GaSb-Based Solar Cells for Full Solar Spectrum Energy Harvesting

DOI: 10.1002/aenm.201700345 conversion efficiency. Devices comprising these alloys (and, in some cases, the group IV element, germanium) have held the world record for conversion efficiency under concentrated sunlight for more than the last thirty years with steady increases in efficiency year-on-year.[2] Efficiency improvements result from many aspects of MJSC design, but among the most important of these are improving the distribution of light between the subcells of the MJSC, increasing the number of subcells, and increasing the fraction of the solar spectrum being captured. The ideal MJSC would harvest the entire solar spectrum extending into the midinfrared wavelength range using a very large number of closely spaced bandgaps, and the theoretical upper-limit of conversion efficiency is ≈86%, assuming full solar concentration of 45900 suns.[3] The Earth’s atmosphere filters the solar spectrum, such that ≈99% of the power contained in the direct-beam airmass 1.5 (AM1.5D) reference spectrum is contained within the spectral band covering 300–2500 nm. This filtering impacts the optimal bandgaps for MJSCs, and recent calculations showed that the optimum lowest energy bandgap for practical MJSC solutions with 4–7 junctions is ≈0.5 eV (2500 nm).[4] These calculations assume more realistic device performance than the idealized, detailed-balance models,[5] and a more practical solar concentration of 1000X, which yields an efficiency projection of 54.6% for a 7 junction (7J) device. Therefore, to achieve virtually full spectrum energy harvesting of the direct-beam component of the terrestrial spectrum, 0.5 eV is the best practical target for the lowest bandgap absorber in an advanced MJSCs with four or more junctions. It should be noted that concentrator photovoltaic (CPV) solutions are typically unable to capture the diffuse portion of the irradiation, which can be a significant fraction of the global irradiation in terrestrial applications. However, recent advancements in hybrid approaches which combine CPV cells with larger area solar cells on the module back-plane to capture diffuse light[6] offer a potential route to even higher efficiency with respect to the total global irradiation incident on a photovoltaic module. No single III–V or group IV substrate offers direct-bandgap, lattice-matched (LM) III–V alloys which span the entire spectral range at favorable bandgap intervals for producing MJSCs In this work, a multijunction solar cell is developed on a GaSb substrate that can efficiently convert the long-wavelength photons typically lost in a multijunction solar cell into electricity. A combination of modeling and experimental device development is used to optimize the performance of a dual junction GaSb/InGaAsSb concentrator solar cell. Using transfer printing, a commercially available GaAs-based triple junction cell is stacked mechanically with the GaSb-based materials to create a four-terminal, five junction cell with a spectral response range covering the region containing >99% of the available direct-beam power from the Sun reaching the surface of the Earth. The cell is assembled in a mini-module with a geometric concentration ratio of 744 suns on a two-axis tracking system and demonstrated a combined module efficiency of 41.2%, measured outdoors in Durham, NC. Taking into account the measured transmission of the optics gives an implied cell efficiency of 44.5%.