Phase evolution during the electrodeposition of Cu–Sn–Zn metal precursor layers and its role on the Cu2S impurity phase formation in Cu2ZnSnS4 thin films

Abstract

Abstract Metal precursor layers in the sequence of Cu(A)–Sn–Cu(B)–Zn (CTCZ) were electrodeposited on FTO–coated glass substrates to yield Cu2ZnSnS4, post sulphurization. Raman mapping and XRD analysis reveal the presence of Cu2S inclusions in the CZTS thin film obtained from the metal precursor in which the molar ratio of Cu(A)/Cu(B) is 0.33 (CTCZ–0.33). Such Cu2S inclusions are not present in the CZTS film obtained from the precursor with Cu(A)/Cu(B) molar ratio 3 (CTCZ–3). Systematic potentiodynamic and XRD studies of the component metal precursor layers reveals a significantly larger amount of unalloyed Sn in the CTCZ–0.33 metal precursor compared to that in the CTCZ–3 precursor. Sulphurization of the precursors thus results in a higher amount of SnS in the CTCZ–0.33 precursor. Subsequently, vaporization of SnS moves the overall composition of the sulphurized CTCZ–0.33 precursor outside of the CZTS formation window in the ternary phase diagram. This compositional shift leads to Cu2S + CZTS as the equilibrium phase assemblage for the film obtained from the CTCZ–0.33 precursor.