McMaster plan aims for a solar breakthrough
I n d u s t r y T h e M o n t h i n O p t o e l e c t r o n i c s
Materials scientists in Canada are to receive
$4.1 million to develop a novel solar cell that
combines silicon with a secret compound
semiconductor to double the efficiency of
conventional cells.
Solar Energy firm ARISE Technologies
and the Ontario Centres of Excellence
are funding the work of John Preston and
Rafael Kleiman, who head up the McMaster
University team.
They are aiming to combine the ubiquity
and low cost of silicon-based solar cells
with the high efficiencies that are associated
with more expensive compound semiconductor
technologies.
?We are aiming to develop cells suited for
one-sun applications ? to create what appear
to be regular silicon panels, but which have
a much higher efficiency because of the
novel materials approach taken,? Kleiman
told Compound Semiconductor.
The exact nature of that materials system
is being kept a closely guarded secret,
although it is expected to double the efficiency
of typical silicon cells. However,
Kleiman did reveal that the team would be
making use of McMaster?s in-house MBE
facility to deposit single-crystal layers of
a compound semiconductor on top of the
silicon host.
The professor added that GaAs will not
be the material used because, at 1.45 eV,
its bandgap is not wide enough to provide
the best conversion efficiency in a double-
junction
device alongside silicon.
Kleiman and colleagues have done some
theoretical modeling to work out what the
best match would be. ?Our plot tells us
clearly that for a double-junction device
with silicon as the substrate, we would like
our second (upper) junction to have a bandgap
of about 1.68 eV,? explained Kleiman,
adding that the design would have a maximum
theoretical efficiency of 43.5%.
While triple-junction cells designed for
high-concentration photovoltaic systems
have already been measured to deliver a
real-world efficiency close to that theoretical
mark (at a 240-sun concentration), the
maximum figure for triple-junctions under
unfocused sunlight is much lower, and ? in
theory ? is comparable to that of the silicon/
III-V hybrid.
?We are targeting a more modest 30%
efficiency,? Kleiman said, on the assumption
that it would be possible to make a cell
work at three-quarters of the theoretical
maximum. He believes that the approach
will only add a modest incremental cost to
the processes currently used to make singlejunction
crystalline silicon cells.
Kleiman freely admits, however, that
MBE will not be the ideal deposition
method for the intended focus on high-
volume,
low-cost applications: ?The later
part of the project will focus on transferring
the technology to a manufacturable process,
such as MOCVD.?
Although the practical side of the research
is only at a very early stage right now, the
team will be able to draw on the experience
gained during the photonics boom, where
Canadian researchers and companies
were
at the cutting edge of compound semiconductor
chip technologies designed for fiber
optic communication networks.
Ambitiously, the team is hoping to transfer
the technology out of the lab and into a
commercial fab in just three years.
However, interfacing a compound semiconductor
with silicon is notoriously difficult,
as Kleiman acknowledges: ?I think that
the central goal is a detailed microscopic
understanding of the III-V-to-silicon interface,?
concluded the researcher. ?Structurally,
chemically and electronically.?
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