UCLA’s Yang Yang on Upping the Efficiency of Polymer Solar Cells

Scientist Interview: May 2011

YangThe sun has always seemed like the ideal place to get clean, renewable energy; a power source that won’t cause more problems than it solves. For scientists and entrepreneurs trying to take advantage of these solar attributes, it means either coming up with more efficient solar cells based on silicon—so far, the best by a country mile in efficiency and reliability, but still expensive and unwieldy—or figuring out ways to make solar cells out of other materials—polymers in particular—that can compete with silicon, but be significantly cheaper, lighter, and more user friendly.

This is the challenge that Yang Yang of the University of California, Los Angeles, undertook in the early 2000s, as what could be called an educational lark. Since then the pursuit of high-efficiency, reliable polymer solar cells has grown into a significant portion of his research efforts, while making Yang one of this publication’s featured “hot” scientists for 2010, as reported in the March/April 2011 issue. Last year, eight recent reports by Yang and colleagues registered as highly cited according to the Hot Papers database—in particular a November 2009 article in Nature Photonics, “Polymer solar cells with enhanced open-circuit voltage and efficiency,” which has been cited more than 200 times in just 18 months (see table below) and which currently ranks at #9 in this issue’s Physics Top Ten.

Yang, 52, received his Bachelor of Science degree from National Cheng-Kung University in Taiwan in 1982. He then went on to get his master’s and, in 1992, his Ph.D. in physics and applied physics from the University of Massachusetts at Lowell.  Yang spent the next four years as a staff scientist at the UNIAX Corporation (now Du Pont Display) in Santa Barbara. He joined the faculty at UCLA in 1997 and is now a professor in the Materials Science and Engineering Department and is faculty director of the Nano Renewable Energy Center at the California NanoSystems Institute. Yang also serves as Director of the Center for Organic Opto-electronics Technologies at Zhejiang University in China.

 
Yang spoke to ScienceWatch.com from his office in Los Angeles.

SW: How and when did you first start working on polymer solar cells? Given that the technology is so new, what prompted the research?

We started working on polymer solar cells about eight or nine years ago. I had a student who was about to graduate, and I challenged him to work on a project that was not associated with his Ph.D. thesis. I was working on silicon-based solar cells, and he chose polymer solar cells. That’s how we got started. And that’s part of our educational goal—to constantly challenge students to find new frontiers of science on which to work. My student got some interesting results, and this prompted us to send a proposal to the University of California Energy Institute, which gave us $30,000 to continue the research.

SW: $30,000 is not a lot of money these days for research. Was it enough to do anything substantial?

It was an honor simply that they gave the money. I still remember that the referee said he didn’t believe in organic solar cells, but he respected our track record and said that the proposal should be funded to prove he’s wrong. That’s the beauty of this country. Anywhere else, if someone says they don’t believe the work, that’s the end of it. Here, they say, “Prove I’m wrong.” And since then we’ve been steadily pushing the technology forward.

SW: What was the state of polymer solar cell technology when you first started?

At that time, polymer solar cells could achieve maybe 1% or 2% efficiency, although there was no standard way of measuring the efficiency to get an accurate number. The reason is that all solar cell measurements were based on silicon devices. That’s how the industry was established. So there was an inherent error that nobody discussed at the time, called the mismatch factor, which affected the different absorption properties of silicon-based and polymer solar cells.

If the conversion efficiency was maybe 2%, I do know our number was below that. The lifetime of these devices wasn’t very good, either. Either way, we took advantage of that $30,000 and hired a post-doc, and the post-doc found a way to push the efficiency up to about 4.4% using the same material.

SW: What’s an ideal efficiency for a marketable polymer solar cell?

To have a reasonable product on the market, I’d say the efficiency should be 10% with a 10-year lifetime.

SW: What was the lifetime of your earliest cells?

To be honest, it was so small we didn’t even bother to measure.

SW: Once you got the efficiency up to 4.4%, what was your approach?

We started to understand the material better, and to apply our understanding from silicon-based solar cells to polymers: in particular, how to make the polymer solar cells have better transport properties—that is, how to make the electrons and holes move faster.

SW: You’ll have to bear with me for what might be considered naïve questions, but what’s a hole? And how do you make them move faster?

Solar cells have two charge carriers—positive and negative carriers. Electrons are negatively charged, holes are positive. For solar cells you want these two types of carriers to move at the same speed. You don’t want electrons, say, to move faster than holes. And to do this we have to understand the polymer morphology, which means in essence understanding how to arrange the polymer to make this happen and also maximize all the other characteristics we need.

