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Solar cell

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A solar cell, or photovoltaic cell, is a semiconductordevice consisting of a large-areap-n junctiondiode, which, in the presence ofsunlightis capable of generating usable electricalenergy. Thisconversion is called the photovoltaic effect. The field of research related to solar cells is known as

photovoltaics.

Solar cells have many applications. They are particularly well suited to, and historically used insituations where electrical power from the gridis unavailable, such as in remote area powersystems, Earth orbiting satellites, handheld calculators, remote radiotelephones, waterpumpingapplications, etc. Solar cells (in the form of modules orsolar panels) are appearing on buildingroofs where they are connected through an inverterto the electricity grid in anet metering

arrangement.

A solar cell, made from a poly-crystalline silicon wafer

Contents

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1 Introductiono

1.1 Etymologyo 1.2 History

o 1.3 Materials and efficiency

o 1.4 Interconnection and modules

2 Theoryo 2.1 Background

o 2.2 Light generation of carriers

o 2.3 The p-n junction

o 2.4 Separation of carriers by the p-n junction

o 2.5 Connection to an external load

o 2.6 Equivalent circuit of a solar cell

3 Manufacture and deviceso 3.1 Energy conversion efficiency

4 Applications and implementations
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5 Cost analysis 6 Current research

o 6.1 Thin-film solar cells

o 6.2 Exotic materials

7 Solar cells and energy payback 8 See also

9 References 10 External links

o 10.1 Yield data

o 10.2 Theory

o 10.3 Cost Benefit

o 10.4 Do-it-yourself

o 10.5 Indexes

o 10.6 Newsgroups

o 10.7 Patents

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Introduction

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Etymology

The etymology of the term "photovoltaic" comes from the Greekphotos meaning light and thename of theItalian physicist Volta, after whom the volt (and consequently voltage) are named. Itmeans literally of light and electricity.

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History

Main article: Timeline of solar cells

The photovoltaic effect was first recognised in 1839 by French physicist Alexandre-EdmondBecquerel. However it was not until 1883 that the first solar cell was built, by Charles Fritts whocoated the semiconductorselenium with an extremely thin layer ofgoldto form the junctions. The

device was only around 1% efficient. Russell Ohl is generally recognized for patenting the modernsolar cell in 1946 (US2402662, "Light sensitive device"). Sven Ason Berglund had a prior patentconcerning methods of increasing the capacity of photosensitive cells.

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Materials and efficiency

Various materials have been investigated for solar cells. There are two main criteria - efficiency andcost. Efficiency is a ratio of the electric power output to the light power input. Ideally, near theequator at noon on a clear day, the solar radiation is approximately 1000 W/m. So a 10% efficient

module of 1 square meter can power a 100 W light bulb. Costs and efficiencies of the materials varygreatly.
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By far the most common material for solar cells (and all other semiconductor devices) is crystallinesilicon. Crystalline silicon solar cells come in three primary categories:

Single crystal or monocrystalline wafers made using the Czochralski process. Mostcommercial monocrystalline cells have efficiencies on the order of 14%; the SunPower cellshave high efficiencies around 20%. Single crystal cells tend to be expensive, and because

they are cut from cylindrical ingots, they cannot completely cover a module without asubstantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at thecorners of four cells. Sunpowerand Shell Solarare among the main manufacturers of thistype of cells.

Poly or multi crystalline made from cast ingots - large crucibles of molten silicon carefullycooled and solidified. These cells are cheaper than single crystal cells, but also somewhatless efficient. However, they can easily be formed into square shapes that cover a greaterfraction of a panel than monocrystalline cells, and this compensates for their lowerefficiencies. See GT Solar HEM Furnace, BP Solar, Sharp Solarand Kyocera Solar.

Ribbon silicon formed by drawing flat thin films from molten silicon and has amulticrystalline structure. These cells are typically the least efficient, but there is a cost

savings since there is very little silicon waste since this approach does not require sawingfrom ingots. See Evergreen Solar, and RWE Schott Solar.

