Synthesis of CZTS/Se and Their Solid Solution from Electrodeposited Cu-Sn-Zn Metal Precursor: A Study of S and Se Replacement Reaction

Selenization, sulfurization, and sulfo-selenization of electrodeposited metal precursor (Cu-Sn-Zn) at high temperature (500-600 degrees C) in S, Se, or S + Se (mixed) atmospheres are used to understand the thermodynamics of chalcogenide incorporation reaction. Phase-pure CZTSe and CZTS were obtained after annealing at 500 degrees C for 1 min in Se (selenization) and 600 degrees C for 10 min in S (sulfurization) atmospheres, respectively. CZTSSe solid solutions are synthesized by the sequential annealing of metal precursors in S and Se atmosphere separately or in the mixed (S + Se) atmosphere. In the S-rich mixed atmosphere, S-rich CZTSSe solid solution is formed at all annealing conditions. Surprisingly, in a Se-rich mixed atmosphere, longer annealing at 600 degrees C yields S-rich CZTSSe. The CZTSSe film formed by annealing in near equimolar S/Se atmosphere exhibits a compositional gradient across the thickness. These results suggest that the crystallinity, composition, and hence the bandgap of CZTSSe can be precisely controlled by the proper choice of annealing temperature, duration, and atmosphere.