"Shadow state" discovery could dramatically boost solar power efficiency
December 22, 2011
Researchers at the University of Texas say it is possible to hike the energy yield of solar cells by exploiting what they call a photon's "shadow state", doubling the number of electrons that may be harvested in the process. They claim the discovery could up the theoretical maximum efficiency of silicon solar cells from 31 to 44 percent.
Prior research led by chemist Xiaoyang Zhu demonstrated that a theoretical increase in efficiency to as high as 66 percent would be possible if solar cells could be made to additionally harvest so-called "hot electrons", residual heat energy that is lost within about a picosecond after a cell absorbs a photon. Zhu then found that this was possible, but only when harvesting photons from "highly focused" sunlight, impractical in real-world applications.
But the team's latest findings point to an alternative means of boosting efficiency. The organic plastic semiconductor pentacene, when absorbing a photon, creates an exciton (an electron paired with an electron hole) which is quantum mechanically coupled to a dark "shadow state" multiexciton from which an additional electron can be harvested. This way, a photon provides double the electrons. Zhu says that the process could see solar cell efficiency increase to 44 percent without the need for a focused solar beam.
"Plastic semiconductor solar cell production has great advantages, one of which is low cost," said Zhu. "Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies."
The latest University of Texas findings were published on December 16 in Science.