Controlled -Type Doping of Pyrite FeS.

Pyrite FeS has extraordinary potential as a low-cost, nontoxic, sustainable photovoltaic but has underperformed dramatically in prior solar cells. The latter devices focus on heterojunction designs, which are now understood to suffer from problems associated with FeS surfaces. Simpler homojunction cells thus become appealing but have not been fabricated due to the historical inability to understand and control doping in pyrite. While recent advances have put S-vacancy and Co-based -doping of FeS on a firm footing, unequivocal evidence for bulk -doping remains elusive. Here, we demonstrate the first unambiguous and controlled -type transport in FeS single crystals doped with phosphorus (P) during chemical vapor transport growth. P doping is found to be possible up to at least ∼100 ppm, inducing ∼10 holes/cm at 300 K, while leaving the crystal structure and quality unchanged. As the P doping is increased in crystals natively -doped with S vacancies, the majority carrier type inverts from to near ∼25 and ∼55 ppm P, as detected by Seebeck and Hall effects, respectively. Detailed temperature- and P-doping-dependent transport measurements establish that the P acceptor level is 175 ± 10 meV above the valence band maximum, explain details of the carrier inversion, elucidate the relative mobility of electrons and holes, reveal mid-gap defect levels, and unambiguously establish that the inversion to -type occurs in the bulk and is not an artifact of hopping conduction. Such controlled bulk -doping opens the door to pyrite homojunctions, unveiling new opportunities for solar cells based on this extraordinary semiconductor.