Study of the fundamental phase formation mechanism of sol-gel sulfurized Cu2ZnSnS4 thin films using in situ Raman spectroscopy

In the current work, the results of the in situ Raman monitoring of phase formation of copper zinc tin sulfide (Cu2ZnSnS4, CZTS) material from room temperature up to 350 °C are presented. Cu-Zn-Sn oxide precursors were sulfurized for up to 180 min using a 100 ppm H2S+4%H2+N2 gas mixture (by volume). The results showed that CZTS phase formed in about 30 min via a reaction between the metal oxide precursor film and the H2S-H2 gas mixture at intermediate temperature of 350 °C and remained stable upon extended exposure. In comparison, at lower temperature such as 170 °C, the oxide precursor film had to be reduced first (e.g., in 4% H2+N2 forming gas) and then the CZTS phase emerged. However, continued sulfurization at lower temperature (e.g., 170 °C) led to the disintegration of CZTS and the formation of CuS impurity, which remains stable upon cooling the sample down to room temperature. The correlation between the in situ Raman monitoring data and the information from other ex situ characterization techniques provided useful information regarding the reaction pathways leading to the formation of sol-gel based CZTS thin films as well as other related phases under different sulfurization conditions. The outcomes of the current study also set the directions for optimizing the CZTS film structure and stoichiometry toward developing high performance CZTS solar cells in future.