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LW1DSE > TECH 08.02.08 00:01l 338 Lines 17785 Bytes #999 (0) @ WW
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º °±² Solar Cells ²±° º
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To be viwed in CP 437
A solar or photovoltaic cell is a device that converts light energy
into electrical energy by the photovoltaic effect. Photovoltaics is the field
of technology and research related to the application of solar cells as solar
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 source is unspecified.
A solar cell fulfills 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). Solar cells have many applications. Individual cells
are used for powering small devices such as electronic calculators. Assemblies
of cells are used to make solar modules, which may in turn be linked in
photovoltaic arrays. These generate a form of renewable electricity,
particularly useful in situations where electrical power from the grid is
unavailable such as in remote area power systems, Earth-orbiting satellites
and space probes, remote radiotelephones and water pumping applications.
Photovoltaic electricity is also increasingly deployed in grid-tied electrical
systems.
Contents
1) History
1.1) Four generations of solar cells
2) Applications and implementations
3) Theory
3.1) Simple explanation
3.2) Photogeneration of charge carriers
3.3) Charge carrier separation
3.4) The p-n junction
3.5) Connection to an external load
3.6) Equivalent circuit of a solar cell
4) Solar cell efficiency factors
4.1) Maximum-power point
4.2) Energy conversion efficiency
4.3) Fill factor
4.4) Quantum efficiency
4.5) Comparison of energy conversion efficiencies
4.5.1) Watts peak
4.5.2) Solar cells and energy payback
5) Light-absorbing materials
5.1) Bulk
5.1.1) Silicon
5.2) Thin films
5.2.1) CdTe
5.2.2) Copper-Indium Selenide
5.2.3) Gallium arsenide (GaAs) multijunction
5.2.4) Light-absorbing dyes
5.2.5) Organic/polymer solar cells
5.2.6) Silicon
5.3) Nanocrystalline solar cells
6) Concentrating photovoltaics (CPV)
7) Silicon solar cell device manufacture
8) Current research on materials and devices
8.1) Silicon processing
8.2) Thin-film processing
8.3) Polymer processing
8.4) Nanoparticle processing
8.5) Transparent conductors
8.6) Silicon wafer based solar cells
8.7) Sliver cells
9) Makers
10) My own experince
1) History:
The term "photovoltaic" comes from the Greek "phos" meaning "light",
and "voltaic", meaning electrical, from the name of the Italian physicist
Volta, after whom the measurement unit volt is 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 wasn't 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. Russia launched the
first artificial satellite in 1957, and the United States' first artificial
satellite was launched in 1958 using solar cells created by Peter Iles in an
effort spearheaded by Hoffman Electronics. The first spacecraft to use solar
panels was the US satellite Explorer 1 in January 1958. This milestone created
interest in producing and launching a geostationary communications satellite,
in which solar energy would provide a viable power supply. This was a crucial
development which stimulated funding from several governments into research
for improved solar cells.
In 1970 the first highly effective GaAs heterostructure solar cells were
created by Zhores Alferov and his team in the USSR. Metal Organic Chemical
Vapor Deposition (MOCVD, or OMCVD) production equipment wasn't developed until
the early 1980's, limiting the ability of companies to manufacture the GaAs
solar cell. In the United States, the first 17% efficient air mass zero (AM0)
single-junction GaAs solar cells were manufactured in production quantities in
1988 by Applied Solar Energy Corporation (ASEC). The "dual junction" cell was
accidentally produced in quantity by ASEC in 1989 as a result of the change
from GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The
accidental doping of Ge with the GaAs buffer layer created higher open circuit
voltages, demonstrating the potential of using the Ge substrate as another
cell. As GaAs single-junction cells topped 19% AM0 production efficiency in
1993, ASEC developed the first dual junction cells for spacecraft use in the
United States, with a starting efficiency of approximately 20%. These cells
didn't utilize the Ge as a second cell, but used another GaAs-based cell with
different doping. Eventually GaAs dual junction cells reached production
efficiencies of about 22%. Triple Junction solar cells began with AM0
efficiencies of approximately 24% in 2000, 26% in 2002, 28% in 2005, and in
2007 have evolved to a 30% AM0 production efficiency, currently in
qualification. In 2007, two companies in the US, Emcore Photovoltaics and
Spectrolab, produce 95% of the world's 28% efficient solar cells.
