The mechanism of crystallization of Cu₂ZnSnS₄ (CZTS) made by the sulfurization of a sputtered Zn/CuSn stack was studied by transmission electron microscopy. At 250 °C, the Zn buried at the bottom of the metallic stack was found to be driven to the upper layer by alloying with Cu and reacting with S to form ZnS. At 500 °C, CZTS was found to be formed and elemental Sn was observed in the vicinity of the back contact region, while large quantities of Cu₂S and ZnS were segregated at the film surface. At 575 °C, Cu₂SnS₃ was found, while all the elemental Sn had vanished. Upon extending the duration of the annealing to 10 min, at 575 °C, all the secondary phases except ZnS were consumed and the CZTS formation was completed, while around 200 nm of MoS₂ was formed between the CZTS layer and Mo back contact. The finished solar cell exhibits an efficiency of 2.66%, an open-circuit voltage of 666.9 mV and a short-circuit current density of 9.14 mA cm⁻². The solar cell performance is possibly limited by the thick MoS₂ layer and the large density of voids in the back contact region due to the Sn loss at high sulfurization temperatures.