"Long before it's in the papers"
November 29, 2011

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“Fool’s gold” may lead to real treasure for solar cell developers

Nov. 29, 2011
Courtesy of Oregon State University
and World Science staff

Py­rite, bet­ter known as “fool’s gold,” was known to the an­cient Ro­mans and has led prospec­tors astray for cen­turies. But it has now al­so helped some re­search­ers dis­cov­er re­lat­ed sub­stances that they say of­fer new, cheap and prom­is­ing op­tions for so­lar en­er­gy.

The new com­pounds, un­like some so­lar cell ma­te­ri­als made from rare, ex­pen­sive or tox­ic el­e­ments, would be be­nign and could be pro­cessed from some of the most plen­ti­ful el­e­ments, say the sci­en­tists. Find­ings have been pub­lished in the re­search jour­nal Ad­vanced En­er­gy Ma­te­ri­als.

Pyrite. (Image courtesy Oregon State U.)
Ap­par­ently ev­er the un­wanted sib­ling, py­rite it­self is as con­sid­ered about as use­less for so­lar en­er­gy as it is as a sub­sti­tute for the yel­low metal, which it de­cep­tively re­sem­bles. But for more than 25 years, the min­er­al, al­so called iron py­rite, was known to have some qual­i­ties that made it of in­ter­est for so­lar en­er­gy. That spurred the re­cent re­search. 

The re­sults have been an­y­thing but fool­ish, said Doug­las Kes­zler, a chem­ist at Or­e­gon State Uni­vers­ity, where the U.S. En­er­gy De­part­ment-funded in­ves­ti­ga­t­ions are un­der way.

“We’ve known for a long time that py­rite was in­ter­esting for its so­lar prop­er­ties, but that it did­n’t ac­tu­ally work,” he ex­plained. “We did­n’t really know why, so we de­cid­ed to take an­oth­er look at it. In this pro­cess we’ve dis­cov­ered some dif­fer­ent ma­te­ri­als that are si­m­i­lar to py­rite, with most of the ad­van­tages but none of the prob­lems.

“The­re’s still work to do in in­te­grat­ing these ma­te­ri­als in­to ac­tu­al so­lar cells,” Kes­zler said. “But fun­da­men­tal­ly, it’s very prom­is­ing.”

Py­rite drew at­ten­tion early in the so­lar en­er­gy era be­cause it had an enor­mous ca­pacity to ab­sorb so­lar en­er­gy, was abun­dant, and could be used in lay­ers 2,000 times thin­ner than some of its com­peti­tors, such as sil­i­con. But it did­n’t ef­fec­tively con­vert the so­lar en­er­gy in­to elec­tricity. The new study pro­poses a rea­son why. In the pro­cess of cre­at­ing so­lar cells, which takes a good deal of heat, py­rite starts to de­com­pose and forms prod­ucts that pre­vent the crea­t­ion of elec­tricity.

Based on their new un­der­stand­ing of ex­actly what the prob­lem was, Kes­zler and col­leagues iden­ti­fied com­pounds that had the same ca­pa­bil­i­ties of py­rite but did­n’t de­com­pose. One was iron sil­i­con sul­fide.

“Iron is about the cheap­est el­e­ment in the world to ex­tract from na­ture, sil­i­con is sec­ond, and sul­fur is vir­tu­ally free,” Kes­zler said. “These com­pounds would be sta­ble, safe, and would not de­com­pose. The­re’s noth­ing here that looks like a show-stopper in the crea­t­ion of a new class of so­lar en­er­gy ma­te­ri­als.”

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Pyrite, better known as “fool’s gold,” was known to the ancient Romans and has led prospectors astray for centuries. But it has now also helped some researchers discover related substances that they say offer new, cheap and promising options for solar energy. The new compounds, unlike some solar cell materials made from rare, expensive or toxic elements, would be benign and could be processed from some of the most plentiful elements, say the scientists. Findings have been published in the research journal Advanced Energy Materials. Apparently ever the unwanted sibling, pyrite itself is as considered about as useless for solar energy as it is as a substitute for gold, which it deceptively resembles. But for more than 25 years, the mineral, also called iron pyrite, was known to have some desirable qualities that made it of interest for solar energy. That spurred the recent research. The results have been anything but foolish, said Douglas Keszler, a chemist at Oregon State University where the investigations are under way, funded by the U.S. Energy Department. “We’ve known for a long time that pyrite was interesting for its solar properties, but that it didn’t actually work,” he explained. “We didn’t really know why, so we decided to take another look at it. In this process we’ve discovered some different materials that are similar to pyrite, with most of the advantages but none of the problems. “There’s still work to do in integrating these materials into actual solar cells,” Keszler said. “But fundamentally, it’s very promising.” Pyrite drew attention early in the solar energy era because it had an enormous capacity to absorb solar energy, was abundant, and could be used in layers 2,000 times thinner than some of its competitors, such as silicon. But it didn’t effectively convert the solar energy into electricity. The new study proposes a reason why. In the process of creating solar cells, which takes a good deal of heat, pyrite starts to decompose and forms products that prevent the creation of electricity. Based on their new understanding of exactly what the problem was, Keszler and colleagues identified compounds that had the same capabilities of pyrite but didn’t decompose. One was iron silicon sulfide. “Iron is about the cheapest element in the world to extract from nature, silicon is second, and sulfur is virtually free,” Keszler said. “These compounds would be stable, safe, and would not decompose. There’s nothing here that looks like a show-stopper in the creation of a new class of solar energy materials.”