Transport Evidence for Sulfur Vacancies as the Origin of Unintentional n-Type Doping in Pyrite FeS₂

Pyrite FeS₂ has long been considered a potential earth-abundant low-cost photovoltaic material for thin-film solar cells but has been plagued by low power conversion efficiencies and open-circuit voltages. Recent efforts have identified a lack of understanding and control of doping, as well as uncontrolled surface conduction, as key roadblocks to the development of pyrite photovoltaics. In particular, while n-type bulk behavior in unintentionally doped single crystals and thin films is speculated to arise from sulfur vacancies (V_S), proof remains elusive. Here, we provide strong evidence, from extensive electronic transport measurements on high-quality crystals, that V_S are deep donors in bulk pyrite. Otherwise identical crystals grown via chemical vapor transport under varied S vapor pressures are thoroughly characterized structurally and chemically, and shown to exhibit systematically different electronic transport. Decreased S vapor pressure during growth leads to reduced bulk resistivity, increased bulk Hall electron density, reduced transport activation energy, onset of positive temperature coefficient of resistivity, and approach to an insulator-metal transition, all as would be expected from increased V_S donor density. Impurity analyses show that these trends are uncorrelated with metal impurity concentration and that extracted donor densities significantly exceed total impurity concentrations, directly evidencing a native defect. Well-controlled, wide-range n-doping of pyrite is thus achieved via the control of V_S concentration, with substantial implications for photovoltaic and other applications. The location of the V_S state within the gap, the influence of specific impurities, unusual aspects to the insulator-metal transition, and the influence of doping on surface conduction are also discussed.