There is no cheaper way to generate electricity today than with the sun. Power stations are being built in sunny locations that will provide solar power for less than two cents per kilowatt hour. The commercially available solar cells based on crystalline silicon make this possible with efficiencies of up to 23%. Therefore, they have a global market share of around 95%. With even higher yields of over 26%, costs could come down further. An international working group led by photovoltaic researchers from Forschungszentrum Jülich now plans to achieve this goal with a nanostructured and transparent material for the solar cell front and a sophisticated design. Scientists report their success from many years of research in the scientific journal Energy of nature.
Silicon solar cells have been steadily improved over the past decades and have already reached a very high level of development. However, the disruptive effect of recombination always occurs after absorption of sunlight and photovoltaic generation of electric charge carriers. In this process, the negative and positive charge carriers that have already been generated combine and cancel out before they can be used for the flow of solar electricity. This effect can be countered by special materials that have a special property – passivation.
“Our nanostructured layers provide precisely this desired passivation,” says Malte Köhler, former doctoral student and first author at the Jülich Institute for Energy and Climate Research (IEK-5), who has since obtained his doctorate. In addition, the ultra-thin layers are transparent – so that the incidence of light is hardly reduced – and exhibit high electrical conductivity.
“No other approach to date combines these three properties – passivation, transparency, conductivity – along with our new design,” says Dr Kaining Ding, head of the Jülich working group. A first prototype of the Jülich TPC solar cell achieved a high efficiency of 23.99% (+ – 0.29%) in the laboratory. This value was also confirmed by the independent CalTeC laboratory of the Solar Energy Research Institute in Hamelin (ISFH). This means that the Jülich TPC solar cell still ranks slightly below the best crystalline silicon cells made in laboratories to date. But simulations performed in parallel have shown that efficiencies of over 26% are possible with TPC technology.
“In addition, we only used manufacturing processes that can be integrated relatively quickly into series production,” Ding emphasizes compared to other research approaches. With this strategy, the Jülich scientists pave the way for their development from the laboratory on a large scale into the industrial production of solar cells without much effort.
Several process steps were required to produce the layers for the TPC solar cell. On a thin layer of silicon dioxide, the researchers deposited a double layer of tiny pyramid-shaped silicon carbide nanocrystals – applied at two different temperatures. Finally, a transparent layer of indium tin oxide followed. Ding and his colleagues used wet chemical processes, chemical vapor deposition (CVD), and a spray process.
For their success, researchers Jülich from IEK 5 and the Jülich Ernst Ruska Center for Electron Microscopy worked closely with several institutes in the Netherlands, China, Russia and Ecuador. Partners include researchers from RWTH Aachen University, University of Duisburg-Essen, Technical Universities of Delft and Eindhoven, University of San Francisco de Quito, University and Institute of Kutateladze thermophysics in Novosibirsk and Sun Yat-Sen University in Guangzhou. In other steps, the Kaining Ding research group plans to further optimize the energy efficiency of its TPC solar cells. “We expect solar cell manufacturers to show great interest in our technology,” says Ding.
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