These technologies are wafer based manufacturing. In other words, in each of the aboveapproaches, self supporting wafers of ~300 micrometres thick are fabricated and then solderedtogether to form a module.

Thin film approaches are module based. The entire module substrate is coated with the desiredlayers and a laser scribe is then used to delineate individual cells. Two main thin film approachesare amorphous silicon and CIS:

Amorphous silicon films are fabricated using chemical vapor deposition techniques,typically plasma enhanced (PE-CVD). These cells have low efficiencies around 8%.

CIS stands for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2. While these filmscan achieve 11% efficiency, their costs are still too high.

There are additional materials and approaches. For example, Sanyo has pioneered the HIT cell. Inthis technology, amorphous silicon films are deposited onto crystalline silicon wafers.

The chart below illustrates the various commercial large area module efficiencies and the bestlaboratory efficiencies obtained for various materials and technologies.

Image:PVModuleLabEffic.jpg

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Interconnection and modules

Usually, solar cells are electrically connected, and combined into "modules", orsolar panels. Solarpanels have a sheet of glass on the front, and a resin encapsulation behind to keep thesemiconductorwafers safe from the elements (rain, hail, etc). Solar cells are usually connected inseries in modules, so that theirvoltages add.

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Theory

[edit]

Background

In order to understand how a solar cell works, a little background theory in semiconductorphysicsis required. For simplicity, the description here will be limited to describing the workings of singlecrystalline silicon solar cells.

Silicon is a group 14 (formerly, group IV) atom. This means that each Si atom has 4 valenceelectrons in its outershell. Silicon atoms can covalentlybond to other silicon atoms to form a solid.There are two basic types of solid silicon, amorphous (having no long range order) and crystalline(where the atoms are arranged in an ordered three dimensional array). There are various other termsfor the crystalline structure of silicon; poly-crystalline, micro-crystalline, nano-crystalline etc, andthese refer to the size of the crystal "grains" which make up the solid. Solar cells can be, and are

made from each of these types of silicon, the most common being poly-crystalline.

Silicon is a semiconductor. This means that in solid silicon, there are certain bands of energieswhich the electrons are allowed to have, and other energies between these bands which areforbidden. These forbidden energies are called the "band gap". The allowed and forbidden bands ofenergy are explained by the theory ofquantum mechanics.

At room temperature, pure silicon is a poor electrical conductor. In quantum mechanics, this isexplained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a betterconductor, it is "doped" with very small amounts of atoms from eithergroup 13 (III) orgroup 15(V) of theperiodic table. These "dopant" atoms take the place of the silicon atoms in the crystal

lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do.However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent

bonds around each atom. Since these extra electrons, or lack of electrons (known as "holes") are notinvolved in the covalent bonds of the crystal lattice, they are free to move around within the solid.Silicon which is doped with group 13 atoms (aluminium,gallium) is known asp-typesilicon

because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group15 atoms (phosphorus, arsenic) is known asn-type silicon because the majority charge carriers(electrons) are negative. It should be noted that both n-type and p-type silcion are electricallyneutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-typesilicon, some of the negative charges are free to move around, while the converse is true for p-type

silicon.

[edit]

Light generation of carriers
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The absorption of photons creates electron-hole pairs, which diffuse to the electrical contacts andcan be extracted to power electrical devices

When aphoton oflight hits a piece of silicon, one of two things can happen. The first is that thephoton can pass straight through the silicon. This (generally) happens when the energy of thephoton is lower than the bandgap energy of the silicon semiconductor. The second thing that can

happen is that the photon is absorbed by the silicon. This (generally) happens if the photon energyis greater than the bandgap energy of silicon. When a photon is absorbed, its energy is given to anelectron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound incovalent bonds between neighbouring 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 thesemiconductor. The covalent bond that the electron was previously a part of now has one lesselectron - this is known as a hole. The presence of a missing covalent bond allows the bondedelectrons of neighboring atoms to move into the "hole," leaving another hole behind, and in thisway a hole can move through the lattice. Thus, it can be said that photons absorbed in thesemiconductor create mobile electron-hole pairs.

