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Amorphous silicon

From Wikipedia, the free encyclopedia

Amorphous silicon:

Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.

Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films onto a variety of flexible substrates, such as glass, metal and plastic. Amorphous silicon cells generally feature low efficiency.

As a second-generation thin-film solar cell technology, amorphous silicon was once expected to become a major contributor in the fast-growing worldwide photovoltaic market, but has since lost its significance due to strong competition from conventional crystalline silicon cells and other thin-film technologies such as CdTe and CIGS.[citation needed] Amorphous silicon is a preferred material for the thin film transistor (TFT) elements of liquid crystal displays (LCDs) and for x-ray imagers.

Amorphous silicon differs from other allotropic variations, such as monocrystalline silicon—a single crystal, and polycrystalline silicon, that consists of small grains, also known as crystallites.

YouTube Encyclopedic

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  • Light Induced Degradation in amorphous silicon solar cells: Part 1
  • Amorphous silicon
  • Amorphous silicon
  • thin film solar cells: CIGS and amorphous silicon
  • Thin-Film Silicon PV Technology

Transcription

Hey, guys. So as you all know, we expect the solar cells to produce electricity. You know, convert sunlight into electricity for a long period of time. So for example, usually you assume that these panels will work for 25 years or even more than that. But I want to discuss this, this special issue which is of light induced degradation and it's, it's a special concern for this amorphous silicon kind of solar cell. So, thin-film solar cells which are made out of amorphous silicon. So when you make these solar cells out of of amorphous silicon, what is often observed is that if you expose them to sunlight, you know, if you just make these cells and expose them to sunlight, you get a certain efficiency. In this case, efficiency of 8.5%. But as you keep on, keep it exposed to light, the efficiency, it starts degrading, and you can see that you know, just in a period of five hours, the efficiency has dropped from 8.5 to, you know, close to 7 point you know, close To, 6.8%. And if you, can, keep on exposing it to light even more the efficiency it starts to degrade. Thankfully it, the rate of degredation slows down, but nevertheless it's still degrading. And, you know, it reduces substantially as compared to what you started at, at t equal to zero. Now, even more interestingly what people have observed, is now if you take this solar cell and anneal it or bake it for a couple of hours. So you know, you put it in a chamber which has a temperature let's say equal 250 degrees centigrade. And then you take it out, and again expose it to sunlight, you know, expose it to sun. And measure its efficiency. So, miraculously what occurs is that the efficiency recovers back. You know, you get part of the efficiency you lost back, and you again have higher efficiency. But unfortunately, again, when you expose it to light again, you know, it starts to degrade again and the efficiency starts to fall. If again, you expose it to, or you anneal it again this efficiency rises up. And it starts to fall again if you keep on exposing it to light. So you get the idea that you know, essentially the efficiency degrades if you expose it to light. It improves if you anneal it or, you know, if you subject this solar cell to this high temperature. And or you are usually efficiency, it can degrade very easily to around 30%. Or, efficiency can degrade by 30%, you know your efficiency can degrade by 30% off where you started off from. And it starts to stabilize after awhile, and, you know, the rate of the decrease, or the rate of this, degradation slows down and, it reaches a stable state, stable state after sometime. But nonetheless you degrade, you know you are operating at an efficiency which is 30 percent lower than what you started at when the cell just came out from the factory. In fact, many of these these [INAUDIBLE] lines, they measure, you know, they measure these cells when after they're manufactured for the purpose of bending them into the different categories. And they observed that as soon as, you know, they measured the cell, they, it starts to degrade. So again, that does not bode very well with what I said earlier, that these things you know, need to work for more than 25 years. But it's not that, you know, when people you know, started when this amorphous silicon became really hot and people were selling these. A lot of these solar panels back in back in 2008 or 2009 kind of time frame. It was not that they, you know, they just discovered about this after they started manufacturing these cells. In fact, this phenomena is very well known for the case of amorphous silicon. It was discovered all the way back in 1977 by these two bright at that time, young scientists Staebler and Wronski, who used to work at RC Labs at that time. And what they observed was they measured the