Thin Film
 |
Rather than growing,
slicing, and treating a crystalline ingot as in crystalline silicon
manufacturing, thin-film technology uses the semi-conductor
characteristics of PV components to create PV material. Thin-film
products are made by sequentially depositing thin layers of the
different materials into a very thin structure, a process that
requires very little semiconductor material. |
Deposition is easier and
less expensive than crystalline silicon ingot-growth techniques.
The three principal
thin-film technologies are Amorphous Silicon (a-Si), Cadmium Telluride (CdTe)
and Copper Indium Gallium diSelenide (CIGS).
Amorphous Silicon
(a-Si)
Amorphous solids, like
common glass, are materials in which the atoms are not arranged in any
particular order. They do not form crystalline structures at all, and they
contain large numbers of structural and bonding defects. Amorphous silicon
applies to PV technology if the deposition conditions are properly
controlled and if composition is carefully modified.
Amorphous silicon absorbs
solar radiation 40 times more efficiently than single-crystal silicon, so
a film only at 1 micron (one one-hundredth of a centimeter) thick can
absorb 90 percent of the usable solar energy. Today, amorphous silicon is
the most common form of thin-film PV and is ideal for low-powered consumer
devices.
Cadmium Telluride (CdTe)
Cadmium Telluride,
another thin-film technology, has cell efficiencies of over 16% in the
laboratory.
CdTe exhibits certain
limitations that keep CdTe from full market acceptance, including
Cadmium’s heavy metal characteristics and tendency to degrade electronic
contacts outdoors. Also, CdTe deposition and crystal formation requires
high processing temperatures. CdTe is only manufactured in a superstrate
configuration in which sunlight must pass through the substrate to get to
the PV material. Glass is the only material that can withstand the
temperature and still be adequately transparent. Due to its fragile
nature, the glass must be thick and heavy to endure the stresses found
during product life in the field.
Copper Indium
Gallium diSelenide (CIGS)
Copper Indium diSelenide
(CuInSe2) has an extremely high absorption that allows 99 percent of
available light to be absorbed in the first micron of the material. This
makes it an optimal, effective PV material. Adding small amounts of
Gallium to the CuInSe2 boosts its light-absorbing band gap, which makes it
more closely match the solar spectrum, thereby improving the voltage and
the efficiency of the PV cell. Consistently creating more electricity from
the same amount of sunlight as other thin-film PVs, CIGS cells have
reached conversion efficiencies of more than 19 percent - much higher than
other thin-film PV. This high conversion efficiency remains stable over
time for reliable performance. CIGS also passes environmental
certification and waste-handling requirements.
