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Wearables: A boost for flexible solar cells


2020-10-24... / / Mobile,Electronics
Wearables: A boost for flexible solar cells image

Luminescent solar concentrators are sheets that collect light at various wavelengths and re-emit it at a wavelength better matched to a solar cell. They can be created in various geometries – for example a flat sheet can be arranged to collect light over its large surface, and then re-emit it from one edge to a small solar cell – effectively giving that cell a larger collecting surface, according to Swiss laboratory Empa. Now Empa as worked with fellow lab ETH Zurich to develop a flexible material that works as a luminescent solar concentrator and can even be applied to textiles – with the aim of improving the efficiency of wearable solar cells, perhaps printed onto clothes. In this case, the concentrator is a flat sheet that fits flat on the surface of the cell. “By means of a new polymer that is applied on textile fibres, jackets, T-shirts and the like could soon function as solar collectors and thus as a mobile energy supply,” according to Empa. This new material is based on amphiphilic polymer co-networks (APCN) – a class of material already used in silicone-hydrogel contact lenses. APCNs consist of two polymers: one hydrophobic and one hydrophylic. As such, they don’t want to mix, but chosen carefully the two can be made to covalently cross-linked, resulting in a self-assembling patchwork of the two polymers. In this case, the polymers form nano-scale domains, and the total interface area between two polymers is consequently huge. If two different luminescent materials are introduced into this arrangement, one hydrophobic and one hydrophilic, these will naturally gravitate to one polymer or the other, forming a matrix where the two luminescent materials face each other, only nanometres apart, across the many small interfaces that total a large area in a small space. There is a non-radiative energy transfer mechanism called Forster resonance energy transfer (FRET), that only works across narrow (sub-wavelength) gaps. empa-flexible-solar-concentratorTwo luminescent materials (red and green) in an APCN form a material that is flexible, vapour permeable and acts as a solar concentrator for photovoltaics The APCN structure sets up good conditions for this, so if the two luminescent materials above are further chosen so that one is a donor and the other an acceptor in a FRET pairing (red and green dots in the artists impression right), then energy will couple well from one to the other. The overall effect is that “these pairs can expand the absorption range of an acceptor to the solar spectrum and increase the optical efficiency under solar irradiation,” according to to the ScienceDirect paper ‘Nano-domains assisted energy transfer in amphiphilic polymer conetworks for wearable luminescent solar concentrators’. Fluorescein (donor) and Lumogen Red 305 (acceptor) were the chosen luminescent materials, and a 200μm layer of the doped APCN placed over a solar cell pushed energy absorption up by between 8% and 24% across 550nm to 375nm. One last characteristic of this APCN is that it is permeable to water vapour. “The luminescent materials capture a much wider spectrum of light than is possible with conventional photovoltaics,” according to Empa. “The novel solar concentrators can be applied to textile fibres without the textile becoming brittle and susceptible to cracking or accumulating water vapour in the form of sweat.” Nano-domains assisted energy transfer in amphiphilic polymer conetworks for wearable luminescent solar concentrators is available in full without payment.



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