AMORPHOUS SILICON FOR SOLAR CELL Will it contribute to energy independence? Mustafa KOCAMAN, Taha ÖNGEL May 08, 2013

General information about solar cells Solar cells are devices which convert solar light energy directly into electricity and function by the photovoltaic effect.  Photo- means light and -voltaic means electrical current or electricity  (light-electricity).  A solar cell provides direct current (DC) electricity that can be used to power DC motors and light bulbs among other things.  Solar cells can even be used to charge rechargeable batteries so that electricity can be stored for later use when the sun is not available. The fully charged batteries are portable energy that can be used whenever and wherever they are needed. In addition, they require little energy to manufacture and use less raw materials, and thus are truly environmentally friendly devices. 2

How do solar cells work? Photovoltaic cells are made of special materials called semiconductors such as silicon, which is currently used most commonly. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off for external use, say, to power a calculator. This current, together with the cell's voltage, defines the power (or wattage) that the solar cell can produce.

Types of PV Cells monocrystalline silicon amorphous silicon The Types of PV cells polycrystalline silicon cadmium telluride monocrystalline silicon

AMORPHOUS SILICON for SOLAR CELLS Solar cells are classified according to the material employed, i.e., crystal silicon, amorphous silicon, and compound semiconductor solar cells “Amorphous” refers to objects having no definite shape and is defined as non-crystal material. Unlike crystal silicon, in which atomic arrangements are regular, amorphous silicon features irregular atomic arrangements as shown in the figures on the right. As a result, the reciprocal action between photons and silicon atoms occurs more frequently in amorphous silicon than in crystal silicon, allowing much more light to be absorbed. Thus, an ultra-thin amorphous silicon film of less than 1µm can be produced and used for power generation. Also, by utilizing metal or plastics for the substrate, flexible solar cells can be produced. 5

An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. (Solids in which there is long-range atomic order are called crystalline solids or morphous). Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous ceramic, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids. 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. Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates. It offers some unique capabilities for a variety of electronics. 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.

Likewise, 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. In early studies of amorphous silicon, it was determined that plasma-deposited amorphous silicon contained a significant percentage of hydrogen atoms bonded into the amorphous silicon structure. These atoms were discovered to be essential to the improvement of the electronic properties of the material. Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of defects to be used within devices. However, hydrogenation is unfortunately associated with light-induced degradation of the material, termed the Staebler–Wronski effect.

The Staebler–Wronski Effect (SWE) refers to light-induced metastable changes in the properties of hydrogenated amorphous silicon. The defect density of hydrogenated amorphous silicon(a-Si:H) increases with light exposure, causing an increase in the recombination current and reducing the efficiency of the conversion of sunlight into electricity. It was discovered by David L. Staebler and Christopher R. Wronski in 1977. They showed that the dark conductivity and photoconductivity of hydrogenated amorphous silicon can be reduced significantly by prolonged illumination with intense light. However, on heating the samples to above 150 °C, they could reverse the effect.

Amorton is an integrated amorphous silicon solar cell which has been developed by SANYO. Amorton uses silane (SiH4) as its source gas and is fabricated using a plasma CVD method. Three amorphous silicon layers -- p-layer, i-layer, and n-layer -- are formed consecutively on a glass substrate. This p-i-n junction corresponds to the p/n junction of a crystal silicon solar cell. In the process of this junction formation, a number of cells are connected in series on a substrate at one time. This allows any desired voltage to be obtained for a variety of equipment operation.

The figure shows the relationship between illumination level and output. There is an enormous difference between the illumination levels indoors and outdoors. SANYO provides two types of products, indoor products for use in the low illumination levels common in indoor environments and outdoor products for the high illumination levels common outdoors.

Amorton Film Amorton Film is an exceptionally thin, light and flexible amorphous silicon solar cell fabricated on plastic film. In addition to these advantages, Amorton Film is also resistant to crack. Its standard configuration includes protective film covering the amorphous silicon solar ceII which measures about 0.3mm in overaII thickness. When an external load is connected, electricity flows through the load. In this way, an a-Si solar cell converts light energy in to electricity and supplies power to external loads. 11

Manufacturing of thin film solar cells from amorphous silicon by PECVD technology The main components of a a-Si:H solar cell are the three layers of p-doped, intrinsic and n-doped material (p-i-n). Usually, they are deposited by plasma enhanced chemical vapour deposition (PECVD). This process is operated at rather low temperature. The main gas is silane SiH4. When using silane diluted by hydrogen, the properties of the deposited layers can be improved. By adding phosphine PH3 or diborane B2H6 doping of the two outer amorphous silicon layers can be achieved. Addition of methane CH4 will generate an alloyed front layer a-SiC:H, while the addition of germane GeH4 will alloy the absorber layer of stacked solar cells and form a-SiGe:H.

Advantages of amorphous silicon solar cells 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. Amorphous silicon can be deposited at temperatures below 300°C, making it a good candidate for flexible substrates and roll-to-roll manufacturing processes.

Amporphous silicon can be produced in a variety of shapes and sizes (e 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 powering electronic calculators, solar wristwatches, garden lights, and to power car accessories. Small solar cells used in pocket calculators have been made with a-Si for many years. 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 (e.g, for use in solar battery rechargers). The human eye is sensitive to light with wavelengths of 400 nm to 700 nm. Since amorphous silicon solar cells are sensitive to light with essentially the same wavelengths, this means that in addition to be used as solar cells they can also be used as light sensors (e.g., outdoor sensor lights, etc).

Disadvantages 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. 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. From reading through the literature, it appears that the expected life is still in the order of 25 years or so.

Amorphous Photosensors Application Examples System ligting control Adjusting Luminosity LCD Black light Expose control Street light Garden light Camera Cellular Phone Mobile Products Car Navigation

References http://www1.eere.energy.gov/solar/sunshot/pv_asi.html http://www.iws.fraunhofer.de/en/business_fields/chemical_surface_reaction_tec hnology/plasma_reaction_technology/equipment/mikrowellen-ap-pecvd- laboranlage.html http://panasonic.net/energy/amorton/en/solar_battery/ http://answers.yahoo.com/question/index?qid=20070226181243AAmD6AV https://www.crystec.com/triasie.htm http://en.wikipedia.org/wiki/Amorphous_silicon http://www.ics.ele.tue.nl/~akash/maartje/searchSystemsBySystem.php?ID=1002 http://www.nobleled.com/en/showroom/system_solution/solar-street-light- system.html http://www.solar-facts-and-advice.com/amorphous-silicon.html http://www.physics.org/article-questions.asp?id=51 http://science.howstuffworks.com/environmental/energy/solar-cell1.htm