| AVS 66th International Symposium & Exhibition | |
| Thin Films Division | Thursday Sessions |
| Session TF+EM+NS+SS-ThM |
| Session: | Thin Films for Energy Harvesting and Conversion |
| Presenter: | Bryan Voigt, University of Minnesota, Minneapolis |
| Authors: | B. Voigt, University of Minnesota, Minneapolis W. Moore, University of Minnesota, Minneapolis D. Ray, University of Minnesota, Minneapolis M. Manno, University of Minnesota, Minneapolis J.D. Jeremiason, Gustavus Adolphus College L. Gagliardi, University of Minnesota, Minneapolis E.S. Aydil, University of Minnesota, Minneapolis C. Leighton, University of Minnesota, Minneapolis |
| Correspondent: | Click to Email |
Pyrite FeS2 has long been considered an ideal absorber material for low-cost and sustainable thin film solar cells because it is composed of earth-abundant, non-toxic, inexpensive elements, has a suitable band gap (0.95 eV), and absorbs light so strongly that a 100-nm-thick film absorbs >90 % of photons with energies above the band gap. Lack of doping control, however, has presented a barrier to realization of the p-n pyrite homojunction, i.e., the simplest route to a pyrite solar cell. Heterojunction pyrite solar cells have proven to have disappointingly low efficiencies (~3%), surface conduction and leaky surface inversion layers being implicated as the culprit. While mitigation of pyrite surface conduction remains a challenge, doping has begun to yield to understanding, renewing optimism for a p-n pyrite homojunction solar cell. In particular, we have shown that rigorously phase-pure pyrite single crystals and thin films are exclusively n-type, due to a common dopant. Most recently, we have identified sulfur vacancies as this unintentional n-dopant, enabling robust control over n-doping levels in single crystals grown by chemical vapor transport (CVT). Progressing towards a p-n pyrite homojunction, here we demonstrate effective p-type doping in crystals by introducing phosphorus in the vapor phase during CVT growth. Increasing the phosphorus concentration from <0.1 ppm to 30 ppm evolves electronic conduction from n-type to p-type, with a clear and reproducible majority carrier inversion for concentrations >10 ppm. Typical transport properties of phosphorus-doped, p-type pyrite crystals include a hole thermal activation energy, room temperature resistivity, hole density, and mobility of ~170 meV, 3 Ω cm, 2 × 1018 cm-3, and 1 cm2 V-1s-1, respectively. Density functional theory calculations confirm that phosphorus substituted on the S site is an acceptor, predicting a defect level at 200 meV above the valence band maximum, in good agreement with experiment. With both n- and p-type doping control achieved, attempts at p-n pyrite homojunction solar cells become possible.
This work was supported by the customers of Xcel Energy through a grant from the Renewables Development Fund and in part by the National Science Foundation through the University of Minnesota MRSEC under DMR-1420013.