Acronym Definition
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A solar cell or photovoltaic cell is a device that converts light energy into
electrical energy. Sometimes the term solar cell is reserved for devices
intended specifically to capture energy from sunlight, while the term
photovoltaic cell is used when the light source is unspecified.
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 (simply put, carrying electrons off through a metal contact into a
wire or other circuit). 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. They have long been used in situations where
electrical power from the grid is unavailable, such as in remote area power
systems, Earth-orbiting satellites and space probes, consumer systems, e.g.
handheld calculators or wrist watches, remote radiotelephones and water pumping
applications. More recently, solar cells are starting to be used in assemblies
of solar modules (photovoltaic arrays) connected to the electricity grid through
an inverter, often in combination with a net metering arrangement.
Four generations of development
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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 sunlight. These cells are typically made
using a 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.
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. There are currently (2007) a number of
technologies/semiconductor materials under investigation or in mass production.
Examples include Amorphous silicon, Polycrystalline silicon, micro-crystalline
silicon, Cadmium telluride, copper indium selenide/sulfide. Typically, the
efficiencies of thin-film solar cells are lower compared with silicon
(wafer-based) solar cells, but manufacturing costs are also lower, so that a
lower cost per watt can be achieved. Another advantage of the reduced mass is
that less support is needed when placing panels on rooftops and it allows
fitting panels on light or flexible materials, even textiles.
Third
Third generation photovoltaics are very different from the previous
semiconductor devices as they 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.
Dye-sensitized solar cells are now in production.
Fourth
Fourth generation Composite photovoltaic technology with the use of polymers
with nano particles can be mixed together to make a single multispectrum layer.
Then the thin multi spectrum layers can be stacked to make multispectrum solar
cells more efficient and cheaper based on polymer solar cell and multi junction
technology used by NASA on Mars missions. The layer that converts different
types of light is first, then another layer for the light that passes and last
is an infra-red spectrum layer for the cell - thus converting some of the heat
for an overall solar cell composite.
Companies working on fourth generation photovoltaics include Xsunx, Konarka
Technologies, Inc., Nanosolar, Dyesol and Nanosys. Research is also being done
in this area by the USA's National Renewable Energy Laboratory (http://www.nrel.gov/).
History
M Timeline of solar cells
The term "photovoltaic" comes from the Greek φ??:phos meaning "light", and
"voltaic", meaning electrical, from the name of the Italian physicist Volta,
after whom the measurement unit volts are named. The term "photo-voltaic" has
been in use in English since 1849.
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 (U.S. Patent
2,402,662 , "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
1958, 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.
In 1970 first highly effective GaAs heterostructure solar cells were created by
Zhores Alferov and his team in the USSR.
Applications and implementations
Polycrystaline PV cells laminated to backing material in a PV module
Polycrystalline PV cellsM 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 , 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 current.
Modules are then interconnected, in series or parallel, or both, to create an
array with the desired peak DC voltage and current.
The power output of a solar array is measured in watts or kilowatts. In order to
calculate the typical energy needs of the application, a measurement in
watt-hours, kilowatt-hours or kilowatt-hours per day is often used. A rule of
thumb commonly used is that peak power times 20% gives average power, equating
to one kW peak producing 4.8 kWh per day.
Theory
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 240-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.
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 fewer 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.
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 solar cells, the dominant mode of charge carrier
separation is by drift. However, in non-p-n-junction 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.
The p-n junction
M 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".
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.
Equivalent circuit of a solar cell
The equivalent circuit of a solar cell
The schematic symbol of a solar cellTo understand the electronic behavior 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 resulting equivalent circuit of
a solar cell is shown on the left. Also shown, on the right, is the schematic
representation of a solar cell for use in circuit diagrams.
Solar cell efficiency factors
Maximum-power point
A solar cell may operate over a wide range of voltages (V) and currents (I). By
increasing the resistive load on an irradiated cell continuously from zero (a
short circuit) to a very high value (an open circuit) one can determine the
maximum-power point, the point that maximizes V×I, that is, the load for which
the cell can deliver maximum electrical power at that level of irradiation.
The maximum power point of a photovoltaic varies with incident illumination. For
systems large enough to justify the extra expense, a maximum power point tracker
tracks the instantaneous power by continually measuring the voltage and current
(and hence, power transfer), and uses this information to dynamically adjust the
load so the maximum power is always transferred, regardless of the variation in
lighting.
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.
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):
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.
