TECHNOLOGY
Figure1. Multi-junction
cells deliver higher
efficiencies than
single-cell equivalents
by using materials
with differing
bandgaps to absorb
different parts of the
solar spectrum. This
approach reduces
thermalization losses
connected by external circuits to obtain a larger mechanically stacked triple-junction
DC output voltage, which can then be converted to InGaP/GaAs/Ge cell is under development,
an AC source for the grid with an inverter). exploiting the same sub-cells employed in current
However, this does not eliminate the need to state-of-the-art monolithic cells.
integrate electrical leads on every cell in the stack,
which has a major impact on cell and stack In the proposed configuration, the InGaP and
development and technology. This is especially a GaAs subcells are processed so the germanium
concern for cell interconnects that need to be substrate, where the epitaxial layers are deposited,
placed on one of the intermediate cell surfaces is removed. In this way, an InGaP cell with a typical
within the stack. thickness of about 1 mm and a 3-4 mm thick GaAs
cell can be stacked on a separately realized
Compared to its monolithic cousin, a mechanically germanium bottom cell. This architecture will allow
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stacked multi-junction solar cell has additional the extraction of the full current generated by the
surfaces and interfaces, and it requires additional germanium cell, which is not the case in a
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adhesive layers in between the different cells. monolithically stacked triple-junction cell.
.solar
These increase the number of sources of optical
loss, which might have an important impact on the Removing the substrates from the top and middle
-pv-management.com
final performance of middle and bottom cells in cells should lead to easier transport of excessive
such a stack. Moreover, in order to fully transmit heat towards the heat sink. A further benefit is that
the non-absorbed light from an upper cell to the it allows interconnection of the contact grids of the
ones beneath it in the stack, the higher cell must different cells from the stack’s front or rear side.
be optically transparent to its sub-bandgap This makes the electrical design less complex,
radiation. opening the door to stacked cell processing on the
wafer scale, which is a key element in upscaling
Issue I 2010
Efforts at IMEC this technology for the production of high-
At IMEC, which is based in Belgium, I am working efficiency concentrator cells.
with several other researchers to develop
mechanically-stacked solar cells that address the Final integration of the individual cells into a
above-mentioned problems. We hope that our mechanical stack also requires know-how and
efforts will ultimately enable these devices to fulfill tools from the semiconductor manufacturing
their high-efficiency potential and deliver the industry, specifically 3D-stacking expertise, an area
benefits associated with their inherent robustness of technology where IMEC has considerable
to spectral variations. The technologies used to strength. Adopting this approach will produce
produce these cells are compatible with high- high-quality bonding, integration and
throughput manufacturing. Specifically, a interconnection processes, while also offering the
Bulkiness is to a certain extent inevitable, due to the use of different solar cells
and their associated substrates in a mechanical stack. In addition,
multiple substrates push up costs. Specifically for CPV applications,
the thermal mass of the full mechanical stack also offers
a major challenge concerning heat dissipation
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