Phase evolution pathways of kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 thin films during the annealing of sputtered Cu-Sn-Zn metallic precursors

•Cu2ZnSnS4 and Cu2ZnSnSe4 were prepared by annealing of sputtered Cu-Sn-Zn precursor in chalcogenides atmosphere.•The formation of secondary phases in kesterite compounds critically affects the device performance.•The phase evolution process is studied as a function of annealing temperature using ex situ TEM characterizations.•The possible phase evolution pathways are proposed for Cu2ZnSnS4 and Cu2ZnSnSe4 compounds.