Stoichiometry effect on Cu2ZnSnS4 thin films morphological and optical properties

Abstract

Thin films of Cu2ZnSnS4 (CZTS) were prepared by sulfurization of multilayered precursors of ZnS, Cu, and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray spectroscopy, and SEM were used for structural, compositional, and morphological analyses, respectively. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric measurements and Photothermal Deflection Spectroscopy. These films show a clear dependence of the optical and microstructural properties on the tin content. As the tin content increases we found: (i) an increase in both crystalline domain and grain size, (ii) an abrupt increase of the energy gap of about 150 meV, from 1.48 to 1.63 eV, and (iii) a decrease of sub-gap absorption up to two orders of magnitude. The results are interpreted assuming the formation of additional defects as the tin content is reduced.