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Power Cell
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HHO Dry Cells Hydrogen Fuel Cells
What Hydrogen on Demand really means is the Hydrogen Generator produces HHO Gas which is used as it is made, the hydrogen gas is not stored for later use. This method of HHO Fuel production is safer then storing huge amounts of hydrogen.
We simple focus on the problem and over look the solution until this solution is so obvious it can long be avoided. The oil crisis is a perfect example of overlooking the obvious answer, which amazingly surrounds us.
With water making up around two-thirds of this planet, and is the answer to our current oil crisis. The technology to convert water in to hydrogen for use as a gas fuel has been around for the past 25 or so years thanks to Stanley Meyer, which who patented the HHO Generator.
This HHO generator is also known as a HHO Fuel Cell, the fuel cell is used to convert water into a combustible gas known as Hydrogen Hybrid Oxide, Brown's Gas, or Hydroxy Gas, named after Dr. Yull Brown, which is the one who discovered this gas compound.
Simply adding a hydrogen generator to a battery and making a few adjustments any internal combustion engine can run on HHO gas. For the ones placing a hydrogen converter on a fuel injected car. You will also need a device called EFIE, (Electronic Fuel Injection Enhancer). Since HHO gas is made simply from water this makes it safe and effective.
Recent research has proved the by adding a hydrogen generator to your car you will get at least a 40% increase in gas mileage and a slight horse power increase, thus drastically reducing the emissions. You can bet the government is adding a few benefits to the users of a hydrogen generator by adding in little incentives. One incentive to using green technology is a tax break on this coming years taxes.
With in a few years the auto industry may be eliminating our demand for oil based fuels, this including the ethanol fuels. This is making our auto giants begin to redefine the automotive industry with the use of Hydrogen on Demand systems. The Hydrogen 7 was recently introduced by BMW which will be able to run on both hydrogen and gas.
The Hydrogen that is used on the Hydrogen 7 is a liquid hydrogen which has to be kept cold. Many companies like Hydrogen Power Inc have installed hydrogen on demand systems in many vehicles. Hypower Fuel Inc has a new hydrogen reactor called H2R. The H2R retrofit insertion device can be placed in almost any car of vehicle.
With commercial made hydrogen on demand systems becoming a reality for the auto giants, but these same units can be built at home by any one with simple hardware you can find at any local hardware store. Hydrogen Generator kits are sold all over the internet today like Run Auto On Water. Some of the kits available produce better then others although one should put in a little bit of research before purchasing a kit over the internet.
With the acceptance of this new technology being a slow process, much like any new technology. People really hate change that has an economic impact on so many companies is dimmed upon or simply put not feasible. At-least not until the energy behind it is so strong it will finally be recognized as a solution that every one knows is right.
Since Yull Brown and Stanley Meyer did not live long enough to see this dream of theirs turn into the answer for fossil fuel consumption, but they did know this would be a reality that every one would accept. The time for Hydrogen on Demand is now for many purposes in cars, at home, businesses, schools and so may others.
Thank you for taking the time to read this article. If you are looking for ways to save money or build you own water fuel cell for your car. To purchase a Hydrogen on demand system visit the HHO Gas Experts.
5 Ways to Make Money From Using Solar Power Cells
Using renewable energy is obviously great for the environment, that's no secret. But a lot of people don't realize just how profitable it can be for you, as well. These days with a DIY energy guide you can even build your own solar power generator, which is another term for solar cells, on a budget for next to nothing, using a number of materials that you likely have around your own home. But getting back to the monetary aspects, here are 5 ways to make money using a solar power generator.
Making your own solar and wind power for less than $200
Utility Expenses - This is pretty straight forward, but when you keep in mind that the average homeowner spends upwards of $2500 on their utility bills alone each year, you realize that there is a great deal of money to be saved from creating your own power from a solar power generator.
Abundance of Power - If you create more energy than you use in a given month, you can sell back whatever natural energy you don't use to the power company as the government pays them to buy your power as it alleviates the national energy crisis. Imagine making an extra $2500 or more each year without doing a thing. The more solar power generators you have, the greater your natural energy output obviously, so many homeowners who build their first couple of solar cells using a DIY energy guide go on to make multiple cells and significantly increase the amount of energy which they can sell back.
Tax Breaks - Particularly these days with the Obama administration in office, homeowners who have embraced natural energy early on and live off their own solar power generator or generators receive considerable tax breaks from the government as again this is helping to alleviate the energy crisis.
