Suppression of the Secondary Phases in CZTS Films Through Controlling the Film Elemental Composition

Kesterite Cu2 ZnSnS4 (CZTS) thin films were grown by the sulfurization of stacked metal precursors deposited using radio-frequency magnetron sputtering on Mo-coated soda-lime glass substrates. In this paper, we report the role of the film chemical composition in the evolution of Cu_2-xS phases and how to avoid their development through controlling the film composition. Furthermore, the effect of the elemental concentration on the structural and morphological properties of the final CZTS films has been investigated. The prepared CZTS films have a composition ratio M = Cu/(Zn + Sn) varying from 0.81 (Cu-poor) to 1.05 (Cu-rich). X-ray diffraction and Raman scattering studies revealed the presence of Cu_2-xS phases in films with a Cu/(Zn + Sn) ratio higher than 1.00 and/or in films with a Sn/Cu ratio close to or less than the stoichiometric value of 0.50. However, Cu_2-xS-phases-free CZTS films were achieved with Sn/Cu ratios sufficiently above 50% without regard to the Cu/(Zn + Sn) ratio. Plan and cross-sectional scanning electron microscopy showed compact films, in general. Electron back-scattered diffraction revealed randomly oriented CZTS films.