Environmental Engineering Reference
In-Depth Information
(4.31)
The end product is a high-purity silicon rod with diameters of up to 30 cm
(about 12 inches) and lengths up to 2 m (about 80 inches). These rods can be
used for the production of polycrystalline solar cells, which consist of a
number of crystals, rather than a single crystal. The crystals of polycrystalline
silicon are differently oriented and separated by grain boundaries. They
introduce some efficiency losses.
To increase solar cell efficiency, monocrystalline material can be produced
from polycrystalline material applying the Czochralski or float zone process.
Seeding a single crystal at high temperatures transforms the polycrystalline
silicon to the desired monocrystalline silicon. No grain boundaries are present
in the resulting material and thus losses within a solar cell are reduced.
Wire saws or inner diameter saws cut the crystalline silicon rods into 200-
µ
m to 500-
µ
m slices. This process causes relatively high cutting losses of up to
50 per cent.
The silicon slices, or so-called
wafers
, are cleaned and doped in the
following steps. Hydrofluoric acid removes any saw damage. Phosphorus and
boron are used for doping silicon to create the p-n junction. Gaseous dopants
are mixed with a carrier gas such as nitrogen (N
2
) or oxygen (O
2
) for gas
diffusion, and this gas mixture flows over the silicon wafers. The impurity
atoms diffuse into the silicon wafer depending on the gas mixture, temperature
and flow velocity. Etching cleans the surface of the doped semiconductor.
Finally, cell contacts are added. A screen printing process adds the
front
and rear contacts
. Materials for the contacts are metals or alloys of aluminium
or silver. The rear contact usually covers the whole cell area. Thin contact
fingers are used for the front contacts, because they obstruct and reflect
sunlight. Only a minimum of the cell's surface should be covered by contacts
in order to optimize light capture.
Finally, an
antireflective coating
is added to the solar cell. This coating
reduces reflection at the metallic silicon surface. Titanium dioxide (TiO
2
) is
mostly used for the coating and gives the solar cell its typical blue colour.
Nowadays, it is also possible to produce antireflective coatings in other
colours, allowing architects to better integrate solar modules with buildings.
Figure 4.11 shows the structure of a crystalline solar cell.
Various other methods can be employed to increase the efficiency. For
example, the solar cell's surface can be structured with miniature pyramids.
The pyramids are shaped in such a way that any reflection of the light is
directed onto the cell, hence producing a second incident beam. Furthermore,
buried front contacts can reduce the reflection losses. A more detailed
description of the production methods can be found in Goetzberger et al, 1998;
Green, 1994; Lasnier and Ang, 1990.
Solar modules with crystalline cells
Single unprotected solar cells can be damaged rapidly as a result of climatic
