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Design of sub-wavelength dielectric antireflective grading for multijunction concentrator photovolatics

In III-V concentrator applications, sunlight is focused with 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 planar layer ARCs for III-V multi-junction concentrator cells in case of with/without protective cover glass in conjunction with wide optical aperture angles, a systematic analysis of design parameters and angular dependent antireflective properties of dielectric gratings has been undertaken, through the implementation of sub-wavelength 2D pyramidal gratings of ZnS and TiO2. The study indicated limited improvement for devices operated with SiO2 like cover glass. In the absence of SiO2 like cover glass, the evaluation indicated that reflection-loss related current losses can be reduced by 2-3 fold compared to their doublelayer ARC counterparts. i.e. for a 3J metamorphic device this lead to a current improvement of 0.7 mA/cm2 per concentration for a 60 degree aperture angles