Selected, Recent Papers by Yang Yang
and Colleagues on Polymer Solar Cells
(Listed by citations)
Rank Paper Citations
1 H.Y. Chen, et al., "Polymer solar cells with enhanced open-circuit voltage and efficiency," Nature Photonics, 3(11): 649-53, 2009.

224

2 G. Li, et al., "’Solvent annealing’ effect in polymer solar cells based on poly(3-hexylthiophene) and methanofullerenes," Adv. Functional Materials, 17(10: 1636-44, 2007.   

200

3 J.H. Hou, et al., "Synthesis, characterization, and photovoltaic properties of a low band gap polymer based on silole-containing polythiophenes and 2,1,3-benzothiadiazole," J. Am. Chem. Soc., 130(48): 16144, 2009.

198

4 L.M. Chen, et al., "Progress in polymer solar cells. Manipulation of polymer: Fullerene morphology and the formation of efficient inverted polymer solar cells," Adv. Materials, 21(14-15): 1434-49, 2009.

124

5 J.H Hou, et al., "Synthesis of a low band gap polymer and its applications in highly efficient polymer solar cells," J. Am. Chem. Soc., 131(43): 15586, 2009.

90

SOURCE: Thomson Reuters Web of Science®.

You have to understand that solar cells are both optical and electronic devices. They pick up light and turn it into electricity. So that’s a dual function, and a lot of materials are very good in the optical but not so good in the electrical, while some are good in the electrical but not so good with optical. So those materials have to be rejected. When we published this 4.4% efficiency in late 2005 we had managed to maximize the performance of this particular polymer both electrically and optically.

SW: And where did you go from there? What was the next step?

Yang:  The next step was to understand different interfaces and material properties and make new materials by working with chemists. We’re collaborating, for example, with Luping Yu at the University of Chicago, and we have a start-up company called Solarmer Energy Inc., which has licensed our patents and hired our graduate students and post-docs to work on pushing the polymer solar cells to commercialization. That’s where a lot of the effort has come from in understanding how to make the devices more efficient. I should add that my post-doc who had the breakthrough of 4.4% efficiency is named Gang Li, and he moved to Solarmer Energy and is now vice-president. [Note: for more coverage of research from Yu and Li, see the Chemistry Top Ten in this issue.]

SW: What are the key factors determining solar cell efficiency, and how do you work to maximize them?

A few things determine solar cell efficiency. One is called the open-circuit voltage. Let’s start with basic electronics: Power is equal to current times voltage. To get higher power, you want to get higher current and voltage. The higher current in our case is coming from photons—the more you pick up, the more electrons and holes you generate. You need some driving force to push the charge out, and that’s the open-circuit voltage.

So one of the things we’ve been doing is engineering the material to pick up a larger portion of the solar spectra. To do this, we’ve synthesized different materials here at UCLA, at Prof. Yu’s laboratory in Chicago, and at Solarmer Energy. We’ve also created different device architectures that utilize a higher open-circuit voltage to make this possible.

SW: Are you experimenting with existing materials or creating your own?

We invent most of the polymers by modifying existing structures. That’s the beauty of polymer solar-cell materials—you can use chemistry to come up with new compounds. With silicon-based solar cells, you can increase the purity and improve the crystallization process, but you have only one material: silicon. In polymers, you have an unlimited choice of materials if you have chemists who can make them for you. So we start from scratch and design the materials to have higher and higher efficiency.

SW: Where do you stand today?

Today the peak levels are at 8.1% to 8.3%. One material is from a company called Konarka in Lowell, Massachusetts. The other one is our material from Solarmer Energy.

SW: And what about the reliability of these compounds? What’s their average lifespan?

We’re at several years now, which is still not enough, of course, for the 15 or 20 years required for a viable product.

SW: So what will it take to get to 10% efficiency?

I’m asking that same question every day. I think we need new materials and better engineering. We need to push materials to pick up even more photons and convert those photons into electrons. Right now we’re picking up part of the infrared spectrum, but not yet all of it. The other thing we’re doing in our group is building solar cells with two layers—a tandem solar cell—one on top of the other. So we put solar cells with two different absorption spectra on top of each other. And we’re hoping that this will provide a broader absorption range in total. We’ve done some of that, but haven’t yet achieved results that satisfy us.

SW: And what about getting the lifespan up from several years to 15 or 20?

