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Fig. 4: InP nanowire array solar cell concept based on resonant light-trapping in 180-nm diameter nanowires. the nanowires were only covering 12% of the cell’s surface. The Swedish lab also demonstrated that with these optically optimized structures, the share of sunlight converted into a photocurrent was about six times the limit in a single ray optics description (the light being optically trapped into the nanostructures). Samuelson then unveiled a new process to grow these nanowires, which he said could bring low-cost volume manufacture into the equation. Aerotaxy, as he named it, consists in flowing gold aerosol seed particles through a furnace containing a mixture of precursor gases (trimethylgallium TMGa and arsine AsH3). This continuous gas-phase synthesis boasts a very fast growth rate of around 1μm/s for structurally and optically tunable nanowire diodes. With this process, Samuelson estimates that the nanowires could be produced at less than 5% the cost of nanowires epitaxially grown on GaAs, and under a quarter of what it would cost to build those on silicon. Using these nanowires in a “carrier ink”, those could be spread and aligned vertically on a micro-patterned surface which would then be pulled off as a thin film. With only 10 to 12% of nanowire surface coverage, 95 to 100% of the light is absorbed claims the professor whose roadmap for the future is to achieve between 20 and 25% of light conversion efficiency with dual bandgap III-V nanowires embedded in flexible thin film. One way to limit light losses in standard solar cells is to rely on light scattering inside the cells, as Dr. Nicholas Hylton explained in his presentation about broadband absorption enhancements. A research associate in the experimental solid state group of the Blackett Laboratory at Imperial College, London, Hylton experimented with aluminium nanoparticle arrays to perform surface-based nanostructural light trapping and increase absorption efficiency – see figure 5. The lab grew thin GaAs photodiodes to probe the effects of scattering/absorption, with gold, silver and aluminium nanoparticles with a diameter of 100nm and distributed at various pitches. Over the 400 to 900nm frequency range, the results recently published in the Scientific Reports under the title: Loss mitigation in plasmonic solar cells (Sci. Rep. 3, 2874) show an integrated efficiency enhancement of 6 and 22 % when using Al nanoparticle arrays of 200nm and 400nm pitches, respectively, whereas the overall external quantum efficiency was reduced when using Au or Ag nanoparticles of the same size (due to the excitation of localised plasmon resonances, and interband transitions leading to energy absorption in Au). Another highlight of the conference were on the advances made by Empa (the Swiss Federal Laboratories for Materials Science and Technology) on Copper indium gallium (di)selenide (CIGS) for thin film solar cells (using a liquid deposition process on flexible substrates). The lab reported a record efficiency of 20.4% for its foil-based solar cells and aims to reach 25% in the medium term. Photovoltaics under attack At a round table that followed the scientific opening, Claude Turmes, Member of the European Parliament, Vice-Chair of the Group of the Greens/EFA and Member of the Committee on Industry, Research and Energy emphasized the need for European countries to cooperate on research and development, to better synchronise the energy production mix across the different member states. One idea put on the table was to standardize PV module sizes to further reduce production costs across all European countries. Turmes highlighted that Europe’s commitment to have 20% of its energy coming from renewable sources by 2020 would mean more PV capacity and decreasing PV costs to match gridparity, though he noted that the current energy market had to be changed to better reflect the costs of pollution and the negative impact of non-renewable energies on the environment. “Our challenge is to change the energy market so photovoltaics can fly, not how to integrate PV into a rotten energy market” he said. “Photovoltaics are becoming successful enough to provide electricity at a cost that threatens the oleogarchies” said Turmes, adding that the biggest obstacle to PV is the market domination by a handful of power companies and their lobbies. “The benefits of photovoltaics for the democratization of energy are so compelling that nobody will be able to stop this energy revolution, we just need to create a societal majority” he added. According to Turmes, the increase of renewables will come as a bottom-up societal revolution, not from large companies centrally managing complex power plants and defending their selfish interests. As millions of consumers decide to become prosumers, they will also become decision partners on energy management and distribution and will accelerate the societal change required for renewables to grow. The costs of PV modules is less than 30% of the total cost of photovoltaic power, the reminder being split between the costs of grid-connection and financing. Hence, research is also needed to store energy efficiently so as to get rid of grid infrastructure restrictions. In the future, traditional power plants will have to change their business model, only to provide energy reserves in case of renewable energy shortages. “We must define a new grid not just for high voltages, but also medium and low voltages, capable of better managing the mix of local energy generation and autonomy, encouraging partnerships between consumers and producers”, said Dominique Ristori, General Director for the Joint Research Centre of the European Commission. “Turning customers from passive to active prosumers should be profitable to the whole society” he concluded. Fig. 5: Concept of nanostructural light trapping using nanoparticles on top of regular cells. 8 Electronic Engineering Times Europe November 2013 www.electronics-eetimes.com


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