technology photovoltaics
rhombus4
Fig. 2. costs. Specifically for CPV applications, the thermal mass
Mechanically of the full mechanical stack also offers a major challenge
stacked concerning heat dissipation.
junctions
employ an The second issue, the complex electrical architecture,
electrical stems from the need to provide individual electrical
contact for interconnections to every cell. This can be addressed at
every junction. the system level with an intelligent string and inverter
Thanks to this design (a string is a number of individual cells that are
approach, there series connected by external circuits to obtain a larger DC
is no need to output voltage, which can then be converted to an AC
current-match source for the grid with an inverter). However, this does
the junctions, not eliminate the need to integrate electrical leads on
or use tunnel every cell in the stack, which has a major impact on cell
diodes and stack development and technology. This is especially
imposes very stringent requirements on cell design for a concern for cell interconnects that need to be placed on
specific operating conditions and locations. The latter of one of the intermediate cell surfaces within the stack.
these also needs to be very well documented.
Compared to its monolithic cousin, a mechanically
Stacking cells: pros and cons stacked multi-junction solar cell has additional surfaces
A promising alternative to these monolithic cells is a and interfaces, and it requires additional adhesive layers in
mechanically stacked multi-junction architecture. With this between the different cells. These increase the number of
approach, different single-junction solar cells are sources of optical loss, which might have an important
integrated by mechanically placing them into a stack, such impact on the final performance of middle and bottom
that each cell absorbs a different part of the incident cells in such a stack. Moreover, in order to fully transmit
spectrum (figure 2). Each of these cells has a separate the non-absorbed light from an upper cell to the ones
electrical contact. This means that they do not have to be beneath it in the stack, the higher cell must be optically
connected in series, which is a massive benefit because it transparent to its sub-bandgap radiation.
removes the need for tunnel junctions and current
matching. Efforts at IMEC
At IMEC, which is based in Belgium, I am working with
Eliminating the need for current matching also produces several other researchers to develop mechanically-stacked
additional, important advantages – it allows full solar cells that address the above-mentioned problems.
exploitation of the power generated by every cell within We hope that our efforts will ultimately enable these
the device, and it creates an inherent robustness against devices to fulfill their high-efficiency potential and deliver
variations in the spectral distribution of the incident light. the benefits associated with their inherent robustness to
What’s more, this approach offers the freedom to realize spectral variations.
any combination of cells with different energy bandgaps,
without the need to worry about lattice-matching issues. The technologies used to produce these cells are
compatible with high-throughput manufacturing.
However, all of these advantages have to be weighed Specifically, a mechanically stacked triple-junction
against three specific challenges associated with InGaP/GaAs/Ge cell is under development, exploiting the
mechanically-stacked solar cells that have hampered same sub-cells employed in current state-of-the-art
progress by their developers, and prevented monolithic cells.
commercialization: bulkiness; a complex electrical
architecture; and optical coupling requirements. In the proposed configuration, the InGaP and GaAs
subcells are processed such that the germanium
Bulkiness is to a certain extent inevitable, due to the use substrate, onto which the epitaxial layers are deposited, is
of different solar cells and their associated substrates in a removed. In this way, an InGaP cell with a typical
mechanical stack. In addition, multiple substrates push up thickness of about 1 µm and a 3-4 µm thick GaAs cell
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
28
www.compoundsemiconductor.net January/February 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87