| AVS 59th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+TF-MoA |
| Session: | Chalcogenide Solar Cells I |
| Presenter: | X. Zhang, University of Minnesota |
| Authors: | X. Zhang, University of Minnesota M. Manno, University of Minnesota A. Baruth, University of Minnesota M. Johnson, University of Minnesota E.S. Aydil, University of Minnesota C. Leighton, University of Minnesota |
| Correspondent: | Click to Email |
Pyrite FeS2 is undergoing a tremendous resurgence of interest as a candidate thin-film solar absorber based on abundant, low-cost, non-toxic elements. Historically, FeS2-based Schottky solar cells have suffered from low open circuit voltages (~ 100 mV), and thus low efficiency, although the origins of this behavior are not entirely clear. In fact, even the electronic properties of FeS2 are not well understood, including the conduction mechanisms and doping behavior. Understanding these issues could contribute significantly to improvements in FeS2-based solar cells, particularly if doping can be understood and controlled. In this work, we present a comprehensive study of the conduction mechanism in FeS2 thin films synthesized via the ex situ sulfidation of Fe films in a S vapor at sulfidation temperatures in the range 100 oC ≤ TS ≤ 700 oC . The resultant films were characterized structurally, using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and confocal Raman microscopy; electrically, via transport and magnetoresistance measurements between 4.5 and 300 K; and magnetically with high-sensitivity dc magnetometry. At TS around 500 ° C we observe a crossover in the conduction mechanism from some form of hopping conduction to a more conventional band transport-type mechanism. Through detailed analysis of the hopping parameters, measurement of the Fe spin-state, and simple calculations based on S diffusion in Fe, we demonstrate that intergranular hopping occurs via highly conductive, S-deficient, nanoscopic grain cores separated by nominally stoichiometric FeS2 shells. We find that the approach towards more conventional band transport as TS is increased above 500 oC is due to an increase of S diffusion into the FeS2 grains. Moreover, this conduction mechanism crossover is found to be accompanied by a sign reversal of the Hall coefficient, from hole-like (in the hopping regime) to electron-like. In addition to placing hard constraints on the conditions under which useful properties can be obtained from FeS2 synthesized under diffusion-limited conditions, these results also highlight potential problems with prior conclusions on the dominance of p-type behavior.
The work was supported by the Initiative for Renewable Energy & the Environment, IREE ((RL-0004-11) . Part of this work was carried out in the University of Minnesota Characterization Facility, a member of the NSF-supported Material Research Facilities Network.