TECHNOLOGY
cell. Performance is also compromised by the
absence of an anti-reflective coating at the rear of
the GaAs cell that limits the transmittance of
infrared radiation to the bottom cell. But the good
news is that the applied technologies for thinning
down the GaAs cell and bonding it on top of an
active germanium solar cell are readily applicable
in an optimized, stacked-cell design.
We have found that thinned down, one side
contacted GaAs cells integrated in the mechanical
stack exhibit identical results to those obtained
previously on similar stand-alone cells. They can
Issue I 2010
produce conversion efficiencies exceeding 23
percent (1 sun, AM1.5), close to our institution’s
best results for regular GaAs solar cells on a
germanium substrate of 24.7 percent. The main
limiting factor for the one side contacted GaAs
cells is the relatively low fill factor. This is due to
Figure 2. Mechanically possibility to perform high accuracy alignment of the two-point measurement method during ‘I-V’
-pv-management.com stacked junctions the different cells in the stack. This should result in characterization, which is imposed by the small
employ an electrical a good alignment of the contact grids applied to available area for contacting the rear-side grid.
.solar
contact for every the front and backside of the individual cells, such
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junction. Thanks to that optical coupling between the different cells in The separately contacted germanium bottom cell
this approach, there is the stack is not hampered by unwanted shadowing in our stack exhibits a conversion efficiency of
24
no need to current- losses due to the contact structures. almost 2 percent (1 sun, AM1.5). The low efficiency
match the junctions, can be attributed to the absence of an anti-
or use tunnel diodes Prototype progress reflective coating at the rear of the GaAs cell, and
One significant step towards the fabrication of this the use of a non-optimized coating on the front
triple-junction stacked cell has been the fabrication side of the germanium cell. Applying an optimized
of a dual-junction version based on GaAs and coating to both these surfaces should lift the
germanium. This has been made by combining 4 efficiency of the germanium bottom cell to
mm, one-side-contacted GaAs solar cells and approximately 3-3.5 percent, which is well above
separately connected germanium solar cells. The the contribution of the germanium cell in
thinned-down GaAs solar cells were bonded on conventional triple-junction cells.
top of the germanium solar cells using silicone
sealant. Transmission measurements on a layer of Future work
this sealant with relevant thickness (~20 mm) Efforts will now be directed at increasing solar cell
revealed that the transmission loss through this performance through the reduction of reflective
layer is limited to 3.5 percent in the 900-1800 nm losses at different cell surfaces and matching of
wavelength range. the area and contact grid of the III-V and
germanium cells. The bonding process demands
The mechanical stack (figure 3) employs high accuracy alignment of the different cells’
individual GaAs and germanium cells that have a contact grids, which can be realized with flip-chip
non-matched cell area and contact grid design. bonding, according to scanning acoustic
This means that the longer-wavelength portion of microscopy measurements. When an ultra-thin,
incident light that passes through the GaAs cell is one-side contacted InGaP top cell is added to the
not optimally coupled to the underlying germanium structure, this should yield mechanically stacked
triple-junction solar cells for CPV applications that
feature efficiencies of around 40 percent and
Figure 3. IMEC has enhanced spectral robustness.
already built a
mechanically stacked Finally, it should be noted that the application of a
dual-junction cell contacting layout that makes the contacts of all
based on GaAs and cells in the stack accessible from the stack’s front
germanium. and rear surfaces is instrumental in allowing wafer-
The next goal scale processing of the full mechanical stack right
is the addition of an up to the final step. This way, dicing to individual
InGaP top cell solar cells can be performed as a last step, right
before cell laydown and interconnection.
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