conductivity, so they measured the conductivity of this amorphous silicon. And they measured it you know right after it was made. And they exposed it to light for some time. And what they saw was that the conductivity it fell to a much lower level after you, after you expose it to light. So this, this phenomena of degradation of this amorphous silicon has been known for for you know quite some time. And people have proposed different theories for it. So you know as is with this degration or these reliability kind of phenomena. People propose different models, and there are ways you, you can be assured that there will be more than one competing models to explain the observed experimental behavior. So the first of these models which is used to explain this degradation is what is called is a hydrogen bond. Switching model to hydrogen bound, switching model. And this was proposed it was one of the earliest models to explain this degradation, so it's more widely accepted as well. And the way this model tries to explain this degradation is that it says that suppose the others amorphous materials, amorphous silicon, in this case. So you have these you know you have these silicon items and they are bonded to each other. Now what happens when you shine light on this material? So when you shine shine light on this material, you generate these electron in whole pairs which are generated throughout throughout the bulk of this absorbing material. And now these electron and hole pairs, they can essentially either get connected, they can recombine and they can release out a photon, out from the system. So, they can relatively recombine. More commonly, they essentially, they can recombine, and they can give away this energy to you, to the lattice itself. So they can recombine and they can [INAUDIBLE] give away this energy to the lattice. Now if such an electron and hole pair, it recombines very close to this bond which is there between these two silicon atoms, it can essentially give out that energy to this bond. And that may result in the breaking of this bond. So that's how essentially you, you break this bond, because you get [UNKNOWN] energy from this recombination of this electron and whole. And that essentially creates, that essentially leads to creation of the dangling bond on each of these silicon atoms. Now, what this model says is that now, this this bond is essentially switched by hydrogen, so essentially, what what happened is that this hydrogen, which is in [INAUDIBLE], which is, you know, always, almost always present in this amorphous silicon material, it opportunistically comes in, and it binds with one of these One of these silicon atoms. But this other bond on the other silicon atom is essentially still still unsatisfied, so it results in creation of this dangling bond on one of the silicon atoms. But overall this bond between the silicon these two silicon atoms is essentially switched by this hydrogen hydrogen molecule. So this is one way to explain, y'know, how this, how this how this degradation occurs, it occurs because as you shine light, your density of these are dangling bond. It increases because, because of the recombination of these electron and hole pairs, which are generated because you shine light. They result, in breaking of these bonds. And it results in creation of these, dangling bind states. So another model which can, explain the creation of these, dangling bind, states as well, is, this hydrogen, collision model. And this is a model which has been recently become which has recently become more popular to explain this phenomena. And the way it says that these dangling bonds are created. It says that you have this amorphous material, so you have a lot of these silicon items, which are, you know, which are bonded to hydrogen anyway, because, you know, not all the, all the bonds in amorphous material are not satisfied with other silicon items. You have a lot of these silicon items which are bonded, which are bonded with the hydrogen. So now what happens is that when you shine light it alerts, it alerts in the breaking of these binds, so what the picture that alerts is that it creates these dangling binds on each of these silicon atoms. And this hydrogen, which is subsequently set free, or these hydrogen atoms, which are subsequently set free, because they are now released, they essentially they essentially, you know, go and form a metastable state somewhere within the lattice array. So these two models are, you know, often frequently used to explain this phenomena.

Description

Silicon is a fourfold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. In crystalline silicon (c-Si) this tetrahedral structure continues over a large range, thus forming a well-ordered crystal lattice.

In amorphous silicon this long range order is not present. Rather, the atoms form a continuous random network. Moreover, not all the atoms within amorphous silicon are fourfold coordinated. Due to the disordered nature of the material some atoms have a dangling bond. Physically, these dangling bonds represent defects in the continuous random network and may cause anomalous electrical behavior.

