|
Along the road towards ubiquitous and low-cost solar cells, solutions to the seemingly mutually exclusive targets
of reducing material consumption while increasing the efficiency has to be found. One potential solution seems
to lie in thin film tandem solar cells. It offers the promise of moderate efficiencies combined with the advantage
of relying on a well-established thin film fabrication technology at reasonable low costs. To finally make them
serious competitors, various structures to be exploited for photon management may be incorporated into these
solar cells with the aim of increasing their efficiency. Besides reducing reflection losses at the entrance facet via
textured surfaces and eliminating the dissipation in the metallic backside reflector, the efficiencies of tandem cells
can be significantly boosted by a wavelength-dependent steering of the spatial domain where light gets absorbed,
i.e. either the top or the bottom cell. This is mainly possible by placing a spectrally selective intermediate
reflector in between both cells. In the present contribution we apply well-adapted numerical routines, which
solve Maxwell's equations rigorously, to quantitatively explore various intermediate reflector concepts for thin
film solar cells from an optical point of view. The solar cells we focus on are silicon based, where the top layer
is made of amorphous and the bottom layer of microcrystalline silicon, respectively. We explore state-of-the-art
concepts for the intermediate reflector, such as homogenous layers based on dielectrics characterized by a lower
permittivity as well as new photonic (such as, e.g., photonic crystals) and plasmonic concepts. Most notably we
will address the issue how randomly textured interfaces, present in thin film solar cells, affect the performance of
each intermediate reflector and how the randomness may contribute to the absorption enhancement. Guidelines
for designing optimized systems will be given.
|