A photon only needs to have energy greater than the band gap energy to excite an electron from thevalence band into the conduction band. However, the solarfrequency spectrum approximates a

black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth iscomposed 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 andthe silicon band gap is converted into heat (via lattice vibrations - calledphonons) rather than intousable electrical energy.

[edit]

The p-n junction

A solar cell is a large-area semiconductorp-n junction. To understand the workings of a p-njunction it is convenient to imagine what happens when a piece of n-type silicon is brought intocontact with a piece of p-type silicon. In practice, however, the p-n junctions of solar cells are notmade in this way, but rather, usually, by diffusing an n-type dopant into one side of a p-type wafer.

If we imagine what happens when a piece of p-type silicon is placed in intimate contact with a pieceof n-type silicon, then what occurs is a diffusion of electrons from the region of high electronconcentration - the n-type side of the junction, into the region of low electron concentration - p-typeside of the junction. When the electrons diffuse across the p-n junction, they recombine with holes

on the p-type side. This diffusion of carriers does not happen indefinitely however, because of theelectric field which is created by the imbalance of charge immediately either side of the junctionwhich this diffusion creates. Electrons from donoratoms on the n-type side of the junction are
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crossing into the p-type side, leaving behind the (extra) positively charged nuclei of the group 15donor atoms, leaving an excess of positive charge on the n-type side of the junction. At the sametime, these electrons are filling in holes on the p-type side of the junction, becoming involved incovalent bonds around the group 13 acceptoratoms, making an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric fieldwhich opposes further diffusion of charge carriers across the junction.

This region where electrons have diffused across the junction is called thedepletion region becauseit no longer contains any mobile charge carriers. It is also known as the "space charge region".

The electric field which is set up across the p-n junction creates a diode, allowingcurrent to flow inonly 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. But since the sign of the charge onelectrons and holes is opposite, conventional currentmay only flow in one direction.

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Separation of carriers by the p-n junction

Once the electron-hole pair has been created by the absorption of a photon, the electron and hole areboth free to move off independently within the silicon lattice. If they are created within a minoritycarrier diffusion length of the junction, then, depending on which side of the junction the electron-hole pair is created, the electric field at the junction will either sweep the electron to the n-type side,or the hole to the p-type side.

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Connection to an external load

Ohmicmetal-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, orhave 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-metalcontact. 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 createdthere.

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Equivalent circuit of a solar cell

The equivalent circuit of a solar cell
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The schematic symbol of a solar cell

To understand the electronic behaviour of a solar cell, it is useful to create a model which iselectrically equivalent, and is based on discrete electrical components whose behaviour is wellknown. An ideal solar cell may be modelled by a current source in parallel with a diode. In practiceno 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 theschematic representation of a solar cell for use in circuit diagrams.

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Manufacture and devices

Because solar cells are semiconductor devices, they share many of the same processing andmanufacturing techniques as other semiconductor devices such as computerand memorychips.

However, the stringent requirements for cleanliness and quality control of semiconductorfabrication are a little more relaxed for solar cells.

Most large-scale commercial solar cell factories today make screen printed poly-crystalline siliconsolar cells. Single crystalline wafers which are used in the semiconductor industry can be made in toexcellent 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(250 to 350 micrometre) slices or wafers. The wafers are usually lightly p-type doped.

To make a solar cell from the wafer, an n-type diffusion is performed on the front side of the wafer,forming a p-n junction a few hundred nanometres below the surface.

Antireflection coatings, which increase the amount of light coupled into the solar cell, are typicallyapplied next. Over the past decade, silicon nitride has gradually replaced titanium dioxide as theantireflection coating of choice because of its excellent surface passivation qualities (i.e., it preventscarrier recombination at the surface of the solar cell). It is typically applied in a layer severalhundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD).

