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Crystal growth mechanism of Cu2ZnSn(S,Se)4 thin films fabricated from nanoparticles

In this paper, we study the reaction mechanisms involved in the transformation from the precursor prepared with nanoparticles to the Cu2ZnSn(S,Se)4 phase during solid-phase sulfurization and selenization. The top-view of the film observed by scanning electron microscopy showed that crystal grains suddenly grow at temperatures above 550 °C. For all samples, impurity phases were not detected by X-ray diffraction and Raman spectroscopy, which indicates that intermediate phases such as binary and ternary compounds are not necessary to produce the Cu2ZnSn(S,Se)4 phase. During sintering, the film was first sulfurized because of the difference between the vapor pressures of S and Se. Subsequently, the S of Cu2ZnSn(S,Se)4 was replaced by Se, causing the generation of a defective layer. For the sample sintered at 600 °C, Cu-rich, Zn-rich, and Sn-rich phases were detected by transmission electron microscopy analysis. Additionally, a diagrammatic Cu2ZnSn(S,Se)4 film growth model was proposed to illustrate the detailed reaction mechanism during precursor sintering.