By Peter Weiss
One frustration of solar energy is that although it’s free, clean, and inexhaustible, it’s a major challenge to harvest efficiently. Consider what happens when photons of sunlight hit a solar cell: They strike electrons in semiconductor material and send them on their way as an electric current. Although many solar photons carry enough energy to theoretically unleash several electrons, they almost never free more than one.
The complex physics behind that limitation boils down to this: An electron loosed by absorbing a photon often collides with a nearby atom. But when it does, it’s less likely to set another electron free than it is to create atomic vibrations that squander the electron’s excess energy on heat.