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Material and Optical Design Rules for High Performance Luminescent Solar Concentrators

Noah Bronstein

 

Dr. Noah Bronstein

Advisor: Professor Paul Alivisatos, Department of Chemistry

This dissertation highlights a path to achieve high photovoltaic conversion efficiency in luminescent solar concentrators, devices which absorb sunlight with a luminescent dye and then re-emit it into a waveguide where it is ultimately collected by a photovoltaic cell. Luminescent concentrators have been studied for more than three decades as potential low-cost but not high efficiency photovoltaics. Astute application of the black body radiation law indicates that photonic design is necessary to achieve high efficiency: a reflective filter must be used to trap luminescence at all angles while allowing higher energy photons to pass through. In addition, recent advances in the synthesis of colloidal nanomaterials have created the possibility for lumophores with broad adsoption spectra, narrow-bandwidth emission, high luminescence quantum yield, tunable Stokes shifts and tunable Stokes ratios. Together , these factors allow luminescent solar concentrators to achieve the optical characteristics necessary for high efficiency. The first generation of these devices was fabricated and tested. The devices achieved the highest luminescent concentration factors yet recorded in literature while maintaining high photon collection efficiency. 

This dissertation highlights a path to achieve high photovoltaic conversion efficiency in luminescent solar concentrators, devices which absorb sunlight with a luminescent dye and then re-emit it into a waveguide where it is ultimately collected by a photovoltaic cell. Luminescent concentrators have been studied for more than three decades as potential low-cost but not high efficiency photovoltaics. Astute application of the blackbody radiation law indicates that photonic design is necessary to achieve high efficiency: a reflective filter must be used to trap luminescence at all angles while allowing higher energy photons to pass through. In addition, recent advances in the synthesis of colloidal nanomaterials have created the possibility for lumophores with broad absorption spectra, narrow-bandwidth emission, high luminescence quantum yield, tunable Stokes shifts and tunable Stokes ratios. Together, these factors allow luminescent solar concentrators to achieve the optical characteristics necessary for high efficiency. The first generation of these devices was fabricated and tested. The devices achieved the highest luminescent concentration factors yet recorded in literature while maintaining high photon collection efficiency.

Further, luminescent concentrators with dielectric reflectors are explored theoretically as the high-bandgap top-junctions in two-junction devices. Simple thermodynamic calculations indicate that this approach can be nearly as good as a traditional vertically stacked tandem. The major barriers to such a device are the optical design of narrow-bandwidth, angle-insensitive reflectors with near-unity reflectivity in the reflection band and near unity transmissivity in the pass-band. Additionally, lumophores with narrow emission line widths and carefully controlled Stokes shifts are required. If new lumophores and optical designs can be created that meet the demanding needs of this application, high performance two-junction photovoltaics that collect both the direct and diffuse light could be achieved. 

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