Structural and optical properties of Cu₂ZnSnS₄ (CZTS) grown by SILAR method

The quest for a cheap renewable source of energy has led to extensive research on solar cells. Hence, the development of the solar absorber material is one important area always under focus. Copper Zinc Tin Sulphide (CZTS) is an earth abundant and non toxic solar absorber but its synthesis is accompanied by secondary phases and defect states which has kept the device efficiency limited within 12%. Investigation into its synthesis has led to a relatively simple and cost effective chemical bath deposition (CBD) method. But formation of thick precipitate prevents the thin film growth. Our work tests the SILAR (Successive Ionic Layer Adsorption and Reaction) method which keeps all the advantages of CBD and also allows formation of thin films of varying thickness. Additionally, no complexing agent is needed as is used in CBD. The process involves dipping the substrate separately in an anionic and cationic solution and minimizing excess deposition by washing it alternately in water. SILAR was performed for varying cycles (20, 50,100). The UV-VIS data shows the energy band gap of 1.54eV for 100 cycles which is very close to the value obtained for CBD (1.41eV). The Tauc plot for both gives a strong evidence of the formation of CZTS. The XRD plot of CZTS formed with 100 cycles SILAR shows peaks that confirm the kesterite phase of CZTS. XRD for the films annealed at 200°C gives sharper peaks as expected. We have found that water as a solvent gives thicker deposition than ethanol (10, 20, 30, 40, and 50 cycles). Rod like structures as seen already for CBD samples are also evident for SILAR ones grown using ethanol solvent when seen under optical microscope (40X magnification). It is observed that an alkaline environment of pH 10 is necessary for the deposition on glass substrate; otherwise no deposition occurs. (author)