| AVS 62nd International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+AS+EM+NS+SE+SS+TF-MoA |
| Session: | Solar Cells II |
| Presenter: | Eray Aydil, University of Minnesota |
| Authors: | X. Zhang, University of Minnesota M. Li, University of Minnesota L. O'Brien, University of Minnesota J. Walter, University of Minnesota M. Manno, University of Minnesota F. Mork, University of Minnesota J. Kakalios, University of Minnesota E.S. Aydil, University of Minnesota C. Leighton, University of Minnesota |
| Correspondent: | Click to Email |
In principle, pyrite FeS2 is one of the most suitable photovoltaic materials for sustainable low-cost, large-scale solar cell manufacturing because it has high absorbance in the visible and comprises earth-abundant inexpensive elements. However, current efficiencies of solar cells based on pyrite FeS2 have not exceeded 2.8%. Early research on this material concluded that unintentionally doped FeS2 thin films are p-type and subsequent solar cell work evolved based on this presumption. In fact, it is now widely accepted that FeS2 thin films almost always exhibit p-type conduction even though single crystals are typically found to be n-type. This discrepancy between single crystals and thin films is perplexing and to date this puzzle remains unexplained. In this talk we reexamine the conclusion that undoped FeS2 films are predominantly p-type and provide an explanation for this “doping puzzle” in pyrite. Using a combination of Hall effect, thermopower, and temperature-dependent resistivity measurements on a large set of well characterized single crystals and thin films, we show that the widely accepted predominant p-type behavior in pyrite films may, in fact, be an artifact of hopping conduction and should be revisited. Specifically, both Hall effect and thermopower measurements establish that all of our high-mobility (>1 cm2V-1s-1) films and single crystals are n-type. Temperature-dependent resistivity measurements on these high mobility films and crystals establish diffusive electronic transport. We find that films with lower mobility (4x10-3-1 cm2V-1s-1) also show n-type Hall effect but exhibit a p-type Seebeck coefficient, leading to a discrepancy in the measured carrier type. Temperature-dependent resistivity measurements on these intermediate mobility films show a transition from diffusive to hopping transport. Finally, both Hall and Seebeck coefficients are strongly suppressed and invert in the lowest mobility thin films (<4x10-3 cm2V-1s-1) indicating apparent p-type conduction. Temperature-dependent resistivity measurements establish unambiguous hopping behavior in these lowest mobility films. Based on this evolution of Hall and Seebeck coefficients with carrier mobility, and the well-known suppression of the Hall and Seebeck effects in conductors with hopping electronic transport, we conclude that the apparent crossover from n-type to p-type with decreasing mobility is, in fact, an artifact of hopping conduction.
Work supported by the NSF under DMR-1309642, in addition to the University of Minnesota NSF MRSEC under DMR-1420013.