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º °±² Solar Cells ²±° º
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To be viwed in CP 437
(From 3 of 1)
4) Solar cell efficiency factors
4.1) 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.
4.2) Energy conversion efficiency
A solar cell's energy conversion efficiency (ï, "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
(E, in W/mý) under standard test conditions (STC) and the surface area of the
solar cell (Ac in mý).
STC specifies a temperature of 25øC and an irradiance of 1000 W/mý with
an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and
spectrum of sunlight incident on a clear day upon a sun-facing 37ø-tilted
surface with the sun at an angle of 41.81ø above the horizon. This condition
approximately represents solar noon near the spring and autumn equinoxes in
the continental United States with surface of the cell aimed directly at the
sun. Thus, under these conditions a solar cell of 12% efficiency with a 100
cm2 (0.01 m2) surface area can be expected to produce approximately 1.2 watts
of power.
4.3) 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):
FF = Pmax / (Voc * Isc)
4.4) Quantum efficiency
As described above, when a photon is absorbed by a solar cell it is
converted to an electron-hole pair. This electron-hole pair may then travel
to the surface of the solar cell and contribute to the current produced by the
cell; such a carrier is said to be collected. Alternatively, the carrier may
give up its energy and once again become bound to an atom within the solar
cell without reaching the surface; this is called recombination, and carriers
that recombine don't contribute to the production of electrical current.
Quantum efficiency refers to the percentage of photons that are
converted to electric current (i.e., collected carriers). External quantum
efficiency is the fraction of incident photons that are converted to
electrical current, while internal quantum efficiency is the fraction of
absorbed photons that are converted to electrical current. Mathematically,
internal quantum efficiency is related to external quantum efficiency by the
reflectance of the solar cell; given a perfect antireflection coating, they
are the same.
Quantum efficiency shouldn't be confused with energy conversion
efficiency, as it doesn't convey information about the power collected from
the solar cell. Furthermore, quantum efficiency is most usefully expressed as
a spectral measurement (that's, as a function of photon wavelength or energy).
Since some wavelengths are absorbed more effectively than others in most
semiconductors, spectral measurements of quantum efficiency can yield
information about which parts of a particular solar cell design are most in
need of improvement.
4.5) Comparison of energy conversion efficiencies
At this point, discussion of the different ways to calculate efficiency
for space cells and terrestrial cells is necessary to alleviate confusion. In
space, where there is no atmosphere, the spectrum of the sun is relatively
unfiltered. However on earth, with air filtering the incoming light, the solar
spectrum changes. To account for the spectral differences, a system was
devised to calculate this filtering effect. Simply, the filtering effect
ranges from Air Mass 0 in space, to approximately Air Mass 1.5 on earth.
Multiplying the spectral differences by the quantum efficiency of the solar
cell in question will yield the efficiency of the device. For example, a
Silicon solar cell in space might have an efficiency of 14% at AM0, but have
an efficiency of 16% on earth at AM 1.5. Terrestrial efficiencies typically
are greater than space efficiencies.
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 14-19%. The highest efficiency cells haven't 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.
However, there is a way to "boost" solar power. By increasing the light
intensity, typically photogenerated carriers are increased, resulting in
increased efficiency by up to 15%. These so-called "concentrator systems" have
only begun to become cost-competitive as a result of the development of high
efficiency GaAs cells. The increase in intensity is typically accomplished by
using concentrating optics. A typical concentrator system may use a light
intensity 6-400 times the sun, and increase the efficiency of a one sun GaAs
cell from 31% at AM 1.5 to 35%.
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
isn't 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$/kWh
(0.50 î/kWh) (central Europe) down to ÷ 0.30 US$/kWh (0.25 î/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.)
4.5.1) 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:
1. insolation (solar irradiance) 1000 W/mý;
2. solar reference spectrum AM (airmass) 1.5;
3. cell temperature 25øC.
4.5.2) Solar cells and energy payback
In the 1990s, when silicon cells were twice as thick, efficiencies were 30%
lower than today and lifetimes were 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.
5) 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:
5.1) Bulk: This 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.
5.1.1) Silicon: 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.
a. 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, don't 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.
b. 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.
c. 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 doesn't require sawing
from ingots.
5.2) 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.
5.2.1) 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.
5.2.2) Copper-Indium Selenide: 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 below 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.
When gallium is substituted for some of the indium in CIS, the material
is sometimes called CIGS, or copper indium/gallium diselenide, a solid
mixture of the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the
chemical formula CuInxGa(1-x)Se2. 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 pure CIS, thus increasing 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. Conversion efficiency
values on metallic flexible foils were reported by AbuShama et al in the
proceedings of the IEEE 4th World Conference on Photovoltaic Energy
Conversion 2006 in Hawaii, USA.
That being said, indium can easily be recycled from decommissioned PV
modules. The recycling program in Germany would be is an example that
highlights the 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. Nanosolar, a California
based company, will soon be producing over 400 megawatts worth of
CIGS-based solar arrays per year. If this production is reached, they will
be one of the world's largest producer of solar cells.
5.2.3) 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.[18] 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 0.26685ÿcmý will generate a power of 2.6ÿW. They estimate that this
technology could eventually produce electricity at a mere 8 ÷ 10 cents/kWh.
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 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.
Solar arrays made with a material which contains gallium arsenide GaAs
and germanium Ge is seeing demand rapidly rise. In just the past 12 months
(12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350
per kg to $680 per kg. Additionally, germanium metal prices have risen
substantially to $1000-$1200 per kg this year. Although some Chinese producers
of these materials may be able to offset some of the price increases with
their lower labor costs. Those materials include gallium (4N, 6N and 7N Ga),
arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles
for growing crystals, and boron oxide, these products are critical to the
entire substrate manufacturing industry.
Companies involved in this type of solar technology include: AXT -
(AXTI) & Emcore - (EMKR) Triple-junction GaAs solar cells were also being
used as the power source of the Dutch four-time World Solar Challenge winners
Nuna in 2005 and 2007.
5.2.4) Light-absorbing dyes
Typically a ruthenium metalorganic dye (Ru-centered) is 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 mý/g TiO2, as compared to approximately 10 mý/g 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.
5.2.5) 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.
5.2.6) 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:
1. Amorphous silicon (a-Si or a-Si:H)
2. protocrystalline silicon or
3. 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 (1.7ÿeV) 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 pacify 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.
5.3) Nanocrystalline solar cells
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).
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º Compilled from Wikipedia.com in the Intenet. Translatted to ASCII by º
º LW1DSE Osvaldo F. Zappacosta. Banfield (1832), Buenos Aires, Argentina. º
º Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. º
º January 28, 2008 º
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