Nanowires created from indium phosphide have shown potential for drastically improving solar cell efficiency and costs. Research on solar cell nanowires is on the rise globally. The dream figure had been 10 percent efficiency, but Magnus T. Borgström and colleagues at Lund University have reported 13.8 percent efficiency using an array of indium phosphide nanowires assembled on surfaces of 1 mm² that each house 4 million nanowires. The cell converted 71 percent of available sunlight to energy. Computer simulation of the absorption in five nanowires. The sunlight comes in from the top. The dark red areas, near the top, have the strongest absorption, while the dark blue areas have the weakest. The simulation was done in three dimensions, but the figure shows a cross section. Images courtesy of Wallentin et al. “Our findings are the first to show that it really is possible to use nanowires to manufacture solar cells,” said Borgström, a semiconductor physics researcher and principal author of the research, which appeared in Science (doi: 10.1126/science.1230969). Nanowire solar cells have not made it out of lab testing just yet, but the scientists hope that, with the recent identification of the ideal diameter for nanowires and how to synthesize them, they could be the perfect material for solar cells in sunny locations such as the southwestern US, southern Spain and Africa. “The right size is essential for the nanowires to absorb as many photons as possible,” Borgström said. “If they are just a few tenths of a nanometer too small, their function is significantly impaired.” Optical microscope image of four nanowire solar cells. Each cell is a slightly lighter shade of purple in color, while the darker areas in between are inactive. The yellow areas are gold metal pads, which are used for connecting the solar cells to an external load. Each cell contains about 4.5 million nanowires. Current solar cells used to supply electricity to homes are made from silicon, which is relatively inexpensive but significantly inefficient because the cells use only a limited portion of the sunlight that reaches them. Nanowires, however, work at much higher efficiency with the same sort of material combinations, keeping the costs low. The process also is less complicated. Nanowires can generate power at the same level as thin film of the same material, even if they cover only about 10 percent of the surface rather than 100 percent, the researchers say. The research was carried out as part of a European Union-funded project, AMON-RA, coordinated by Lund professor Knut Deppert. Further investigation is needed to explore how to make these cells larger and to determine cost and durability. This scanning electron microscopy image shows a side-view of nanowires that have been coated with a transparent, conductive oxide. The sunlight comes in from the top, which is why the top contact must be transparent for light. The substrate is used for the bottom contact. “I am very proud of such a great result — it has well exceeded our expectations,” Deppert said. “We will, of course, continue the research on nanowire solar cells and hope to achieve an even higher level of efficiency than the 13.8 percent that we have now reported.” For more information, visit: www.lunduniversity.lu.se