Intermediate reflectors in thin film solar cells comprising randomly textured surfaces

Abstract

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.