Spray‐pyrolyzed Cd‐substituted kesterite thin‐films for photovoltaic applications: Post annealing conditions and property studies

Abstract

Kesterite materials were investigated for their suitability as absorber layers for thin‐film photovoltaic cells. Thin-films of copper cadmium zinc tin sulfide (Cu2CdxZn1‐xSnS4) were prepared by spray pyrolysis on soda‐lime glass. The obtained thin‐films were subjected to post annealing treatment at 525 °C in a sulfur atmosphere to study the effects on morphology, stoichiometry, phase formation and optical properties by scanning electron microscopy (SEM), energy‐dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and ultraviolet visible spectroscopy (UV–Vis). The XRD patterns revealed the presence of prominent kesterite peaks in the unannealed and annealed thin‐films, with enhanced crystallinity in the annealed thin‐films. The crystal sizes of the unannealed and annealed thin‐films were estimated from the XRD data and ranged from 5 nm to 27 nm for unannealed thin‐films and 13 nm–25 nm for annealed thin‐films. The unannealed thin‐films exhibited a non‐uniformly shaped morphology, which was a mixture of cube‐shaped and rod‐shaped nanoparticles, while the annealed thin‐films exhibited a uniformly shaped morphology of flake‐shaped nanoparticles, indicating enhanced crystallinity. Chemical composition analyses revealed a divergent stoichiometry with high Cu content and deficient S content for the unannealed thin‐films, while the annealed thin‐films exhibited the desired stoichiometry, compensated by the sulfur atmosphere during the post‐annealing treatment. The UV–Vis absorption spectra showed a high absorption coefficient of more than 104 cm−1 for both the unannealed and annealed thin‐films. The bandgap energies for the unannealed thin‐films deviated from the optimum bandgap value for PV application and ranged from 2.38 eV to 1.46 eV with increasing Cd ratio, while the annealed thin‐films exhibited bandgap energies of about 1.77 eV–1.28 eV with increasing Cd ratio, indicating red‐shift absorption. Although other optical properties such as refractive index and dielectric properties were improved with the addition of Cd for unannealed thin‐films, these properties were better in the annealed thin‐films. The enhanced properties of the spray pyrolyzed Cd‐substituted thin‐films and their treatment after annealing can be explored as absorber layer deposition method and treatments to improve the performance and commercialization of kesterite thin‐film photovoltaic devices.