Quantifying point defects in Cu_2ZnSn(S,Se)_4 thin films using resonant x-ray diffraction

摘要

Cu_2ZnSn(S,Se)_4 is an interesting, earth abundant photovoltaic material, but has suffered from low open circuit voltage. To better understand the film structure, we have measured resonant x-ray diffraction across the Cu and Zn K-edges for the device quality thin films of Cu_2ZnSnS_4 (8.6% efficiency) and Cu_2ZnSn(S,Se)_4 (3.5% efficiency). This approach allows for the confirmation of the underlying kesterite structure and quantification of the concentration of point defects and vacancies on the Cu, Zn, and Sn sublattices. Rietveld refinement of powder diffraction data collected at multiple energies is used to determine that there exists a high level of Cu_(Zn) and Zn_(Cu) defects on the 2c and 2d Wyckoff positions. We observe a significantly lower concentration of Zn_(Sn) defects and Cu or Zn vacancies.