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Ternary Pyrite Photovoltaics

December 15, 2011

Materials scientists are always on the lookout for cheaper, and less toxic, photovoltaic materials. One possible contender for a next generation photovoltaic material is iron pyrite, which I reviewed in a previous article (Pyrite Photovoltaics, January 24, 2011). Iron pyrite, FeS2, called "fool's gold" because its color resembles gold's luster, is a common mineral on Earth since iron and sulfur are both common.

Pyrite crystalsPyrite crystals.

(Oregon State University photograph).

Pyrite is not that interesting chemically, but it's a semiconductor, and it can be used to make photovoltaic diodes with a bandgap of 0.95 eV. This bandgap is quite close to that of silicon (see table). Only photons with energy greater than a semiconductor's bandgap can produce a current in a photovoltaic device. In the table, this translates to all wavelengths below the listed values. Nearly all the solar spectrum is accessible to pyrite.

Semiconductor Formula Bandgap (eV) Wavelength (nm)
Germanium Ge 0.67 1853
Iron Pyrite FeS2 0.95 1307
Silicon Si 1.11 1118
Gallium Arsenide GaAs 1.43 868
Silicon Carbide SiC 2.86 434

Scientists at the University of California (Irvine) have investigated production of nanocrystalline iron pyrite for thin-film solar cells.[1-2] They developed a process for making high quality nano-colloidal inks of iron pyrite. Using the inks, they prepared polycrystalline pyrite thin films.

A group from the University of California (Berkeley) and Lawrence Berkeley National Laboratory has investigated hydrothermal synthesis of 100 to 500 nm pyrite nanocrystals.[3-5] Berkeley team member, Cyrus Wadia, says that the theoretical efficiency of iron sulfide is 31 percent, and 20 nanometers of pyrite can absorb as much light as 300 micrometers of silicon.[5]

However, there are some problems with pyrite as a photovoltaic. It's hard to prepare the material defect-free and with high efficiency. A recent research collaboration between Oregon State University and the National Renewable Energy Laboratory focused on the problems with pyrite and how they could be overcome.[7]

One problem is the decomposition of pyrite at processing temperatures. At lower temperatures, iron pyrite will oxidize, and at temperatures above 550°C it will decompose into iron sulfide (FeS) and sulfur. The research team found that chemical relatives of pyrite, Fe2SiS4 and Fe2GeS4, had less of a problem. These ternary materials also have a higher bandgap than iron pyrite and can harvest more of the solar spectrum.[6]

Douglas Keszler, a distinguished professor of chemistry at Oregon State University and director of its Center for Green Materials Chemistry, had this summary of the team's work.
"Iron is about the cheapest element in the world to extract from nature, silicon is second, and sulfur is virtually free... 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... The beauty of a material such as this is that it is abundant, would not cost much and might be able to produce high-efficiency solar cells. That's just what we need for more broad use of solar energy."[7]

This work was performed at Oregon State University's Center for Inverse Design, funded in 2009 by a $3 million grant from the US Department of Energy. It's one of the Energy Frontier Research Centers funded under a $777 million US government energy program.[7]

References:

  1. Tiffany Hsu, "Fool's Gold Catches Eye Of Solar Energy Researchers," Los Angeles Times, January 14, 2011.
  2. James Puthussery, Sean Seefeld, Nicholas Berry, Markelle Gibbs, and Matt Law, "Colloidal Iron Pyrite (FeS2) Nanocrystal Inks for Thin-Film Photovoltaics," J. Am. Chem. Soc., DOI: 10.1021/ja1096368, December 22, 2010.
  3. Cyrus Wadia, Yue Wu, Sheraz Gul, Steven K. Volkman, Jinghua Guo and A. Paul Alivisatos, "Surfactant-Assisted Hydrothermal Synthesis of Single phase Pyrite FeS2 Nanocrystals," Chem. Mater., vol. 21, no. 13 (June 16, 2009), 2009, 21 (13), pp 2568-2570.
  4. Cyrus Wadia, A. Paul Alivisatos and Daniel M. Kammen, "Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment," Environ. Sci. Technol. vol. 43, no. 6 (February 13, 2009), pp 2072-2077.
  5. "Mining fool's gold for solar: Cyrus Wadia is using abundant materials to grow nanocrystals for cheaper photovoltaics,"Technology Review, November 1, 2009 (via ecnext.com).
  6. Liping Yu, Stephan Lany, Robert Kykyneshi, Vorranutch Jieratum, Ram Ravichandran, Brian Pelatt, Emmeline Altschul, Heather A. S. Platt, John F. Wager, Douglas A. Keszler and Alex Zunger, "Iron Chalcogenide Photovoltaic Absorbers," Advanced Energy Materials, vol. 1, no. 15 (October, 2011), pp. 748-753.
  7. David Stauth, "Fool's gold leads to new options for cheap solar energy," Oregon State University Press Release, November 23, 2011.