Comparison of energy conversion efficiencies
M Photovoltaics
Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to
42.8% with multiple-junction research lab cells. Solar cell energy conversion
efficiencies for commercially available multicrystalline 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 about 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 energy 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 ~0.60 US$/kWh (0.50 €/kWh) (central
Europe) down to ~0.30 US$/kWh (0.25 €/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)
Watts 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
measure of watts peak (Wp) is used. It is the output power under these
conditions known as STC:
insolation (solar irradiance) 1000 W/m2
solar reference spectrum AM (airmass) 1.5
cell temperature 25°C
Solar cells and energy payback
In the 1990s, when silicon cells were twice as thick, efficiencies 30% lower
than today and lifetimes shorter, it may well have cost more energy to make a
cell than it could generate in a lifetime. The energy payback time of a modern
photovoltaic module 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, meaning they generate more energy over
their lifetime than the energy expended in producing them. .
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. 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 configured as nanocrystals and 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. The following is a current list of light absorbing
materials, listed by configuration and substance-name:
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.
Silicon
Main articles: silicon and list of silicon producers
Basic structure of a silicon based solar cell and its working mechanism.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.
New Structures: These new compounds are special arrangements of silicon that can
dramatically improve efficiency such as ormosil.
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.
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 and started
building another 100 MW plant in Malaysia (2007).
The perception of the toxicity of CdTe is based on the toxicity of elemental
cadmium, a heavy metal that is a cumulative poison. 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.
CIGS
CIGS are multi-layered thin-film composites. The abbreviation stands for copper
indium gallium diselenide. Unlike the conventional silicon based 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
absorber layer. Higher efficiencies (around 30%) can be obtained by using optics
to concentrate the incident light. The use of gallium increases the optical
bandgap of the CIGS layer as compared to CIS thus increase the open-circuit
voltage. In another point of view, gallium is added to replace as much indium as
possible due to gallium's relative availability to indium. Approximately 70% 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 2 GW of CIGS cells (roughly the
amount of silicon cells produced in 2006) would use about 10% of the indium
produced in 2004. For comparison, silicon solar cells used up 33% of the world's
electronic grade silicon production in 2006! Nanosolar claims to waste only 5%
of the indium it uses. As of 2006, the best conversion efficiency for flexible
CIGS cells on polyimide is 14.1% by Tiwari et al, at the ETH, Switzerland.
That being said, indium can easily be recycled from decommissioned PV modules.
The recycling program in Germany would be one good example to follow. It also
highlights the new regenerative industrial paradigm: "From cradle to cradle".
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.
CIS
Possible combinations of I III VI elements in the periodic table that have
photovoltaic effect
The materials based on CuInSe2 that are of interest for photovoltaic
applications include several elements from groups I, III and VI in the periodic
table. These semiconductors are especially attractive for thin film solar cell
application because of their high optical absorption coefficients and versatile
optical and electrical characteristics which can in principle be manipulated and
tuned for a specific need in a given device. CIS is an abbreviation for general
chalcopyrite films of copper indium selenide (CuInSe2), CIGS mentioned above is
a variation of CIS. While these films can achieve 13.5% efficiency, their
manufacturing costs at present are high when compared with a silicon solar cell
but continuing work is leading to more cost-effective production processes.
There are more plans by AVANCIS and Shell in a joint effort to build another
plant in Germany with a capacity of 20 MW. Honda in Japan has finished its
pilot-plant testing and is launching its commercial production. In North
America, Global Solar has been producing pliable CIS solar cell in smaller scale
since 2001. Apart from Daystar Technologies and Nanosolar mentioned in CIGS,
there are other potential manufacturers coming on line such as Miasole using a
vacuum sputtering method and also a Canadian initiative CIS Solar attempting to
make solar cells by low cost electroplating process.
Gallium arsenide (GaAs) multijunction
High-efficiency cells have been developed for special applications such as
satellites and space exploration. 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. 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
a record high of 40.7% efficiency under solar concentration and laboratory
conditions. These devices use 20 to 30 different semiconductors layered in
series. At the National Renewable Energy Laboratory, a new cell of area 0.26685
cm2 will generate a power of 2.6 W. They estimate that this technology could
eventually produce electricity at a mere 8-10 cents/ kW/ hr. This is similar to
the price of electricity today. Thus, this breakthrough could ultimately result
in increased consumer use of solar cells.
This technology is still months away from commercial manufacture, but similar
technology is being used right now on the Mars rover missions. The rovers have
outlived their predicted life spans and have worked for over two years. Their
success in the dust-ridden Martian environment is a strong testament to the
durability and longevity of these types of solar cells.
Light absorbing dyes
M 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 is typically 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 forecast within this decade.
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 6% efficiency for the best cells to date.
