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Monocrystalline solar cells are made out
of silicon ingots, which are cylindrical
in shape. To optimise performance,
four sides are cut out of the cylindrical
ingots to make silicon wafers. Unlike
monocrystalline-based solar panels, for
poly-crystalline solar panels raw silicon is
melted and poured into a square mould,
which is cooled and cut into perfectly
square wafers. Some crystalline materials
have been used on UAVs but it tends to
be relatively heavy, as the cells have to
be thicker, and the effciency is typically
only in the 15-18% range.
The second category is CIGS
technology, part of the class of what
would be called amorphous solar cells,
and there is also amorphous silicon. They
are not crystalline, as amorphous implies,
and because of that they tend to have
much lower effciency. So where CIGS
cells are typically in the 10-12% effciency
range, amorphous silicon would be even
lower than that, perhaps 6-8%.
Despite that limited effciency CIGS
cells are relatively light and inexpensive,
so they are suitable where effciency
is not the key factor, but as soon as
something more effcient is needed
a third category of cells, like a GaAs
type or a III-V type cell which can give
effciencies of 25-35%, is required.
Silicon is in column 4 in the periodic
table which means that silicon atoms
can bond to silicon atoms and make a
perfect crystal structure. If you move one
column left, and one column right, you
have a 3 and a 5 which can also bond
together to make a perfect crystal. This
is how Gallium and Arsine form GaAs.
There are two benefts of using a 3-5
(III-V) instead of a 4 (or silicon). First –
they work better for converting light to
electricity. This has to do with something
called the band gap of the material. GaAs
is a direct band gap material and silicon
is an indirect band gap material. The
second beneft has to do with junctions.
You can mix and match various atoms to
create new 3-5 junctions while keeping
the crystal intact. This allows for better
overall performance.
Multi-junction cells
There are two ways to make solar cells
work more effciently. One is to make each
junction better. The other is to add more
and more junctions. A junction is a layer
in the solar cell that captures a certain
portion of light better.
A typical solar cell has only a single
junction, and captures effciently only
in the wavelength that’s closest to the
properties of the materials used; for
silicon that’s infrared. Any wavelength
in the solar spectrum that has higher
energy – be it visible red, orange, yellow,
green or blue – will have progressively
higher energies per photon.
It is possible to convert all those
photons into electrical power, but with
blue photons about half the energy is
wasted in conversion in a silicon cell
because the cell is converting it at the
infrared photon energy. It’s a quantum
conversion: a cell’s quantum effciency
June/July 2016 |
Unmanned Systems Technology
The layers of an inverted
metamorphic multi-junction cell
(Courtesy of ML Devices)
Solar panels integrated into the wings
of a UAV (Courtesy of ML Devices)