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CZTS nanoparticles produced by a wet-chemical method were mixed with different proportions of Selenium powder to observe the sulfur selenium exchange reaction during selenization and the effect of the varying Selenium supply on the tetragonal ordering and recrystallization of the originally disordered cubic structure. Evaluation of the diffraction angle shift of the reflections in in-situ XRD measurements during the selenization process yields the temperature dependent composition of the CZTSSe. The presence of selenium in small to medium amounts seems to benefit the ordering of the cations as well as the crystallite growth, whereas very high proportions seem to reverse the positive effects.

The constitution of secondary phases in kesterite Cu2ZnSnSe4 (CZTSe) thin films is still a limiting factor for their application in solar cells. Therefore an enhanced understanding of phase formation processes during the fabrication of CZTSe films is required. In this study we present a temperature and film-depth dependent phase analysis of Zn/Sn/Cu precursors on ZnO substrates selenized at different temperatures. A special sample preparation step using a focused ion beam was applied to prepare shallow angle cross sections for depth-resolved Raman profiling of the thin films. At low selenization temperatures multiphase structures are demonstrated and a first formation of CZTSe besides secondary phases at only 250 °C is detected. At high selenization temperatures an accumulation of ZnSe at the interface of CZTSe and ZnO substrates is observed. Furthermore indications for the formation of a thin SnO2 interface layer were found by X-ray diffraction, secondary electron microscopy and energy dispersive X-ray spectrometry.

Cu2ZnSnS4 nanoparticles have been produced by a facile and high-yield wet-chemical method at low synthesis temperature. In order to remove the organic residues the synthesis was followed by a pre-annealing procedure at 250 °C in nitrogen atmosphere. Data evaluation of ex-situ XRD measurements on the pre-annealed sample showed, that the diffraction pattern of the as-synthesized powder can be well modeled by diffraction of a hexagonal and a disordered cubic CZTS structure. The CZTS nanoparticles were annealed under nitrogen atmosphere up to 550 °C without and with the addition of selenium powder and diffraction patterns were recorded in-situ as a function of temperature in order to investigate the structural phase transitions during the re-crystallization and selenization of the powders. The in-situ XRD measurements showed a structural phase transition into the tetragonal CZTS structure for pure CZTS and with the addition of selenium. With the addition of selenium, an exchange reaction between sulfur and selenium during the heating process was observed by the shift in the diffraction peaks and subsequent refinement of the lattice parameters of the crystal structures.

We present a study on the temperature and film depth dependent phase formation in Cu-Zn-Sn-Se thin films. Zn/Sn/Cu precursors were selenized at different temperatures, followed by depth-resolved Raman profiling. A high depth resolution for Raman analysis was achieved by a special sample preparation step using a focused ion beam to prepare shallow angle cross sections (SACS) of the absorber. Multi-phase structures were observed at low selenization temperatures with a first formation of the quaternary Cu 2 ZnSnSe 4 at only 250 °C and the existence of Sn x O y in films annealed at 330 °C. At high selenization temperatures up to 560 °C Cu 2 ZnSnSe 4 was the main phase with some traces of ZnSe and a MoSe 2 interface layer at the back contact. Furthermore, compositional gradients were investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy measurements on sample cross sections. The combination of these results allows for observation of both elemental composition and phase composition of the films and their dynamics during the annealing procedure.