Setback for fool's gold photovoltaics

Transmission electron microscopy images of iron pyrite. New work found that the conductivity of the crystals was too high for the material to be any use in photovoltaics. Credit: ACS

Extensive tests by US researchers on nanocrystals of the pyrite phase of iron sulfide – also known as fool’s gold – suggest that the material is unlikely to be a good candidate for photovoltaic (PV) applications, contrary to some predictions. Not all experts agree, however, and believe that there could yet be some genuine sparkle in the material.

‘There were early data in the 1980s and 90s looking at pyrite photovoltaics which found that this was possible since pyrite was a semiconductor,’ says Brian Korgel of the University of Texas at Austin, who led the research. ‘But the devices performed poorly and in a sense it was dropped.’ However, interest has been revived recently, given the relatively low cost of preparing Fe₂S. ‘It is made of Earth-abundant materials and is not toxic and, because it is dark, it is a really strong light-absorber,’ Korgel notes.

The team used an established technique to create nanocrystals of pyrite and incorporated the crystals into four different types of photovoltaic device. ‘The pyrite nanocrystals simply did not work,’ Korgel says. ‘There was no PV activity. When we measured the electrical properties we found they were very electrically conductive, like a metal. You can’t make a PV with a metal as an absorber layer.’

The conductivity is a result of high doping of the crystals, Korgel says. ‘Theory has shown that surfaces of pyrite can sustain defects that give rise to doping like this. Therefore it might be possible make pyrite PVs if someone figures out how to passivate the surfaces, but I’m convinced, based on our studies, that it won’t be easy. Someone must do something quite special for it to work. I think it is a dead-end.’

However, not everyone is as pessimistic. Cyrus Wadia, of the Lawrence Berkeley National Laboratory in the US and currently assistant director of Clean Energy and Materials R&D in the US government Office of Science and Technology Policy, says: ‘In general, I think the work is an important step in our understanding of pyrite.’ Wadia adds: ‘To balance my optimism for future pyrite work, I will be the first to state that pyrite has great promise but we are just beginning to learn about the many practical challenges of using pyrite in a photoelectrical device. So any results, good or bad, feed our understanding and we must keep expanding that knowledge base. That said, I wouldn't jump to conclusions based on these results alone. Korgel's work draws on existing best practices for growing pyrite nanocrystals which we already believe to be problematic. I think we are still a long way from knowing the true potential of pyrite in a photovoltaic application.’