Well, let’s shoot for 10 years first and then go to 15 or 20. The question is why these solar cells fail. It’s not clear yet, but we have several theories. The first is that ultraviolet light tends to degrade polymer chains. UV light doesn’t account for a lot of photons in the solar spectra, but it does a lot of damage, in the same way that UV light causes skin cancer. So one idea is to block UV light by using the equivalent of sunscreen on the solar cell. At UCLA we’ve put a layer of molecules in front of the solar cell that picks up the UV light and converts it into visible light, so we actually recycle the energy. That’s quite a promising idea.

SW: Is that going to be enough to keep these solar cells going for another decade?

"I would hope that in 10 years we’ll have polymer solar cells incorporated into buildings," says Yang Yang of the University of California, Los Angeles.

No—we still have to have better protection for the polymers, and that’s got to be by some other means. Another reason polymer solar cells degrade is because of contact with oxygen and moisture from the air. So we want to encapsulate the elements of the solar cell and enclose them in a protective layer or cage so they don’t come into contact with moisture and oxygen. If we can do all these things well, then a lifespan of 10 years for these solar cells is doable.

SW: Assuming all goes well, how long do you think it will take to reach 10% efficiency and a 10-year lifespan? And will you be able to compete with silicon when you get there?

I’d say maybe in three to five years. I hope the efficiency will be there then, and we’ll have more room to be competitive. Silicon-based solar cells now fall into three categories. One is a single crystal, and that’s used, for example, for solar panels on rooftops. Another is called a polysilicon solar cell—or polycrystalline silicon—and it’s crystalline, but multiple small crystals, not a single large one. And then there’s amorphous silicon, which is just randomly oriented silicon molecule domains. Amorphous silicon also suffers from lifetime issues, and its efficiency is now about 8%. That’s respectable.

If we reach 10% and a 10-year lifespan, we can certainly replace amorphous silicon. And if we can reach 10 or 12% efficiency we can compete with or replace polysilicon.

SW: I assume this means that these other silicon technologies have uses different from the single-crystal variety, and that’s what you’ll aim for with polymer solar cells. Is that a reasonable assessment?

Yes and no. Let me give you an example: There are companies that now produce flexible solar panels that are not like a thick solar panel on a roof, but laminated flexible solar cells laid on top of metal rooftops—usually in industrial warehouses. Those are polysilicon solar cells, and they’re competing for rooftop application. The problem with the traditional silicon panel is that it’s pretty heavy. If you want to put it on your roof, you’d better have a contractor do some reinforcement of the roof structure beforehand. So with silicon solar panels, installation costs are half the total costs. If we can make a thin film and reduce the weight significantly, then the way we can use solar cells will change.

That would be our first target. What we’ve been discussing so far would be ultimate uses for polymer solar cells, but until then most people think they’ll be useful for other things, too—for example, charging portable electronics. You could put the solar cells on a ladies’ purse or the outside lining of a briefcase and then use them to charge your cell phone or notebook computer and prolong the battery life. The military is interested in having a solar cell tent, so when troops go where no electricity is available they’ll always have a way to charge their electronics. If you look at the packs that soldiers now carry, the heaviest items are not the weapons but batteries for operating radios and other electronics.

SW: Okay, while acknowledging all the pitfalls of making predictions, what would you hope to see for polymer solar cells 10 years from now?

I would hope that in 10 years we’ll have polymer solar cells incorporated into buildings. This is what’s called building-integrated photovoltaics. Let me give you an example: Right now I have blinds on my window to block out the sunlight. Too much sunlight causes the room to heat up, and then I have to turn on the air conditioner.

So rather than just using blinds to block out the sunlight, why don’t we use plastic solar cells to turn that sunlight into electricity? That would be quite an interesting application. Polymer solar cells would be light enough in weight and flexible enough that they could be used in many of these places that people don’t think solar cells are practical. You could take some nails and attach these solar cell panels to the outside of the house in the same way you’d put up a poster. Then you could turn that sunlight into electricity. To achieve that we’d really have to make solar cells cheap so people would feel like they’re putting paper up on the wall.

And then an obvious application is to use these solar cells in places like India, China, and Africa, where in some areas there’s no electricity available at all. You could have a battery and solar panels and generate electricity during the day to be used for light at night. It’s probably being done already in places, but the challenge is to make the solar cells so inexpensive that this can be done on a wide scale. Another advantage of plastic polymer solar cells is that it’s really the only technology that allows you to print the solar cell like you’re printing a newspaper. So if we need energy in large quantities, it would help to be able to print these out in large quantities at low cost and quickly. How many rooftop solar panels do you see today? Not too many. Maybe we can change all that in the future.

 
 

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