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Cyrus Wadia makes pure pyrite nanocrystals in his lab at the University of California, Berkeley.

Fool’s gold, also called pyrite or iron sulfide, can be unearthed just about anywhere, from the hills of California to the villages of Yunnan Province in China. But instead of digging pyrite up, researcher Cyrus Wadia is making pure nano­particles of the compound from iron and sulfur salts in his lab at the University of California, Berkeley. His ultimate goal is to turn fool’s gold into real treasure: an inexpensive solar cell.

Today, most solar cells are made of silicon, but they are expensive: though silicon is abundant, turning it into photovoltaics requires extensive, energy-intensive processing. Materials such as cadmium telluride and copper indium gallium diselenide are simpler to process, yielding thin-film cells that cost less to produce. But the elements needed to make these compounds, such as tellurium and gallium, are too rare to meet global energy demands.

So Wadia did a study of possible solar-cell materials, examining not only their chemistry and physics but also their availability. One of the standouts was fool’s gold: it is abundant and cheap, and it has optical properties that allow it to efficiently convert sunlight into electricity. “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 micro­meters of silicon. Because it absorbs so much more light, it can be made into thinner cells, which require less raw material.

Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden, CO, thinks that Wadia and his colleagues “present a compelling case for pursuing these materials.” Although the rarity of the elements used in newer thin films isn’t currently an issue, it will be one in the long term, Beard says. Meanwhile, they pose a more immediate problem: some of them are toxic. These drawbacks make alternatives such as pyrite worth developing.

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.

Crystalline Creations
Pyrite’s crystal structure can take several forms. Only one of them has the electrical properties that make pyrite a good solar material, though, and it takes just the right pH and temperature to generate a solution of nanocrystals that exist solely in that form. To make the crystals, Wadia pipettes chalky-orange iron salts, clear sulfide salts, and a bubbly, iridescent surfactant into a Teflon-lined metal cylinder. The surfactant keeps the particles from clumping as they grow. He seals the cylinder inside an autoclave container and bakes it at 200 °C for four hours. After he takes it out, Wadia unscrews the canister, revealing a clear liquid with a black layer at the bottom: pure pyrite nano­crystals about 100 to 500 nanometers across.

To convert sunlight into usable electricity, solar cells require two different types of semiconductors. When photons hit the iron sulfide, electrons in the compound are excited–but those negative charges can’t flow out of the cell and into an external circuit unless a compound with different electrical properties pulls away the positive charges, called holes. One candidate for the job is copper sulfide, another cheap and abundant material that Wadia has made into nanocrystals in collaboration with Yue Wu, now an assistant professor of chemical engineering at Purdue University.

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Credit: Jen Siska
Video by Katherine Bourzac, edited by Brittany Sauser

Tagged: Energy, Materials, renewable energy, solar cells, photovoltaics, nanocrystals, Fool's gold

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