Chen, Eric Yilluen2019-05-082019-05-082018http://udspace.udel.edu/handle/19716/24143To exceed current solar cell efficiency limits while maintaining low costs, new material paradigms must be explored, and their properties understood. In this dissertation, I introduce a photon upconversion platform using semiconductor nanocrystals, i.e. quantum dots (QDs), with inherently broadband absorption and high luminescence efficiencies. An upconverter placed behind a host solar cell allows the host cell to exceed the efficiency limit of a traditional single-junction solar cell due to the upconversion of low-energy light to high-energy light that is then used by the solar cell. To calculate the net solar conversion efficiency achievable by a solar cell backed by a semiconductor upconverter nanostructure, I first create a kinetic rate model and calculate the maximum internal upconversion quantum efficiency (iUQE). Considering real upconverter material properties and performance metrics, I calculate that up to 96% iUQE is achievable, with up to 39% solar cell efficiency with a GaAs host cell. Next, colloidal coupled-QD CdSe(Te)/CdS/CdSe core/rod/emitter upconverter nanostructures were synthesized, closely implementing the modeled structure. I compare the upconverted light using below-emitter-bandgap excitation to traditional, above-emitter-bandgap photoluminescence (PL) to confirm that the upconverted light originates in the emitter QD. Under single-wavelength cw illumination, I demonstrate the first instance of visible upconversion PL (UCPL) in semiconductor upconverters with as low as 100 mW/cm^2 (1-sun) cw laser power. To determine whether the quadratic power-dependence of UCPL is due to Auger upconversion or sequential upconversion, I combine the single-color, sub-emitter-bandgap cw beam with a longer-wavelength 980nm cw beam (which does not itself induce UCPL) and observe an increase in UCPL relative to the UCPL generated from single-wavelength excitation. To improve upconverter performance, CdSe(Te)/CdS(1-x)Se(x)/CdSe alloyed core/rod/emitter nanostructures were synthesized. I demonstrate that alloying the CdS(1-x)Se(x) rod of the full structure creates more charge separation (i.e. funneling to the emitter) compared to that of uniform CdS rods as well as spherical nanostructures using time-resolved PL spectroscopy. I measure an iUQE of 0.002% in the graded rod structures, double that of the flat rod structures, suggesting carriers more efficiently recombine in the emitter via a graded rod. To ensure that the higher iUQE is not purely due to improved PL quantum yield (PLQY) of the emitter, I normalize the iUQE to the measured PLQY and calculate a ratio of 6%, which is nearly two orders of magnitude higher than that of the flat rod structure. The results show that engineering a tunable, low-dimensional material system with heterostructure device engineering techniques guided by understanding of carrier transfer and relaxation kinetics provides a pathway for creating novel, high-efficiency, energy generation devices.Thesis1100472231https://doi.org/10.58088/7pkz-tq702019-02-14en