Making your own solar and wind power for less than $200
Home Business - As I mentioned a couple of times earlier in this article, it's entirely possible to build your own solar power generator using inexpensive materials and a DIY energy guide which details how to go about doing this. To have the same tools professionally build and installed would cost approximately $3000, so it's clear people are willing to spend top dollar for these energy creating, money making tools. Many people who use these DIY guides go on to build multiple generators, not for themselves, but to give or sell to their friends, relatives, or even start their own business.
Home Value - Having a solar power generator attached to your home significantly adds to the value of your home. This is because other homeowners want to save money just like you, so keep that in mind when it comes time to sell your home.
Making your own solar and wind power for less than $200
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Best Residential Solar Energy Systems and other Related Resources:
Magniwork Magnetic Energy Generator - How would you like to create a generator which creates free electric energy? Using our easy-to-follow guide, you will be able create a Magnetic Power Generator which creates absolutely free energy, and doesn’t require any resource like wind or solar energy to function, the magniwork generator creates energy by itself and powers your home for free. The generator works fully off the grid.
GreenDIYEnergy is a guide on how to start generating your own green energy at home. Developed by Ultimate Results, LLC, GreenDIYEnergy's page promises to give users a step-by-step video and manual on building and installing their own personal solar panels. GreenDIYEnergy's site also gives users a snek peek at the video that shows them how to buld and install their own solar panels.
Homemade Energy is fast becoming the buzz word in environmental circles. Consumers are becoming fed up with the problems associated with using conventional energy resources. Tired of rising costs, changing our lifestyle and in some cases being subjected to rationing, many home owners are turning to the newest trend in energy conservation DIY Solar Power Kits. A number of companies are now producing homemade energy guides, available for instant download on the internet, which give you step by step instructions on how to assemble the materials for your own DIY solar power kits.
What is the maximum power a solar cell can provide?
Assuming light of arbitrary intensity is available, what is the limitation of the maximum power a solar cell can provide for a given surface area? What causes this limitation?
Assuming light of arbitrary intensity is available, what is the limitation of the maximum power a solar cell can provide for a given surface area? What causes this limitation? I'm not talking about the 1000 W/m^2 standard solar heat flux. I've seen fresnel lenses intensify this to nearly a point, putting a large area of solar energy into a small cell at a heat flux of hundreds of suns. What are the limits of doing this? Heat is one obvious problem, but are there others?
This has nothing to do with efficiency. Assume an arbitrary (or infinite) number of photons are cheaply available.
A solar cell (or a "photovoltaic" cell) is a semiconductor device that converts photons from the sun (solar light) into electricity. In general a solar cell that includes both solar and non-solar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell. Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.
Solar cells have many applications. Historically solar cells have been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, consumer systems, e.g. handheld calculators or wrist watches, remote radiotelephones and water pumping applications. Recently solar cells are particularly used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement.
Solar cells are regarded as one of the key technologies towards a sustainable energy supply.
Three generations of development
[edit] First
The first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of solar light. These cells are typically made using silicon wafer. First generation photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market.
[edit] Second
The second generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. These devices were initially designed to be high-efficiency, multiple junction photovoltaic cells. Later, the advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. This contributed to a prediction of greatly reduced costs for thin film solar cells. However, most of the assembly costs for depositing thin film solar cells are still significantly higher than for bulk silicon technologies. Another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light materials, even textiles.
[edit] Third
Third generation photovoltaics are very different from the other two, broadly defined as semiconductor devices which do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include photoelectrochemical cells, Polymer solar cells, and nanocrystal solar cells.
[edit] History
Main article: Timeline of solar cells
The term "photovoltaic" comes from the Greek φώς:phos meaning "light", and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) are named. It means literally of light and electricity.
The photovoltaic effect was first recognised in 1839 by French physicist Alexandre-Edmond Becquerel. However it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 (US2402662, "Light sensitive device"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light.
This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent. This milestone created interest in producing and launching a geostationary communications satellite by providing a viable power supply. Russia launched the first artificial satellite in 1957, and the United States' first artificial satellite was launched in 1958. Russian Sputnik 3 ("Satellite-3"), launched on 15 May 1957, was the first satellite to use solar arrays. This was a crucial development which diverted funding from several governments into research for improved solar cells.
