Highly Efficient Solar Cells Could Result from Quantum Dot Research

June 17, 2010

AUSTIN, Texas — Conventional solar cell efficiency could be increased from the current limit of 30 percent to more than 60 percent, suggests new research on semiconductor nanocrystals, or quantum dots, led by chemist Xiaoyang Zhu at The University of Texas at Austin.

Zhu and his colleagues report their results in this week's Science.

Xiaoyang Zhu
Dr. Xiaoyang Zhu, professor of chemistry, has discovered a method to capture the higher energy sunlight that is lost as heat in conventional solar cells.Photo: Marsha Miller

The scientists have discovered a method to capture the higher energy sunlight that is lost as heat in conventional solar cells.

The maximum efficiency of the silicon solar cell in use today is about 31 percent. That's because much of the energy from sunlight hitting a solar cell is too high to be turned into usable electricity. That energy, in the form of so-called "hot electrons," is lost as heat.

If the higher energy sunlight, or more specifically the hot electrons, could be captured, solar-to-electric power conversion efficiency could be increased theoretically to as high as 66 percent.

"There are a few steps needed to create what I call this 'ultimate solar cell,'" says Zhu, professor of chemistry and director of the Center for Materials Chemistry. "First, the cooling rate of hot electrons needs to be slowed down. Second, we need to be able to grab those hot electrons and use them quickly before they lose all of their energy."

Zhu says that semiconductor nanocrystals, or quantum dots, are promising for these purposes.

As for the first problem, a number of research groups have suggested that cooling of hot electrons can be slowed down in semiconductor nanocrystals. In a 2008 paper in Science, a research group from the University of Chicago showed this to be true unambiguously for colloidal semiconductor nanocrystals.

Zhu's team has now figured out the next critical step: how to take those electrons out.

They discovered that hot electrons can be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide.

"If we take the hot electrons out, we can do work with them," says Zhu. "The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility."

Hot Electron TransferThe researchers used quantum dots made of lead selenide, but Zhu says that their methods will work for quantum dots made of other materials, too.

He cautions that this is just one scientific step, and that more science and a lot of engineering need to be done before the world sees a 66 percent efficient solar cell.

In particular, there's a third piece of the science puzzle that Zhu is working on: connecting to an electrical conducting wire.

"If we take out electrons from the solar cell that are this fast, or hot, we also lose energy in the wire as heat," says Zhu. "Our next goal is to adjust the chemistry at the interface to the conducting wire so that we can minimize this additional energy loss. We want to capture most of the energy of sunlight. That's the ultimate solar cell.

"Fossil fuels come at a great environmental cost," says Zhu. "There is no reason that we cannot be using solar energy 100 percent within 50 years."

Funding for this research was provided by the U.S. Department of Energy. Coauthors include William Tisdale, Brooke Timp, David Norris and Eray Aydil from the University of Minnesota, and Kenrick Williams from The University of Texas at Austin.

For more information, contact: Lee Clippard, College of Natural Sciences, 512-232-0675; Dr. Xiaoyang Zhu, professor of chemistry, 512-471-9914.

16 Comments to "Highly Efficient Solar Cells Could Result from Quantum Dot Research"

1.  rezzouk said on June 21, 2010

"There is no reason that we cannot be using solar energy 100 percent within 50 years."

It is a great dream, but I think it is possible, so we must work more. Thanks to you, and it sounds like a good job. Can you send me more information about that?

2.  joe harris said on June 22, 2010

I had been thinking that we are not utilizing infrared wavelengths and had asked chemist son-in-law who said it can't be done. I hope DOE increases funding.

3.  George Edwards said on June 24, 2010

This is so exciting. The relative high cost of current silicone cells to the cost of fossil electricity makes the switch to solar expensive without substantial subsidies from the government. With a higher efficiency cells, and hopefully, a relatively even or reduced cost of production, we truly can advance into the solar era. One way or another we will finally have to go there, whether we choose to or are forced to is the dilemma but the sooner, the better.

