Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu11In9 precursors to be employed as light absorbing CuInS2 (CIS) layers in CIS–CdS heterojunction thin-film solar cells has been investigated. The Cu11In9 precursor films were produced by DC-sputtering from a single-phase Cu11In9 target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In2S compound, the formation of which is favoured by the phase transition of Cu11In9 to Cu16In9 at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuSx surface phase.