[edit] Applications and implementations
Polycrystaline PV cells laminated to backing material in a PV moduleMain article: photovoltaic array
Solar cells are often electrically connected and encapsulated as a module. PV modules often have a sheet of glass on the front (sun up) side with a resin barrier behind, allowing light to pass while protecting the semiconductor wafers from the elements (rain, hail, etc.). Solar cells are also usually connected in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher amperage. Modules are then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and amperage.
[edit] Theory
[edit] Simple explanation
Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.
Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel.
An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.
Optionally:
The DC current enters an inverter.
The inverter turns DC electricity into 120 or 230-volt AC (alternating current) electricity needed for home appliances.
The AC power enters the utility panel in the house.
The electricity is then distributed to appliances or lights in the house.
The electricity that is not used will be recycled and reused in other facilities.
[edit] Photogeneration of charge carriers
When a photon hits a piece of silicon, one of three things can happen:
the photon can pass straight through the silicon - this (generally) happens for lower energy photons,
the photon can reflect off the surface,
the photon can be absorbed by the silicon which either:
Generates heat, OR
Generates electron-hole pairs, if the photon energy is higher than the silicon band gap value.
Note that if a photon has an integer multiple of band gap energy, it can create more than one electron-hole pair. However, this effect is usually not significant in solar cells. The "integer multiple" part is a result of quantum mechanics and the quantization of energy.
When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one less electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.
A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy.
[edit] Charge carrier separation
There are two main modes for charge carrier separation in a solar cell:
drift of carriers, driven by an electrostatic field established across the device
diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential).
In the widely used p-n junction designed solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n junction designed solar cells (typical of the third generation of solar cell research such as dye and polymer thin-film solar cells), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion.
[edit] The p-n junction
Main article: semiconductor
The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).
If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. The electric field established across the p-n junction creates a diode that promotes current to flow in only one direction across the junction. Electrons may pass from the n-type side into the p-type side, and holes may pass from the p-type side to the n-type side. This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region".
[edit] Connection to an external load
Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load. Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there.
[edit] Equivalent circuit of a solar cell
The equivalent circuit of a solar cell
The schematic symbol of a solar cellTo understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode. In practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The result is the "equivalent circuit of a solar cell" shown on the left. Also shown on the right, is the schematic representation of a solar cell for use in circuit diagrams.
[edit] Solar cell efficiency factors
[edit] Maximum power point
A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load (voltage) in the cell from zero (indicating a short circuit) to infinitely high values (indicating an open circuit) one can determine the maximum power point (the maximum output electrical power, Vmax x Imax; or Pm, in watts).
[edit] Energy conversion efficiency
A solar cell's energy conversion efficiency (η, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of Pm, divided by the input light irradiance under "standard" test conditions (E, in W/m2) and the surface area of the solar cell (Ac in m²).
At solar noon on a clear March or September equinox day, the solar radiation at the equator is about 1000 W/m2. Hence, the "standard" solar radiation (known as the "air mass 1.5 spectrum") has a power density of 1000 watts per square meter. Thus, a 12% efficiency solar cell having 1 m² of surface area in full sunlight at solar noon at the equator during either the March or September equinox will produce approximately 120 watts of peak power.
[edit] Fill factor
Another defining term in the overall behavior of a solar cell is the fill factor (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc):
[edit] Quantum efficiency
Quantum efficiency refers to the percentage of absorbed photons that produce electron-hole pairs (or charge carriers). This is a term intrinsic to the light absorbing material, and not the cell as a whole (which becomes more relevant for thin-film solar cells). This term should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell.
[edit] Comparison of energy conversion efficiencies
Main article: Photovoltaics
Silicon solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 30% or higher with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available mc-Si solar cells are around 14-16%. The highest efficiency cells have not always been the most economical -- for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering a little under four times the electrical power.
To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the electricity that is not needed immediately.
A common method used to express economic costs of electricity-generating systems is to calculate a price per delivered kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Using the commercially available solar cells (as of 2006) and system technology leads to system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity generation costs ranged from ~ 50 eurocents/kWh (0.60 US$/kWh) (central Europe) down to ~ 25 eurocents/kWh (0.30 US$/kWh) in regions of high solar irradiation. This electricity is generally fed into the electrical grid on the customer's side of the meter. The cost can be compared to prevailing retail electric pricing (as of 2005), which varied from between 0.04 and 0.50 US$/kWh worldwide. (Note: in addition to solar irradiance profiles, these costs/kwh calculations will vary depending on assumptions for years of useful life of a system. Most c-Si panels are warrantied for 25 years and should see 35+ years of useful life.)
