photovoltaics,” says Professor Patrick Görrn of the University of Wuppertal. “Concentrated sunlight can be efficiently divided up into different wavelengths. This would allow for effective bandgap matching, in which different wavelengths of the sunlight are guided to solar cells optimised for that wavelength. The theoretical efficiency limit of such a device would be increased from 34 per cent to 86 per cent.” To make these devices a reality, planar


collectors that both efficiently collect the light hitting them and transport it over large planar distance are needed. However, efforts to achieve this have revealed new challenges. “Essentially, there is a trade- off between efficient collection of sunlight and guiding it over large distances with low optical loss,” says Görrn. “At present, the most studied approach to getting around this has been to use luminescent solar concentrators (LSCs). However, work on these has been going on for 30 years and they are still a long way off being practically useful.”


“Once we can show that we are quickly achieving high excitation and propagation, then we can start to get


excited that we have found something that can really change the world of solar power”


Unlike LSCs, which use energetic


separation to get around the trade-off between efficiently collecting sunlight and efficiently guiding it, Görrn’s idea uses spatial separation. The planarity (flatness) of the concentrator is broken using structures that are separated from the guided modes, thus allowing for both efficient


concentration and propagation.


Waveguides have discreet modes, some of which have nodes that exhibit “blindspots” for the light inside the waveguide. When scattering bodies at the point of the node excite a wave laterally,


the wave doesn’t


“see” the scattering bodies and so propagation is not affected (see figure 1). The project, which began in March 2015,


“In just six hours,


the world’s deserts receive more energy from the sun than humankind


consumes in a year” Görrn is now suggesting an entirely new


approach to solar concentration. “Our idea uses a planar concentrator which has a new type of planar waveguide.” says Görrn. “Ideally, our device will take the form of a black plastic foil that that concentrates the solar energy to a point at which the resulting immense light intensity can be used for efficient small-area solar cells or solar-to- fuel reactions. We hope to eventually be able to produce this foil for under €1/m2. Unlike CSP, the device will work with light shining at any angle and so will remove the need for expensive moving parts.” The proposed device could theoretically


concentrate incident solar power to a concentration factor of more than 104. “This level of light concentration would be a game-changer,”


has seen exciting developments in recent months. “We started with collection efficiencies of 10-8 (meaning 10-6 per cent of the light is excited laterally). We have now reached 10 per cent, propagating over a distance of 8mm. These are good signs that what we thought we could do is achievable.” The film used to excite the waves laterally


is made up of silver nanoparticles, which have the strongest interaction with light of any particle known today, due to plasmonic interactions. “This film — which appears to the eye as being extremely black — is then placed in the middle of the waveguide at the point of the node. The film is special because it is able to excite efficiently but is also thin enough to fit within the node.” Part of


the work has been to find the says Görrn. “These


devices could be created cost efficiently and used in households for passive lighting and even passive heat transfer.”


www.projectsmagazine.eu.com


optimal material to make the waveguides (which should be as transparent as possible). This has produced some unexpected and welcome findings. “Our original plan was to use dielectrics produced by atomic layer deposition – an expensive and lengthy process. In fact, what we have found is that one of the best materials for making the waveguides is actually a commercially produced polymer. So not only have we managed to make the device more efficient, we have also managed to make it more cost efficient.” Research now continues with trying to optimise the excitation efficiency, which is


what Görrn believes will be the crux of their work. “Once we can show that we are quickly achieving high excitation and propagation, then we can start to get excited that we have found something that can really change the world of solar power.”


★ AT A GLANCE Project Information


Project Title: HyMoCo- Hybrid Node Modes for Highly Efficient Light Concentrators


Project Objective: Luminescent solar concentrators have been developed for over thirty years, but due to waveguide losses their maximum size is still limited to a few centimetres. HyMoCo suggests the exploitation of novel waveguides in order to realize passive planar concentrators of unsurpassed collection efficiency, size, concentration, lifetime and costs.


Project Duration and Timing: 03/15 – 01/20


Project Funding: ERC Starting Grant – €1.485m


MAIN CONTACT


Patrick Görrn Patrick Görrn is professor and holds the Chair of Large Area Optoelectronics at the University of Wuppertal. During 2009-2011 he was a Feodor Lynen Research Fellow of the Alexander von Humboldt Foundation at Princeton University. In 2008 he received his PhD in Electrical Engineering from the University of Braunschweig.


Contact: Tel: +49-202-439-1424 Email: goerrn@uni-wuppertal.de Web: www.lgoe.uni-wuppertal.de


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