Pyrite-structure iron disulfide, in addition to being an important mineral, is also an $\sim 1$-eV-band-gap semiconductor with substantial application potential. In particular, pyrite's earth-abundant, nontoxic, low-cost constituents, combined with its high visible light absorption, high carrier mobility, good minority carrier diffusion length, and other attributes, have led to it being pursued as a photovoltaic absorber material. An effort in this direction from the 1980s was not particularly successful, however, with pyrite-based solar cells not exceeding $\sim 3\%$ power conversion efficiency. Subsequent to that effort, increased focus on sustainable energy technologies drove a substantial resurgence of interest in ${\mathrm{FeS}}_2$, more closely focused on the fundamental obstacles to high performance as a photovoltaic absorber. This second wave of effort took place over the last 15 years or so, quite strongly focused on high-quality ${\mathrm{FeS}}_2$ single crystals as model systems. This Research Update aims to summarize the knowledge and understanding gained through this recent effort, focusing specifically on the electronic (mostly transport) properties of single-crystal pyrite. Highlighted advances include comprehensive study of surface conduction in ${\mathrm{FeS}}_2$, greatly improved understanding of doping, and consequent deeper insight into fundamental behavior such as conduction mechanisms, the insulator-metal transition, and doping-induced magnetism. Through such advances, substantial insights into the origins of the underperformance of pyrite ${\mathrm{FeS}}_2$ as a photovoltaic material have been obtained, to the point that fresh strategies for redesigned solar (and other) devices mitigating these problems can now be formulated and tested.

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• Doping effects
• Hopping transport
• Metal-insulator transition
• Photovoltaic absorbers
• Surface states
• Transport phenomena
• Doped semiconductors
• Semiconductors