Secondary Phases, Morphology and Band Tails of Third Generation Photovoltaic Absorber Layer CZTS Annealed at Different Temperatures

Abstract

In this work, we have successfully fabricated Cu₂ZnSnS₄ (CZTS) thin films using the sol-gel spin-coating technique. Thin films were annealed in an argon atmosphere at different temperatures of 350, 375, 400, 425, and 450 ℃ for an hour. The influence of annealing temperature on structural, morphological, and optical properties was investigated. x-ray diffraction measurement confirmed the formation of secondary phases in as-deposited and annealed thin films at 350 and 375 ℃. A pure kesterite phase was obtained at an annealing temperature of 400 ℃, but degradation occurred at higher temperatures. The morphology study of the surface structure showed that thin films have homogeneous surfaces. The Volmer-Weber mode was the dominating growth mode. The as-deposited and annealed CZTS thin films exhibited a high absorption coefficient of the order of 10⁴ cm^-1, and the optical energy band gap E_g was red-shifted with increasing annealing temperature. The optical study showed a decrease in Urbach tail energy with an increase in annealing temperature and reached 0.39 eV at 400 ℃. The refractive index and dielectric constants of CZTS thin films were calculated.