  Solar cells based on kesterite-type $Cu_2ZnSnS_4$ (CZTS) thin films were
fabricated using a chemical route to prepare the $CZTS$ films, consisting in
sequential deposition of $Cu_2SnS_3$ (CTS) and $ZnS$ thin films followed by
annealing at $550^\circ C$ in nitrogen atmosphere. The $CTS$ compound was
prepared in a one-step process using a novel chemical procedure consisting of
simultaneous precipitation of $Cu_2S$ 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 $Cu_2SnS_3$ compound have not yet been reported in
literature, special emphasis was put on finding the parameters that allow
growing the $Cu_2SnS_3$ thin films by simultaneous precipitation of $Cu_2S$ 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 $SnCl_2$, $Na_3C_6H_5O_7\cdot 2H2O$, $CuCl_2$ and
$Na_2S_2O_3\cdot5H_2O$. The formation of thin films of $CTS$ and $CZTS$ free of
secondary phases grown with a stoichiometry close to that corresponding to the
$Cu_2SnS_3$ and $Cu_2ZnSnS_4$ 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.
