excite electrons to a potential somewhat lower than in the first
cell. Spectrum splitting concept is the alternative solution where
laterally arranged multiple bandgap solar cells are placed in parallel.
In this case light is separated into spectral components by
a spectrum splitting device and is directed onto individual cells
of corresponding bandgap energies such that each spectral band
is absorbed by the optimized bandgap materials [33]. Spectrum
splitting systems and multi-junction CPV systems both attempt
to reduce thermalization and non-absorption losses by employing
cells with different bandgaps. However, spectrum splitting systems
have a number of advantages over multi-junction CPV systems.
Multi-junction systems face difficulties with cooling issues since
the top cell is normally cooled via the connection to the bottom
heat-sinked cell. Multi-junction cells are constrained by currentmatching,
since a common current must flow through all junctions.
This constraintleads to reduced currentthrough the multi-junction
cell if the individual junction currents are not matched. Further,
lattice-matching is required to avoid degradations in performance,
which adds additional constraints to what materials can be used
in the cell and ultimately how many junctions can be stacked [34].
These constraints are completely eliminated in spectrum splitting
systems, which can use a larger number of effective junctions to
further reduce the thermalization and non-absorption losses. In
addition, single junction cells, used in spectrum splitting systems,
are much cheaper to manufacture than multi-junction cells.