The wafer is then metallised, 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 thefront surface using a silverpaste. The rear contact is also formed by screen-printing a metal paste,typically aluminum. Usually this contact covers the entire rear side of the cell, though in some cell
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designs it is printed in a grid pattern. The metal electrodes will then require some kind of heattreatment 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) byflat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheetof temperedglass on the front, and apolymerencapsulation on the back.

Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase theamount of light coupled into the cell. Such surfaces can usually only be formed on single-crystalsilicon, though in recent years methods of forming them on multicrystalline silicon have beendeveloped.

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Energy conversion efficiency

Typical module efficiencies for commercially available screen printed multicrystalline solar cells

are around 12%. A solar module's energy conversion efficiency, (or just efficiency) is the ratio ofthe maximum output electrical power divided by the input light power under "standard" testconditions. The "standard" solar radiation (known as the "air mass 1.5 spectrum") has a powerdensity of 1000watts per squaremetre. Thus, a typical 1 m solar panel in direct sunlight will

produce approximately 120 watts of peak power. A more technical description of efficiency is themaximum power, made up of the fill factorx the open circuit voltage x the short circuit current,divided by the input power.

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Applications and implementations

See the article solar panel for information about applications and implementations of solar cells andpanels.

[edit]

Cost analysis

The US retail module costs are in the $3.50 to $5.00/Wp range (see SolarBuzz). Additional

installation costs for a residential rooftop retrofit in California (CA) is around $3.50/Wp or more.So on the low side, installed system costs are about $7.00/Wp in CA, and probably higher in placeswith less experience. Federal, state, utility, and othersubsidies combined pay about half the cost. SoCA rule of thumb is that the installed system PV will cost you at the low end, $3.50/Wp.

Under net metering, one offsets regular retail utility rate which for CA is about 11 cents/kWh.Knowing installed system costs, amount of sunshine, and the utility rates, one can figure out theyears till payback with or without financing costs. Assuming no financing costs and a $6/Wpinstalled system cost (lower than current $7), one can take sunshine and utility rate informationfrom around the globe and come up with a payback graph such as shown below. The addition ofsubsidies brings down the years to payback proportionately. For example, if the years to payback

were 24 years at $6/Wp, and subsidies brought that down to $3/Wp, the years to payback would be12.
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[edit]

Current research

There are currently many researchgroups active in the field ofphotovoltaicsat universitiesandresearch institutions around the world.

Much of the research is focussed on making solar cells cheaper and/or more efficient, so that theycan more effectively compete with other energy sources, including fossil energy. One way of doingthis is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a verycommon element, but is normally bound in silica sand. Another approach is to significantly reducethe amount of raw material used in the manufacture of solar cells. The various thin-film

technologies currently being developed make use of this approach to reducing the cost of electricityfrom solar cells.

The invention ofconductive polymers, (for which Alan Heegerwas awarded aNobel prize) maylead to the development of much cheaper cells that are based on inexpensive plastics, rather thansemiconductor grade silicon. However, all organic solar cellsmade to date suffer from degradationupon exposure toUV light, and hence have lifetimes which are far too short to be viable.

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Thin-film solar cells

The next step in reducing the cost of solar cells and panels seems certain to come from thin-filmtechnology. Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer
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based solar cells, leading to a significant price drop per kWh. There are many research groupsaround the world actively researching different thin-film approaches and/or materials.

Thin Film solar cells are mainly deposited by PECVD from silane gas and hydrogen. This processproduces a material without crystalline orientation : amorphous silicon. Depending on thedeposition's parameters bothprotocrystalline silicon, which has been shown to exhibit the most

stability, and nanocrystalline silicon can also be obtained. These types of silicon present dandlingand twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) aswell as in the deformation of the valence and conduction bands (band tails). This contributes toreduce theefficiencyof Thin-Film solar cells by reducing the number of collected electron-hole pair

by incident photon.

Amorphous silicon (a-Si) 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 collectan important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgapas c-Si, the two material can be combined by depositing two diodes on top of each other : thetandem 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.

