photovoltaics technology
rhombus4
can be stacked on a separately realized germanium Fig.3. IMEC
bottom cell. This architecture will allow the extraction of has already
the full current generated by the germanium cell, which is built a
not the case in a monolithically stacked triple-junction cell. mechanically
stacked
Removing the substrates from the top and middle cells dual-junction
should lead to easier transport of excessive heat towards cell based on
the heat sink. A further benefit is that it allows GaAs and
interconnection of the contact grids of the different cells germanium.
from the stack’s front or rear side. This makes the The next goal
electrical design less complex, opening the door to is the addition
stacked cell processing on the wafer scale, which is a key of an InGaP
element in upscaling this technology for the production of top cell
high-efficiency concentrator cells.
identical results to those obtained previously on similar
Final integration of the individual cells into a mechanical stand-alone cells. They can produce conversion
stack also requires know-how and tools from the efficiencies exceeding 23 percent (1 sun, AM1.5), close to
semiconductor manufacturing industry, specifically 3D- our institution’s best results for regular GaAs solar cells
stacking expertise, an area of technology where IMEC has on a germanium substrate of 24.7 percent. The main
considerable strength. Adopting this approach will limiting factor for the one-side contacted GaAs cells is the
produce high-quality bonding, integration and relatively low fill factor. This is caused by the use of the
interconnection processes, while also offering the two-point measurement method during ‘I-V’
possibility to perform high-accuracy alignment of the characterization, which is imposed by the small available
different cells in the stack. This should result in a good area for contacting the rear-side grid in the present
alignment of the different contact grids applied to the configuration.
front and backside of the individual cells, such that optical
coupling between the different cells in the stack is not The separately contacted germanium bottom cell in our
hampered by unwanted additional shadowing losses due stack exhibits a conversion efficiency of almost 2 percent
to the contact structures. (1 sun, AM1.5). The low efficiency can be attributed to the
absence of an anti-reflective coating at the rear of the
Prototype progress GaAs cell, and the use of a non-optimized coating on the
One significant step towards the fabrication of this triple- front side of the germanium cell. Applying an optimized
junction stacked cell has been the fabrication of a dual- coating to both these surfaces should lift the efficiency of
junction version based on GaAs and germanium. This has the germanium bottom cell to approximately 3-3.5
been made by combining 4 µm, one-side-contacted GaAs percent, which is well above the contribution of the
solar cells and separately connected germanium solar germanium cell in conventional triple-junction cells.
cells. The thinned-down GaAs solar cells were bonded on
top of the germanium solar cells using silicone sealant. Future work
Transmission measurements on a layer of this sealant with Efforts will now be directed at increasing solar cell
relevant thickness (~20 µm) revealed that the performance through the reduction of reflective losses at
transmission loss through this layer is limited to 3.5 different cell surfaces and matching of the area and
percent in the 900-1800 nm wavelength range. contact grid of the III-V and germanium cells. The bonding
process demands high-accuracy alignment of the different
The presently realized mechanical stack (figure 3) employs cells’ contact grids, which can be realized with flip-chip
individual GaAs and germanium cells that have a non- bonding, according to scanning acoustic microscopy
matched cell area and contact grid design. This means measurements. When an ultra-thin, one-side contacted
that the longer-wavelength portion of incident light that InGaP top cell is added to the structure, this should yield
passes through the GaAs cell is not optimally coupled to mechanically stacked triple-junction solar cells for CPV
the underlying germanium cell. Performance is also applications that feature efficiencies of around 40 percent
compromised by the absence of an anti-reflective coating and enhanced spectral robustness.
at the rear of the GaAs cell that limits the transmittance of
infrared radiation to the bottom cell. But the good news is Finally, it should be noted that the application of a
that the applied technologies for thinning down the GaAs contacting layout that makes the contacts of all cells in
cell and bonding it on top of an active germanium solar the stack accessible from the stack’s front and rear
cell are readily applicable in an optimized, stacked-cell surfaces - as demonstrated in the present work - is
design. instrumental in allowing wafer-scale processing of the full
mechanical stack right up to the final step. This way,
We have found that thinned-down, one-side-contacted dicing to individual solar cells can be performed as a last
GaAs cells integrated in the mechanical stack exhibit step, right before cell laydown and interconnection.
January/February 2010
www.compoundsemiconductor.net 29
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