UNS: Collaborative Research: 30%-Efficient III-V/Silicon Tandem Solar Cells

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PI: Minjoo Larry Lee / Zachary C. HolmanProposal Number: 1509687/ 1509864The sun represents the most abundant potential source of sustainable energy on earth. Currently, solar cells based on crystalline silicon materials dominate the photovoltaics market for production of electricity from the sun because they offer the highest solar energy conversion efficiency at the lowest manufacturing cost. However, to accelerate the penetration of solar energy in the renewable electricity market, the solar energy conversion efficiency of silicon-based solar cells must ultimately increase beyond its practical limit of 24%. The goal of this project is develop a silicon-based solar cell which contains Group III and V elements from the Periodic Table, arranged in layers which have the potential to increase the solar energy conversion efficiency to 30%. The fundamental science underlying the performance of this the III-V/silicon tandem cell will be used to develop the best strategy for eventual manufacture. As part of the educational activities of this project, the principal investigators will be actively involved in an outreach program that seeks to broaden the participation of under-represented groups in engineering by using solar research as a platform for involvement in both technical and career-development sessions at Veterans meetings and the Society of Hispanic Professional Engineers conference.Photovoltaic devices that contain multiple p-n junctions are currently the only route to achieve solar energy conversion efficiencies that exceed the Shockley-Queisser single p-n junction limit that caps the theoretical performance of crystalline silicon solar cells currently in commercial use. The overall goal of this proposed research is to develop a fundamental understanding of two-terminal tandem solar cell performance through controlled growth of Group III-V elements on silicon. The fabrication strategy is guided by fundamental studies and is designed to optimize the material and device architecture to achieve 30% solar energy conversion efficiency, which is beyond the 24% practical limit of single-junction crystalline silicon solar cells. Towards this end, the model Group III-V material selected for study is GaAsP, since it has a direct and tunable bandgap, and can be grown on a transparent, compositionally graded buffer on a GaP/Si template. The bottom cell of the tandem device will be based on an amorphous silicon/crystalline silicon heterojunction solar cell, where the front amorphous silicon layers will be replaced with the GaP template layer upon which the top cell is grown. The research plan has three objectives. The first objective is to understand and control the formation of threading dislocations in the GaAsP absorber, and to develop optimized window and back-surface field layers for the top cell that will both increase transmission into the GaAsP absorber and reduce surface recombination. The second objective is to understand and improve the passivation of GaP on silicon and the transport of electrons across the conduction band offset. The third objective is to maximize the conversion of infrared light into current in the bottom cell by designing single-side light-trapping textures and dielectric/metal rear reflectors, and then tune the thicknesses, doping densities, and bandgaps of the III-V supporting layers to both form a recombination junction between the sub-cells and match their currents. The research outcomes will advance fundamental scientific understanding of multi-junction solar cell performance while developing fabrication strategies that will enable for scalable industrial manufacture of devices potentially capable of delivering 30% solar energy conversion efficiency. The principal investigators will also use the research outcomes to enhance instructional materials in photovoltaics course offerings at Yale University and Arizona State University.