Iron pyrite (cubic FeS2) seems a nearly ideal absorber layer for scalable thin-film solar cells due to its suitable band gap (widely accepted to be ~0.95 eV), high absorption coefficient (a > 105 cm-1 for h? > 1.3-1.4 eV), sufficient minority carrier diffusion length, practically infinite elemental abundance, and low toxicity. Although large short-circuit photocurrent densities (> 30 mA cm-2) have been reported from pyrite single crystals, all pyrite photocells to date suffer from a low open-circuit voltage (VOC) that limits their power conversion efficiency to ~3%. Proposed explanations for the low VOC include bulk phase impurities, a high concentration of bulk point defects (particularly sulfur and iron vacancies), a metallic FeS-like surface layer, and surface states that generate a hole-rich surface inversion/accumulation layer, possibly having a reduced band gap compared to bulk pyrite. We have recently argued that the only explanation consistent with the main experimental data (i.e., high photocurrents but low photovoltages from pyrite single crystals, as well as the ubiquitous high p-type conductivity of pyrite thin films regardless of composition or preparation method) is that pyrite has a hole-rich surface inversion/accumulation layer that results in a leaky and/or small potential energy barrier.In this talk, I describe how numerical modeling of Hall effect data has been used to demonstrate the existence of a conductive inversion layer at the surface of high-quality n-type single crystals of iron pyrite (cubic FeS2) grown by a flux technique. The presence of the inversion layer is corroborated by photoemission spectroscopy and Hall measurements as a function of crystal thickness. This hole-rich surface layer can explain both the low photovoltage of pyrite solar cells and the near-universal high p-type conductivity of polycrystalline pyrite thin films that have together perplexed researchers for the past thirty years. We find that the thickness and conductivity of the inversion layer can be modified by mechanical and chemical treatments of the pyrite surface, suggesting that it is possible to eliminate this hole-rich layer by passivating surface states and subsurface defects. Furthermore, modeling of the high-temperature electrical conductivity shows that the electronic band gap is 0.8 � 0.05 eV at room temperature (compared to 0.94 eV according to optical transmission data), confirming that photovoltages of ~500 mV should be attainable from pyrite under solar illumination.

Inversion layer at the surface of n-type iron pyrite

247th National Meeting (2014)

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