Controlled p -Type Doping of Pyrite FeS 2 .

Pyrite FeS 2 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 2 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 n -doping of FeS 2 on a firm footing, unequivocal evidence for bulk p -doping remains elusive. Here, we demonstrate the first unambiguous and controlled p -type transport in FeS 2 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 18 holes/cm 3 at 300 K, while leaving the crystal structure and quality unchanged. As the P doping is increased in crystals natively n -doped with S vacancies, the majority carrier type inverts from n to p 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 p -type occurs in the bulk and is not an artifact of hopping conduction. Such controlled bulk p -doping opens the door to pyrite p-n homojunctions, unveiling new opportunities for solar cells based on this extraordinary semiconductor.