Influences of oxygen incorporation on the structural and optoelectronic properties of Cu2ZnSnS4 thin films

Abstract

This study used the sol–gel method to prepare Cu2ZnSnS4 thin films containing oxygen and explored the composition, structural, and optoelectronic properties of the films. The non-vacuum process enabled the oxygen content of the Cu2ZnSnS4 films to be 8.89at% and 10.30at% for two different annealing conditions. In the crystal structure, oxygen was substituted at the positions of sulfur and appeared in the interstitial sites of the lattice. The compositions of the thin films deviated from the stoichiometric ratio. Both films had kesterite structures with no secondary phase structure. The kesterite CZTS film possessed a composite microstructure of crystallite and crystalline states. The microstructure of the Cu2ZnSnS4 film with higher oxygen content was denser and the average grain size was smaller. Incorporating oxygen atoms into crystalline Cu2ZnSnS4 changed the energy band structure: the direct energy band gaps were, respectively, 2.75eV and 2.84eV; the thin films mainly adsorbed photons with wavelengths less than 500nm; and the absorption coefficients increased from 104cm−1 to 105cm−1. The films had a comparatively high absorptive capacity for photons less than 350nm. Increasing the oxygen content of the film lowered the resistivity. Thus, the oxygen-containing Cu2ZnSnS4 thin film could be a candidate for the p-type absorber layer material required in multi-junction solar cells.