Coevaporated Cu–In films as precursors for solar cells

We investigate Cu–In thin films used as precursors for the production of ${\mathrm{CuInS}}_2$ and ${\mathrm{Se}}_2$ solar-cell absorber material via reactive annealing. The films are produced by coevaporation of Cu and In onto glass substrates and are characterized by means of Rutherford backscattering and x-ray diffraction (XRD). The interplay of phase composition, morphology, and surface topography is studied as a function of chemical composition, substrate temperature, and annealing processes. The analysis of the XRD data is based on known crystallographic data for the phases ${\mathrm{Cu}}_7{\mathrm{In}}_3$ $\mathrm{(\delta }$ phase), ${\mathrm{Cu}}_{16}{\mathrm{In}}_9$ ${\mathrm{(\eta }}^{\prime }$ phase), ${\mathrm{Cu}}_{11}{\mathrm{In}}_9,$ and In. Refined crystallographic data for ${\mathrm{CuIn}}_2$ are presented, and the low-temperature modification of ${\mathrm{Cu}}_{16}{\mathrm{In}}_9$ $\mathrm{(\eta }$ -phase) is investigated by means of bulk powder samples. These data and the inclusion of texture effects allow us to perform a complete RIETVELD type analysis of the Cu–In precursors. It is shown that, in contrast to sequentially evaporated films, all known Cu–In equilibrium phases can be formed during film deposition. These are ${\mathrm{Cu}}_7{\mathrm{In}}_3,$ ${\mathrm{Cu}}_{16}{\mathrm{In}}_9,$ ${\mathrm{Cu}}_{11}{\mathrm{In}}_9,$ and ${\mathrm{CuIn}}_2.$ Moreover, it is found that single-phase films of all these phases can be produced. Film roughness is shown to increase with deposition temperature and In content. The results presented offer new prospects for sulfurization and selenization processes in solar-cell production.