Finding a material that exhibits the bulk photovoltaic effect for visible light would greatly simplify solar cell construction. Moreover, it would be a way around an inefficiency intrinsic to interfacial solar cells, known as the Shockley-Queisser limit, where some of the energy from photons is lost as electrons wait to make the jump from one material to the other.
"Think of photons coming from the sun as coins raining down on you, with the different frequencies of light being like pennies, nickels, dimes and so on. A quality of your light-absorbing material called its 'bandgap' determines the denominations you can catch," Rappe said. "The Shockley-Queisser limit says that whatever you catch is only as valuable as the lowest denomination your bandgap allows. If you pick a material with a bandgap that can catch dimes, you can catch dimes, quarters and silver dollars, but they'll all only be worth the energy equivalent of 10 cents when you catch them.
"If you set your limit too high, you might get more value per photon but catch fewer photons overall and come out worse than if you picked a lower denomination," he said. "Setting your bandgap to catch only silver dollars is like only being able to catch UV light. Setting it to catch quarters is like moving down into the visible spectrum. Your yield is better even though you're losing most of the energy from the UV you do get."
As no known materials exhibited the bulk photovoltaic effect for visible light, the research team turned to its materials science expertise to devise how a new one might be fashioned and its properties measured.
Starting more than five years ago, the team began theoretical work, plotting the properties of hypothetical new compounds that would have a mix of these traits. Each compound began with
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University of Pennsylvania