Single Crystals*

Pyrite FeS₂ has long been considered an ideal semiconductor for low-cost solar cells as it is composed of earth-abundant, non-toxic, inexpensive elements, has a suitable band gap (0.95 eV), and absorbs sunlight strongly. Disappointing power conversion efficiencies, however, have plagued pyrite solar cells. This failing is now widely acknowledged to be due to problems with pyrite’s surface, and a lack of doping control. An important unanswered question is the origin of the n-type behavior seen in unintentionally-doped pyrite single crystals and thin films. Here, we present the first substantial electronic transport evidence that sulfur vacancies are this n-type dopant by varying sulfur vapor pressure (P_S) during single crystal growth. Crystals grown under high P_S exhibit semiconducting behavior, with transport activation energies of 225 meV and 300 K electron densities (n_300K) of 10¹⁶ cm^-3. Decreasing P_S increases n_300K to >10¹⁷ cm^-3 and decreases the activation energy ten-fold, evidencing an evolution towards an insulator-metal transition. These trends are independent of metal impurity concentrations and, moreover, n_300K is too large to be explained by these impurities.