Scientists get to the heart of fool's gold as a solar material

November 18, 2014

MADISON, Wis. - As the installation of photovoltaic solar cells continues to accelerate, scientists are looking for inexpensive materials beyond the traditional silicon that can efficiently convert sunlight into electricity.

Theoretically, iron pyrite -- a cheap compound that makes a common mineral known as fool's gold -- could do the job, but when it works at all, the conversion efficiency remains frustratingly low. Now, a University of Wisconsin-Madison research team explains why that is, in a discovery that suggests how improvements in this promising material could lead to inexpensive yet efficient solar cells.

"We think we now understand why pyrite hasn't worked," says 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."

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

A film of iron pyrite -- 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 the current online edition of the Journal of the American Chemical Society, Jin and first author Miguel Cabán-Acevedo, a chemistry Ph.D. student, together with other scientists at UW-Madison, explain how they identified defects in the body of the iron pyrite material as the source of inefficiency. The research was supported by the U.S. Department of Energy.

In a photovoltaic material, absorption of sunlight creates oppositely charged carriers, called electrons and holes, that must be separated in order for sunlight to be converted to electricity. The efficiency of a photovoltaic solar cell can be judged by three parameters, Jin says, and the solar cells made of pyrite were almost totally deficient in one: voltage. Without a voltage, a cell cannot produce any power, he points out. 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 says.

"We did a lot of different measurements and studies to look comprehensively at the problem," says 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," says Cabán-Acevedo, "and 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 says. "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."
-end-
CONTACT: Song Jin, 608-262-1562, jin@chem.wisc.edu (prefers email for first contact)

--David Tenenbaum, 608-265-8549, djtenenb@wisc.edu

University of Wisconsin-Madison

Related Iron Articles from Brightsurf:

How stony-iron meteorites form
Meteorites give us insight into the early development of the solar system.

Bouillon fortified with a new iron compound could help reduce iron deficiency
Iron fortification of food is a cost-effective method of preventing iron deficiency.

Iron nanorobots go undercover
Customizable magnetic iron nanowires pinpoint and track the movements of target cells.

Iron deficiency in corals?
When iron is limited, the microalgae that live within coral cells change how they take in other trace metals, which could have cascading effects on vital biological functions and perhaps exacerbate the effects of climate change on corals.

Blocking the iron transport could stop tuberculosis
The bacteria that cause tuberculosis need iron to survive. Researchers at the University of Zurich have now solved the first detailed structure of the transport protein responsible for the iron supply.

Observed: An exoplanet where it rains iron
Nature magazine is publishing today a surprising study about the giant, ultra-hot planet WASP-76b in which researchers from the Instituto de Astrofísica de Canarias (IAC) have taken part.

An iron-clad asteroid
Mineralogists from Jena and Japan discover a previously unknown phenomenon in soil samples from the asteroid 'Itokawa': the surface of the celestial body is covered with tiny hair-shaped iron crystals.

It's Iron, Man: ITMO scientists found a way to treat cancer with iron oxide nanoparticles
Particles previously loaded with the antitumor drug are injected in vivo and further accumulate at the tumor areas.

The brain may need iron for healthy cognitive development
Iron levels in brain tissue rise during development and are correlated with cognitive abilities, according to research in children and young adults recently published in JNeurosci.

The regulators active during iron deficiency
Iron deficiency is a critical situation for plants, which respond using specific genetic programmes.

Read More: Iron News and Iron Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.