TECHNOLOGY
cells. In the latter case, cell compositions are into a stack, such that each cell absorbs a different
modified to yield a superior combination of part of the incident spectrum (figure 1). Each of
bandgap energies. The downside of this approach these cells has a separate electrical contact. This
is that each cell differs in its crystal lattice spacing, means that they do not have to be connected in
so additional buffer layers are needed to pin the series, which is a massive benefit because it
crystallographic defects and prevent them from removes the need for tunnel junctions and current
degrading the performance of the active layers. matching.
Regardless of the form of monolithic multi-junction Eliminating the need for current matching also
solar cell, current matching between different cells produces additional, important advantages – it
is essential, due to the inherent series connection allows full exploitation of the power generated by
of these integrated devices. In addition, there is a every cell within the device, and it creates an
need for tunnel junctions, which are applied to inherent robustness against variations in the
electrically connect the different cells in the stack. spectral distribution of the incident light. What’s
These junctions can handle the high peak more, this approach offers the freedom to realize
tunneling currents. any combination of cells with different energy
bandgaps, without the need to worry about lattice-
In real-life CPV applications there is an additional matching issues.
complication too - non-uniform illumination levels
on the solar cell. This may lead to local current However, all of these advantages have to be
densities exceeding the tunnel junction design weighed against three specific challenges
value, and ultimately higher resistances and associated with mechanically-stacked solar cells
21
voltage losses. Deviations in the spectral that have hampered progress by their developers,
distribution of the incident sunlight occur all the and prevented commercialization: bulkiness; a
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time, because they depend on changes in complex electrical architecture; and optical
.solar
geographical, seasonal, daily and climatic coupling requirements.
conditions. This makes it very tough to current
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match cells for optimal energy yield, and imposes Bulkiness is to a certain extent inevitable, due to
very stringent requirements on cell design for the use of different solar cells and their associated
specific operating conditions and locations. The substrates in a mechanical stack. In addition,
latter of these also needs to be very well multiple substrates push up costs. Specifically for
documented. CPV applications, the thermal mass of the full
mechanical stack also offers a major challenge
Stacking cells: concerning heat dissipation.
Issue I 2010
Pros and cons
A promising alternative to The second issue, the complex electrical
these monolithic cells is a architecture, stems from the need to provide
mechanically stacked multi-junction individual electrical interconnections to every cell.
architecture. With this approach, This can be addressed at the system level with an
different single-junction solar cells are intelligent string and inverter design (a string is a
integrated by mechanically placing them number of individual cells that are series
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