“Green” Aqueous Synthesis, Structural, and Optical Properties of Quaternary Cu2ZnSnS4 and Cu2NiSnS4 Nanocrystals

Element substitution in Cu2ZnSnS4‐like chalcogenides offers the potential to create alternative low‐cost photovoltaic and thermoelectric materials with tunable properties. In this work, the “green” synthesis of colloidal cation‐substituted Cu–Ni–Sn–S nanocrystals (CNTS NCs) in aqueous solutions using thioglycolic acid as a stabilizer is reported for the first time. The structural and optical properties of CNTS NCs are studied in colloidal solutions and thin films, and are compared with those of Cu–Zn–Sn–S (CZTS) NCs obtained under similar conditions. The NC sizes of both compounds are estimated to be in the range of 1.5–2.5 nm. Both NCs exhibit strongly non‐stoichiometric composition and a structure corresponding to cationically disordered kesterite Cu2ZnSnS4, which are common features of such quaternary metal‐based chalcogenides. The phonon Raman spectra of CNTS and CZTS NCs exhibit very similar lineshapes, but the CNTS phonon band has a larger width and lower frequency, presumably due to stronger cation disorder. The absorption of both types of NCs extends continuously through the visible range with an estimated bandgap of ≈2.2 eV and sub‐bandgap absorption due to an Urbach tail. The absorption coefficient of CNTS is determined to be α > 102 cm−1 at 700 nm and α > 104 cm−1 at 400 nm.