The material can be passivated by hydrogen, which bonds to the dangling bonds and can reduce the dangling bond density by several orders of magnitude. Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of defects to be used within devices such as solar photovoltaic cells, particularly in the protocrystalline growth regime.[1] However, hydrogenation is associated with light-induced degradation of the material, termed the Staebler–Wronski effect.[2]

Schematic of allotropic forms of silicon: monocrystalline, polycrystalline, and amorphous silicon

Amorphous silicon and carbon

Amorphous alloys of silicon and carbon (amorphous silicon carbide, also hydrogenated, a-Si1−xCx:H) are an interesting variant. Introduction of carbon atoms adds extra degrees of freedom for control of the properties of the material. The film could also be made transparent to visible light.

Increasing the concentration of carbon in the alloy widens the electronic gap between conduction and valence bands (also called "optical gap" and bandgap). This increases the light efficiency of solar cells made with amorphous silicon carbide layers. On the other hand, the electronic properties as a semiconductor (mainly electron mobility), are adversely affected by the increasing content of carbon in the alloy, presumably due to the increased disorder in the atomic network.[3]

Several studies are found in the scientific literature, mainly investigating the effects of deposition parameters on electronic quality, but practical applications of amorphous silicon carbide in commercial devices are still lacking.

Properties

The density of ion implanted amorphous Si has been calculated as 4.90×1022 atom/cm3 (2.285 g/cm3) at 300 K. This was done using thin (5 micron) strips of amorphous silicon. This density is 1.8±0.1% less dense than crystalline Si at 300 K.[4] Silicon is one of the few elements that expands upon cooling and has a lower density as a solid than as a liquid.

Hydrogenated amorphous silicon

Unhydrogenated a-Si has a very high defect density which leads to undesirable semiconductor properties such as poor photoconductivity and prevents doping which is critical to engineering semiconductor properties. By introducing hydrogen during the fabrication of amorphous silicon, photoconductivity is significantly improved and doping is made possible. Hydrogenated amorphous silicon, a-Si:H, was first fabricated in 1969 by Chittick, Alexander and Sterling by deposition using a silane gas (SiH4) precursor. The resulting material showed a lower defect density and increased conductivity due to impurities. Interest in a-Si:H came when (in 1975), LeComber and Spear discovered the ability for substitutional doping of a-Si:H using phosphine (n-type) or diborane (p-type).[5] The role of hydrogen in reducing defects was verified by Paul's group at Harvard who found a hydrogen concentration of about 10 atomic % through IR vibration, which for Si-H bonds has a frequency of about 2000 cm−1.[6] Starting in the 1970s, a-Si:H was developed in solar cells by David E. Carlson and C. R. Wronski at RCA Laboratories.[7] Conversion efficiency steadily climbed to about 13.6% in 2015.[8]

Deposition processes

CVD PECVD Catalytic CVD Sputtering
Type of film a-Si:H a-Si:H a-Si:H a-Si
Unique application Large-area electronics Hydrogen-free deposition
Chamber temperature 600C 30–300C 30–1000C
Active element temperature 2000C
Chamber pressure 0.1–10 Torr 0.1–10 Torr 0.001–0.1 Torr
Physical principle Thermolysis Plasma-induced dissociation Thermolysis Ionization of Si source
Facilitators W/Ta heated wires Argon cations
Typical drive voltage RF 13.56 MHz; 0.01-1W/cm2
Si source SiH4 gas SiH4 gas SiH4 gas Target
Substrate temperature controllable controllable controllable controllable

Applications

While a-Si suffers from lower electronic performance compared to c-Si, it is much more flexible in its applications. For example, a-Si layers can be made thinner than c-Si, which may produce savings on silicon material cost.

One further advantage is that a-Si can be deposited at very low temperatures, e.g., as low as 75 degrees Celsius. This allows deposition on not only glass, but on plastic or even on paper[9][10] substrates as well, making it a candidate for a roll-to-roll processing technique. Once deposited, a-Si can be doped in a fashion similar to c-Si, to form p-type or n-type layers and ultimately to form electronic devices.

Another advantage is that a-Si can be deposited over large areas by PECVD. The design of the PECVD system has great impact on the production cost of such panel, therefore most equipment suppliers put their focus on the design of PECVD for higher throughput, that leads to lower manufacturing cost[11] particularly when the silane is recycled.[12]

Arrays of small (under 1 mm by 1 mm) a-Si photodiodes on glass are used as visible-light image sensors in some flat panel detectors for fluoroscopy and radiography.

