Reaction pathways for the formation of Cu2ZnSn(Se,S)4 absorber materials from liquid-phase hydrazine-based precursor inks

Kesterite Cu2ZnSn(Se,S)(4) (CZTSSe) is rapidly becoming an important photovoltaic material due to the abundance and industrial compatibility of its constituent elements. Hydrazine-based slurry deposition has taken a leading role in producing high efficiency devices from this material system, outperforming even high vacuum deposition methods. In this paper, we study the reaction mechanisms involved in the overall transformation from the precursor ink to the solid-state framework and finally to the CZTSSe phase during deposition and subsequent thermal treatment. X-ray diffraction and Raman spectroscopy have been employed to track the various stages of the reaction pathway, and to mark the formation and consumption of precursor phases as they interact to form the final material. It was found that drying the precursor ink at room temperature results in the integration of copper and tin chalcogenide complexes to form a bimetallic framework, with hydrazine and hydrazinium molecules as spacers. After mild thermal annealing, the spacers are removed and the Cu2Sn(Se,S)(3) + Zn(Se,S) -> Cu2ZnSn(Se,S)(4) reaction is triggered. This reaction pathway contains far fewer steps than most deposition processes, which typically start with elemental or binary chalcogenides. As the formation of secondary phases such as Cu2-xS, SnSe, and SnSe2 is no longer necessary to produce the final Cu2ZnSn(Se,S)(4) phase, the relative simplicity of this formation mechanism is likely beneficial for the performance of the resulting solar cells.