Intro to Photovoltaics :~)

Geeze, i’m new at this… was wondering what Bill and Donald were
talking about with photovoltaics and looked some stuff up… duh…
they’re solar panels :~)

I’ve copied and pasted the stuff below, but here’s the link in case you’d like to explore.  

Solar cell

From Wikipedia, the free encyclopedia

A solar cell is a semiconductor device that converts photons from the sun (solar light) into electricity.
The general term for a solar cell including both solar and non-solar
sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell. Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes)
in a light-absorbing material, and separation of the charge carriers to
a conductive contact that will transmit the electricity. This
conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.

A solar cell, made from a monocrystalline silicon wafer

The most common configuration of this device, the first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of solar light. These cells are typically made using silicon.
However, successive generations of photovoltaic cells are currently
being developed that may improve the photoconversion efficiency for
future photovoltaics. The second generation of photovoltaic
materials is based on multiple layers of p-n junction diodes. Each
layer is designed to absorb a successively longer wavelength
of light (lower energy), thus absorbing more of the solar spectrum and
increasing the amount of electrical energy produced. The third generation
of photovoltaics is very different from the other two, and is broadly
defined as a semiconductor device which does not rely on a traditional
p-n junction to separate photogenerated charge carriers. These new
devices include dye sensitized cells, organic polymer cells, and
quantum dot solar cells.

Solar cells have many applications. They are particularly well
suited to, and historically used in, situations where electrical power
from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones and water pumping applications. Assemblies of solar cells (in the form of modules or solar panels) on building roofs can be connected through an inverter to the electricity grid in a net metering arrangement.

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History

Main article: Timeline of solar cells

The term “photovoltaic” comes from the Greek phos meaning “light”, and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) are named. It means literally of light and electricity.

The photovoltaic effect was first recognised in 1839 by French physicist Alexandre-Edmond Becquerel. However it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 (US2402662, ”Light sensitive device”). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells.

Applications and implementations

Main article: solar panel

Solar cells are often electrically connected and encapsulated as a module, termed a solar panel.
Solar panels often have a sheet of glass on the front (sun up) side
with a resin barrier behind, allowing light to pass while protecting
the semiconductor wafers from the elements (rain, hail, etc). Solar cells are also usually connected in series in modules, creating an additive voltage.

Theory

Simple explanation

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 (like bubbles) are 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.

Optionally:

1. The DC current enters an inverter.
2. The inverter turns DC electricity into 120 or 240-volt AC (alternating current) electricity needed for home appliances.
3. The AC power enters the utility panel in the house.
4. The electricity is then distributed to appliances or lights in the house.

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 when the energy of the photon is lower than the bandgap energy of the silicon semiconductor.
2. the photon can reflect off the surface - this (generally) happens
   if the photon energy is greater than the bandgap energy of silicon.
3. the photon can be absorbed by the silicon - this happens if the photon energy is within the bandgap energy of silicon.

Note that if a photon has an integer multiple of bandgap
energy, it can create more than one electron-hole pair. However, this
effect is usually not significant in solar cells. The “integer
multiple” part is a result of quantum mechanics and the quantization of energy.

When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band,
and is tightly bound in covalent bonds between neighboring atoms, and
hence unable to move far. The energy given to it by the photon
“excites” it into the conduction band,
where it is free to move around within the semiconductor. The covalent
bond that the electron was previously a part of now has one less
electron - this is known as a hole. The presence of a missing covalent
bond allows the bonded electrons of neighboring atoms to move into the
“hole,” leaving another hole behind, and in this way a hole can move
through the lattice. Thus, it can be said that photons absorbed in the
semiconductor create mobile electron-hole pairs.

A photon need only have greater energy than that of the band gap in
order to excite an electron from the valence band into the conduction
band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth
is composed of photons with energies greater than the band gap of
silicon. These higher energy photons will be absorbed by the solar
cell, but the difference in energy between these photons and the
silicon band gap is converted into heat (via lattice vibrations -
called phonons) rather than into usable electrical energy.

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 designed solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n junction
designed solar cells (typical of the third generation of solar cell
research such as dye and polymer thin-film solar cells), a general
electrostatic field has been confirmed to be absent, and the dominant
mode of separation is via charge carrier diffusion.

The p-n junction

Main article: semiconductor

The most commonly known solar cell is configured as a large-area p-n junction
made from silicon. As a simplification, one can imagine bringing a
layer of n-type silicon into direct contact with a layer of p-type
silicon. In practice, p-n junctions of silicon solar cells are not made
in this way, but rather, by diffusing an n-type dopant into one side of
a p-type wafer (or vice versus).

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 cell

To understand the electronic behaviour of a solar cell, it is useful to create a model
which is electrically equivalent, and is based on discrete electrical
components whose behaviour is well known. An ideal solar cell may be
modelled by a current source in parallel with a diode. In practice no
solar cell is ideal, so a shunt resistance and a series resistance
component are added to the model. The result is the “equivalent circuit
of a solar cell” shown on the left. Also shown on the right, is the
schematic representation of a solar cell for use in circuit diagrams.