Photovoltaics

The "Teal Photon" solar-powered calculator produced in the late 1970s

Hydrogenated amorphous silicon (a-Si:H) has been used as a photovoltaic solar cell material for devices which require very little power, such as pocket calculators, because their lower performance compared to conventional crystalline silicon (c-Si) solar cells is more than offset by their simplified and lower cost of deposition onto a substrate. Moreover, the vastly higher shunt resistance of the p-i-n device means that acceptable performance is achieved even at very low light levels. The first solar-powered calculators were already available in the late 1970s, such as the Royal Solar 1, Sharp EL-8026, and Teal Photon.

More recently, improvements in a-Si:H construction techniques have made them more attractive for large-area solar cell use as well. Here their lower inherent efficiency is made up, at least partially, by their thinness – higher efficiencies can be reached by stacking several thin-film cells on top of each other, each one tuned to work well at a specific frequency of light. This approach is not applicable to c-Si cells, which are thick as a result of its indirect band-gap and are therefore largely opaque, blocking light from reaching other layers in a stack.

The source of the low efficiency of amorphous silicon photovoltaics is due largely to the low hole mobility of the material.[13] This low hole mobility has been attributed to many physical aspects of the material, including the presence of dangling bonds (silicon with 3 bonds),[14] floating bonds (silicon with 5 bonds),[15] as well as bond reconfigurations.[16] While much work has been done to control these sources of low mobility, evidence suggests that the multitude of interacting defects may lead to the mobility being inherently limited, as reducing one type of defect leads to formation others.[17]

The main advantage of a-Si:H in large scale production is not efficiency, but cost. a-Si:H cells use only a fraction of the silicon needed for typical c-Si cells, and the cost of the silicon has historically been a significant contributor to cell cost. However, the higher costs of manufacture due to the multi-layer construction have, to date, made a-Si:H unattractive except in roles where their thinness or flexibility are an advantage.[18]

Typically, amorphous silicon thin-film cells use a p-i-n structure. The placement of the p-type layer on top is also due to the lower hole mobility, allowing the holes to traverse a shorter average distance for collection to the top contact. Typical panel structure includes front side glass, TCO, thin-film silicon, back contact, polyvinyl butyral (PVB) and back side glass. Uni-Solar, a division of Energy Conversion Devices produced a version of flexible backings, used in roll-on roofing products. However, the world's largest manufacturer of amorphous silicon photovoltaics had to file for bankruptcy in 2012, as it could not compete with the rapidly declining prices of conventional solar panels.[19][20]

Microcrystalline and micromorphous silicon

Microcrystalline silicon (also called nanocrystalline silicon) is amorphous silicon, but also contains small crystals. It absorbs a broader spectrum of light and is flexible. Micromorphous silicon module technology combines two different types of silicon, amorphous and microcrystalline silicon, in a top and a bottom photovoltaic cell. Sharp produces cells using this system in order to more efficiently capture blue light, increasing the efficiency of the cells during the time where there is no direct sunlight falling on them. Protocrystalline silicon is often used to optimize the open circuit voltage of a-Si photovoltaics.

Large-scale production

United Solar Ovonic roll-to-roll solar photovoltaic production line with 30 MW annual capacity

Xunlight Corporation, which has received over $40 million of institutional investments,[citation needed] has completed the installation of its first 25 MW wide-web, roll-to-roll photovoltaic manufacturing equipment for the production of thin-film silicon PV modules.[21] Anwell Technologies has also completed the installation of its first 40 MW a-Si thin film solar panel manufacturing facility in Henan with its in-house designed multi-substrate-multi-chamber PECVD equipment.[22]

Photovoltaic thermal hybrid solar collectors

Aerospace product with flexible thin-film solar PV from United Solar Ovonic

Photovoltaic thermal hybrid solar collectors (PVT), are systems that convert solar radiation into electrical energy and thermal energy. These systems combine a solar cell, which converts electromagnetic radiation (photons) into electricity, with a solar thermal collector, which captures the remaining energy and removes waste heat from the solar PV module. Solar cells suffer from a drop in efficiency with the rise in temperature due to increased resistance. Most such systems can be engineered to carry heat away from the solar cells thereby cooling the cells and thus improving their efficiency by lowering resistance. Although this is an effective method, it causes the thermal component to under-perform compared to a solar thermal collector. Recent research showed that a-Si:H PV with low temperature coefficients allow the PVT to be operated at high temperatures, creating a more symbiotic PVT system and improving performance of the a-Si:H PV by about 10%.