The chart at the right illustrates the various commercial large-area module energy conversion efficiencies and the best laboratory efficiencies obtained for various materials and technologies.
Reported timeline of solar cell energy conversion efficiencies (from National Renewable Energy Laboratory (USA)
[edit] Peak watt (or Watt peak)
Since solar cell output power depends on multiple factors, such as the sun's incidence angle, for comparison purposes between different cells and panels, the peak watt (Wp) is used. It is the output power under these conditions:[1]
solar irradiance 1000 W/m²
solar reference spectrum AM (airmass) 1.5
cell temperature 25°C
[edit] Solar cells and energy payback
There is a common conception that solar cells never produce more energy than it takes to make them. While the expected working lifetime is around 40 years, the energy payback time of a solar panel is anywhere from 1 to 20 years (usually under five) depending on the type and where it is used (see net energy gain). This means solar cells can be net energy producers and can "reproduce" themselves (from just over once to more than 30 times) over their lifetime.[2][3]
This is disputed, however, by some researchers who object that such analysis doesn't take into account waste, inefficiency, and related energy costs that would come with a real-world solar cell.[4]
[edit] Light-absorbing materials
All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface; however, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms (listed in alphabetical order). Many currently available solar cells are configured as bulk materials that are subsequently cut into wafers and treated in a "top-down" method of synthesis (silicon being the most prevalent bulk material). Other materials are configured as thin-films (inorganic layers, organic dyes, and organic polymers) that are deposited on supporting substrates, while a third group are used as quantum dots (electron-confined nanoparticles) embedded in a supporting matrix in a "bottom-up" approach. Silicon remains the only material that is well-researched in both bulk and thin-film configurations.
[edit] Bulk
These bulk technologies are often referred to as wafer-based manufacturing. In other words, in each of these approaches, self-supporting wafers between 180 to 240 micrometers thick are processed and then soldered together to form a solar cell module. A general description of silicon wafer processing is provided in Manufacture and Devices.
[edit] Silicon
Main articles: silicon and list of silicon producers
By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.
monocrystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the corners of four cells.
Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots - large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient.
Ribbon silicon: formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.
[edit] Thin films
The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.
[edit] CdTe
Cadmium telluride is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production. Despite much discussion of the toxicity of CdTe-based solar cells, this is the only technology (apart from amorphous silicon) that can be delivered on a large scale, as shown by First Solar and Antec Solar. There is a 40 megawatt plant in Ohio (USA) and a 10 megawatt plant in Germany. First Solar is scaling up to a 100 MW plant in Germany.
The perception of the toxicity of CdTe is based on the toxicity of elemental cadmium. However, it is possible for toxic elements to combine to form a harmless compound, as in the example of sodium chloride, better known as common salt, which consists of the highly reactive metal sodium and the corrosive and toxic gas chlorine.
Scientific work, particularly by researchers of the National Renewable Energy Laboratories (NREL) in the USA, has shown that the release of cadmium to the atmosphere is lower with CdTe-based solar cells than with silicon photovoltaics and other thin-film solar cell technologies.
[edit] CIGS
CIGS are multi-layered thin-film composites. The abbreviation stands for copper indium gallium selenide. Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as of December 2005 was 19.5% with CIGS. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. As of 2006, the best conversion efficiency for flexible CIGS cells on polyimide is 14.1% by Tiwari et al, at the ETH, Switzerland.
The use of indium increases the bandgap of the CIGS layer, gallium is added to replace as much indium as possible due to gallium's relative availability to indium. Approximately 70%[5] of Indium currently produced is used by the flat-screen monitor industry. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium. Producing 2GW of CIGS cells (roughly the amount of silicon cells produced in 2006) would use about 10% of the indium produced in 2006.[citation needed] Nanosolar claims to waste only 5% of the indium it uses and suggests that Daystar's vacuum sputtering technology may tend to waste about 60% of the indium.
Selenium allows for better uniformity across the layer and so the number of recombination sites in the film are reduced which benefits the quantum efficiency and thus the conversion efficiency.
[edit] CIS
CIS is an abbreviation for general chalcogenide films of copper indium selenide (CuInSe2). While these films can achieve 11% efficiency, their manufacturing costs are high at present but continuing work is leading to more cost-effective production processes. A manufacturing plant was built in Germany by Würth Solar. It was inaugurated in October 2006. Full production is expected by end of 2006.
