Optimizing Design of 2D Sub-wavelength Gratings for Antireflection Coatings of Multi-junction III-V Concentrator Cells
Wang, Wei 1987-
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In III-V concentrator applications sunlight is focused onto the surface of cell and consequently light arrives with a wide angular distribution that limits the effectiveness of conventional thin-film antireflective coatings (ARC). Furthermore, the transmission properties are generally degraded non-uniformly over the electromagnetic spectrum, which in the case of multi-junction solar cells, leads to additional subcell current matching-related losses. Here, and in an attempt to identify a better alternative to the conventional dual layer ARCs, we have undertaken a systematic analysis of design parameters and angular dependent antireflective properties of dielectric grating formed through the implementation of sub-wavelength arrays of 2D pyramidal and hemispherical textures. The study includes evaluation of these properties for several common dielectrics i.e. SiO2, Si3N4, SiC, TiO2, and ZnS, through a careful selection of dielectric material and design. These structures can significantly surpass the performance of planar double-layer ARCs (i.e. MgF2/ZnS), and the total number of reflected photons over 380-2000 nm wavelength range can be reduced to less than 2%, by use of single material textured dielectric. It is also shown that the implementation of these structures for a typical concentrated 3 or 4 junction solar cell with acceptance angles ranging from 0-60 degrees, reduces total losses of reflected photons for each subcell to less than 4%, hence reduces current degradation.