Amorphous silicon

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Amorphous silicon:

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

Silicon Chemical element with atomic number 14

Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard and brittle crystalline solid with a blue-grey metallic lustre; and it is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic table: carbon is above it; and germanium, tin, and lead are below it. It is relatively unreactive. Because of its high chemical affinity for oxygen, it was not until 1823 that Jöns Jakob Berzelius was first able to prepare it and characterize it in pure form. Its melting and boiling points of 1414 °C and 3265 °C respectively are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth's crust. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust after oxygen.

Thin-film transistor field-effect transistor device

A thin-film transistor (TFT) is a special kind of MOSFET made by depositing thin films of an active semiconductor layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, because the primary application of TFTs is in liquid-crystal displays (LCDs). This differs from the conventional bulk MOSFET transistor, where the semiconductor material typically is the substrate, such as a silicon wafer.

Liquid-crystal display display that uses the light-modulating properties of liquid crystals

A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome. LCDs are available to display arbitrary images or fixed images with low information content, which can be displayed or hidden, such as preset words, digits, and seven-segment displays, as in a digital clock. They use the same basic technology, except that arbitrary images are made up of many small pixels, while other displays have larger elements. LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.

Contents

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, but are one of the most environmentally friendly photovoltaic technologies, since they do not use any toxic heavy metals such as cadmium or lead.[ citation needed ]

A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.

Toxic heavy metal

A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts. The term has particular application to cadmium, mercury, lead and arsenic, all of which appear in the World Health Organization's list of 10 chemicals of major public concern. Other examples include manganese, chromium, cobalt, nickel, copper, zinc, selenium, silver, antimony and thallium.

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 ]

Thin-film solar cell type of second-generation solar cell

A thin-film solar cell is a second generation solar cell that is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate, such as glass, plastic or metal. Thin-film solar cells are commercially used in several technologies, including cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin-film silicon.

Growth of photovoltaics Worldwide growth of photovoltaics. History, current status and forecast.

Worldwide growth of photovoltaics has been close to exponential between 1992 and 2018. During this period of time, photovoltaics (PV), also known as solar PV, evolved from a niche market of small scale applications to a mainstream electricity source. When solar PV systems were first recognized as a promising renewable energy technology, subsidy programs, such as feed-in tariffs, were implemented by a number of governments in order to provide economic incentives for investments. For several years, growth was mainly driven by Japan and pioneering European countries. As a consequence, cost of solar declined significantly due to experience curve effects like improvements in technology and economies of scale. Several national programs were instrumental in increasing PV deployment, such as the Energiewende in Germany, the Million Solar Roofs project in the United States, and China's 2011 five-year-plan for energy production. Since then, deployment of photovoltaics has gained momentum on a worldwide scale, increasingly competing with conventional energy sources. In the early 21st Century a market for utility-scale plants emerged to complement rooftop and other distributed applications. By 2015, some 30 countries had reached grid parity.

Crystalline silicon

Crystalline silicon (c-Si) is the crystalline forms of silicon, either multicrystalline silicon (multi-Si) consisting of small crystals, or monocrystalline silicon (mono-Si), a continuous crystal. Crystalline silicon is the dominant semiconducting material used in photovoltaic technology for the production of solar cells. These cells are assembled into solar panels as part of a photovoltaic system to generate solar power from sunlight.

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.

Monocrystalline silicon is the base material for silicon-based discrete components and integrated circuits used in virtually all modern electronic equipment. Mono-Si also serves as a photovoltaic, light-absorbing material in the manufacture of solar cells.

Polycrystalline silicon high purity, polycrystalline form of silicon

Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.

Crystallite

A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. The orientation of crystallites can be random with no preferred direction, called random texture, or directed, possibly due to growth and processing conditions. Fiber texture is an example of the latter. Crystallites are also referred to as grains. The areas where crystallites meet are known as grain boundaries. Polycrystalline or multicrystalline materials, or polycrystals are solids that are composed of many crystallites of varying size and orientation.

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.

Tetrahedron Polyhedron with 4 faces

In geometry, a tetrahedron, also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ordinary convex polyhedra and the only one that has fewer than 5 faces.

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.

In chemistry, a dangling bond is an unsatisfied valence on an immobilized atom. An atom with a dangling bond is also referred to as an immobilized free radical or an immobilized radical, a reference to its structural and chemical similarity to a free radical.

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 Schematic of allotropic forms of silcon horizontal plain.svg
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 can potentially increase 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, due to the increased disorder in the atomic network.

