Spectrum-splitting photovoltaics: Holographic spectrum splitting in eight-junction, ultra-high efficiency module
To achieve photovoltaic energy conversion with ultra-high module efficiency, the number of junctions can be increased beyond the 3-5 used in conventional lattice-matched multi-junction photovoltaics. We demonstrate a photovoltaic design that incorporates eight III-V semiconductor junctions arranged laterally as four dual-junction subcells operating electrically and optically in an independent manner. An integrated holographic optical element is used to split the incoming solar spectrum into four different spectral bands, each diffracted onto the appropriate subcell. In addition, two-axis concentration is used to achieve total concentration of 672 suns. A preliminary design for this holographic spectrum splitter composed of twelve simple, commercially available holograms predicts an achievable 76.1% optical efficiency and 37.1% two-terminal module efficiency; opportunities for improved efficiency are discussed.