Solar cells []

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1 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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Jordan University of Science and Technology

Department of applied Physics

Solar cells

[
Operation principles and testing
]

Advisor:

Dr.

Adnan Shariah

Ghassan Mohammad Masadeh

Table of content



Introduction

Semiconductors

p
-
n junction

Solar cell system work

Performance of solar cells

Silicon solar cells

I
-
V Characteristic of solar cells

Testing and result

Introduction



Solar cells are devices in Which sun light releases
electric charges so they can move freely in a
semiconductor and ultimately flow through an electric
load, such as a light bulb or a motor .



The phenomenon of producing voltages and currents in
this way is known as the photovoltaic effect (PV e ffect).


The PV effect was discovered in
1839
by French physicist
Becquerel. It remained in the laboratory until
1954
. When Bell
laboratories produced the first silicon solar cell.


Solar cells are already being used in terrestrial applications
where they are economically competitive with alternative
sources.



Examples are powering communications equipment ,pumps,
and refrigerators located far from existing power lines .


The first of the economic forces the rising price of
conventional sources particularly those employing fossil fuels.
continues automatically, in part because the resource is
limited.


The second reducing the cost of electricity from solar cell
system is the subject of world wide research and
development efforts today.

To increase the economic attractiveness of the solar cell option:

-

increase cell efficiencies

-
-

reduce cost of producing cells modules.

-

devise new cell or system designs for lower total cost
per unit power out put.

Semiconductors:


Semiconductors are crystals that in their pure state
are resistive, but when the proper impurities are added
this process is called doping in trace amounts often
measured in parts per billion, display much lower
resistance along with other interesting and useful
properties. Depending on the selection of impurities
added, semiconductor materials of two electrically
different types:


n
-
type and p
-
type
.

p
-
n junction:


The basic structure formed by the intimate contact of
p
-
type and n
-
type semiconductors

n
-
type semiconductor:


A semiconductor type in which the density of holes in
the valence band is exceeded by the density of electrons
in the conduction band.

P
-
type semiconductor :


A semiconductor type in which the density of electrons
in the conduction band is exceeded by the density of
holes in the valence band.

The solar cell system work


The most important physical phenomena employed in all
solar cells are very similar to the classical p
-
n junction.
When light is by the junction the energy of the absorbed
photons is transferred to the electron and hole both free to
move. These particles diffuse through the semiconductor
and ultimately encounter an energy barrier that permits
charged particles of one sign to pass but reflects those of
the other sign.


The charge carriers in the junction region create a
potential gradient, get accelerated under the electric field
and circulate as the current through an external circuit.

The current from the cell may pass directly through the load
or it may be changed first by the power, conditioning
equipment from those provided by the cell, other subsystems
that may also be used include energy storage devices such
as batteries and concentrating lenses or mirror that focus the
sunlight onto a smaller to and hence less costly
semiconductor cell.

Performance of solar cells


An important feature of solar cells is that the voltage of
the cell does not depend on its size, and remains fairly
constant with changing light intensity. However, the current
in a device is almost directly proportional to light intensity
and size.

Figure below shows example I / V curves for a single cell as a function
of light input



A solar cell's power output can be characterized by two
numbers a maximum Open Circuit Voltage Voc measured
at zero output current and a short circuit current Isc


Where:


Voc = k T/ q ln [(I
L

/I
o
)+
1
]

And


I = I
o

[ exp.(qv/kT)
-
1
]
-

I

L


I
L

= q AG (L
e
+ W + L
h
)

And the power can be computed via this equation:

P = I * V


As you might then expect, a combination of less than
maximum current and voltage can be found that
maximize the power produced. This condition is called
"maximum power point”

the single crystal silicon cell

Silicon has some special chemical properties, especially in
its crystalline form. An
atom

of silicon has
14
electrons,
arranged in three different shells. The outer shell, however,
is only half full, having only four electrons. A silicon atom will
always look for ways to fill up its last shell. To do this, it will
share electrons with four of its neighbor silicon atoms.
except that in this case, each atom has four hands joined to
four neighbors. That's what forms the crystalline structure,
and that structure turns out to be important to this type of PV
cell.

A solar cell has silicon with impurities other atoms mixed
in with the silicon atoms, changing the way things work
a bit. We usually think of impurities as something
undesirable, but in our case, our cell would not work
without them. These impurities are actually put there on
purpose. Consider silicon with an atom of phosphorous
here and there, maybe one for every million silicon
atoms. It still bonds with its silicon neighbor atoms, but
in a sense, the phosphorous has one electron that
doesn't have anyone to hold hands with. It doesn't form
part of a bond, but there is a positive proton in the
phosphorous nucleus holding it in place.

This diagram shows a typical crystalline silicon solar cell

In solar cells applications this characteristic is usually
drawn inverted about the voltage axis. The cell generates
no power in short
-
circuit or open
-
circuit. The cell delivers
maximum power
P
max

when operating at a point on the
characteristic where the product
IV

is maximum. This is
shown graphically below where the position of the
maximum power point represents the largest area of the
rectangle shown.

Testing and result



THANK YOU