photovoltaics technology
rhombus4
Stacking sidesteps the strain
in multi-junction cells
If terrestrial concentrator photovoltaics are to enjoy significant commercial success
then electricity generation costs must fall. One way to do this is to improve the
design of the cell with a stacked architecture that eliminates strain and current
matching issues, according to IMEC’s Giovanni Flamand.
T
he solar industry is changing. Although silicon is cells. In the latter case, cell compositions are modified to
still the dominant material for solar cell yield a superior combination of bandgap energies. The
manufacture, alternative technologies like thin-film and downside of this approach is that each cell differs in its
concentrator photovoltaics (CPV) are emerging. The latter crystal lattice spacing, so additional buffer layers are
of these promises to excel in sunnier climes and involves needed to pin the crystallographic defects and prevent
the focusing of incident sunlight onto cells with a typical them from degrading the performance of the active layers.
area of just 10-100 mm
2
. By focusing the light by a factor
of several hundred, it is possible to minimize the total Regardless of the form of monolithic multi-junction solar
expenditure on these relatively expensive cells, and cell, current matching between different cells is essential,
ultimately realize an acceptable cost-per-Watt at the due to the inherent series connection of these integrated
system level. devices. In addition, there is a need for tunnel junctions,
which are applied to electrically connect the different cells
Generating costs for CPV systems are also influenced by in the stack. These junctions can handle the high peak
the efficiency of the cell. Single-junction solar cells are tunneling currents.
inappropriate, because they are limited to a theoretical
maximum efficiency of about 30 percent, due to In real-life CPV applications there is an additional
thermalization and transmission losses. In comparison, complication too - non-uniform illumination levels on the
multi-junction cells can produce far higher efficiencies, solar cell. This may lead to local current densities
because they cut thermalization losses by using several exceeding the tunnel junction design value, and ultimately
cells to absorb different parts of the solar spectrum. higher resistances and voltage losses. Deviations in the
spectral distribution of the incident sunlight occur all the
Today the dominant multi-junction solar cell technology is time, because they depend on changes in geographical,
the monolithic triple-junction (InGaP/(In)GaAs/Ge) solar seasonal, daily and climatic conditions. This makes it very
cell that was originally developed and commercialized for tough to current match cells for optimal energy yield, and
space applications. These devices feature In
0.5
Ga
0.5
P,
In
0.01
Ga
0.99
As and germanium cells that are lattice-matched
to a germanium substrate to ensure excellent material
quality and photovoltaic performance.
The conversion efficiency record for a multi-junction cell
under concentrated irradiation has been broken on several
occasions over the last few years. Spectrolab has made
the most recent claim for the record, and in August 2009
it announced an efficiency of 41.6 percent, which was
achieved under a concentration of 364 suns. But this
record may not last for long because further
improvements in monolithic multi-junction cells are Fig.1. Multi-junction cells deliver higher efficiencies than single-cell
expected. These could come through either adding more equivalents by using materials with differing bandgaps to absorb different
junctions to the stack, or using lattice-mismatched solar parts of the solar spectrum. This approach reduces thermalization losses
January/February 2010
www.compoundsemiconductor.net 27
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