October 25, 2009

Status and Potential of Pyrite Solar Power

An analysis showed that low cost materials are needed to scal solar power and Pyrite (FeS2) is a leading candidate.

Solar photovoltaics have great promise for a low-carbon future but remain expensive relative to other technologies. Greatly increased penetration of photovoltaics into global energy markets requires an expansion in attention from designs of high-performance to those that can deliver significantly lower cost per kilowatt-hour. To evaluate a new set of technical and economic performance targets, we examine material extraction costs and supply constraints for 23 promising semiconducting materials. Twelve composite materials systems were found to have the capacity to meet or exceed the annual worldwide electricity consumption of 17000 TWh, of which nine have the potential for a significant cost reduction over crystalline silicon. We identify a large material extraction cost (cents/watt) gap between leading thin film materials and a number of unconventional solar cell candidates including FeS2, CuO, and Zn3P2. We find that devices performing below 10% power conversion efficiencies deliver the same lifetime energy output as those above 20% when a 3/4 material reduction is achieved. Here, we develop a roadmap emphasizing low-cost alternatives that could become a dominant new approach for photovoltaics research and deployment.

From MIT Technology Review: so far, the pyrite-based cells have proved dis?appointing in their performance, though the Berkeley researcher?s have used copper sulfide in combination with cadmium sulfide to make cells that have a 1.6 percent efficiency

Previous efforts to build solar cells with pyrite produced devices that, at best, converted only 2.8 percent of sunlight into electricity. Wadia thinks the low efficiency is due to inconsistencies in the crystal structure of the pyrite. He is the first to make pyrite nanoparticles, and his method results in pyrite crystals with a uniform, favorable structure. The resulting material, he believes, will outperform conventional pyrite in solar cells.

Cells incorporating pyrite would be preferable because the material is less toxic and cheaper to recover than cadmium compounds. When the pyrite nanoparticles are spun onto the chip, however, nanoscale pinholes tend to form. To electrons, such minuscule gaps look like the Grand Canyon--they cannot cross and migrate into the external electrical circuit. Instead, the electrons tunnel down to the bottom electrode, causing the cell to short-circuit.

It's difficult to make good pyrite films because the nanocrystals tend to sink to the bottom of any liquid. The better a particle is suspended, the smoother the film it will form. Wadia believes that smaller particles might lead to better suspensions: the pyrite particles are 20 to 100 times the size of the copper sulfide particles, which are about five nano?meters across. Wadia is trying everything he can to make them smaller, including mechanically pressing or grinding them and tinkering with reaction conditions. He's also collaborating with bio?engineers at the Lawrence Berkeley National Laboratory to genetically engineer viruses so that they accumulate pyrite nanoparticles on their coats; the next step would be to get the viruses to line up into uniform films.

Wadia acknowledges that he's still many years away from making an efficient solar cell with pyrite nanocrystals. Ultimately, though, his goal is to produce a cell that's cheap enough to make solar energy the dominant power source. He says, "I just need the science to work."

"The theoretical efficiency of iron sulfide is 31 percent. That's as good as silicon," says Wadia. What's more, 20 nanometers of pyrite can absorb as much light as 300 micro?meters of silicon. Because it absorbs so much more light, it can be made into thinner cells, which require less raw material.

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