Photonic crystals with an "inverse opal" structure could increase the light-trapping capability of thin-film solar cells and help bring about lower-cost solar energy. Researchers at Purdue University’s Birck Nanotechnology Center and School of Electrical and Computer Engineering used the 3-D crystals to improve thin-film light absorption compared with conventional thin-film cells. Illustrations (a) through (d) depict the creation of 3-D photonic crystals. The "inverse opal" structure is formed by treating the crystals with hydrofluoric acid (d). Scanning electron microscope images show the small-scale crystal structure from three different angles (e and g), and a photograph (h) shows how the same membrane can be wrapped around a glass pipette. Courtesy of Leo Tom Varghese, Purdue University. The synthetic crystals' inverse opal structure makes use of and enhances properties found in the gemstones to reflect, diffract and bend incoming sunlight. Researchers created the inverse opals using a process called meniscus-driven self-assembly. "Usually, in thin-film silicon solar cells, much of the sunlight comes right back out,” said Peter Bermel, an assistant professor at Purdue. “Using our approach, the light comes in, and it is diffracted, causing it to propagate in a parallel path within the film." Compared to solar cells made of silicon wafers, thin-film cells cost as much as 100 times less. However, they are less efficient. The new approach to light trapping indicates roughly a 10 percent increase in efficiency over conventional silicon thin films, with further potential for improvement. Researchers say the technology is better at absorbing and harvesting near-infrared light. "Light in the near-infrared range is important because there is a lot of solar energy in that wavelength range and also because silicon can convert near-infrared light to energy if it can absorb it, but thin films don't fully absorb it," Bermel said. Applications for thin-film solar cells include generating electricity for utilities and homes, amd smaller-scale applications such as mobile charging of electronic devices. The research was published in Advanced Optical Materials. For more information, visit engineering.purdue.edu/ECE.