However, these cells could be beneficial for some applications where mechanical
flexibility and disposability are important.
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 absorbs the visible part of the solar spectrum more
strongly than 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.
Recently, solutions to overcome the limitations of thin film crystalline silicon
have been developed. Light trapping schemes where the incoming light is
obliquely coupled into the silicon and the light traverses the film several
times enhance the absorption of sunlight in the films. Thermal processing
techniques enhance the crystallinity of the silicon and passify electronic
defects. The result is a new technology — thin film Crystalline Silicon on Glass
(CSG) . CSG solar devices represent a balance between the low cost of thin films
and the high efficiency of bulk silicon.
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.
Nanocrystalline solar cells
M 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).
Concentrating photovoltaics (CPV)
Concentrating photovoltaic systems use a large area of lenses or mirrors to
focus sunlight on a small area of photovoltaic cells. If these systems use
single or dual-axis tracking to improve performance, they may be referred 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 and also allows for
the use of high-performance materials such as gallium arsenide. Despite the
advantages of CPV technologies their application has been limited by the costs
of focusing, tracking and cooling equipment. On October 25, 2006, the Australian
federal government and the Victorian state government together with photovoltaic
technology company Solar Systems announced a project using this technology,
Solar power station in Victoria, planned 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.
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 paste is then fired at
several hundred °C to form metal electrodes in 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.
Current research on materials and devices
M 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.
Silicon processing
One way of reducing the cost 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 - at current efficiencies, it takes over two years
for a conventional solar cell to generate as much energy as was used to make the
silicon it contains. 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).
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 Professor Andrew Blakers at the Australian National University with
their "Sliver" cells, by micromachining wafers into very thin, virtually
transparent layers that could be used as transparent architectural coverings.
Using this technique, one silicon wafer is 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.
Another novel approach employed by Evergreen Solar is to grow silicon ribbons
from specialized 'string puller' furnaces. They claim to be able to produce
thinner cells without machining waste plus the resulting cells are naturally
rectangular in shape.
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 [vague] 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.
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.
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. In addition, thin films of many of these materials on
conventional silicon solar cells can increase the optical coupling efficiency
into the silicon cell, thus boosting the overall efficiency. 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(MEG).
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 efficiency.
Silicon wafer based solar cells
Despite the numerous attempts at making better solar cells by using new and
exotic materials, the reality is that the photovoltaics market is still
dominated by silicon wafer-based solar cells (first-generation solar cells).
This means that most solar cell manufacturers are equipped to produce these type
of solar cells. Therefore, a large body of research is currently being done all
over the world to create silicon wafer-based solar cells that can achieve higher
conversion efficiency without an exorbitant increase in production cost. The aim
of the research is to achieve the lowest $/watt solar cell design that is
suitable for commercial production.
Sliver cells
Professor Andrew Blakers and Dr Klaus Weber, working at Australian National
University and Origin Energy have developed a technique for slicing a single
silicon wafer, which allows a significantly larger collector surface area from
each wafer, compared to usual solar cells. The technique involves taking a
silicon wafer, typically 1 to 2mm thick, and making a multitude of parallel,
transverse slices across the wafer, creating a large number of slivers that have
a thickness of 50 micrometres and a width equal to the thickness of the original
wafer. These slices are rotated 90 degrees, so that the surfaces corresponding
to the faces of the original wafer become the edges of the slivers. The result
is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an
exposed silicon surface area of about 175 cm2 per side into about 1000 slivers
having dimensions of 100 mm x 2 mm x 0.1 mm, yielding a total exposed silicon
surface area of about 2000 cm2 per side. As a result of this rotation, the
electrical doping and contacts that were on the face of the wafer are located
the edges of the sliver, rather than the front and rear as is the case with
conventional wafer cells. This has the interesting effect of making the cell
sensitive from both the front and rear of the cell (a property known as
bifaciality).[
Are you interested in
mult-player online internet games? Such as runescape and neopets?Internet
Game Online-games, tips, cheats and kids forumsAnother good forum is
the Internet Junction For Gamers IJFG.COM Internet
Junction For Gamers, Runescape Market and More IJFG.COM Jokes, Pranks,
Runescape and other cool games at IJFG.COM. RuneScape is set in a medieval
fantasy world, similar to "Guild Wars" or "EverQuest", where players control
character representations of themselves. As with most massive multiplayer online
roleplaying games (MMORPG), there is no overall objective or end to the game.
Players explore, form alliances, perform optional tasks, and complete quests for
rewards and to build character's skills.
RuneScape has often been one of
the top massive online role playing games. It is a unique game. But, with a
unique game, comes unique players. Players get bored, and then try to develop
cheats....autos or bots that will help them achieve success in their beloved
games of Runescape 2.
