How a solar cell works

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2 Νοε 2013 (πριν από 4 χρόνια και 7 μέρες)

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How a solar cell works


Photovoltaic (PV) are solid
-
state semiconductor devices that convert light directly into
electricity. They are usually made of silicon with traces of other elements and are first
cousins to transistors, liquid electronic devices
(LEDs), computer chips and other electronic
devices.

A photovoltaic device (generally called a solar cell) consists of layers of semiconductor
materials with different electronic properties. In a typical WAAREE Solar crystalline silicon
cell, the bulk of t
he material is silicon, doped with a small quantity of boron to give it a
positive or p
-
type character. A thin layer on the front of the cell is doped with phosphorous
to give it a negative or n
-
type character. The interface between these two layers contai
ns an
electric field and is called a junction.

Light consists of particles called photons. When light hits the solar cell, some of the photons
are absorbed in the region of the junction, freeing electrons in the silicon crystal. If the
photons have enough
energy, the electrons will be able to overcome the electric field at the
junction and are free to move through the silicon atoms in the cell and into an external
circuit as energy. As they flow through the external circuit they give up their energy as
usef
ul work (turning motors, charging batteries, for example.) and return to the solar cell.

The photovoltaic process is completely solid
-
state and self
-
contained. There are no moving
parts and no materials are consumed or emitted.

During a typical sunny day,
an array of solar cells one
-
meter square exposed to the sun at
noon will receive approximately 1 kilowatt (kW) of power. WAAREE Solar’s multicrystalline
cells convert roughly 15% of this into electricity; hence 1m² of cells generate 150 electric
Watts in f
ull sunshine.

There are a number of solar cell technologies which have varying conversion rates:
amorphous silicon thin film 6%
-

7%; cadmium telluride (CdTe) thin film 8%
-

10%;
multi (or poly) crystalline silicon 12%
-

15%; monocrystalline silicon (SiN)
13%
-

16%.

WAAREE Solar only uses multicrystalline and monocrystalline silicon technologies and
research scientists in our technology group are actively looking at alternative methods to
grow silicon, which is more efficient.


How solar cells are made



Cast
ing

The most important material for making solar cells is silicon, which is derived from silica
(quartz). There is a host of other solar cell technologies that rely on other semiconductors
like amorphous silicon, cadmium telluride, copper indium diselenide

and organic cells,
besides gallium arsenide and indium phosphide.



Silica (silicon dioxide) is reduced in an arc blast furnace and then further purified to obtain
high purity silicon, equivalent to electronic grade silicon used in computers. This silicon

is
used as the raw material (feedstock) to make the silicon wafer.



Silicon feedstock is melted in a crucible to form either monocrystalline or multicrystalline
silicon, depending on the production process used.



During the melting process, a small quan
tity of boron is mixed to make the silicon p
-
type (to
give the material a positive characteristic). It is made into ingots or bricks, which are cut to
a more appropriate shape. After shaping, ingots or bricks are sawn into thin slices called
wafers using w
ire saws.



Etching and texturing

Wafers are cleaned with industrial soaps and then etched to remove saw damage.
Monocrystalline wafers are further etched in a hot solution of sodium hydroxide and
isopropanol to form square
-
based pyramids also called texture
. The texturization helps
reduce the reflection of sunlight. The same process doesn’t work as well in the case of
multicrystalline wafer, yet other texturing methods are also available for multicrystalline
wafers to further enhance light capture.



Diffusion

and edge isolation

Wafers that have been pre
-
doped with boron during the casting process, are then given a
negative (n
-
type) surface characteristic by diffusing them with a phosphorous source at
high temperature, which in turn creates the negative/positiv
e (n
-
p) junction.



Phosphorous diffuses not only into the desired wafer surface but also into the side and the
opposite surface, to some extent. This gives a shunting path between the cell front and rear.
Removal of the path around the wafer edge, edge ju
nction isolation, is performed by coin
stacking the cells and exposing them to a plasma etching chamber to etch exposed edges
and remove the short
-
circuit.



Anti
-
reflection coating

To further reduce surface reflection, the wafers are coated with an anti
-
ref
lection coating
(ARC) such as silicon nitride or titanium oxide. One of the best ARCs is silicon nitride,
deposited by the plasma enhanced chemical vapor deposition (PECVD) technique. This
process not only deposits a layer of ARC but also improves the elec
tronic properties of the
silicon by injecting hydrogen, which further improves silicon quality.



Metallization

The cells are now capable of generating electricity. The front and the rear surfaces need to
have contacts added (usually in the form of metal str
ips made out of a good metal
conductor) to collect the electricity.



Silver is the most widely used metal for contact formation owing to its solderability. Silver
in the form of a paste is screen printed onto the front and the rear. In addition,
aluminum

paste is applied to the rear to achieve back surface field (BSF
), which

improves the
performance of the solar cell. These metal pastes are subsequently heated above the
alloy
ing temperature to form a good O
hmic

(
Ω
)

c潮tact.




Solar modules

Each solar cell
produces roughly half a Volt when exposed to light. Therefore, many cells are
connected in series to add voltage. Solar modules comprise several individual solar cells
that are connected together and encapsulated in a protective envelope behind a sheet of
protective glass. Combined with a metal frame and equipped with connectors, solar
modules can be transported and connected in the field in a safe and practical manner.

[Add Solar Module


Dispatch Picture]


How solar modules are rated

The output of a solar

module is measured and rated in the factory at standard test
conditions (STC). For example a WAAREE WE
-
170 is rated at 170 Watts under full sunshine.
This rating is used to size systems as well. An array of 20 WAAREE WE
-
170 on a home
comprises a 3,400
-
Wat
t system, commonly referred to as a 3.4
-
kilowatt (kW) system. Any
combination is possible within the limits of local electrical codes and multi
-
megawatt arrays
have been designed and installed worldwide.


What affects the performance of solar systems?



Weat
her and temperature

Weather naturally affects the performance of solar modules but not entirely as you might
expect. The amount of sunlight, of course, is most important in determining the output a
solar electric system will produce at a given location, bu
t temperature is also important.



Contrary to most people's intuition, solar electric panels actually generate more power at
lower temperatures with other factors being equal. This is because solar cells are electronic
devices and generates electricity fr
om light, not heat. Like most electronic devices, solar
cells operate more efficiently at cooler temperatures. In temperate climates, solar panels
will generate less energy in the winter than in the summer but this is due to the shorter
days, lower sun ang
les and greater cloud cover, not the cooler temperatures.


Solar modules’ output is proportional to the sun’s intensity, so cloud cover will reduce the
system output. Typically, the output of any industrial solar module is reduced to five to 20%
of its ful
l sun output when it operates under cloudy conditions.



Cell configuration

Solar cells are wired in series to increase the voltage levels. Typically, 36 cells are wired in
series to make a 12V battery
-
charging module. Any number of cells can be connected in

series and most commercial modules sold today incorporate 72 cells.


Product features from:
http://www.bp.com/sectiongenericarticle.do?categoryId=9019590&conte
ntId=7036
980