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 (voltage) in the cell from zero (indicating a short circuit) to infinitely high values (indicating an open circuit) one can determine the maximum power point (the maximum output electrical power, V_max x I_max; or P_m, in W).

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 connected to an
electrical circuit. This term is calculated using the ratio of P_m, divided by the input light irradiance under “standard” test conditions (E, in W/m²) and the surface area of the solar cell (A_c 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, as 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 (V_oc) and the short circuit current (I_sc):

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

Main article: Solar panel

Silicon solar cell efficiencies vary from 6% for amorphous
silicon-based solar cells to 30% or higher with multiple-junction
research lab cells. Solar cell energy conversion efficiencies for
commercially available mc-Si solar cells are around 12%. The
highest efficiency cells have not always been the most economical –
for example a 30% efficient multijunction cell based on exotic
materials such as gallium arsenide or indium selenide and produced in
low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering a little under four times the electrical power.

To make practical use of the solar-generated energy, the electricity
is most often fed into the electricity grid using inverters
(grid-connected PV systems); in stand alone systems, batteries are used
to store the electricity that is not needed immediately.

A common method used to express economic costs of
electricity-generating systems is to calculate a price per delivered
kilowatt-hour (kWh). The solar cell efficiency in combination with the
available irradiation has a major influence on the costs, but generally
speaking the overall system efficiency is important. Using the
commercially available solar cells (as of 2005) and system technology
leads to system efficiencies between 5 and 15%. As of 2005, electricity
generation costs ranged from ~ 50 eurocents/kWh (0.60 US$/kWh) (central
Europe) down to ~ 25 eurocents/kWh (0.30 US$/kWh) in regions of high
solar irradiation. This electricity is generally fed into the
electrical grid on the customer’s side of the meter. The cost can be
compared to prevailing retail electric pricing (as of 2005), which
varied from between 0.04 and 0.50 US$/kWh worldwide.

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)

Solar cells and energy payback

There is a common myth that solar cells never produce more energy
than it takes to make them. While the expected working lifetime is
around 40 years, the energy payback time of a solar panel is anywhere
from 1 to 30 years (usually under five) depending on the type and where
it is used (see net energy gain). This means solar cells can be net
energy producers and can “reproduce” themselves (from 6 to more than 30
times) over their lifetime.^[1]

Light Absorbing Materials

All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect.
The materials used in solar cells tend to have the property of
preferentially absorbing the wavelengths of solar light that reach the
earth surface; however, some solar cells are optimized for light
absorption beyond Earth’s atmosphere as well. Light absorbing materials
can often be used in multiple physical configurations to take
advantage of different light absorption and charge separation
mechanisms (listed in alphabetical order). Many currently available
solar cells are configured as bulk materials that are
subsequently cut into wafers and treated in a “top-down” method of
synthesis (silicon being the most prevalent bulk material). Other
materials are configured as thin-films (inorganic layers, organic dyes, and organic polymers) that are deposited on supporting substrates, while a third group are used as quantum dots
(electron-confined nanoparticles) embedded in a supporting matrix in a
“bottom-up” approach. Silicon remains the only material that is
well-researched in both bulk and thin-film configurations.

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.

Germanium

Germanium
is also being investigated as a light absorbing material. Germanium has
a smaller band gap than silicon, making it a better material for
absorption of longer wavelengths of light (infrared). Wafers can be
useful as a multi-layered thin film substrate in this case.

Silicon

By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si).
Bulk silicon is separated into multiple categories according to
crystallinity and crystal size in the resulting ingot, ribbon, or wafer.

Main article: silicon

1. 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.
2. 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.
3. 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.

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.

CIGS

CIGS are multi-layered thin-film composites. The abbreviation stands for copper indium gallium selenide. Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor),
these cells are best described by a more complex heterojunction model.
The best efficiency of a thin-film solar cell as of December 2005 was
19.5% with CIGS. Higher efficiencies (around 30%) can be obtained by
using optics to concentrate the incident light. As of 2006, the best
conversion efficiency for flexible CIGS cells on polyimide is 14.1% by
Tiwari et al, at the ETH, Switzerland.

CIS

CIS is an abbreviation for general chalcogenide films of Cu(In_xGa_1-x)(Se_xS_1-x)₂.
While these films can achieve 11% efficiency, their manufacturing costs
are high at present but continuing work is leading to more cost
effective production processes.

CdTe

Cadmium telluride is an efficient light absorbing material for thin-film solar cells. However, Cd is also regarded as a toxic heavy metal in the USA, reducing the incentive for development in that country.

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 and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are very low.

