Half-cycle growth of Cu2ZnSnS4 thin films by successive ionic layer adsorption and reaction method: Initial growth mechanism

The adsorption and reaction mechanisms of Cu2ZnSnS4 (CZTS) quaternary compound deposited by the Successive Ionic Layer Adsorption and Reaction (SILAR) method are discussed. The understanding of nucleation and growth mechanisms of this material will improve the control of composition, homogeneity and final properties of the obtained thin films. Therefore, a half-cycle study of CZTS deposited by the SILAR method over CdS films was carried out, since the interface plays a critical role for its application in solar cells. In this way, X-ray photoelectron spectroscopy (XPS) and UV–Vis spectroscopy were used to correlate the metal elemental composition in the thin film and the population of the metal ion complexes in a solution. This analysis revealed that copper adsorbs at the surface to a greater extent than Zn and Sn, achieving a stoichiometry ratio close to Cu: Zn: Sn: S to 3:1:1:5, even when the copper concentration is five times smaller. The growth rate, determined by XPS ultra-thin film analysis, was about 0.7 nm/cycle, which is consistent with an atomic layer by layer mode of growth. •A half-cycle study of CZTS thin films deposited by SILAR on CdS films was done.•Chemical equilibria of Cu, Zn and Sn in a citrate solution at different pH was resolved.•The mechanism is based on the adsorption of metal-citrate and an anion exchange reaction.•The pH suitable to obtain a stable cationic solution for the CZTS deposition is 6–7.•XPS results showed Cu adsorbs at the surface in a greater extent than Zn and Sn.