Surface conduction in n-type pyrite FeS2 single crystals

Pyrite FeS2 has long been recognized as a high potential photovoltaic material, due to its exceptionally high optical absorption, low toxicity, and the abundance and low cost of its constituents. Despite the suitable band gap (0.95 eV), minority carrier diffusion length (100-1000 nm), and short-circuit current density, power conversion efficiencies in FeS2-based solar cells have never exceeded 3% however, primarily due to low open circuit voltages (V-oc similar to 0.1 V). Surface phenomena have been implicated as the root cause of this low V-oc, recent experiments on n-type crystals providing evidence for surface conduction, including p-type surface inversion. Here we report a detailed study of electronic transport in a large set (similar to 120 samples) of thoroughly characterized vapor-transport-grown n-FeS2 single crystals, with both as-prepared and mechanically polished surfaces. Abundant evidence for surface conduction is obtained from the temperature dependence of the resistance and its anisotropy, the thickness dependence of the resistivity, the sensitivity to surface preparation, and the nature of an observed surface insulator-metal transition. While the bulk transport is relatively reproducible, as-grown crystals display striking diversity in surface behavior, which is suppressed by polishing. Via detailed analyses, we demonstrate that the FeS2 surface conduction is truly two dimensional, that it can influence in-plane transport even at room temperature, and that a p-type surface inversion layer can be unambiguously established, with no possibility of artifacts from hopping conduction. A nonlinear Hall effect is also observed, allowing us to constrain a two-channel conduction model we show capable of describing all field-and temperature-dependent transport data. Combined with simple arguments, these results place limits on the thickness of the surface conduction layer, which lie below similar to 3 nm. Finally, in some crystals, for unknown reasons, the as-grown surface is definitively n type. These results highlight that while surface conduction is clearly important in pyrite FeS2, and is gradually yielding to understanding, additional work is clearly warranted to further understand and control it.