There's a limit on the conversion efficiency of a conventional solar cell. No matter how it's tweaked, it can only convert 31 percent of the light that hits it into usable electrical current. That's because there's a broad spectrum of wavelengths in sunlight, and some of it has more energy than the active material in the solar cell can handle. High-energy light hits the active material in a solar cell and knocks loose electrons that have a similarly high energy--then these electrons rapidly lose that excess energy as heat.
Physicists know that if they could capture "hot electrons", they could more than double the efficiency of solar cells. The problem is that they lose their energy in a picosecond. Now, researchers have for the first time demonstrated that it's possible to capture hot electrons while they're still in their high energy state, before that heat loss happens.
Careful design at the nanoscale is key. Instead of a conventional bulk semiconductor, the researchers used quantum dots, because these nanomaterials can confine electrons over a longer timescale. "Nanomaterials can keep electrons electrons hot for a longer period of time, so that you can get them out," says Xiaoyang Zhu, professor of chemistry at the University of Texas, Austin.The confinement is great--until you want to get the hot electrons out. "The electron likes to stay inside the nanomaterial, so you need to make an extremely strong interaction with another material" that will conduct the electrons out of the quantum dot, Zhu says. His group coated the quantum dots with a very thin layer of an electrical conductor, and were meticulous about the quality of the interface between that material and the quantum dots.
So now it's possible to get hot electrons out, but one major problem remains. Those hot electrons require new device designs that prevent them from simply losing their energy to heat once they enter the metal wire of an electrical circuit. "We hope to inspire people to work on the engineering," says Zhu.