Capturing More Light with a Single Solar Cell

“In a conventional semiconductor material, it takes a certain amount of energy to free an electron and generate electricity. Photons that have less energy—say, the photons in infrared light—don’t generate electricity. And if a photon has more than the minimum—for instance a photon in energetic ultraviolet light—the extra energy is wasted as heat.
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The new semiconductor material is based on gallium arsenide. Normally, this material requires high-energy photons to generate electricity. But the researchers modified it so that the energy from more than one photon is used to free an electron—energy adds up until an electron is freed. Replacing some of the arsenic atoms in the material with nitrogen atoms creates regions that act as stepping stones for electrons that have absorbed some energy from low-energy photons, where they can wait to receive energy from more photons, says Wladek Walukiewicz, who leads the Solar Energy Materials Research Group at the Lawrence Berkeley lab, and also led the project.”

Capturing More Light with a Single Solar Cell – Technology Review