4.  Charles Curtis said on June 24, 2010

As one who has benefited from the petrolem industry and all of its traditional downstream industries, I applaud Dr. Zhu and his associates. This is the type of research that needs to be done. Define a practical goal and set about solving the problems that stand in the way.

5.  Cary Michael Cox said on June 24, 2010

Great work. I'll be happy with any source of energy in which we can tell the Middle East to go play in their sandbox.

News like yours always brings out my University of Texas at Austin pride. All the best with your continued research.

6.  Sarah Northington Fox said on June 24, 2010

Are there demonstration fields in operation to observe the solar set-up? In Texas?

7.  William Rhea said on June 24, 2010

Has spectrolab been involved with this process? They are a major leader in high efficiency triple junction solar cells. Very interesting work! Has this gone from theory to practical application yet (i.e. a higher efficiency cell)? From what I am reading it appears the heat loss has moved from the cell to the conducting wire junction at this point. If this can increase the efficiency up to 51 percent on a cost efficient cell and transfer wire this would be a MASSIVE discovery worth billions. Keep at it. This is awesome stuff. Thanks, solardog

8.  William Rhea said on June 24, 2010

Question: Evolution Solar (symbol EVSO on the pink sheets OTC) stock climbed 91 percent at the close when they broke your news earlier today. That's about $4.6 million USD added to their market cap in a day! Wow. Are they affiliated with your program/college in any way?

9.  ArmandoObledo@UTSA said on June 24, 2010

Awesome development in solar research! From the sound of the 66 percent theoretical limit, Zhu must be looking into "Auger generation." Hot electrons are a conventional PV cells enemy since they greatly contribute to cell degradation and are usually not harvested, but if you can use hot electrons in narrow bandgap QDs like PbSe in a superlattice type architecture, then they donate kinetic energy to other adjacent QDs and cause an "avalanche" effect energetically speaking. Is there research being done on cell architecture that leads to Auger generation within a PbSe QD superlattice?

10.  John Coltrane said on June 25, 2010

But what about the hot "holes" left behind after the electron transfer to TiO2? Won't they simply oxidize the PbSe lattice, destroying the QDs? This is a major problem with all polar semiconductors like PbSe, CdSe etc. Not as much a problem with covalent semiconductors like SI, so you'd need to demonstrate this with Si to have any real impact on PVs. Also TiO2 is a widegap semiconductor and will likely absorb a lot of the near UV sunlight instead of the PbSe and it will still be wasted as heat.

Solar thermal is just much more practical then PV devices, except in space.

11.  Logan Cravens said on June 25, 2010

As an architect I am interested if this technology could be used integrally with a panel used to clad a building.

Also interested in the toxicity of the materials used and their eventual potential to be recovered and reused.

12.  Claudio said on June 30, 2010

Could you share the paper with me? Could be good news for the solar industry.

13.  Vik said on June 30, 2010

@Cary, "Great work. I'll be happy with any source of energy in which we can tell the Middle East to go play in their sandbox."

Nobody in the Middle East is asking you to play with their sandbox. You yourself have built an economy and society that require more resources than available in your country. Hence you need to import oil. The long-term solution to America's energy problems remains conservation. There is no guarantee of technology providing all the answers.

And, bear in mind, the sun shines much brighter in these "sandbox" countries. They are best placed to make use of solar technology.

14.  VIjay Ramasamy said on Aug. 3, 2010

Great news for solar cell technology and downstream industries. Being in the TiO2 industry, I am interested to know more about the type of TiO2 used and its theory. Keep up the good work!

15.  ashwani said on Aug. 27, 2010

It's very good news, but I think in the total energy of the sun we have 60 percent IR radiations. How you use them is a major factor to increase the efficiency of solar cells.

16.  Ronald A. Wood said on Sept. 7, 2011

I was wondering if any further progress has been made on this research ?
Ron