Spray deposition of Cu₂ZnSnS₄ thin films

Full text: Cu₂ZnSnS₄ (CZTS) is an excellent candidate dor the absorbing layer in photovoltaic devices [1]. It has excellent optical properties (a ~10⁵ cm^-1) and a band gap matching the solar spectrum (1.3< E_g>1.7 eV). Also, it does not contain toxic or scarce elements. Spray pyrolysis has been used before to deposit thin films [2]. It is cost effective, does not require high vacuum, can copy irregular geometries and suitable for flexible substrates. Thin films of CZTS have been deposited on glass and on conductive glass (FTO) using spraying aqueous precursors. Temperatures ranged from 325 to 425 deg C. The effect of annealing in sulfur vapor on the structural, morphological, and optical properties of CZTS films has been studied. XRD revealed the formation of polycrystalline CZTS films, where the crystalline degree increased with substrate temperature and with sulfurization. Raman maps show a homogenous distribution of the CZTS phase and a random distribution of a Cu_XS binary phase. Raman signals attributed to this secondary phase seem to be reduced after sulfurization. Band gap values vary between 1.3 and 1.5 eV. Sulfurization has no significant effect on the composition of the deposits or on the band gap energy. The crystallite size increased after annealing in sulfur. Films deposited at 425 deg C presented a nearly stoichiometric composition, suitable optical properties and p-type conductivity. CZTS is suitable for absorbent layers in solar cells, even in the as-deposited condition [3]. References: [1] D.B. Mitzi, O. Gunawan, et al. Sol. Energy Mater. Sol. Cells 95 (2011) (6) 1421 [2] P.S. Patil, Mater. Chem. Phys. 59 (3) (1999) 185. [3] M. Valdés, G. Santoro, M. Vázquez, J Alloys Comp 585 (2014) 776-782. (author)