PHOTOVOLTAICS Semiconductor India
into the substrate, resulting in a loss of short low defect density, a seed layer consisting concentrators. The primary goal of this
circuit current, which can amount to as of grains with small aspect ratio is prepared activity is the development of an innovative
much as 7mA/cm
2
. using the technique of aluminium induced technology to produce 4 terminal high
IMEC develops techniques to improve the crystallisation (AIC), followed by epitaxial efficiency mechanical stacks capable of
efficiency of epitaxial thin film solar cells. thickening. Because thermal CVD is used to efficiencies up to 35%. This activity
For example, a plasma texturing technique grow the active layer, a substrate has to be comprises the manufacturing of thin film
was worked out to texture the front surface chosen that withstand high temperatures, InGaP/GaAs topcells and Ge bottom cells.
of the active layer of the solar cell to such as ceramic and glass ceramic
decrease reflectance and increase surface substrates. The best efficiencies so far are On the longer term, organic solar cells can
light scattering. Apart from the efficiency 8% on ceramic substrates and 6.4% on glass become a good alternative for silicon based
improvement thanks to reduced reflectance ceramic substrates, which are the best solar cells for specific applications. Active
and oblique coupling of the light into the layers of organic solar cells are typically in
cell, plasma texturing also lowers contact the order of 100nm to several µm’s. The low
resistance compared to planar cells. Plasma material consumption and the fact that the
texturing results in an improvement of the
Another adaptation
technologies to deposit these layers
short circuit current with 1.0 to 1.5 mA/cm
2
that improves the
(printing) are compatible with extremely
and an extra improvement of 0.5 to 1.0%. high production throughputs (1 to 2 orders
efficiency of epitaxial
of magnitude in comparison with the
Another adaptation that improves the present solar cell technologies) could result
efficiency of epitaxial thin film silicon solar thin film silicon in costs a factor 5 to 10 lower than the
cells is the incorporation of a porous silicon
mirror at the interface between the active
solar cells is the
present solar cell technologies.
layer and the low cost substrate. This mirror
incorporation of
One of the most promising concepts in the
decreases the transmittance of long field of organic solar cells is that of the bulk
wavelength light into the substrate. In
a porous silicon
donor/acceptor heterojunction. Here, the
practice, the reflector is made by active layer consists of an intimate mixture
electrochemical etching of a porous silicon
mirror at the
of two different conjugated organic
stack of alternating high and low porosity
interface between the
materials sandwiched between metallic
layers. These porous silicon layers have an electrodes.
index of refraction that depends on the
active layer and the
porosity. The multilayers with alternative Conclusion
porosities therefore act as a multiple Bragg low cost substrate The solar cell industry is booming and
reflector. Moreover, the porous silicon research teams worldwide are looking for
retains the crystallographic structure of the ways to improve the cell efficiencies and/or
underlying structures, enabling epitaxial reduce the costs. IMEC has developed an
growth. Solar cells prepared on low cost Si results so far worldwide for thin film polySi evolutionary roadmap which predicts a
substrate with this reflector and screen cells based on a seed layer approach. A gradual transition from thin wafer based
printed contacts reached an excellent crucial part of the device in thin film polySi crystalline Si solar cells towards thicknesses
efficiency of 13.9%, coming close to the solar cells is the emitter. The use of an as low as 40µm or monolithic thin film
efficiencies of multicrystalline Si solar cells. amorphous Si/polycrystalline Si crystalline Si solar cell modules on a low
heterojunction emitter instead of a diffused cost carrier. All this should reduce the cost
Thin film poly silicon homojunction emitter led to a boost in the of crystalline Si solar cells by at least a
Another thin film silicon solar cell approach open circuit voltage by 90mV. factor of 3 compared to nowadays and a
with high cost reduction potential is the thin reduction by an order of magnitude of the
film polySi technology. Here a thin layer Beyond silicon amount of Si/Watt which presently is
(only about 5 µm) of crystalline silicon is Besides its extensive work on silicon based slightly below 10g/Watt. As India develops
deposited on a low cost foreign substrate. In solar cells, IMEC also researches its solar plans the roadmap provides clues a
order to achieve a layer with a relatively photovoltaic stacks for terrestrial to what direction they can take.
26 www.semiconductor-india.com | Autumn 2008
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