Mining Fool's Gold for Solar

Cyrus Wadia is using abundant materials to grow nanocrystals for cheaper photovoltaics.

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.

Wadia synthesizes the nanocrystals of copper sulfide by injecting copper and sulfide salts and a surfactant into a three-neck flask over a hot plate; as a magnetic stir bar spins inside, nano­particles of the compound form. After removing the surfactant and resuspending the nano­particles in chloroform, he transfers them into a glove box. Inside is a glass chip, about 2.5 centimeters square, that has been coated with a thin layer of indium tin oxide, which acts as an electrical contact. Wadia places the glass chip on a small disc and pipettes the inky black suspension of pyrite nano­crystals onto it. He starts the disc spinning rapidly for a minute to spread the nanocrystals in an even layer. Then he sets the chip on a hot plate and heats it for 10 to 15 minutes to fix the particles to its surface.

After Wadia repeats the process with the copper sulfide solution, the bottom electrical contact is covered by the nanoparticle layers. He gives the chip a quick swipe with a plain cotton swab to reëxpose a strip of the indium tin oxide that acts as the bottom electrical contact for the cell. He then covers the chip with a mask, or stencil, that outlines two sets of four squares with rectangular tails. Wadia places the chip and a small piece of solid aluminum inside a thermal evaporator that looks like a metal bell jar. After he seals the jar, he heats it; the aluminum evaporates, and as it cools, it settles on the exposed parts of the chip. This creates eight square electrical contacts with tails that lead to the edge of the chip.

Pyrite Sees the Light
The chip is now ready for testing. Wadia unscrews a solar-cell tester, places the chip inside, and screws it back together. He then illuminates it with light that mimics the distribution of wavelengths found in sunlight. When the light hits the chip, the system measures the current, the voltage across the chip, and other properties. A screen displays a plot of the current running through the cell against the voltage running across it. So far, the pyrite-based cells have proved dis­appointing in their performance, though the Berkeley researcher­s have used copper sulfide in combination with cadmium sulfide to make cells that have a 1.6 percent efficiency. That’s not good enough for practical use, but the results are promising enough to justify continuing work on the technology.

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 nano­meters 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 bio­engineers 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.”

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