Pyrite FeS2 is an outstanding candidate for a low-cost, nontoxic, sustainable photovoltaic material, but efficient pyrite-based solar cells are yet to materialize. Recent studies of single crystals have shed much light on this by uncovering a p-type surface inversion layer on n-type (S-vacancy doped) crystals, and the resulting internal p-n junction. This leaky internal junction likely plays a key role in limiting efficiency in pyrite-based photovoltaic devices, also obscuring the true bulk semiconducting transport properties of pyrite crystals. Here, we demonstrate complete mitigation of the internal p-n junction in FeS2 crystals by fabricating metallic CoS2 contacts via a process that simultaneously diffuses Co (a shallow donor) into the crystal, the resulting heavy n doping yielding direct Ohmic contact to the interior. Low-temperature bulk transport studies of controllably Co- and S-vacancy doped semiconducting crystals then enable a host of previously inaccessible observations and measurements, including determination of donor activation energies (which are as low as 5 meV for Co), observation of an unexpected second activated transport regime, realization of electron mobility up to 2100cm2V-1s-1, elucidation of very different mobilities in Co- and S-vacancy-doped cases, and observation of an abrupt temperature-dependent crossover to bulk Efros-Shklovskii variable-range hopping, accompanied by an unusual form of nonlinear Hall effect. Aspects of the results are interpreted with the aid of first-principles electronic structure calculations on both Co- and S-vacancy-doped FeS2. This work thus demonstrates unequivocal mitigation of the internal p-n junction in pyrite single crystals, with important implications for both future fundamental studies and photovoltaic devices.
Bibliographical noteFunding Information:
This work was supported by the customers of Xcel Energy through the Renewables Development Fund, and in part by the National Science Foundation NSF through the University of Minnesota (UMN) MRSEC under Grant No. DMR-2011401. Work on Co-doped specifically was supported by the U.S. Department of Energy through the UMN Center for Quantum Materials under Grant No. DE-SC-0016371. Parts of this work were carried out in the Characterization Facility, UMN, which receives partial support from NSF through the MRSEC program. B.V. acknowledges the UMN Doctoral Dissertation Fellowship and D.R. acknowledges the Minnesota Supercomputing Institute for computing resources. We thank B. Shklovskii and T. Birol for informative discussions, and J. Batley for Co contact evaporation and discussions.
© 2021 American Physical Society.