Donor-emitter fluorophore pairs in luminescent solar concentrators: from material synthesis to device fabrication
AffiliationSchool of Chemistry
Document TypePhD thesis
Access StatusOpen Access
© 2019 Bolong Zhang
A luminescent solar concentrator (LSC) is a type of light harvesting device, showing potential as an alternative to the traditional photovoltaics (PV). A typical LSC consists of a planar waveguide system embedded with fluorophores, which absorb light incident on the surface and confines the emission to the edges. As the surface area of the LSC is much bigger than the edge area, the incident light can be concentrated. A PV cell attached to the edge will convert the output light into electricity. Although there are many advantages of LSCs, the unsatisfactory efficiency of LSCs still limits their wide applications. There are four major energy loss pathways of LSCs, which are 1. the photoluminescence quantum efficiency (PLQY) loss, 2. the escape cone loss, 3. the re-absorption effect and 4. the transmittance loss. The donor-emitter fluorophore pair system can potentially improve the performance of LSCs via multiple aspects, in particular, reducing the re-absorption effect. The donor-emitter fluorophore pair is a biomimetic system inspired by the light-harvesting antenna from the natural photosynthesis where a light absorbing donor harvests the incident light and transfers the energy to the acceptor (or emitter), via mainly the Forster resonance energy transfer (FRET) process. By carefully tuning the concentration ratio of the donor and the emitter, one can achieve a fluorophore pair system that is mainly comprised of the donor’s absorption spectrum and the emitter’s emission spectrum. Consequently, the spectral overlap between the absorption and emission spectra of the fluorophore pair can be minimized, leading to a reduction of the re-absorption effect. By using specially engineered donor-emitter pairs one can reduce not only the re-absorption effect but also the escape cone loss and the PLQY loss. To build a highly efficient donor-emitter fluorophore pair system, the concentration of both the donor and the emitter are required to be high enough to fulfill the requirement of the FRET process. However, this may lead to the aggregation caused quenching (ACQ) of the fluorophores, resulting in reduced PLQY. Aggregation induced emission (AIE) is one approach to neutralize the ACQ in the donor-emitter fluorophore pair system. The AIE effect allows the PLQY of fluorophores to remain at a reasonable level in high concentration or in solid state. By replacing the common fluorophores by AIE type of fluorophores in the donor-emitter fluorophore pair system, one can expect a better overall PLQY of the fluorophore pair in the required concentration. The photophysics of an AIE donor-emitter pair was characterized in this thesis and the performance in an LSC evaluated. The other approach to avoid the ACQ effect is to stop the intermolecular interaction of the fluorophores by installing some bulky substituents to keep the fluorophores apart in aggregates. Perylene diimide (PDI) derivatives are a class of molecules that show excellent photophysical properties, but often affected by the ACQ effect. When the bulky substituents are added through the imide position on PDI, the resulting molecule should show a better tolerance to the ACQ effect and leave the photophysical properties untouched. In the thesis, a series of PDI donors with bulky substituents were designed and synthesized and paired with a PDI emitter. The performance of a LSC based on the PDI donor-emitter showed considerable improvement due to the increased PLQY and the reduced re-absorption. Inspired by the success of the PDI donor-emitter pair, a large area LSC device was fabricated based on the same materials. To ensure efficient FRET, the fluorophore concentration of a donor-emitter pair is required to be high. Therefore, a thin layer of the fluorophore coated on a transparent substrate was the chosen device structure for the large-area LSC. Doctor blading thin film deposition was applied to print the dye layer on the surface of the substrate. Two different types of substrate materials and three types of solar cells were investigated to reveal the effects on LSC performance. By optimizing the substrate materials and the attached solar cells, the performance of the resulting large-area LSC was among the best LSCs reported in the literature. The escape cone loss is the last major energy loss pathway in LSCs, which can be also eliminated by using donor-emitter fluorophore pair system. The escape cone loss can be minimized by aligning the transition-dipole of the emitter perpendicular to the surface of the waveguide. However, the absorbance of the LSC with the aligned fluorophore will be reduced, because the transition dipole of the fluorophore is then parallel to the direction of the incident light. One way to avoid the absorption reduction in the emitter-aligned LSC is to introduce a donor fluorophore that is isotropically oriented. The isotropic donors will harvest incident light as normal, but the energy will be transported to the perpendicularly aligned emitter. The emitters then emit light in parallel to the surface of the waveguide, leading to a reduced escape cone loss. A prototype LSC was prepared based on the selectively aligned donor-emitter pair and fully characterized in the thesis.
Keywordsluminescent solar concentrator; light harvesting; energy transfer; perylene diimide; diphenyl anthracene; aggregation induced emission
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