This shows an 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. This image relates to a paper that appeared in Jan. 17, 2013, issue of Science Express, published by AAAS. The paper, by Jesper Wallentin at Lund University in Lund, Sweden, and colleagues was titled, "InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit."