An upside-down cake throws a new light on photovoltaics

Do better with less. That is the challenge the researchers from EPFL have set for themselves. Their specialty: manufacturing solar cells that are one thousand times thinner than conventional cells. In order to boost the output of the cells, they have developed a new nanopatterning technique. Even though silicon is one of the most abundant elements, the energy required to make silicon from sand is immense. It is for this reason, but also to reduce manufacturing costs, that Professor Christophe Ballif and his team from the Photovoltaics and Thin-Film Electronics Laboratory at the EPFL have been working for several years on thin-film silicon solar cells that are a thousand times thinner than conventional cells. There's just one catch: the thinner the cells, the less they absorb the rays of the sun and the less electricity they produce. So researchers are trying to trap light in the thin silicon layers to increase their absorption. Traditionally, thin layers of zinc oxide—a material that is very abundant, completely non-toxic, and that grows in the form of small pyramid-shaped crystals—are used for this purpose. These crystals scatter light efficiently into the underlying silicon layer. With such zinc oxide layers, even a new world record cell efficiency was achieved. Layers of zinc oxide seen through an electronic microscope, on the left: natural pyramid structure, on the right: structure when grown on a mould (height of images 5 microns). Reducing costs