Key role of Cu-Se binary phases in electrodeposited CuInSe₂ precursors on final distribution of Cu-S phases in CuIn(S,Se)₂ absorbers

Using micro-Raman spectroscopy, we demonstrate that the formation of the CuS binary phase in CISEL™ cells absorbers is highly determined by the presence of a Cu-Se binary phase in the precursors and depends on the Cu/In ratio. A selective sulphurization mechanism of the Cu-Se phase is proposed as the origin of CuS platelets. For low Cu/In ratio both binaries are associated to the surface and do not affect the device performance. Conversely, when the binary phase in the precursor is present close to the back contact (for high Cu/In ratio), the CuS binary phase is also formed at this region after sulphurization. Strongly associated to this Cu-rich binary phase, a spinel-type phase is also observed in the sulphurized samples. This phase has n-type conductivity, which has a strong impact on the characteristics of the solar cells. The relationship between the presence of these phases and the electrical properties of the final devices is described, showing that the reduction of the V_oc and the increase of the R_s are related to the formation of a back diode by the spinel-type layer, as consequence of an undesirable n-p-n structure.