1.1) Four generations of solar cells
The first generation photovoltaic cell consists of a large-area,
single-crystal, single layer p-n junction diode, capable of generating usable
electrical energy from light sources with the wavelengths of sunlight. These
cells are typically made using a diffusion process with silicon wafers.
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 terrestrial solar cell market.
The second generation of photovoltaic materials is based on the use of
thin epitaxial deposits of semiconductors on lattice-matched wafers. There are
two classes of epitaxial photovoltaics: space and terrestrial. Space cells
typically have higher AM0 efficiencies (28-30%) in production, but have a
higher cost per watt. Their "thin-film" cousins have been developed using
lower-cost processes, but have lower AM0 efficiencies (7-9%)in production and
are questionable for space applications. The advent of thin-film technology
contributed to a prediction of greatly reduced costs for thin film solar cells
that has yet to be achieved. 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.
An advantage of thin-film technology theoretically results in reduced mass so
it allows fitting panels on light or flexible materials, even textiles. The
advent of thin GaAs-based films for space applications (so-called "thin
cells") with potential AM0 efficiencies of up to 37% are currently in the
development stage for high specific power applications. Second generation
solar cells now comprise a small segment of the terrestrial photovoltaic
market, and approximately 90% of the space market.
Third-generation photovoltaics are proposed to be very different from
the previous semiconductor devices as they don't rely on a traditional p-n
junction to separate photogenerated charge carriers. For space applications
quantum well devices (quantum dots, quantum ropes, etc.) and devices
incorporating carbon nanotubes are being studied - with a potential for up to
45% AM0 production efficiency. For terrestrial applications, these new devices
include photoelectrochemical cells, polymer solar cells, nanocrystal solar
cells, Dye-sensitized solar cells and are still in the research phase.
A hypothetical 'fourth-generation' of solar cells may consist of
composite photovoltaic technology, in which polymers with nano particles can
be mixed together to make a single multispectrum layer. Then the thin
multispectrum layers can be stacked to make multispectrum solar cells more
efficient and cheaper based on polymer solar cell and multijunction 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. Current research is being conducted under a
DARPA grant to determine if this technology is viable. 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.
2) Applications and implementations
2.1) Polycrystalline PV cells
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 kWúh per day.
3) Theory
1: Photons in sunlight hit the solar panel and are absorbed by semiconducting
materials, such as silicon.
2: 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 (called holes) and flow in the
direction opposite of the electrons in a silicon solar panel.
3: An array of solar panels converts solar energy into a usable amount of
direct current (DC) electricity, generally used to charge batteries.
3.2) Photogeneration of charge carriers
When a photon hits a piece of silicon, one of three things can happen:
1. the photon can pass straight through the silicon, this (generally) happens
for lower energy photons,
2. the photon can reflect off the surface,
3. 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.
3.3) Charge carrier separation
There are two main modes for charge carrier separation in a solar cell:
1. drift of carriers, driven by an electrostatic field established across the
device;
2. 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.
3.4) The p-n junction
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 aren't 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 doesn't 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".
3.5) 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.
3.6) Equivalent circuit of a solar cell
To 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 behavior 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.
Also shown, on the right, is the schematic representation of a solar cell for
use in circuit diagrams.
Rs Rs = series resistance
+ ÚÄÄÄÄÄÄÄÂÄÄı±±±ÄÄÄÂÄÄÄÄÄÄÄÄo + Gp = parallel conductance
ÄÄÁÄÄ Á ± Io = current source
D \ / Io ± Gp Eo D = P-N Junction
ÄÄÂÄÄ Â ± Eo = ouput voltage
ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄo -
Fig1: Equivalent circuit of a solar cell
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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
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
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