[edit] Gallium arsenide (GaAs) multijunction
High-efficiency cells have been developed for special applications such as satellites and space exploration which require high-performance. These multijunction cells consist of multiple thin films produced using molecular beam epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2.[6] Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible. GaAs multijunction devices are the most efficient solar cells to date, reaching as high as 39% efficiency.[7] They are also some of the most expensive cells per unit area (up to US$40/cm²).
[edit] Light absorbing dyes
Main article: Dye-sensitized solar cells
Typically a Ruthenium metalorganic dye (Ru-centered) used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m²/gram TiO2, as compared to approximately 10 m²/gram of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and typically are manufactured by screen printing, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecasted within this decade.
[edit] Organic/polymer solar cells
Organic solar cells and Polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are low at 4-5% efficiency for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.
[edit] Silicon
Silicon thin-films are mainly deposited by Chemical vapor deposition (typically plasma enhanced (PE-CVD)) from silane gas and hydrogen gas. Depending on the deposition's parameters, this can yield:
Amorphous silicon (a-Si or a-Si:H)
protocrystalline silicon or
Nanocrystalline silicon (nc-Si or nc-Si:H).
These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon.
Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.
[edit] Nanocrystalline solar cells
Main article: Nanocrystal solar cell
These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption).
[edit] Concentrating Photovoltaics (CPV)
Concentrating photovoltaic systems use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. [8] If these systems use single or dual-axis tracking to improve performance, they may be refered to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of CPV systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of general photovoltaic materials [9] and also allows for the use of high-performance materials such as gallium arsenide.[10] Despite the advantages of CPV technologies their application has been limited by the costs of focusing, tracking and cooling equipment. On October 25, 2006, Australia announced it would construct a solar plant using this technology to come online in 2008 and be completed by 2013. This plant, at 154 MW, would be ten times larger than the largest current photovoltaic plant in the world.[11]
[edit] Silicon solar cell device manufacture
Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and memory chips. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells. Single crystalline wafers which are used in the semiconductor industry can be made into excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production.
Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers below the surface.
Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically applied next. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed.
The wafer is then metallized, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminium. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The metal electrodes will then require some kind of heat treatment or "sintering" to make Ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process.
[edit] Current research on materials and devices
Main article: Timeline of solar cells
There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be divided into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as light absorbers and charge carriers.
[edit] Silicon processing
One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO2) to produce silicon is a very high energy process, and more energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole.
The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1.5 tonnes of carbon dioxide.
Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius). [12][13] While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, (with a particle size of a few micrometres), and may therefore offer new opportunities for development of solar cell technologies.
Another approach is also to reduce the amount of silicon used and thus cost, as done by Australian National University in production of their "Sliver" cells, by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings.[14] Using this technique, two silicon wafers are enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output.
Yet another way to achieve cost improvements is to reduce wastes during the crystal formation by improved modelisation of the process, as done by FemagSoft, spin-off of the Université Catholique de Louvain.
[edit] Thin-film processing
Thin-film solar cells use less than 1% of the raw material (silicon or other light absorbers) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials, however it remains to be seen if these solutions can generate the same space-efficiency as traditional silicon processing.
One particularly promising technology is crystalline silicon thin films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach.
Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications.
[edit] Polymer processing
The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. Additionally, most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.
[edit] Nanoparticle processing
Experimental non-silicon solar panels can be made of quantum heterostructures, eg. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot-modified photovoltaics may be able to achieve up to 42 percent energy conversion efficiency due to multiple exciton generation.[15]
[edit] Transparent Conductors
Many new solar cells use transparent thin films that are also conductors of electrical charge. The dominant conductive thin films used in research now are transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al), and indium tin oxide (abbreviated "ITO"). These conductive films are also used in the LCD industry for flat panel displays. The dual function of a TCO allows light to pass through a substrate window to the active light absorbing material beneath, and also serves as an ohmic contact to transport photogenerated charge carriers away from that light absorbing material. The present TCO materials are effective for research, but perhaps are not yet optimized for large-scale photovoltaic production. They require very special deposition conditions at high vacuum, they can sometimes suffer from poor mechanical strength, and most have poor transmittance in the infrared portion of the spectrum (e.g.: ITO thin films can also be used as infrared filters in airplane windows). These factors make large-scale manufacturing more costly.
A relatively new area has emerged using carbon nanotube networks as a transparent conductor for organic solar cells. Nanotube networks are flexible and can be deposited on surfaces a variety of ways. With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve
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