Optimizing design of 2D sub-wavelength ARC gratings for multi-junction III–V concentrator cells

In high X III-V concentrator applications sunlight is focused onto the surface of cell with a wide angular distribution that limits the effectiveness of conventional thin-film AR coatings. 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 sub-cell 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. The evaluation indicates that through a careful selection of the design and dielectric material these structures can significantly surpass the performance of planar double layer ARCs (i.e. MgF₂/ZnS), and the total number of reflected photons over the 380-2000 nm wavelength range can be reduced to less than 2%. Finally it is shown that the implementation of these structures for a typical 3 or 4 junction solar cells (i.e. inverted metamorphic) and for acceptance angles ranging from 0-60 degrees, reduces total losses of reflected photons for each subcell (and to some extent the resulting current degradation) to less than 4%.