The effects of dopant impurities on Cu₂ZnSnS₄ system Raman properties

Impurities in Cu₂ZnSnS₄ (CZTS) can alter the electronic and optical properties, which may reduce photovoltaic device efficiency. Phonon dispersion susceptibility can be used to investigate the effects of foreign dopants on pairing between atoms within lattice. In order to investigate the effect of free holes on the optical phonons and asymmetry in the Raman peaks due to impurities, Sb-, Fe-, and Ag-doped CZTS thin films were prepared and systematically studied. Doping with impurities causes a shift in Raman peak frequency as well as linewidth. An investigation has been made based on the reduced mass of the vibrating CZTS lattice that changes due to dopant addition. For polycrystalline Sb-doped CZTS, the predicted wavenumber shift for Raman ‘A’ mode follows a similar trend as evidenced in experiments and can be explained by the square-root relationship between frequency and the vibrating mass. Raman lineshape for Sb-doped CZTS thin films and nanocrystals becomes wider, asymmetric, and moves toward lower vibrational frequency when laser power density increases. Atomic force microscopy was also performed to examine surface properties.