Design of high-performance luminescent solar concentrators based on aggregation-induced emitters in organic-inorganic hybrid waveguides
Solar-harvesting systems that have the potential to be integrated into the urban built environment have attracted significant interest. However, traditional solar panels are not well-suited for many modern architectures, due to their bulky and rigid structure, and reduced performance in diffuse sunlight conditions. Luminescent solar concentrators (LSCs) provide a realistic solution to such limitations. LSCs are solar-harvesting devices fabricated using a transparent waveguide that is either doped or coated with luminescent species (lumophores). They can collect large areas of solar radiation and spectrally convert it to more useful energy via the process of photoluminescence (PL) of the embedded lumophores. The PL emission is then redirected to the edges of the LSC where strips of solar cells are mounted to convert the light energy into electricity.
LSCs work well under both diffuse and direct sunlight conditions, making them particularly desirable for use in built environment and regions with high cloud coverage. In addition, they are often available in a variety of colours and geometries, ideal for architectural design. Nevertheless, the optical performance of an LSC is often undermined by the effect of aggregation-caused quenching (ACQ), which occurs at high lumophore concentrations in solid waveguides. ACQ can be potentially overcome by using lumophores that exhibit aggregation-induced emission (i.e., AIEgens). The emission of AIEgens becomes enhanced, rather than quenched, in aggregated states due to restricted intramolecular rotation (RIM).
This thesis aims to investigate the use of AIEgens in waveguides that are made using a family organic-inorganic hybrid materials, also known as ureasils to fabricate LSCs with improved efficiencies. In Chapter 3, the first part of this work, a green-emitting conjugated polymer (CP) with AIE characteristics was used in a di-branched ureasil (di-ureasil) at various concentrations. No ACQ was observed for the AIE-active CP at high concentrations and by mixing it with a red-emitting perylene dye at optimised concentration ratio, effective Fӧrster energy transfer (FRET) was observed between the two lumophores. Due to the extended light harvesting window and reduced reabsorption loss provided by the dual-lumophore system, the resulting FRET-based LSC showed a high internal photon efficiency of 20%.
In Chapter 4, a silole-based AIEgen, was adopted as the lumophore in a di-ureasil waveguide. Two methods of incorporation were investigated, covalent grafting and physical dispersion, at different concentrations. Both methods of incorporation led to the AIE behaviour of the lumophore in the di-ureasil host and no ACQ occurred even at the highest doping concentration of 1.4 mM. Compared to physical dispersion, the AIEgen that is covalently grafted to the hybrid matrix showed improved dispersity that reduced scattering losses and enhanced RIM that led to higher photoluminescence quantum yields (PLQY). Moreover, the occurrence of FRET from the photo-active ureasil to the silole AIEgen was confirmed with time-resolved PL measurements as shown by the reduction in the lifetime of the ureasil donor. This synergetic interaction between the host and lumophore may improve the absorption efficiency and hence the overall performance of the corresponding LSC device.
In Chapter 5, the same silole-based AIEgen was employed as the lumophore doped in ureasil matrices with different organic backbones via covalent grafting. The effect of the organic structure of the ureasil matrix on the photophysical properties of the resulting AIEgen-ureasil material was investigated and the chain length of the polymer backbone was found to be the major factor. A shorter organic chain leads to a higher density of emitters in the siliceous and urea domain, and hence higher PLQY of the photo-active hybrid host. The FRET efficiency was also shown to be the highest between the ureasil with the shortest organic chain and the AIEgen. The AIEgen-ureasil system with optimised organic structure and concentration was fabricated then into a prototype LSC device. This was further mixed with a red-emitting perylene to create another FRET system, in order to shift the emission of the AIEgen further to the red region and increase the overall solar-harvesting range of the resulting LSC. The final AIEgen-based FRET LSC exhibited external and internal photon efficiencies of 9.0% and 29.3%, respectively, which are comparable to some of the most recent LSC designs reported in the literature.
In summary, this work demonstrates a viable approach to mitigating the optical losses, including ACQ and reabsorption, for developing a new generation of LSCs with high optical performance. Moreover, the facile and versatile nature of the sol-gel process used to fabricate the hybrid material offers the possibility of designing LSCs with tunable optical and physical properties. Such results have undoubtably shown the great potential of AIE-active lumophores, in combination with ureasil hybrid waveguide, to advance the development of solar-harvesting systems that can be eventually integrated into the urban built environment.