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Maximizing the Efficiency of Luminescent Solar Concentrators by Implanting Resonant Plasmonic Nanostructures (SOLAR-PLUS)

Final Report Summary - SOLAR-PLUS (Maximizing the Efficiency of Luminescent Solar Concentrators by Implanting Resonant Plasmonic Nanostructures (SOLAR-PLUS))

Solar cell system prices have experienced a strong fall since 2008 due to a decrease in the price of pure silicon and a maturing technology. Still, for photovoltaics to become economic without any government subsidies further cost reductions have to be achieved. One potential solution is the luminescent solar concentrator (LSC), the subject of this project, which offers an encouraging means to include solar energy to the built environment; LSCs concentrate sunlight without the need for expensive tracking equipment and their design makes them suitable as windows. LSCs are composed of a transparent matrix material, generally a slab of polymer which is doped with fluorophores to absorb the incoming sunlight. Commonly used fluorophores include organic dyes, quantum dots and rare earths. Depending on the quantum yield of the fluorophore, either the photon is emitted at a longer wavelength or the absorbed energy is dissipated to heat. If emitted, the photon experiences a wavelength shift (Stokes shift) and, given the emission falls outside of the escape cone, is trapped through total internal reflection within the host matrix. Light that is guided towards the sides of the slab is converted into electricity by solar cells. Contingent on the design only one or multiple side facets of the waveguide can be covered by solar cells. The key advantages of LSCs compared to other concentrating solar power devices
- No expensive tracking device is required
- Direct as well as diffuse sunlight is absorbed by the fluorophores which is particularly interesting for the urban environment and countries with a lot of
- There are various options to integrate LSCs into the built environment
- Materials used to create LSCs are inexpensive compared to solar cells which can yield a lower cost per unit of power
- The waveguide can be flexible which increases the number of potential applications

Current LSC designs suffer though from low efficiency, which causes them to be uneconomic. The work in SOLAR-PLUS makes several advances in the field of luminescent concentrators with ultimate target to tackle the problem of low efficiency by combining theoretical, modeling, experimental and fabrication routes.
During the period of the grant the following advances were made in the field of LSCs;
- First, a novel experimental method was proposed to separate the optical efficiency and the most dominant loss channels in LSCs, escape cone and non-unity quantum yield losses. This was the first method ever to be devised to determine all three LSC metrics and provides the community of researcher with a very strong experimental tool to assess the performance of their designs.

- Secondly, a hybrid model was developed, for the first time, that couples a nanoscale simulation method (finite-difference time-domain) with Monte-Carlo ray tracing. This method paved the way for the simulation of large scale LSC systems which contain nanostructures.
Among other systems, the method was used to determine the potential of plasmonic LSCs. It is shown that metallic absorption causes substantial optical losses which limits the applicability of plasmonics for LSCs.

- Thirdly, fluorophore alignment and Forster resonance energy transfer (FRET) was investigated theoretically and experimentally. Ray tracing was again the method of choice to simulate a LSC with aligned and linked fluorophores. Homeotropic alignment improved the trapping efficiency while the linking induced FRET between the fluorophores to circumvent the reduced absorption of homeotropic alignment. As was shown in, the efficiency of a LSC can be strongly enhanced by both, alignment and FRET; also, the quantum yield of the donor in the FRET pair is not as vital due to the e ciency of FRET. This makes quantum dots very suitable as donors due to their spectrally wide absorption. As a proof-of-concept an LSC was fabricated with quantum dots linked to organic dye molecules. The optical efficiency of the LSC was strongly enhanced due to reduced non-unity quantum yield losses.

- Finally, a flexible LSC made of polydimethylsiloxane (commercial silicon) was fabricated to increase the potential applications of LSCs. Crucially, the LSC device remains very efficient when bent. Such flexible LSCs can find application to power up wearable electronic devices and electronics integrated in outdoor equipment like tents and rucksacks among others.

Overall, the project has made contributions in both understanding the physical phenomena underlying the operation of LSCs as well as proposing practical solutions that can result in highly efficient and eventually commercial devices. A follow up project should focus on scaling up the dimensions of the prototypes built during SOLAR-PLUS and measure their performance under real environmental conditions. This would pave the way for eventual commercialisation of LSCs.