Thin-film-transistor liquid-crystal display

Amorphous silicon has become the material of choice for the active layer in thin-film transistors (TFTs), which are most widely used in large-area electronics applications, mainly for liquid-crystal displays (LCDs).

Thin-film-transistor liquid-crystal display (TFT-LCD) show a similar circuit layout process to that of semiconductor products. However, rather than fabricating the transistors from silicon, that is formed into a crystalline silicon wafer, they are made from a thin film of amorphous silicon that is deposited on a glass panel. The silicon layer for TFT-LCDs is typically deposited using the PECVD process.[23] Transistors take up only a small fraction of the area of each pixel and the rest of the silicon film is etched away to allow light to easily pass through it.

Polycrystalline silicon is sometimes used in displays requiring higher TFT performance. Examples include small high-resolution displays such as those found in projectors or viewfinders. Amorphous silicon-based TFTs are by far the most common, due to their lower production cost, whereas polycrystalline silicon TFTs are more costly and much more difficult to produce.[24]

See also

References

  1. ^ Collins, R.W.; Ferlauto, A.S.; Ferreira, G.M.; Chen, Chi; Koh, Joohyun; Koval, R.J.; Lee, Yeeheng; Pearce, J.M.; Wronski, C.R. (2003). "Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry". Solar Energy Materials and Solar Cells. 78 (1–4): 143–180. doi:10.1016/S0927-0248(02)00436-1.
  2. ^ Wronski, C.R.; Pearce, J.M.; Deng, J.; Vlahos, V.; Collins, R.W. (2004). "Intrinsic and light induced gap states in a-Si:H materials and solar cells—effects of microstructure" (PDF). Thin Solid Films. 451–452: 470–475. Bibcode:2004TSF...451..470W. doi:10.1016/j.tsf.2003.10.129.
  3. ^ Catalano, A.; Newton, J.; Trafford, M. (1989). "a-Si1−xCx:H Based Transistor Performance and the relationship to Electrical and Optical Properties". IEEE Transactions on Electron Devices. 36 (12): 2839. doi:10.1109/16.40969.
  4. ^ Custer, J. S.; Thompson, Michael O.; Jacobson, D. C.; Poate, J. M.; Roorda, S.; Sinke, W. C.; Spaepen, F. (January 24, 1994). "Density of amorphous Si". Applied Physics Letters. 64 (4): 437–439. Bibcode:1994ApPhL..64..437C. doi:10.1063/1.111121. ISSN 0003-6951.
  5. ^ Street, R. A. (2005). Hydrogenated Amorphous Silicon. Cambridge University Press. ISBN 9780521019347.
  6. ^ Paul, William; Anderson, David A. (September 1, 1981). "Properties of amorphous hydrogenated silicon, with special emphasis on preparation by sputtering". Solar Energy Materials. 5 (3): 229–316. doi:10.1016/0165-1633(81)90001-0.
  7. ^ Carlson, David; Wronski, C.R. (1976). "amorphous silicon solar cell". Applied Physics Letters. 26 (11): 671–673. Bibcode:1976ApPhL..28..671C. doi:10.1063/1.88617.
  8. ^ File:PVeff(rev170324).png
  9. ^ Águas, Hugo; Mateus, Tiago; Vicente, António; Gaspar, Diana; Mendes, Manuel J.; Schmidt, Wolfgang A.; Pereira, Luís; Fortunato, Elvira; Martins, Rodrigo (June 2015). "Thin Film Silicon Photovoltaic Cells on Paper for Flexible Indoor Applications". Advanced Functional Materials. 25 (23): 3592–3598. doi:10.1002/adfm.201500636. S2CID 94159781.
