4.3.2
PV Cells
The PV phenomenon depends on the electric field created
at a junction plane between two special semiconductor
materials.
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Source : IMEC
Fig 4.2 Polycrystalline Silicon Cell
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Photons
from solar radiation can then energise electrons near
this junction which allows them to be conducted into an
external circuit and generate power. The vital feature
of a PV cell is therefore a solid interface between two
different materials, which might be two crystalline wafers
or a film and a substrate. The key to success lies in
finding a combination of semiconductors which both have
good electrical performance and can also be economically
manufactured.
PV cells
are generally made either of thick crystalline silicon,
sliced from ingots or castings or from grown ribbons,
or of thin films, deposited in thin layers on a low cost
substrate. The majority of module production (84%) has
so far involved the former, whilst future plans centre
on the latter. Thin film technology is eventually expected
to dominate the market for PV modules on buildings because
of its advantages in terms of weight, robustness and visual
appearance. The main types of cell are shown in Table
4.1, demonstrating considerable momentum within solar
cell R&D to meet the range of applications demanded
by a growing PV market.
Table
4.1: Cell types and characteristics
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Cell Type
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Characteristics
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Pictures from ref 4.1
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Crystalline silicon
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Mainstay of most power modules (single- or multi-
crystalline)
Widely available and well understood
Uses technology developed for the electronics industry.
Efficiency: typically 13 - 16% for commercial products,
but values already exceed 25% in the laboratory
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Single
crystal solar cells in Panel
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Thin Films
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Three main types of thin film cells are available
today:
- amorphous silicon (a-Si)
- copper indium diselenide (CIS)
- cadmium telluride (CdTe)
Thin films use thin layers of photosensitive
materials on a low cost substrate such as glass,
stainless steel or plastic. All of these have active
layers in the thickness range of a few microns,
and all are manufactured by continuous processes
which are capable of large volume, low cost production.
They also have the advantage that their operating
characteristics can be adapted by relatively simple
changes to the manufacturing steps.
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Future possibilities
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Concentrators focus light from a large area
on to a small area of photovoltaic material using
an optical concentrator (such as a mirror or Fresnel
lens), thus reducing the quantity of PV cells required.
Two main drawbacks are that they cannot make full
use of diffuse sunlight and they must always be
directed towards the sun with a tracking system.
Organic dye solar cells first developed
in 1991 still have low efficiencies and show a poor
long term stability, but they could become important
in the longer term.
Spheral solar technology uses minute silicon
beads bonded to an aluminium foil matrix. This offers
an important potential cost advantage because of
the reduced need for silicon, but the technology
is still not in full scale commercial production.
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Source: "Solar
Generation" EPIA/Greenpeace, December 2001
Renewable Energy Sources 
