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Narrow-bandwidth solar upconversion: Case studies of existing systems and generalized fundamental limits
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See supplementary material at http://dx.doi.org/10.1063/1.4796092
for (1) a discussion of the energy levels and optimization routine used to model the upconversion process, (2) calculation of currents in each cell, (3) a discussion of the relationship between upconverter relaxation energy and upconverter bandwidth, (4) a table of the spectral parameters used to model upconversion in the bimolecular and lanthanide nanoparticle case studies, and (5) a brief note on solar cell non-idealities. [Supplementary Material]
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Upconversion of sub-bandgap photons is a promising approach to exceed the Shockley-Queisser limit in solar technologies. Calculations have indicated that ideal, upconverter-enhanced cell efficiencies can exceed 44% for non-concentrated sunlight, but such improvements have yet to be observed experimentally. To explain this discrepancy, we develop a thermodynamic model of an upconverter-cell considering a highly realistic narrow-band, non-unity-quantum-yield upconverter. As expected, solar cell efficiencies increase with increasing upconverter bandwidth and quantum yield, with maximum efficiency enhancements found for near-infrared upconverter absorption bands. Our model indicates that existing bimolecular and lanthanide-based upconverters will not improve cell efficiencies more than 1%, consistent with recent experiments. However, our calculations show that these upconverters can significantly increase cell efficiencies from 28% to over 34% with improved quantum yield, despite their narrow bandwidths. Our results highlight the interplay of absorption and quantum yield in upconversion, and provide a platform for optimizing future solar upconverter designs.
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