Optically induced structural transformation in disordered kesterite Cu$_{2}$ZnSnS$_{4}$

The kesterite-structured semiconductor Cu$_2$ZnSnS$_4$ is one of the most promising compound for earth-abundant low-cost solar cells. One of the complex problem on this way deals with its stoichiometry. In this work Raman spectra of Cu-rich Cu$_2$ZnSnS$_4$ crystals are discussed in connection with the non-stoichiometric composition and disordering within the cation sublattice of the kesterite. The shift of the main $A$-peak from 338 to 331 cm$^{-1}$ and its broadening are attributed here to transition from the kesterite (I$\bar{4}$-symmetry) to the disordered kesterite structure (I$\bar{4}$2m-symmetry). It is shown that this transition may also be driven by an intense light, which could stimulate transformation of Cu$^+$-ion to Cu$^{2+}$-ions and facilitates generation of Cu$_{\text{Zn}}$-defects on 2d-crystalographic positions.