying energy necessary to nucleate grain growth. The laser fluence must be carefully controlled in order to induce crystallization without causing widespread melting. Crystallization of the film occurs as a very small portion of the silicon film is melted and allowed to cool. Ideally, the laser should melt the silicon film through its entire thickness, but not damage the substrate. Toward this end, a layer of silicon dioxide is sometimes added to act as a thermal barrier. This allows the use of substrates that cannot be exposed to the high temperatures of standard annealing, polymers for instance. Polymer-backed solar cells are of interest for seamlessly integrated power production schemes that involve placing photovoltaics on everyday surfaces.
A third method for crystallizing amorphous silicon is the use of thermal plasma jet. This strategy is an attempt to alleviate some of the problems associated with laser processing – namely the small region of crystallization and the high cost of the process on a production scale. The plasma torch is a simple piece of equipment that is used to thermally anneal the amorphous silicon. Compared to the laser method, this technique is simpler and more cost effective.
Plasma torch annealing is attractive because the process parameters and equipment dimension can be changed easily to yield varying levels of performance. A high level of crystallization (~90%) can be obtained with this method. Disadvantages include difficulty achieving uniformity in the crystallization of the film. While this method is applied frequently to silicon on a glass substrate, processing temperatures may be too high for polymers.
- Bell Labs Demonstrates the First Practical Silicon Solar Cell
- Journal of Applied Physics - A New Silicon p‐n Junction Photocell for Converting Solar Radiation into Electrical Power
- "Photovoltaics Report". Fraunhofer ISE. 28 July 2014. Archived from the original on 31 August 2014. Retrieved 31 August 2014.
- "Production Process of Silicon". http://www.simcoa.com.au. Simcoa Operations. Archived from the original on 17 September 2014. Retrieved 17 September 2014.
- Fraunhofer ISE, 24th European PV Solar Energy Conference and Exhibition Reaching kerf loss below 100 µm by optimizations. September 2009
- HZDR - Helmholtz-Zentrum Dresden-RossendorfSilicon kerf loss recycling, 4 April 2014
- Green, M. A. (2004), "Recent Developments in Photovoltaics", Solar Energy 76 (1–3): 3–8, doi:10.1016/S0038-092X(03)00065-3.
- Campbell, S. A. (2001), The Science and Engineering of Microelectronic Fabrication (2nd ed.), New York: Oxford University Press, ISBN 0-19-513605-5.
- Streetman, B. G. & Banerjee, S. (2000), Solid State Electronic Devices (5th ed.), New Jersey: Prentice Hall, ISBN 0-13-025538-6.
- Shah, A. V.; et al. (2003), "Material and solar cell research in microcrystalline silicon", Solar Energy Materials and Solar Cells 78 (1–4): 469–491, doi:10.1016/S0927-0248(02)00448-8.
- Technical articles
- S. Y. Myong, S. W. Kwon, J. H. Kwak, K. S. Lim, J. M. Pearce, and M. Konagai, "Good stability of protocrystalline silicon multilayer solar cells against light irradiation originating from vertically regular distribution of isolated nano-sized silicon grains", 4th World Conference on Photovoltaic Energy Conversion Proceedings, p. 492, 2006.
- S. Y. Myong, K. S. Lim, J. M. Pears, "Double amorphous silicon-carbide p-layer structures producing highly stabilized pin-type protocrystalline silicon multilayer solar cells", Applied Physics Letters, 87(19), 193509 (2005); 87, 259901(2005).
- Kishore, R.; Hotz, C.; Naseem, H. A. & Brown, W. D. (2001), "Aluminum-Induced Crystallization of Amorphous Silicon (α-Si:H) at 150°C", Electrochemical and Solid State Letters 4 (2): G14–G16, doi:10.1149/1.1342182.
- Yuan, Zhijun; Lou, Qihong; Zhou, Jun; Dong, Jingxing; Wei, Yunrong; Wang, Zhijiang; Zhao, Hongming; Wu, Guohua (2009), "Numerical and experimental analysis on green laser crystallization of amorphous silicon thin films", Optics & Laser Technology 41 (4): 380–383, doi:10.1016/j.optlastec.2008.09.003.
- Lee, Hyun Seok; Choi, Sooseok; Kim, Sung Woo; Hong, Sang Hee (2009), "Crystallization of Amorphous Silicon Thin Film by Using a Thermal Plasma Jet", Thin Solid Films 517 (14): 4070–4073, doi:10.1016/j.tsf.2009.01.138.