Paper EN+TF-MoA8
Synthesis of Photovoltaic Cu₂ZnSnS₄ via Ex Situ Sulfidation of Co-Sputtered Cu-Zn-Sn Thin Films

Monday, October 29, 2012, 4:20 pm, Room 15

Cu₂ZnSnS₄ (CZTS) is an emerging low-cost solar absorber for thin film photovoltaics based on non-toxic, high earth-abundance elements. While ex situ sulfidation of Cu-Zn-Sn precursor films in S vapor is a popular synthesis route for CZTS, much remains to be understood with regard to the sulfidation mechanisms, microstructural control, and structure-property relationships. In this work, DC magnetron co-sputtered Cu-Zn-Sn films, of varying composition, were sealed with 1.0 mg of S in evacuated (10^-6 Torr) quartz ampoules, and then isothermally heat treated at sulfidation temperatures in the range 100 ^oC ≤ T_S ≤ 700 ^oC. The films were then characterized structurally by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction, and electrically via resistivity measurements between 4.2 K and 300 K. We find that the phase purity of the resultant films depends strongly on T_S, with complete conversion of the precursor film to CZTS occurring only at T_S ≥ 550 ^oC. The final phase purity of ex situ sulfidized CZTS films is however remarkably insensitive to modest amounts of excess Zn and Sn in the Cu-Zn-Sn precursor film. Excess Zn or Sn in the precursor is readily ejected during sulfidation, via elemental or binary sulfide evaporation, respectively, while Sn-deficiency can also be corrected by introducing elemental Sn to the sealed quartz ampoule. However, due to the low melting point of Sn, and the absence of appropriate Sn-Zn alloys, the precursor composition does play a significant role in defining the final CZTS film microstructure, and the lateral homogeneity. In addition to detailed discussion of the physics and chemistry underlying the above observations we will also provide data on the transport properties of such films, including observations of hopping conduction.