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Discovery Could Sharply Boost Solar Cell Efficiency at Low Cost

Chart from Emanuel Sachs of MIT with data that is already two years old. Recent discoveries hold the promise of keeping the cost reductions going past 2020.

New research on solar energy conversion finds, “the efficiency of conventional solar cells could be significantly increased.” The research was led by chemist Xiaoyang Zhu at The University of Texas at Austin. Their news release explains:

it’s possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.

“Plastic semiconductor solar cell production has great advantages, one of which is low cost,” said Zhu, a professor of chemistry. “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 research holds the possibility of increasing the efficiency of solar cells by 50% to 100%.

A recent study found that “Solar Power Is Much Cheaper to Produce Than Most Analysts Realize.” And Climate Progress has laid out a path for further reductions (see Anatomy of a Solar PV System: How to Continue “Ferocious Cost Reductions” for Solar Electricity).

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Advances like this one hold the promise of even further reductions post-2020. The study itself in Science, “Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer” (subs. req’d) is pretty technical stuff.

Here’s a more digestible version of the finding from the news release:

The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent, because much of the sun’s energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of “hot electrons,” is instead lost as heat. Capturing hot electrons could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.

Zhu and his team previously demonstrated that those hot electrons could be captured using semiconductor nanocrystals. They published that research in Science in 2010, but Zhu says the actual implementation of a viable technology based on that research is very challenging.

“For one thing,” said Zhu, “that 66 percent efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel. This creates problems when considering engineering a new material or device.”

To circumvent that problem, Zhu and his team have found an alternative. They discovered that a photon produces a dark quantum “shadow state” from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene.

Zhu said that exploiting that mechanism could increase solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more widespread use of solar technology.

And here’s the basics of the underlying science:

  • Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton.
  • The exciton is coupled quantum mechanically to a dark “shadow state” called a multiexciton.
  • This dark shadow state can be the most efficient source of two electrons via transfer to an electron acceptor material, such as fullerene, which was used in the study.
  • Exploiting the dark shadow state to produce double the electrons could increase solar cell efficiency to 44 percent.

One never knows which advances will ultimately succeed in the marketplace, and new research typically takes many years to be commercialized. But the trend toward a steady reduction in cost is poised to continue for a long, long time.

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