The sun has enough power to supply the whole earth with energy. But as long as renewable energy is more expensive than energy produced by coal or nuclear plants, solar energy won’t be first choice. In Europe photovoltaic cells make only a vanishing small share of renewable energy sources.
Researchers in UK, Switzerland and Germany aim to lower the costs and increase the efficiency. The N2P project is coordinated by the Fraunhofer Institute for Material and Beam Technology in Dresden, Germany. Here researchers developed a process to enhance the absorption qualities of solar cells for an invisible yet important part of sunlight, infrared light. Conventional solar cells hardly make use of this wavelength. Most of it passes through the cell and is lost. By removing the nano structured surface of the wafer on the rear side of the solar cell, using a chemical etching process, it is turns into a “mirror” that reflects the infrared rays back into the cell.
As the light rays are scattered by the glass, they have a longer pathway through the silicon cell and thus generate more electrical current. So far the researchers were able to increase the efficiency by 30 percent if compared to the efficiency of standard thin film solar cells.
Researchers from the École Polytechnique Fédérale de Lausanne (EPFL) in Neuchâtel, Switzerland, are working on thin film solar cells. Thin film solar cells have on the one hand a lot of advantages: Producing them consumes less raw materials and energy than producing common solar cells. Additionally the time they need to pay off is shorter. On the other hand there is a drawback: currently their efficiency is about 40 percent lower than in conventional solar cells. Only seven percent of the sunlight can be exploited.
To maximize the light trapping effect they do the opposite: they roughen the glass surface of thin film solar cells. This is done to diffuse the light. When the light beam has a longer way through the cell it generates more electrons.
To roughen the upper surface Dr. Sylvain Nicolay from EPFL deposits a layer of crystals of a so called transparent conductive oxide onto the glass. “The larger the nano sized pyramids are, the higher is the diffusion”, he says. The efficiency of thin film solar cells is now improved from seven to ten percent.
The nano crystals Dr Nicolay uses were developed at the University of Salford in Manchester, UK. Until recently the nano crystals had to be imported from Japan and made the production of such solar cells very expensive. Now the scientists are testing the crystals they developed on their own. The aim is to produce them much cheaper and thus reduce the costs significantly.
Each single method of improving the solar cells can only make a small difference on their efficiency. But combining both, these nano-tuned solar cells will become considerably more competitive than the modules of the past.
(17 May 2011)
Corinna Luecke