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

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 Eg 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.