  10. ^ Vicente, António; Águas, Hugo; Mateus, Tiago; Araújo, Andreia; Lyubchyk, Andriy; Siitonen, Simo; Fortunato, Elvira; Martins, Rodrigo (June 17, 2015). "Solar cells for self-sustainable intelligent packaging". Journal of Materials Chemistry A. 3 (25): 13226–13236. doi:10.1039/C5TA01752A. ISSN 2050-7496.
  11. ^ Shah, A.; Meier, J.; Buechel, A.; Kroll, U.; Steinhauser, J.; Meillaud, F.; Schade, H.; Dominé, D. (September 2, 2005). "Towards very low-cost mass production of thin-film silicon photovoltaic (PV) solar modules on glass". Thin Solid Films. Elsevier B.V. 502 (1–2): 292–299. doi:10.1016/j.tsf.2005.07.299.
  12. ^ Kreiger, M.A.; Shonnard, D.R.; Pearce, J.M. (2013). "Life cycle analysis of silane recycling in amorphous silicon-based solar photovoltaic manufacturing". Resources, Conservation and Recycling. 70: 44–49. doi:10.1016/j.resconrec.2012.10.002. S2CID 3961031.
  13. ^ Liang, Jianjun; Schiff, E. A.; Guha, S.; Yan, Baojie; Yang, J. (2006). "Hole-mobility limit of amorphous silicon solar cells". Applied Physics Letters. 88 (6): 063512. Bibcode:2006ApPhL..88f3512L. doi:10.1063/1.2170405. S2CID 18053686.
  14. ^ Smith, Z E.; Wagner, S. (1987). "Band tails, entropy, and equilibrium defects in hydrogenated amorphous silicon". Physical Review Letters. 59 (6): 688–691. Bibcode:1987PhRvL..59..688S. doi:10.1103/PhysRevLett.59.688. PMID 10035845.
  15. ^ Stathis, J. H. (1989). "Analysis of the superhyperfine structure and the g-tensor of defects in amorphous silicon". Physical Review B. 40 (2): 1232–1237. Bibcode:1989PhRvB..40.1232S. doi:10.1103/PhysRevB.40.1232. PMID 9991947.
  16. ^ Johlin, Eric; Wagner, Lucas K.; Buonassisi, Tonio; Grossman, Jeffrey C. (2013). "Origins of Structural Hole Traps in Hydrogenated Amorphous Silicon". Physical Review Letters. 110 (14): 146805. Bibcode:2013PhRvL.110n6805J. doi:10.1103/PhysRevLett.110.146805. hdl:1721.1/80776. PMID 25167024.
  17. ^ Johlin, Eric; Simmons, C. B.; Buonassisi, Tonio; Grossman, Jeffrey C. (2014). "Hole-mobility-limiting atomic structures in hydrogenated amorphous silicon" (PDF). Physical Review B. 90 (10): 104103. Bibcode:2014PhRvB..90j4103J. doi:10.1103/PhysRevB.90.104103. hdl:1721.1/89217.
  18. ^ Wesoff, Eric (January 31, 2014) "The End of Oerlikon’s Amorphous Silicon Solar Saga." Greentech Media.
  19. ^ "The End Arrives for ECD Solar". GreentechMedia. February 14, 2012.
  20. ^ "Oerlikon Divests Its Solar Business and the Fate of Amorphous Silicon PV". GrrentechMedia. March 2, 2012.
  21. ^ "Xunlight Completes Installation of its First 25 Megawatt Wide-Web Roll-to-Roll Photovoltaic Manufacturing Equipment". Xunlight. June 22, 2009.
  22. ^ "Anwell Produces its First Thin Film Solar Panel". Solarbuzz. September 7, 2009.
  23. ^ "TFT LCD – Fabricating TFT LCD". Plasma.com. Archived from the original on May 2, 2013. Retrieved July 21, 2013.
  24. ^ "TFT LCD – Electronic Aspects of LCD TVs and LCD Monitors". Plasma.com. Archived from the original on August 23, 2013. Retrieved July 21, 2013.

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