Quantifying Point Defects in Cu2ZnSn(S,Se)4 Thin Films using Resonant X-Ray Diffraction

Cu₂ZnSn(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 Cu₂ZnSnS₄ (8.6% efficiency) and Cu₂ZnSn(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.