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Building a Better Sun-Trap

How does the saying go? Build a better solar cell and the world will beat a path to your efficiently temperature-controlled and environmentally friendly door? Okay, maybe that’s not exactly the saying, but you’ve got to admit that the idea of cheaper, more efficient solar technology is at least as compelling today as the idea of a better mousetrap was way back when. NC State physicists are doing their part to create more efficient solar technology by figuring out why current products aren’t working all that well.

Free the trapped excitons!

Organic solar cells are fairly inexpensive to produce, but the best among them still only converts about 8 percent of the solar energy captured into usable electricity. Generally speaking, organic solar cells are constructed of interpenetrating domains of two materials. One material absorbs photons from the sun, and then the captured energy – known as an exciton – travels through the device to the other material where it is converted into electrical charges. It sounds simple enough, but the reality is that not enough of these excitons are making it across the layers to make the devices truly efficient. In addition, many of the separated charges do not make it out but recombine and get lost.

According to physicist Dr. Harald Ade, one reason for the energy loss is the way that the materials in the domains are mixed. Most scientists assume that each domain of material is “pure” and that the charges created by the excitons should be able to travel freely within neatly self-contained “fingers” to the electrodes. However, Ade’s team has recently demonstrated that where these layers combine they don’t intertwine like fingers. Instead, the device contains pure nanocrystals of each material embedded in an amorphous mixture of the two, turning the neat pathways that scientists believed existed inside the devices into a muddy mess that the charges can’t navigate. The charges stay trapped, and the energy is lost.

By challenging assumptions and figuring out why what we have isn’t doing what we would like it to do, Ade hopes to give the solar-energy industry the tools it needs to improve its devices. And that would mean a brighter future for everyone.