RuneScape is a virtual world which
is divided into two part: Members Areas and Non-Members areas. People who pay to
play (p2p), receive access to the special areas. They also have access to the
free areas. The members' places are much larger, offer "better" items for the
gameplay of rs2, and much, much more. The character that you create when you
first start playing runescape, moves around the game on foot; either by running,
or walking. Players are challenged to their utmost skills by fighting new
monsters, completing difficult quests, and manipulating marketing. As Runescape
2 is an RPG (Role playing game), there is no set path a person must take to play
rs. They can choose what to do, and when, whether it be training their
money-making skills, or fighting another player. Players usually interact with
each other by chatting through public chat, or private chat.Internet
Junction For Gamers, Runescape Market and More IJFG.COM IJFG.com was a
runescape 2 based site. They have now, however, taken another look....
Of course the king of all game
cheating websites is
trick
the trik (otherwise known as RPG Cheats Site), where you can find cheat
forums, mmorpg topsite, arcade games and any mmo game related topics.
The master of massive multiplayer
online role-playing games (MMORPG) cheats can be found at Trik.com
Trik.com; this site is one of the best today. The forum section,
Trik.com forum, originally came from IJFG.com (Internet Junction For
Gamers) , which was one of the best websites that discussed various gamers'
issues. The full name was Internet Junction For Gamers, Runescape Market and
More. This site had Jokes, Pranks, RuneScape and other cool games. RuneScape is
set in a medieval fantasy world, similar to "Guild Wars" or "EverQuest," where
players control character representations of themselves. As with most MMORPG,
there is no overall objective or end to the game. Players explore, form
alliances, perform optional tasks, and complete quests for rewards and to build
characters' skills.
Trik.com continues IJFG.com's
success, but Trik.com has more to offer. Trik Topsite can be found at
Trik Topsite; the TopSite is a great addition if you want to find the best
MMO RPG site(s) or raise your site in the rankings. Trik.com also has a
viciously competitive Arcade. If you want to be the #1 Arcade on Trik, then come
prove yourself at Trik.com arcade:
Trik arcade. Trik.com ?Trik.com/topsite ?Trik.com/forum/arcade.php
With the rising popularity of
commercial MMORPG games came the desire from ardent players of these games to
run their own servers beside the ones run by the game's creator. Since the
original server software is not usually available, the behavior of the server
has to be re-engineered. This can be done by analyzing the data stream with the
original server, or by disassembling and analyzing the client which is
available.
Ultima Online was one of the first
large MMORPGs. Due to its openness in implementation, server emulators arose
very quickly, even during the beta stage of development. The destination to
which the client connects was changeable by simply editing a text file. In beta
stage the client-server data stream was not encrypted yet. The term server
emulator became known through Ultima Online server reimplementation such as UOX,
which was the pioneer. Many forks and reimplementations followed UOX, because
its source code was released under the GNU General Public License relatively
early. RunUO is today the most widely used UO-server emulator. After RuneScape
implemented anti-cheating measures, many gamers left and started their own
private servers. The best place to discuss the private server is at
Trik- The Master of Private Server.
Another useful site is
Rune
Web ruwb.com . This site is about more serious RuneScape gold trading,
account exchange, gold for real life cash and many services. It includes tips on
how to avoid getting lured/scammed while using the marketplace. For programming,
visual basics, java, C/C++, scar and all other languages such as PHP, HTML, ASP,
Delphi. There are also sections for graphics talents, plus many cool videos and
fun stuff.
A defining moment in internet
gaming history was when a group of gamers called (hygo 7) decided to start an
ultimate game forum, which they named
hygo.com. It has the best financial backing, the friendliest game community,
and the highest quality of information. Currently Hygo.com has entered a new
phase...Hygo.com is offering the best private server game. With thousands of
members, Hygo.com is your next place to visit, as they have an amazing game with
a community and economy.
Hygo.com - The Online Adventure Game. is definitely one of the top sites you
want to join right now!
Ezud.com is now the powerhouse of
Runescape bugs and glitches. All and any rs2 bugs that anyone could ever
want are now found at the
Ezud forum. From a range of infinite running in runescape, to rs item
duping, ezud truly is an amazing glitching site.
Ezud has an excellent administration, and a great
moderating team. When everyone strives to make ezud.com a better place….it
becomes just that: a better place. Everyone contributes, and helps
Ezud strive.
So come on down to the new type of runescape 2 cheating:
runescape bugging. This is Ezud…this is
RuneScape 2 Bug Abuse.
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