Information
on Photovoltaic:
Solar Electricity and Solar Cells in Theory and Practice
The word Photovoltaic
is a combination of the Greek word for Light and the name of the physicist
Allesandro Volta. It identifies the direct conversion of sunlight into
energy by means of solar cells. The conversion process is based on the
photoelectric effect discovered by Alexander Bequerel in 1839. The
photoelectric effect describes the release of positive and negative charge
carriers in a solid state when light strikes its surface.
· How Does a
Solar Cell Work?
· Characteristics of Solar Cells
· Different Cell Types
· From the Cell to Module
· Natural Limits of Efficiency
· New Directions |
How
Does a Solar Cell Work?
Solar cells are composed of various semi conducting
materials. Semiconductors are materials, which become
electrically conductive when supplied with light or heat, but which
operate as insulators at low temperatures.
Over 95% of all the solar cells produced worldwide are composed of the
semiconductor material Silicon (Si). As the second most abundant element
in earth's crust, silicon has the advantage, of being available in
sufficient quantities, and additionally processing the material does not
burden the environment. To produce a solar cell, the semiconductor is
contaminated or "doped". "Doping" is the intentional
introduction of chemical elements, with which one can obtain a surplus of
either positive charge carriers (p-conducting semiconductor layer) or
negative charge carriers (n-conducting semiconductor layer) from the
semiconductor material. If two differently contaminated semiconductor
layers are combined, then a so-called p-n-junction results on the boundary
of the layers.
model of a crystalline solar
cell
At this junction, an interior
electric field is built up which leads to the separation of the charge
carriers that are released by light. Through metal contacts, an electric
charge can be tapped. If the outer circuit is closed, meaning a consumer
is connected, then direct current flows.
Silicon cells are
approximately 10 cm by 10 cm large (recently also 15 cm by 15 cm). A
transparent anti-reflection film protects the cell and decreases
reflective loss on the cell surface.
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Characteristics of a Solar Cell
The usable voltage from solar cells depends on the semiconductor
material. In silicon it amounts to approximately 0.5 V. Terminal voltage
is only weakly dependent on light radiation, while the current intensity
increases with higher luminosity. A 100 cm² silicon cell, for example,
reaches a maximum current intensity of approximately 2 A when radiated by
1000 W/m².
current-voltage line of a si-solar
cell
The output (product of
electricity and voltage) of a solar cell is temperature dependent. Higher
cell temperatures lead to lower output, and hence to lower efficiency. The
level of efficiency indicates how much of the radiated quantity of light
is converted into useable electrical energy.
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Different
Cell Types
One can distinguish three cell types according to the type of crystal:
monocrystalline, polycrystalline and amorphous. To produce a
monocrystalline silicon cell, absolutely pure semi conducting material is
necessary. Monocrystalline rods are extracted from melted silicon and then
sawed into thin plates. This production process guarantees a relatively
high level of efficiency.
The production of polycrystalline cells is more cost-efficient. In this
process, liquid silicon is poured into blocks that are subsequently sawed
into plates. During solidification of the material, crystal structures of
varying sizes are formed, at whose borders defects emerge. As a result of
this crystal defect, the solar cell is less efficient.
If a silicon film is deposited on glass or another substrate material,
this is a so-called amorphous or thin layer cell. The layer thickness
amounts to less than 1µm (thickness of a human hair: 50-100 µm), so the
production costs are lower due to the low material costs. However, the
efficiency of amorphous cells is much lower than that of the other two
cell types. Because of this, they are primarily used in low power
equipment (watches, pocket calculators) or as facade elements.
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| Material |
Level
of efficiency in % Lab |
Level
of efficiency in % Production |
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| Monocrystalline
Silicon |
approx.
24 |
14
to17 |
| Polycrystalline
Silicon |
approx.
18 |
13
to15 |
| Amorphous
Silicon |
approx.
13 |
5
to7 |
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From the Cell to the Module
In order to make the appropriate
voltages and outputs available for different applications, single
solar cells are interconnected to form larger units. Cells
connected in series have a higher voltage, while those connected
in parallel produce more electric current. The interconnected
solar cells are usually embedded in transparent
Ethyl-Vinyl-Acetate, fitted with an aluminium or stainless steel
frame and covered with transparent glass on the front side.
The typical power ratings of such
solar modules are between 10 Wpeak and 100 Wpeak. The
characteristic data refer to the standard test conditions of 1000
W/m² solar radiation at a cell temperature of 25° Celsius. The
manufacturer's standard warranty of ten or more years is quite
long and shows the high quality standards and life expectancy of
today's products.
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Natural Limits of Efficiency
In addition to optimising the
production processes, work is also being done to increase the
level of efficiency, in order to lower the costs of solar cells.
However, different loss mechanisms are setting limits on these
plans. Basically, the different semiconductor materials or
combinations are suited only for specific spectral ranges.
Therefore a specific portion of the radiant energy cannot be used,
because the light quanta (photons) do not have enough energy to
"activate" the charge carriers. On the other hand, a
certain amount of surplus photon energy is transformed into heat
rather than into electrical energy. In addition to that, there are
optical losses, such as the shadowing of the cell surface through
contact with the glass surface or reflection of incoming rays on
the cell surface. Other loss mechanisms are electrical resistance
losses in the semiconductor and the connecting cable. The
disrupting influence of material contamination, surface effects
and crystal defects, however, are also significant.
Single loss mechanisms (photons with too little energy are not
absorbed, surplus photon energy is transformed into heat) cannot
be further improved because of inherent physical limits imposed by
the materials themselves. This leads to a theoretical maximum
level of efficiency, i.e. approximately 28% for crystal silicon.
Theoretical
maximum levels of efficiency of various solar cells at
standard conditions
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New Directions
Surface structuring to reduce
reflection loss: for
example, construction of the cell surface in a pyramid structure,
so that incoming light hits the surface several times. New
material: for example, gallium arsenide (GaAs), cadmium telluride
(GdTe) or copper indium selenide (CuInSe²).
Tandem or stacked cells:
in order to be able to use a wide spectrum of radiation, different
semiconductor materials, which are suited for different spectral
ranges, will be arranged one on top of the other.
Concentrator cells:
A higher light intensity will be focussed on the solar cells by
the use of mirror and lens systems. This system tracks the sun,
always using direct radiation.
MIS Inversion Layer cells:
the inner electrical field are not produced by a p-n junction, but
by the junction of a thin oxide layer to a semiconductor.
Grätzel cells:
Electrochemical liquid cells with titanium dioxide as electrolytes
and dye to improve light absorption.
Text and illustrations
used with the permission of the German Foundation for Solar Energy
(Deutschen Gesellschaft für Sonnenenergie e.V.)
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