Efficient Photon Upconversion in Semiconductor Nanostructures: Constraints and Opportunities

Photon upconversion is a promising approach to realizing photovoltaics with efficiency beyond the Shockley-Quessier limit by harvesting low-energy photons and converting them to high-energy photons that can be absorbed by a host single-junction solar cell. Existing upconversion materials have limited potential benefit for solar energy harvesting applications because of their narrow absorption bandwidth and low quantum efficiency. We first present numerical simulations of semiconductor nanostructures designed to overcome these limitations. The computational results demonstrate the potential impact of such upconversion materials and identify critical material parameters that must be achieved for practical realization of upconverters that can have a meaningful impact on solar energy harvesting. We then present experimental progress toward realizing this semiconductor upconversion paradigm. We describe the choice of material composition and structure, growth of the nanostructures, and ultrafast optical characterization of the carrier dynamics that result in these structures. We show that upconversion can be achieved in at least one realization of this semiconductor upconversion paradigm. We conclude with a discussion about the prospects for realizing efficient photon upconversion and the constraints that must be addressed to meet this goal.