A better understanding of reaction pathways of the formation of CZTSSe absorbers using co-sputtered Cu-Zn-Sn-S precursors

The aim of this work is to have a better understanding of the effect of annealing on the crystallization behavior and the formation of secondary phases in Cu₂ZnSn(S_x,Se_1-x)₄ (CZTSSe) absorbers. Cu-Zn-Sn-S precursors were deposited on glass/Mo substrate by co-sputtering of Cu, ZnS, and SnS and then annealed under Se atmosphere. In order to have a better understanding of the mechanisms of formation of CZTSSe and secondary phases during annealing, the effect of the variation of the annealing temperature between 250 and 600°C on structural and compositional properties of the materials has been studied. For that, a detailed material characterization of the bulk and the front and back surface of the CZTSSe absorbers at different temperature, combining x-ray diffraction, Raman, and Glow Discharge Optical Spectroscopy (GD-OES) is performed. These studies revealed that during the selenization, the formation of CZTSSe occurs via two simultaneous and separated reaction paths. A Se-rich CZTSSe is formed for which the reaction rate depends on the Se diffusion through the films, while a S-rich CZTSSe rapidly forms at the CZTSSe/Mo interfaces. When increasing the temperature this leads to the formation of SnSe2 and Cu2Se secondary phases at the surface of the absorbers while ZnS secondary phases are formed in the bulk of the layers.