Gallium Arsenide (GaAs) Multijunction

High-efficiency cells have been developed for special applications such as satellites and space exploration which require high-performance. These multijunction cells consist of multiple thin films produced using molecular beam epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP₂ [1]. Each type of semiconductor will have a characteristic band gap
energy which, loosely speaking, causes it to absorb “light” most
efficiently at a certain “color”, or more precisely, to absorb electromagnetic radiation
over a portion of the spectrum. The semiconductors are carefully chosen
to absorb nearly all of the solar spectrum, thus generating electricity
from as much of the solar energy as possible. GaAs multijunction
devices are the most efficient solar cells to date, reaching as high as
39% efficiency [2]. They are also some of the most expensive cells per unit area (up to US$40/cm²).

Solar-concentrating photovoltaics

Ironically, despite the high cost of multijunction cells, they are
used as part of one of the most cost-effective and promising
technologies, known as solar-concentrating photovoltaics. In these
systems, solar energy is concentrated several hundred times, which
actually increases the cell efficiency, and reduces the
semiconductor area needed per watt of power output. Many reputable
sources are projecting that the cost of concentrating system will soon
drop to US$3/Watt.

For examples of concentrating systems under development, see the recent experimental “Sunflower”, or the “SunCube”.

Light Absorbing Dyes

Main article: Dye-sensitized solar cells

Typically a Ruthenium metalorganic dye (Ru-centered) used as a
monolayer of light-absorbing material. The dye-sensitized solar cell
depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m²/gram TiO₂, as compared to approximately 10 m²/gram of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO₂,
and the holes are passed to an electrolyte on the other side of the
dye. The circuit is completed by a redox couple in the electrolyte,
which can be liquid or solid. This type of cell allows a more flexible
use of materials, and typically are manufactured by screen printing,
with the potential for lower processing costs than those used for bulk
solar cells. However, the dyes in these cells also suffer from
degradation under heat and UV light, and the cell casing is difficult
to seal due to the solvents used in assembly. In spite of the above,
this is a popular emerging technology with some commercial impact
forecasted within this decade.

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 (1.7 eV) than crystalline
silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the
visible part of the solar spectrum, but it fails to collect the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a tandem cell.
The top cell in a-Si absorbs the visible light and leaves the infrared
part of the spectrum for the bottom cell in nanocrystalline Si, as
pioneered by the Sanyo HIT cell. A patented silicon thin film
technology being developed by XsunX, Inc,
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.

Quantum Dots

These dimensionally-confined structures make use of some of the same
light absorbing materials from thin-film configurations, but are
suspended in a supporting matrix of conductive polymer or mesoporous
metal oxide.

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 in to
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 (250 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 aluminum. Usually this contact covers the entire rear
side of the cell, though in some cell designs it is printed in a grid
pattern. The metal electrodes will then require some kind of heat
treatment or “sintering” to make Ohmic contact
with the silicon. After the metal contacts are made, the solar cells
are interconnected in series (and/or parallel) by flat wires or metal
ribbons, and assembled into modules or “solar panels”. Solar panels
have a sheet of tempered glass on the front, and a polymer
encapsulation on the back. Tempered glass cannot be used with amorphous
silicon cells because of the high temperatures during the deposition
process.

Current research on materials and devices

Main article: Timeline of solar cells

There are currently many research groups active in the field of photovoltaics in universities
and research institutions around the world. This research can be
divided into three areas: making current technology solar cells cheaper
and/or more efficient to effectively compete with other energy sources;
developing new technologies based on new solar cell architectural
designs; and developing new materials to serve as light absorbers and
charge carriers.

Silicon Processing

One way of doing this is to develop cheaper methods of obtaining
silicon that is sufficiently pure. Silicon is a very common element,
but is normally bound in silica, or silica sand.
Processing silica (SiO2) to produce silicon is a very high energy
process, and more energy efficient methods of synthesis are not only
beneficial to the solar industry, but also to industries surrounding
silicon technology as a whole.

The current industrial production of silicon is via the reaction
between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius.
In this process, known as carbothermic reduction, each tonne of silicon
(metallurgical grade, about 98% pure) is produced with the emission of
about 1.5 tonnes of carbon dioxide.

Solid silica can be directly converted (reduced) to pure silicon by
electrolysis in a molten salt bath at a fairly mild temperature (800 to
900 degrees Celsius). ^{[2]<a href=”http://en.wikipedia.org/wiki/Photovoltaic_cell#_note-2}

Intro to Photovoltaics :~)

Here's another site that's based here in Indonesia that's got some good animations on how solar panels work, and practical systems.  Great for introductions to solar panel systems. Click on the 'animations' tree on the left, and check out the 'general' section.  This is the company that installed our first solar system.  We're using their S-4 battery system…  

the website is www.sundaya.com (daya is power or force in Malay / Indonesian) 

Re: Intro to Photovoltaics :~)

Hey, that's pretty snazzy - that looks like a well-conceived system.

I want one! That would be a perfect survival/camp system.

I'd have to look, but I don't think you can get something so modular and well designed that easily here in the states. Altho I haven't shopped for PV systems for years.

Nice animations too.