Synthesis of Cu₂(Zn_1−xCo_x)SnS₄ nanocrystals and formation of polycrystalline thin films from their aqueous dispersions

Quintenary Cu2(Zn1−xCox)SnS4 is an analog of the promising solar absorber material Cu2ZnSnS4 (CZTS). The initial rapid progress in CZTS has stalled because the similar sizes of Cu and Zn cations lead to facile formation of antisite defects, which are thought to limit the solar cell performance. Cobalt substitution for Zn may reduce cation disorder. Herein, we report the synthesis of wurtzite Cu2(Zn1−xCox)SnS4 across the entire composition range and a systematic study of the substitution of Co into the wurtzite CZTS lattice. The synthesis is based on microwave heating to only 160 °C and uses metal salts and thiourea as precursors and ethylene glycol as the solvent. The Cu2(Zn1−xCox)SnS4 nanocrystals were phase pure wurtzite within the detection limits of X-ray diffraction and Raman scattering. The wurtzite lattice parameters, nanocrystal sizes, and A1 Raman mode peak positions depend on the Co concentration, x. The lattice parameters follow Vegard's law within the accuracy of our measurements, and the A1 Raman mode shifts nearly linearly with x. The nanocrystal size decreases from 8 nm to 4 nm as x increases from 0 to 1. The absorption band edge blue shifted from 1.1 eV for x = 0 to 1.35 eV for x = 1. These values are lower than those predicted by density functional theory calculations and previous attempts at determining the optical band gap for wurtzite Cu2ZnSnS4 (x = 0) and Cu2CoSnS4 (x = 1). Either the band gaps of wurtzite Cu2ZnSnS4 (x = 0) and Cu2CoSnS4 (x = 1) are lower or these materials have significant band tails due to defects. We also prepared polycrystalline Cu2(Zn1−xCox)SnS4 thin films by thermal annealing, in sulfur, of coatings comprised of Cu2(Zn1−xCox)SnS4 nanocrystals. Upon annealing in sulfur, the wurtzite Cu2(Zn1−xCox)SnS4 nanocrystals transformed into larger grains (100 s of nm to microns) that have a kesterite structure. The films with x ≤ 0.4 were phase pure kesterite within the detection limits of XRD and Raman scattering, but, for x ≥ 0.6, secondary phases such as Cu1.96S and Co0.24Zn0.76S were also detected.