One particularly promising technology is crystalline silicon thin-films on glass substrates. Thistechnology makes use of the advantages of crystalline silicon as a solar cell material, with the costsavings of using a thin-film approach. From thePacific Solarwebsite:

"Crystalline Silicon on Glass (CSG) [is] the photovoltaic technology developed by PacificSolar that is now being commercialised by CSG Solar. A very thin layer of silicon, less thantwo micrometres thick, is deposited directly onto a glass sheet whose surface has beenroughened by applying a layer of tiny glass beads. The silicon is not crystalline when firstdeposited, but becomes so after heat treatment in an oven. The resulting layer is processedusing lasers and ink-jet printing techniques to form the electrical contacts needed to get thesolar-produced electricity out of the thin silicon film."

In 2005, a full-scale production factory is being built in Thalheim, Germany to commercialise thistechnology (project management by IB Vogt GmbH). CSG Solar expects to release its first productfor sale in 2006. Each solar module will have a rated power exceeding 100 watts and will becheaper than competing solar panels.

Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind ofmaterials, including flexible substrates (PETfor example), which opens a new dimension for new

applications.

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Exotic materials

For special applications, such as Deep Space 1, high-efficiency cells can be made from galliumarsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different

band gap energy so that it absorbs its share of the electromagnetic spectrum with very highefficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of eachother, each absorbing a different spectrum of light, efficiency as high as 28% have been achieved.

The multiple junction solar cells may be very efficient, but are prohibitively expensive to make.Cost-effective use of these cells could be achieved with concentrating optics so that less of the arrayconsists of actual semiconductor devices.
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Experimental non-silicon solar panels can be made ofcarbon nanotubes orquantum dots embeddedin a specialplastic. These have only one-tenth the efficiency of silicon panels but could bemanufactured in ordinary factories, not clean rooms which should lower the cost. Whileconventional solar cells only generate electricity from the visible light spectrum, experimental cellshave been made that use the infrared spectrum. By varying the size of the quantum dots, the cellscan be tuned to absorb different wavelengths. If panels that absorb both visible and infrared

spectrums are able to be manufactured, the panels may be able to achieve up to 30 percentefficiency. (McDonald, et al., 2005)

Some of the most efficient solar cell materials are cadmiumtelluride (CdTe) and copperindiumgalliumselenide (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n

junction (see undersemiconductor), these cells are best described by a more complex heterojunctionmodel. The best efficiency of a bare solar cell as of April2003 was 16.5% [Dr IM Dharmadasa,Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by usingoptics to concentrate the incident light.

Polymerororganic solar cells are built from ultra thin layers (typically 100 nm) of organic

semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only acrude description of the functioning of such cells, aselectronhopping and other processes also playa crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, butefficiencies achieved to date are low and cells are highly sensitive to air and moisture, makingcommercial applications difficult. In the reverse mode, the technology has however alreadysuccessfully been commercialised in organic LEDsandorganic displays, also calledpolymerdisplays.

Graetzel cells(sometimes calledphotoelectrochemical cells) have been around for two decades orso. A p/n junction is used here too in the form of a doped solid (normally titanium dioxide) incontact with a solid or liquid electrolyte (for exampleCuI). In contrast to the classical solar cell notthe semiconductor but a dye placed at the p/n interface is used for absorption of radiation,mimicking the process ofphotosynthesis. As a result, this type of cell allows a more flexible use ofmaterials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions.Commercial applications have failed to appear due to the fast degradation occurring in Graetzelcells.

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Solar cells and energy payback

There is a common but mistaken notion that solar cells never produce more energy than it takes tomake them. While the expected working lifetime is around 40 years, the energy payback time of asolar panel is anywhere from 1 to 30 years (usually under five) depending on the type and where itis used (see net energy gain). This means solar cells are net energy producers and can "reproduce"themselves (from 6 to more than 30 times) over their lifetime. For details seeNet Energy AnalysisFor Sustainable Energy Production From Silicon Based Solar Cells.

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See also

Autonomous building Future energy development
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Green technology Photodiode Photovore Renewable energy Solar power Solar panel
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