Increasing solar cell power production on micro and nano satellites using sub-wavelength gratings

Abstract

The application of 2-dimensional periodic arrays of sub-wavelength structures as anti-reflection surfaces at the interface between a vacuum and commercial solar cell coverglass is investigated. It is determined that compared to commercial anti-reflective coatings a single anti-reflective sub-wavelength grating will yield 3.7 to 7.1% total increase in power production during a 24 hour orbital period for a typical earth observing 3-U CubeSat. Sub-wavelength gratings are approximated as a many layered stack of thin films with refractive indices defined by their vertical position in the array and the grating fill factor at that position. The transmission of light between wavelengths of 350nm and 1800nm and incident angles 0–90 degrees is calculated by the transfer matrix method. The optimal periodicity, profile and height of the features for an antireflective sub-wavelength grating applied to a commercial triple junction solar cell in Air Mass 0 are explored by analysis of the transmission coefficient surfaces for various gratings, and two sub-wavelength grating surfaces are benchmarked against the performance of a MgF 2 thin film anti reflective coating, as well as uncoated cover glass. The AM0 solar spectrum is propagated through the transmission coefficients of the SWG surface as well as a model for the external quantum efficiency and heat loss of commercially available Triangular Advanced Solar Cells. The amount of light reaching the cells is then applied to the photo response of each sub cell to determine the expected increase in solar cell power production at incident angles from 0–90 degrees. The incident angles for solar panels mounted on 3-U CubeSats in typical earth observation orbits are determined and used to calculate the expected increase in average power production over orbit which is found to be between 3.7 and 7.1% of the total.