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.