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Sustainable Energy

Solar Cells for Cheap

Not everyone gets a solar cell named after them: but Michael Gratzel did. He says his novel technology, which promises electricity-generating windows and low manufacturing costs, is ready for the market.

Michael Grätzel, chemistry professor at the Ecoles Polytechniques Fédérales de Lausanne in Switzerland, is most famous for inventing a new type of solar cell that could cost much less than conventional photovoltaics. Now, 15 years after the first prototypes, what he calls the dye-sensitized cell (and everyone else calls the Grätzel cell) is in limited production by Konarka, a company based in Lowell, MA, and will soon be more widely available.

Grätzel is now working on taking advantage of the ability of nanocrystals to dramatically increase the efficiency of solar cells.

Technology Review asked him about the challenges to making cheap solar cells, and why new technologies like his, which take much less energy to manufacture than conventional solar cells, are so important.

Technology Review: Why has it been so difficult to make efficient, yet inexpensive solar cells that could compete with fossil fuels as sources of electricity?

Michael Grätzel: It’s perhaps just the way things evolved. Silicon cells were first made for [outer] space, and there was a lot of money available so the technology that was first developed was an expensive technology. The cell we have been developing on the other hand is closer to photosynthesis.

TR: What is its similarity to photosynthesis?

MG: That has to do with the absorption of light. Light generates electrons and positive carriers and they have to be transported. In a semiconductor silicon cell, silicon material absorbs light, but it also conducts the negative and positive charge carriers. An electric field has to be there to separate those charges. All of this has to be done by one material–silicon has to perform at least three functions. To do that, you need very pure materials, and that brings the price up.

On the other hand, the dye cell uses a molecule to absorb light. It’s like chlorophyll in photosynthesis, a molecule that absorbs light. But the chlorophyll’s not involved in charge transport. It just absorbs light and generates a charge, and then those charges are conducted by some well-established mechanisms. That’s exactly what our system does.

The real breakthrough came with the nanoscopic particles. You have hundreds of particles stacked on top of each other in our light harvesting system.

TR: So we have a stack of nanosized particles…

MG: …covered with dye.

TR: The dye absorbs the light, and the electron is transferred to the nanoparticles?

MG: Yes.

TR: The image of solar cells is changing. They used to be ugly boxes added to roofs as an afterthought. But now we are starting to see more attractive packaging, and even solar shingles (see ”Beyond the Solar Panel”). Will dye-sensitized cells contribute to this evolution?

MG: Actually, that’s one of our main advantages. It’s a commonly accepted fact that the photovoltaic community thinks that the “building integrated” photovoltaics, that’s where we have to go. Putting, as you say, those “ugly” scaffolds on the roof–this is not going to be appealing, and it’s also expensive. That support structure costs a lot of money in addition to the cells, and so it’s absolutely essential to make cells that are an integral part.

[With our cells] the normal configuration has glass on both sides, and can be made to look like a colored glass. This could be used as a power-producing window or skylights or building facades. The wall or window itself is photovoltaicly active.

TR: The cells can also be made on a flexible foil. Could we see them on tents, or built into clothing to charge iPods?

MG: Absolutely. Konarka has a program with the military to have cells built into uniforms. You can imagine why. The soldier has so much electrical gear and so they want to boost their batteries. Batteries are a huge problem–the weight–and batteries cost a huge amount of money.

Konarka has just announced a 20-megawatt facility for a foil-backed, dye-sensitized solar cell. This would still be for roofs. But there is a military application for tents, and Konarka is participating in that program.

TR: When are we going to be able to buy your cells?

MG: I expect in the next couple of years. The production equipment is already there. Konarka has a production line that can make up to one megawatt [of photovoltaic capacity per year].

TR: How does the efficiency of these production cells compare with conventional silicon?

MG: With regard to the dye-cells, silicon has a much higher efficiency; it’s about twice [as much]. But when it comes to real pickup of solar power, our cell has two advantages: it picks up [light] earlier in the morning and later in the evening. And also the temperature effect isn’t there–our cell is as efficient at 65 degrees [Celsius] as it is at 25 degrees, and silicon loses about 20 percent, at least.

If you put all of this together, silicon still has an advantage, but maybe a 20 or 30 percent advantage, not a factor of two.

TR: The main advantage of your cells is cost?

MG: A factor of 4 or 5 [lower cost than silicon] is realistic. If it’s building integrated, you get additional advantages because, say you have glass, and replace it [with our cells], you would have had the glass cost anyway.

TR: How close is that to being competitive with electricity from fossil fuels?

MG: People say you should be down to 50 cents per peak watt. Our cost could be a little bit less than one dollar manufactured in China. But it depends on where you put your solar cells. If you put them in regions where you have a lot of sunshine, then the equation becomes different: you get faster payback.

TR: Silicon cells have a head-start ramping up production levels. This continues to raise the bar for new technologies, which don’t yet have economies of scale. Can a brand-new type of cell catch up to silicon?

MG: A very reputable journal [Photon Consulting] just published predictions for module prices for silicon for the next 10 years, and they go up the first few years. In 10 years, they still will be above three dollars, and that’s not competitive.

Yes, people are trying to make silicon in a different way, but there’s another issue: energy payback. It takes a lot of energy to make silicon out of sand, because sand is very stable. If you want to sustain growth at 40-50 percent, and it takes four or five years to pay all of the energy back [from the solar cells], then all of the energy the silicon cells produce, and more, will be used to fuel the growth.

And mankind doesn’t gain anything. Actually, there’s a negative balance. If the technology needs a long payback, then it will deplete the world of energy resources. Unless you can bring that payback time down to where it is with dye-cells and thin-film cells, then you cannot sustain that big growth. And if you cannot sustain that growth, then the whole technology cannot make a contribution.

TR: Why does producing your technology require less energy?

MG: The silicon people need to make silicon out of silicon oxide. We use an oxide that is already existing: titanium oxide. We don’t need to make titanium out of titanium oxide.

TR: An exciting area of basic research now is using nanocrystals, also called quantum dots, to help get past theoretical limits to solar-cell efficiency. Can dye-sensitized cells play a role in the development of this approach?

MG: When you go to quantum dots, you get a chance to actually harvest several electrons with one photon. So how do you collect those? The quantum dots could be used instead of a [dye] sensitizer in solar cells. When you put those on the titanium dioxide support, the quantum dot transfers an electron very rapidly. And we have shown that to happen.

TR: You are campaigning for increased solar-cell research funding, and not just for Grätzel cells.

MG: There’s room for everybody.

I am excited that the United States is taking a genuine interest in solar right now, after the complete neglect for 20 years. The Carter administration supported solar, but then during the Reagan administration, it all dropped down by a factor of 10. And labs like NREL [National Renewable Energy Laboratory in Golden, CO] had a hard time surviving. But I think there is going to be more funding.

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