Discovery of a 'dark state' could increase maximum theoretical efficiency of solar cells from 31 to 44 percent

December 15, 2011, University of Texas at Austin

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.

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 .

"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 , leading to much higher efficiencies."

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

The maximum theoretical 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 to as high as 66 percent.

Zhu and his team previously demonstrated that those hot electrons could be captured using . 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 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.

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.

Explore further: Highly efficient solar cells could result from quantum dot research

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2.5 / 5 (4) Dec 15, 2011
Does this mean a lot of power comes from the dark side, quoth Darth Vader? LOL Keep it up, this and another discovery about 130 percent efficiency from a quantum dot solar cell. Love it! Love it! Love it!!
not rated yet Dec 15, 2011
Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton
Unfortunately it destroys pentacene gradually too. Such way of quantum yield increasing shortens the lifetime of solar cell. In addition, pentacene is prone to oxidation.
5 / 5 (5) Dec 15, 2011
They really need to come up with a consistent efficiency measurement notation for solar cells.

For example, the following cell gets '41% efficiency.'

I think they should always notate efficiency as absolute (energy created from all energy available.) - Rather than calculating the efficiency of a cell from the portion of the spectrum that it utilizes.

3.1 / 5 (11) Dec 15, 2011
Wow! another possible 10-12% increase!.

We're up to how much now? 398% ?
1 / 5 (1) Dec 15, 2011
Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton
Unfortunately it destroys pentacene gradually too. Such way of quantum yield increasing shortens the lifetime of solar cell. In addition, pentacene is prone to oxidation.
Details. DuPont will fix this I am sure.
0.3 / 5 (42) Dec 15, 2011
"We're up to how much now? 398% ?" - GuyTardalikus

Poor Guy. He can't keep track of what type of solar cells are being discussed and has no concept that one improvement may overlap or even negate another.

That is why Guy will never be a scientist or an engineer.

4.2 / 5 (5) Dec 15, 2011
Vendar, can't you figure a joke, seriously nobody believes solar is going up at that rate, each 1% seems to take decades despite all these articles.
3 / 5 (2) Dec 16, 2011
I remember my engineering school back in the 70's when computers first started coming into college campuses in computer rooms on green and black screens with 2 color grafix. Our highest aspiration other than beer was to own a HP35 calculator, and microsoft was not yet an itch in ole Bill's greedy palm. Back then the internet was called 'arpanet' and only colleges had limited access as much of it was 'culllassssified'. Friends of mine were building solar panels out of wood frames with enough copper tubing to make a half dozen moonshine stills, and the bottoms had to be larded with fiberglass insulation to keep the heat in and on the tubes. One idea was to face the collector with one way mirror glass so the light would come through the backwards facing glass and heat the water filled tubes, reflect off the foil facing on top of the insulation layer...and reflect back and forth from the inside of the mirror glass to the foil..lather ...rinse ...repeat. Yeah we cum a long way since.
0 / 5 (34) Dec 16, 2011
"Vendar, can't you figure a joke." - Chippy

In this instance no. I recognize it as a disparaging remark directed at the solar cell researchers.

0 / 5 (34) Dec 16, 2011
I remember playing with an HP35 back in my university days. It was about the size of a microwave oven, had a 4 inch CRT display that showed all three levels of it's internal stack, recorded programs on a magnetic card about the size of a credit card, and had a fertite core memory.

The one I played with was connected to a plotter with a bundle of wires about 3 cm in diameter.

A few months later I purchased a TI59 for $400.
not rated yet Dec 16, 2011

You know I was in chip design for two decades so I also started out with PDP 8s,11s, and the mighty PDP10, all great stuff.

The fact that computers today are literally millions times faster, cheaper, less power, just doesn't apply to solar power in the same way. Computers benefit from the reduction in feature sizes on an exponential curve going from 10u down to 20nm. The reduction in silicon wafer costs is a tiny part of that, silicon substrates are cheap but have very high energy content.

A PV panel is nothing but a very large no of simple diodes on a large substrate using very course features. The cost of making substrates has declined much slower than you think mostly due to China using dirty coal to make this.

Now the extra equipment needed like the grid tie inverters does benefit from the huge sale of PC switching PSUs to get them close to 50c/W peak. Inverters are PSU the other way around. Without PCs, solar panels and inverters would cost far more, but not zillions more.
not rated yet Dec 16, 2011

You are talking about solar thermal collectors which is probably a far better value than solar PV for most home owners, and its a better way to exploit the available insolation, but then peeps want electrical too. The Carter Whitehouse had these on the roof top and not the PVs.

The really good tubes from Germany though don't seem to be that cheap. In Israel I think solar thermal on roofs is mandated on new homes.

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