Transmission electron microscopy analysis for the process of crystallization of Cu2ZnSnS4 film from sputtered Zn/CuSn precursor

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 degrees 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 degrees 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 degrees C, Cu2SnS3 was found, while all the elemental Sn had vanished. Upon extending the duration of the annealing to 10 min, at 575 degrees 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 MoS 2 layer and the large density of voids in the back contact region due to the Sn loss at high sulfurization temperatures.