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

Highlights: • Oxygen incorporation in Cu₂ZnSnS₄ changes the energy band structure. • The material has a comparatively high-absorptive capacity for short wavelength. • Absorption coefficients of the film increase from 10⁴ to 10⁵ cm⁻¹. • The oxygen-containing CZTS film has a mixture of crystallite and crystalline states. • The material could be a candidate as an absorber layer in multi-junction solar cells. - 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 8.89 at% and 10.30 at% 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, 2.75 eV and 2.84 eV; the thin films mainly adsorbed photons with wavelengths less than 500 nm; and the absorption coefficients increased from 10⁴ cm⁻¹ to 10⁵ cm⁻¹. The films had a comparatively high absorptive capacity for photons less than 350 nm. 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.