Sn-rich CZTS films spin-coated from methanol-based sol-gel solution: annealing effect on microstructure and optoelectronic properties

Photovoltaic light absorber Cu 2 ZnSnS 4 (CZTS) with Sn-rich composition is the least studied compound compared with its stoichiometry and Zn-rich/Cu-poor compositions. Sn-rich CZTS films were prepared by spin coating from a nontoxic methanol-based solution. A brief (10 min) annealing in nitrogen was performed in the temperature range of 350–550 °C. The effect of annealing temperature on the film composition, morphology, crystallite size, microstrain, dislocation density, optoelectronic, and transport properties was investigated. For this study, scanning electron microscopy, x-ray diffraction, energy dispersive x-ray analysis, Raman spectroscopy, x-ray photoelectron spectroscopy, photocurrent spectroscopy, and Hall-effect techniques were employed. The annealed films were compact, uniform, and photosensitive. A systematic increase in the crystallites size and decrease in the microstrain and dislocation density was observed as the annealing temperature was increased. The films had a direct band gap energy of 1.47–1.50 eV. The presence of two sub-band gap direct transition energies of 1.04–1.08 eV and 1.16–1.21 eV were detected and their origins were discussed. Films were highly p-type in which the hole concentration increased systematically from 1.8 × 10 18 to 1.5 × 10 19 cm −3 , and the hole mobility decreased steadily from 7.7 cm 2 /V-s to 0.94 cm 2 /V-s with the increase of annealing temperature. This behavior revealed that the ionized-impurity scattering is the dominant mechanism for the transport of holes in Sn-rich CZTS films. These highly p-type Sn-rich films have favorable properties suitable for device applications. The effect of annealing temperature, T A , on the film XRD pattern, crystallite size, L , hole concentration, p , and hole mobility µ p . Surface morphology of an annealed films is also shown. Highlights Highly p-type Sn-rich CZTS films can be spin-coated on glass from a methanol-based solution. Sn-rich CZTS films show photosensitivity and good structural and optoelectronic properties. Density and mobility of holes are controllable in the ranges of 10 18 –10 19 cm −3 and 1–8 cm 2 /V-s. Ionized impurity scattering is the dominant mechanism for the transport of holes in the films. Sn-rich films show a band gap of 1.47–1.50 eV and two sub-band gap electron transition energies.