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 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. [3] 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). [4] 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. [5] Starting in the 1970s, a-Si:H was developed in solar cells by RCA by which steadily climbed in efficiency to about 13.6% in 2015. [6]

Deposition processes

CVD PECVD Catalytic CVD Sputtering
Type of filma-Si:Ha-Si:Ha-Si:Ha-Si
Unique application Large-area electronics Hydrogen-free deposition
Chamber temperature600C30–300C30–1000C
Active element temperature2000C
Chamber pressure0.1–10 Torr0.1–10 Torr0.001–0.1 Torr
Physical principle Thermolysis Plasma-induced dissociationThermolysisIonization of Si source
Facilitators W/Ta heated wires Argon cations
Typical drive voltageRF 13.56 MHz; 0.01-1W/cm2
Si source SiH4 gasSiH4 gasSiH4 gasTarget
Substrate temperaturecontrollablecontrollablecontrollablecontrollable

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 plastic 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 [7] particularly when the silane is recycled. [8]

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 Vintage Teal Photon Solar Powered Electronic Pocket Calculator, LCD With Yellow Filter, One Of The First Solar Powered Calculators, Made In Japan, Circa 1978 (15083726059).jpg
The "Teal Photon" solar-powered calculator produced in the late 1970s

Amorphous silicon (a-Si) 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. 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 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. [9] This low hole mobility has been attributed to many physical aspects of the material, including the presence of dangling bonds (silicon with 3 bonds), [10] floating bonds (silicon with 5 bonds), [11] as well as bond reconfigurations. [12] 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. [13]

The main advantage of a-Si in large scale production is not efficiency, but cost. a-Si 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 unattractive except in roles where their thinness or flexibility are an advantage. [14]

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. [15] [16]

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

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. [17] 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. [18]

Photovoltaic thermal hybrid solar collectors

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. [19] 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. [20]

See also

Related Research Articles

Amorphous solid crystal system

In condensed matter physics and materials science, an amorphous or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal. In some older books, the term has been used synonymously with glass. Nowadays, "glassy solid" or "amorphous solid" is considered to be the overarching concept, and glass the more special case: Glass is an amorphous solid that exhibits a glass transition. Polymers are often amorphous. Other types of amorphous solids include gels, thin films, and nanostructured materials such as glass.

Nanocrystalline 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. The crystalline volume fraction is another criterion to describe the materials in this transition zone.

Photovoltaics Method of generating electrical power by converting solar radiation into direct current electricity

Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.

An epitaxial wafer is a wafer of semiconducting material made by epitaxial growth (epitaxy) for use in photonics, microelectronics, spintronics, or photovoltaics. The epi layer may be the same material as the substrate, typically monocrystaline silicon, or it may be a more exotic material with specific desirable qualities.

Solar cell electrical device that converts the energy of light directly into electricity by the photovoltaic effect

A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices can be combined to form modules, otherwise known as solar panels. The common single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.

Protocrystalline distinct phase occurring during crystal growth which evolves into a microcrystalline form

A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form. The term is typically associated with silicon films in optical applications such as solar cells.

Sharp Solar, a subsidiary of Sharp Electronics, is a solar energy products company owned by Sharp Corporation and based in Osaka, Japan.

The Staebler–Wronski Effect (SWE) refers to light-induced metastable changes in the properties of hydrogenated amorphous silicon.

Organic solar cell

An organic solar cell (OSC) or plastic solar cell is a type of photovoltaic that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. Most organic photovoltaic cells are polymer solar cells.

TEL Solar, formerly Oerlikon Solar, is a manufacturer of production equipment for the manufacturing of thin-film silicon cells, headquartered in Trübbach, Switzerland, near the border to Liechtenstein. The Japanese electronics and semiconductor company Tokyo Electron acquired the company of about 650 employees from OC Oerlikon in November 2012.

Cadmium telluride photovoltaics Type of solar power cell

Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride, a thin semiconductor layer designed to absorb and convert sunlight into electricity. Cadmium telluride PV is the only thin film technology with lower costs than conventional solar cells made of crystalline silicon in multi-kilowatt systems.

Copper indium gallium selenide solar cells direct bandgap semiconductor useful for the manufacture of solar cells

A copper indium gallium selenide solar cell is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper, indium, gallium and selenium on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials.

Solar cell efficiency Ratio of energy extracted from sunlight in solar cells

Solar cell efficiency refers to the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity by the solar cell.

Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures compared to in traditional methods. LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors.

Flisom is a developer and manufacturer of photovoltaic (PV) thin film solar cells, located near Zurich, Switzerland. The company produces high-efficiency CIGS thin film solar modules on flexible plastic foil using proprietary roll-to-roll manufacturing techniques.

References

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