Fabrication of solar cells based on Cu ₂ ZnSnS ₄ prepared from Cu ₂ SnS ₃ synthesized using a novel chemical procedure

Abstract

Solar cells based on kesterite-type Cu ₂ ZnSnS ₄ (CZTS) thin films were fabricated using a chemical route to prepare the CZTS films, consisting in sequential deposition of Cu ₂ SnS ₃ (CTS) and ZnS thin films followed by annealing at 550 °C in nitrogen atmosphere. The CTS compound was prepared in a one-step process using a novel chemical procedure consisting of simultaneous precipitation of Cu ₂ S and SnS ₂ performed by diffusion membranes assisted CBD (chemical bath deposition) technique. Diffusion membranes were used to optimize the kinetic growth through a moderate control of release of metal ions into the work solution. As the conditions for the formation in one step of the Cu ₂ SnS ₃ compound have not yet been reported in literature, special emphasis was put on finding the parameters that allow growing the Cu ₂ SnS ₃ thin films by simultaneous precipitation of Cu ₂ S and SnS ₂ . For that, we propose a methodology that includes numerical solution of the equilibrium equations that were established through a study of the chemical equilibrium of the system SnCl ₂ , Na ₃ C ₆ H ₅ O ₇ ·2H ₂ O, CuCl ₂ and Na ₂ S ₂ O ₃ ·5H ₂ O. The formation of thin films of CTS and CZTS free of secondary phases grown with a stoichiometry close to that corresponding to the Cu ₂ SnS ₃ and Cu ₂ ZnSnS ₄ phases, was verified through measurements of X-ray diffraction (XRD) and Raman spectroscopy. Solar cell with an efficiency of 4.2%, short circuit current of 16.2 mA/cm ² and open-circuit voltage of 0.49 V was obtained.