Pyrite - a solar eclipse of the heart
US researchers say they have found what looks to be a new class of abundant, benign and efficient photovoltaic (PV) materials for solar cells.
The discovery comes as a result of a study conducted at Oregon State University’s Center for Inverse Design into why iron pyrite (FeS2) is unsuitable as a PV material, despite its ability to absorb huge amounts of solar energy.
Work centred on FeS2 films, which are more relevant to PV devices, to establish why they are p-type and why they show no photo-response. The first step was to address the stoichiometry of iron pyrite and resolve the intrinsic nature of this composition.
Using a combination of techniques, including density function theory, the planewave projector augmented-wave method and Hall effect measurements, the researchers found that sulphur deficiencies are a common trait in FeS2, but that they are not bulk vacancies as previously thought. Rather, they are accommodated through coexisting secondary metallike phases, whose presence as largely amorphous forms in the films provides a source of hole carriers that in turn leads to free-carrier absorption at energies below pyrite’s 0.9eV band gap.
Despite annealing samples in excess sulphur vapour for an extended period of time at temperatures of 300–600°C, the films could not be fully converted to stoichiometric FeS2, pointing to an inherent thermal instability of the compound. This explains the challenges in producing high-quality, single-phase FeS2 films required for PV applications.
As a result, the team replaced the previous design rule of avoiding bulk sulphur vacancies with one of selecting compounds that do not spontaneously phase-separate into sulphurdeficient conducting materials with small band gaps. Also, to ensure a sufficiently large band gap, the Fe2+ ion had to be bound by at least six sulphur atoms to provide a ligand-field splitting of enough magnitude for effective solar absorption. This generally requires Fe2+ in an octahedral site, and such a site can be stabilised by adding a third element with an electronegativity that favours strong covalent bonding with sulphur.
These conditions led them to look at iron silicon sulphide and iron germanium sulphide (Fe2SiS4 and Fe2GeS4). Compared with FeS2, they found that phase coexistence is not an issue with either compound, while their higher band gaps (1.4 –1.5eV) allow for more efficient solar radiation absorption.
Now that these compounds have been identified, work is starting in earnest to develop practical solar cells with them. But this is some way off, as team member Professor Douglas Keszler, explains, ‘We are attempting to provide the wider solar-cell community with enough data and information so that they can proceed with integration efforts. In the case of these compounds, we expect that significant effort will likely be required over the next 10 years to develop and integrate appropriate contact layers in a cell.’
This work is a useful and important step forward though, says Professor Ken Durose of the Stephenson Institute for Renewable Energy at the University of Liverpool. ‘These guys have put the marker down. What has to be done now is to show it will work. They have done a good job here though and I’ll certainly be keeping an eye on this.’