Photovoltaics: 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 semiconducting 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. |
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model of a crystalline solar cell |
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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. |
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current-voltage line of a si-solar cell
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| 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
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One can distinguish three cell types according to the type of crystal:
monocrystalline, polycrystalline and amorphous. To produce a monocrystalline
silicon cell, absolutely pure semiconducting 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
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Level of efficiency in % Lab
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Level of efficiency in % Production
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Monocrystalline Silicon
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approx. 24
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14 to17
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Polycrystalline Silicon
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approx. 18
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13 to15
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Amorphous Silicon
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approx. 13
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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 aluminum 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
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In addition to optimizing 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. |
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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 (CdTe) 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.)
Concise and comprehensible explanations of the basic concepts in
solar heating and photovoltaics can be found in our Solar-Lexicon.
Reports on technology, business
and politics, as well as presentations
on innovative systems and products can be found in the Solar
Magazine
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