Real-time investigations on the formation reactions during annealing of sulfurized Cu–Sn precursors

The quaternary compound kesterite Cu2ZnSnS4 (CZTS) is a promising candidate for the production of low-cost thin film solar cells.Depending on the precursor composition and deposition technique several intermetallic precursor phases may appear, affecting the formation reactions during the crystallization process of the thin film absorber. A better understanding of these formation reactions in the system Cu–Zn–Sn–S is required for the optimization of CZTS absorbers and future development of solar modules.The crystallization of CZTS is completed by the reaction of Cu2SnS3 and ZnS. The formation of Cu2SnS3 itself depends on the different available precursor compounds after sulfur evaporation. Incomplete conversion of binary sulfides during annealing may lead to the formation of undesirable compounds, i.e. the transformation of Cu2SnS3 into Cu4SnS4 via reaction with Cu2-xS, which affect or even inhibit the crystallization of CZTS.Therefore a precise knowledge about formation reactions of binary and ternary Cu–Sn sulfides during annealing at low temperatures is important to crystallize a monophase CZTS absorber. Real-time investigations on the formation reactions in the quaternary and also the ternary subsystems of Cu–Zn–Sn–S while annealing stacked elemental layers elucidate the reaction paths of binary and ternary sulfides.We report on results of time-resolved and angle-dispersive XRD experiments during annealing comparing the formation reactions in sulfurized Cu–Sn precursors prepared by different deposition techniques.