Experimental and Theoretical Study of Stable and Metastable Phases in Sputtered CuInS₂

The chalcopyrite Cu(In,Ga)S₂ has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS₂ (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn₅ S₈ and NaInS₂ accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.