Discovery of a ‘Dark State’ Could Mean a Brighter Future for Solar Energy

Dec. 15, 2011

AUSTIN, Texas — The efficiency of conventional solar cells could be significantly increased, according to new research on the mechanisms of solar energy conversion led by chemist Xiaoyang Zhu at The University of Texas at Austin.

Professor Xiaoyang Zhu

Professor Xiaoyang Zhu

Zhu and his team have discovered that 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."

Zhu and his team published their groundbreaking discovery Dec. 16 in Science.

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.

Science Behind the Discovery:

  • 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.

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.

The research team was spearheaded by Wai-lun Chan, a postdoctoral fellow in Zhu’s group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila. The research was supported by the National Science Foundation and the Department of Energy.

For more information, contact: Lee Clippard, Lady Bird Johnson Wildflower Center, 512-232-0104; Xiaoyang Zhu, professor, 512-471-9914.

4 Comments to "Discovery of a ‘Dark State’ Could Mean a Brighter Future for Solar Energy"

1.  bart likes said on Dec. 16, 2011

When could a demonstration cell be produced and of course a viable product?

2.  Brian Collins said on Dec. 16, 2011

Fantastic news that gives hope to the possibility that the world can live on solar power.

3.  James P Louviere said on Dec. 17, 2011

Congratulations to the entire research team. While the charade in Washington and other places where public figures of every political stripe pose and bully their opponents without producing a single milliwatt of energy, serious scientists from every nation and in research centers in universities and great laboratories everywhere are actually addressing the obvious need to convert the nearly limitless flow of energy pouring down upon us everyday into the amazing form of energy we are so dependent upon: Electricity. We are fortunate that the sun is in a very long period of its life history during which life forms like human beings and the animals and plants currently populating planet Earth have been able to thrive. The great strides in technology and our newly developed appreciation for and understanding of Earth's weather systems have progressed hand in hand with out ability to use electricity in exciting new ways. It is heartening that Dr. Zhu and his brilliant team are working on such an efficient means of tapping the abundant flow of free energy from the sun with a technology that can ensure that we are able to sustain our energy-hungry civilization without being dependent on energy sources produced by exploiting the abundant fossil and nuclear fuels we have been dependent on until now. At last discoveries and techniques like those emerging the the University Texas offer hope that the wasteful and environmentally disastrous production of carbon monoxide and the resultant unintended release of enormous quantities of methane from the clathrates that are breaking down as the earth's lands and waters warm up may be a thing of the past within our lifetimes is a thrilling prospect. It is not the expanded harvesting of subterranean fuels that will sustain us through this century and beyond, but the enlightened and burgeoning shift to the superabundant source of totally non-polluting, totally earth friendly, totally free-of-charge energy we will be able to convert into electricity that will guarantee the eventual healing of the great weather systems we have enjoyed prior to the era of carbon fuels and the effects their waste gases have produced.
James P Louviere, Austin Texas - delighted to be so close to where this fine work is in progress!

4.  Brien Seeley M.D. said on Dec. 22, 2011

Dr. Zhu: Congratulations on a great discovery that will have major effects on the future of electric powered aircraft, like those that just completed the Green Flight Challenge sponsored by Google. We hope that you can join us for the 6th Annual CAFE Electric Aircraft Symposium next April 27 in Santa Rosa, CA.