Raman scattering and disorder effect in Cu2ZnSnS4

The Raman spectra of surface regions of bulk Cu2ZnSnS4 (CZTS) samples with different Cu and Zn cation content were obtained and the differences in the spectra are attributed to statistical disorder effects in the cation sublattice. This disorder in the Cu and Zn sublattices may initiate a change of the crystal symmetry from kesterite‐type $({I\bar 4})$ to $({I\bar 42m})$ space group. The investigated CZTS crystals grown at high temperature are characterised by the co‐existence of regions with different composition ratio of Cu/(Zn + Sn) which results in kesterite and disordered kesterite phases. The presence of a disordered phase with ${I\bar 42m}$ symmetry is reflected in the appearance of a dominant broadened A‐symmetry peak at lower frequency than the peak of the main A‐symmetry kesterite mode at 337 cm–1. We suppose that due to a small energy barrier between these phases the transition from one phase to the other can be stimulated by optical excitation of Cu2ZnSnS4. The analysis of the Raman spectra measured under different excitation conditions has allowed obtaining first (to our knowledge) experimental evidence of the existence of such optically induced structural transition in CZTS. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) One of the most important challenges for the CZTS‐type materials used for solar cells is nonstoichiometry. In this Letter, we investigate the relationship of antisite defects in the cation sublattice of CZTS with the peculiarities of its Raman spectra. The change of CZTS crystal symmetry from kesterite‐type I4 to a disordered kesterite‐type $({I\bar 42m})$ manifests itself in a change of frequency and bandwidth of the main Raman A peak. This fact is discussed in the connection with disorder of Cu+ and Zn2+‐ions on 2c and 2d sites in the crystalline lattice.