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The reduction of Cu2+ and crystal growth processes during colloidal synthesis of Cu2ZnSnS4 nanoparticles

The reduction Cu<sup>2+</sup> and the crystal growth processes during colloidal synthesis of Cu<sub>2</sub>ZnSnS<sub>4</sub> nanoparticles are investigated. We observe that ions Cu<sup>2+</sup>, Zn<sup>2+</sup>, Sn2+, and S<sup>2-</sup> from the precursors initially form amorphous-like nanoparticles, maintaining charge balance. To form the ideal kesterite crystal structure, Cu atoms need to be reduced from Cu<sup>2+</sup> in the precursor stage into Cu<sup>+</sup>. The reduction of Cu<sup>2+</sup> into Cu<sup>+</sup> take place as crystal growth of the nanoparticles occur, and coalescence of nanoparticles seems limited. We deduce that it is essential to control the reduction and crystal growth for obtaining stoichiometric Cu<sub>2</sub>ZnSnS<sub>4</sub> crystals, which is important for realizing high quality absorber layers for solar cells. The mechanisms and the implications are discussed.