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Fool’s Gold may be worth its weight in gold when it comes to solar energy.

As the installation of photovoltaic solar cells continues to accelerate, scientists need to find inexpensive materials beyond the traditional silicon that can efficiently convert sunlight into electricity.

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Iron pyrite, or better known as fool’s gold, could do the job, but when it works at all, the conversion efficiency remains low. But that may soon change because a University of Wisconsin-Madison research team knows why that is the case. If they make a few improvements, this material could lead to inexpensive yet efficient solar cells.

“We think we now understand why pyrite hasn’t worked,” said chemistry Professor Song Jin, “and that provides the hope, based on our understanding, for figuring out how to make it work. This could be even more difficult, but exciting and rewarding.”

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Although most commercial photovoltaic cells nowadays use silicon, the light-collecting film must be relatively thick and pure, which makes the production process costly and energy-intensive, Jin said.

A film of iron pyrite, which would be a compound built of iron and sulfur atoms, could be 1,000 times thinner than silicon and still efficiently absorb sunlight.

Like silicon, iron and sulfur are common elements in the Earth’s crust, so solar cells made of iron pyrite could have a significant material cost advantage in large scale deployment. In fact, previous research that balanced factors like theoretical efficiency, materials availability, and extraction cost put iron pyrite at the top of the list of candidates for low-cost and large-scale photovoltaic materials.

In a photovoltaic material, absorption of sunlight creates oppositely charged carriers, called electrons and holes, that must separate in order for sunlight to convert to electricity.

The efficiency of a photovoltaic solar cell ends up judged by three parameters, Jin said, and the solar cells made of pyrite were almost totally deficient in one: Voltage. Without a voltage, a cell cannot produce any power, he said. Yet based on its essential parameters, iron pyrite should be a reasonably good solar material. “We wanted to know, why is the photovoltage so low?” Jin asked.

“We did a lot of different measurements and studies to look comprehensively at the problem,” said Cabán-Acevedo, “and we think we have fully and definitively shown why pyrite, as a solar material, has not been efficient.”

In exploring why pyrite was practically unable to make photovoltaic electricity, many researchers have looked at the surface of the crystals, but Cabán-Acevedo and Jin also looked inside. “If you think of this as a body, many have focused on the skin, but we also looked at the heart,” said Miguel Cabán-Acevedo, a chemistry Ph.D. student and first author of a paper on the subject. “We think the major problems lie inside, although there are also problems on the skin.”

The internal problems, called “bulk defects,” occur when a sulfur atom is missing from its expected place in the crystal structure. These defects are intrinsic to the material properties of iron pyrite and are present even in ultra-pure crystals. Their presence in large numbers eventually leads to the lack of photovoltage for solar cells based on iron pyrite crystals.

Science advances by comprehending causes, Jin said. “Our message is that now we understand why pyrite does not work. If you don’t understand something, you must try to solve it by trial and error. Once you understand it, you can use rational design to overcome the obstacle. You don’t have to stumble around in the dark.”

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