From magnetic cubic pre-kesterite to semiconducting tetragonal kesterite Cu2ZnSnS4 nanopowders via the mechanochemically assisted route

Presented is a study on the mechanochemically assisted synthesis of kesterite Cu2ZnSnS4 from the elements for photovoltaic applications. Initial application of high energy ball milling under selected conditions, including the highest achievable rotation speeds up to 1000 rpm, affords a pure cubic nanophase with the nominal kesterite composition. The phase is tentatively called a pre-kesterite and it is shown by EPR to be magnetic. Accordingly, it does not produce either Cu-65 or Sn-119 MAS NMR spectra. Also, the material shows no well-defined absorption in the UV-vis range and, based on this, does not exhibit definite semiconducting properties. The highly disordered and defected structure with random metal site occupation originated in the mechanochemical synthesis step is proposed to account for the observed properties of pre-kesterite. Upon subsequent pyrolysis under argon at temperatures above 300 degrees C, preferably at around 500 degrees C, the cubic phase is converted to the tetragonal phase of kesterite, apparently, by metal site reconfiguration. The annealed nanopowders show the expected Cu-65 and Sn-119 MAS NMR characteristics. The Raman spectra support similar bonding environment and lattice phonon characteristics for both related phases as well as the eventual formation of kesterite nanopowders whereas their UV-vis spectra provide the direct band gap in the range of 1.35-1.48 eV, typical for semiconducting kesterite. (C) 2018 Elsevier B.V. All rights reserved.