Evolution of the microstructural, electrical and optical characteristics of sol-gel derived Cu2ZnSnS4 thin films during sulfurization

Abstract

The present study reports the evolution of microstructural, electrical and optical characteristics of sol-gel derived Cu2ZnSnS4 (CZTS) thin-films during sulfurization process performed at 530 °C. The precursor films prior to sulfurization were largely amorphous although the presence of CZTS phase was apparent in the corresponding X-ray diffraction (XRD) pattern. For the sulfurization process, as the temperature of the film arrived at 300 °C during ramp-up stage the secondary phase copper sulfide (CuxS) (x = 1–2) was detected in the corresponding XRD pattern. This phase was subsequently assimilated into the CZTS phase as the film reached at 450 °C. When the film arrived at 530 °C, the sulfurization temperature, it had completely converted into a phase pure CZTS with a close to stoichiometric composition and uniform surface morphology. Beyond this stage the structural characteristics did not show significant changes. The grain size increased from 4 to 200 nm during the sulfurization process. The electrical conductivity of the films was p-type at all the stages of the sulfurization process, while the resistivity first increased to 165 Ω⋅cm as the film reached 530 °C and from there it decreased continuously to a value of 0.66 Ω⋅cm at the end of the sulfurization process. The optical band gap and the Urbach energy decreased from 2.0 to 1.5 eV and from 1.2 to 0.25 eV, respectively. The disorder in CZTS was studied using Raman spectroscopy and its implications on the electrical properties have been discussed. Further, a post sulfurization heating of the CZTS film at 300 °C for 1 h in an inert ambient resulted in two orders of magnitude decrease in the charge carrier density from 2.1 × 1019 cm–3 to 1.35 × 1017 cm–3 which can be attributed to an increase in the density of donor-type defects.