AVS 61st International Symposium & Exhibition
    Energy Frontiers Focus Topic Wednesday Sessions
       Session EN+AS+EM+SE-WeM

Paper EN+AS+EM+SE-WeM5
Phase Transformation, Surface States, and Electronic Structures of Pyrite Thin Films Under In Situ Heating and Oxygen Gas Exposure

Wednesday, November 12, 2014, 9:20 am, Room 315

Session: Thin Film Photovoltaics
Presenter: Yu Liu, University of California Irvine
Authors: Y. Liu, University of California Irvine
N. Berry, University of California Irvine
Y.N. Zhang, University of California Irvine
C.-C. Chen, Argonne National Laboratory
H. Bluhm, Lawrence Berkeley National Laboratory
Z. Liu, Lawrence Berkeley National Laboratory
R.Q. Wu, University of California Irvine
M. Law, University of California Irvine
J.C. Hemminger, University of California Irvine
Correspondent: Click to Email
Iron pyrite (cubic FeS2) with its exceptional optical absorption and suitable band gap is a promising earth-abundant semiconductor for thin film solar cells. Using ambient pressure synchrotron x-ray spectroscopies, we report the nanoscale depth profiles of surface and electronic structures for phase-pure pyrite thin films under in situ heating and oxygen gas exposure. Polarized x-ray absorption spectra show that the absorption edge of Fe L2-edge shifts closer to the Fermi surface with increasing temperature. The XAS line shapes of Fe and S L-edge provide the information of ligand crystal field environment and the phases of the FeS2 particles. We also report the non-destructive photoemission depth distributions of sulfur defects, vacancies, impurities and oxide as a function of temperature and oxygen dose. Valence band spectra indicate a band gap narrowing related to the creation of surface states at elevated temperature. An irreversible phase transition from pyrite (FeS2) to pyrrhotites (Fe1-xS) occurs above 430 °C. In addition, our results under in situ oxygen gas exposure suggest that the surface monosulfide species is oxidized first, and the reduction in the total density of states near the Fermi surface is caused by oxide layers of sulfate like and iron oxide products on the top ~2 nm.