Spectral downshifting by molecular aggregates in planar concentrators for solar energy conversion

© 2016 Dr. James Earl Patrick L. BanalThe luminescent solar concentrator (LSC) is a planar concentrator that is composed of a large sheet of waveguide with embedded dyes to harvest light. The planar configuration of the LSC alleviates the necessity for constant solar tracking and enables integration into an urban environment, which are attractive features in realizing net zero energy infrastructures. However, the efficiencies of current LSCs are still below expectations for commercial viability. Reabsorption of the waveguided photons in an LSC is a key loss mechanism attenuating the light output of the LSC. Concentration quenching limits the amount of chromophores that can be added in the waveguide thus limiting the light absorbed by the LSC. Current endeavors to maximize the performance of LSCs have been mainly focused on increasing the Stokes shift of chromophores while efforts to reduce concentration quenching are rare in literature. Aggregates of chromophores that exhibit aggregation-induced emission (AIE) have markedly different photophysical behavior compared to isolated chromophores. The twisted structures of AIE chromophores, as evidenced by their crystal structures, reduced fluorescence quantum efficiency losses of the AIE chromophores even as pure thin-films. The presence of rotating phenyl rings, a molecular feature common in tetraphenylethene and its extended derivatives investigated in this thesis, imparts the AIE photophysical behavior. The investigated AIE chromophores were found to exhibit Stokes shifts exceeding 1 eV. Monte Carlo ray tracing simulations suggest that LSCs comprised of AIE chromophores, particularly tetraphenylethene, show low reabsorption losses at large LSC dimensions leading to high concentration factors compared to highly fluorescent quantum dots. The absorption range of tetraphenylethene in the UV region is limiting towards total light harvesting of the entire terrestrial solar spectrum but still useful in specialized applications, such as transparent photovoltaic devices or visible-blind photodetectors. Pyrene derivatives of tetraphenylethene, gem-pyrene ethenes, were synthesized and characterized with the aim to extend the absorption of the diphenylethene core to the entire UV region while maintaining a wavelength cutoff at 400 nm and shift the emission energy of the chromophore towards the wavelength region where the external quantum efficiency of a photovoltaic device is high. Regioisomerism in pyrene attachment onto the diphenylethene structure resulted to profound differences in the photophysical properties of the gem-pyrene ethenes. The gem-pyrene ethenes were measured to have a large Stokes shift > 1 eV in the aggregated state with one of the gem-pyrene ethenes having a quantum yield in excess of 50% even at 50% w/w in PMMA. A transparent solar concentrator was fabricated and characterized using one of the gem-pyrene ethenes with power conversion efficiency comparable with the state-of-the-art phosphor-based transparent solar concentrator. The gem-pyrene ethene-based transparent solar concentrator was found to be stable in air over long periods in ambient conditions compared to the phosphor-based transparent solar concentrator, which can only operate under oxygen-free conditions. This was the first demonstration of an organic-based transparent solar concentrator. An AIE chromophore, DPATPAN, was used as the donor in an energy migration-trapping approach with the highly fluorescent red dye DCTJB as the energy trap to circumvent reabsorption due to its practically lossless quantum yield at high concentration values in PMMA. Fluorescence anisotropy measurements of DPATPAN revealed energy migration dynamics that may have contributed to the efficient energy transfer between DPATPAN and DCJTB. The LSCs using the DPATPAN-DCJTB pair showed low reabsorption loss while maintaining high film absorption. It was identified that separation between the absorption of the donor and emission of the acceptor is critical in maximizing the reabsorption reduction possible with the energy migration-trapping approach. While there was a spectral mismatch between the maximum absorp- tion of the DPATPAN-DCJTB LSC with peak solar flux, the current density derived from external quantum efficiency measurements still showed that the migration-trapping LSC is clearly superior in harvesting light. A novel molecularly insulated perylene diimide was then synthesized as an energy transfer donor in a migration-trapping LSC with perylene red/Lumogen® R 305 (LR305) as trap to match the maximum absorption of the LSC with the peak solar flux while maintaining adequate separation between the donor absorption and acceptor emission bandwidths. The quantum yield and fluorescence lifetime of the insulated perylene diimide was found to be constant up to 58 mM in PMMA without any indication of either H-aggregate or excimer species. The diffusion length of energy migration for the insulated perylene diimide was determined to be in excess of 200 Å, an order of magnitude higher than simple perylene diimides like perylene orange. Resulting LSC devices using the insulated perylene diimide donor-LR305 energy transfer pair resulted in efficiencies exceeding that of an LSC with only LR305, the state-of-the-art dye for dye-based LSCs. A low reabsorption loss was observed for this energy transfer LSC compared to the pure LR305-based LSC without sacrificing light absorption