Iron Sulfide for Photovoltaics

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Abstract
In recent years, iron sulfide (FeS2), otherwise known as pyrite, has received some attention as a possible candidate for thin-film photovoltaic applications. This interest stems in part from the fact that it has a relatively large absorption coefficient of α > 105 cm−1 for photon energies larger than about 1.3 eV (see Figure 1), such that a thin layer of less than 1,000 Å should effectively absorb the visible part of the solar spectrum. The bandgap of pyrite of 0.95 eV is probably on the borderline of what might still be considered acceptable for a high-efficiency single-junction device, but can be considered ideal in combination with a 1.8 eV material for tandem structures. Another important advantage of this compound semiconductor is that it is composed of cheap, abundant, and non-toxic elements, the latter being of particular importance in times of growing environmental consciousness.Pyrite is a naturally occurring mineral (also known as “fool's gold” because of its golden color) which shows both n- and p-type conduction. This could indicate possible electronic viability of this material. The study of synthetic crystals has been severely impeded by the fact that pyrite undergoes a peritectic decomposition into the more stable FeS and liquid sulfur at 743°C and that therefore the melt growth of high-purity large single-crystalline material is not possible. Nevertheless, it has been shown that polycrystalline pyrite layers could be prepared with different thin-film preparation techniques (see Table I), such that in principle a cost-effective large-scale application could be envisaged.