A technique that uses special molecules as light antennas to harvest the energy from weak infrared light has been found to amplify the process 3300 times and could lead to improved solar cells and medical imaging techniques. Materials scientists and chemists from the University of Groningen and from the FOM Foundation harvested infrared light — which has too little energy to release electrons in solar cells — more efficiently by modifying an organic dye that acts as light antennas to transmit the energy to the nanoparticles to which they are attached. These particles subsequently convert two weak captured photons into a single strong, energy-rich photon in a process called upconversion. Diagram of the nanocrystal with infrared-absorbing antennas. NIR = near infrared, ET = energy transfer, VIS = emission of visible light. (Images: University of Groningen) “There are inorganic materials made from rare-earth metals that can facilitate this upconversion process,” said Kees Hummelen, a University of Groningen professor of organic chemistry and leader of the FOM-focus group on next-generation organic photovoltaics. “However, these materials absorb very few infrared photons. We have therefore attached organic molecules to them as antennae that can capture these photons and transmit the energy to the upconversion material.” Because of this, the entire infrared absorption process, upconversion and the emission of visible light is increased by a factor of 3300, Hummelen said. Even with the antennas, his group can capture only a limited amount of infrared light. He predicts that an even better yield can be obtained, but because the upconversion process inside the nanocrystal is still inefficient, it is not yet possible to achieve. Inspiration from nature. (left) A natural photosynthesis system with light-harvesting molecules (LH) and a reactive center (RC); (right) a schematic representation of the nanocrystal that realizes the upconversion (UC) with the attached antennas in green. “Two photons must come together in the material within a short space of time, he said. “In practice, the efficiency of this process is still very low. However the harvest is already much better, so step one has been achieved.” The scientists’ work is most applicable for solar cells, as about half of all the solar energy reaching the Earth’s surface consists of infrared light. By capturing more infrared light, solar cells will be able to pass the Shockley-Queisser efficiency limit; for solar cells that consist of one color, the limit is 32 percent. Spectrum of sunlight at sea level. About 49 percent of the radiation is in the infrared or near-infrared. A German group plans to incorporate the nanocrystals with antennas into solar cells to test them in practice, Hummelen said. The upconversion system also could be applicable for medical imaging techniques. “Infrared light penetrates further into biological tissues than visible light,” he said. “If you allow compounds that carry out upconversion to bind to specific cells in tissues, then you can make images using infrared light.” The research was published online July 15 in Nature Photonics. For more information, visit: www.rug.nl