Spin-coated $$\hbox {Cu}_2\hbox {ZnSnS}_{4}$$ solar cells: A study on the transformation from ink to film

In this paper, we study the DMSO/thiourea/chloride salt system for synthesis of pure-sulfide $$\hbox {Cu}_2\hbox {ZnSnS}_{4}$$ Cu 2 ZnSnS 4 (CZTS) thin-film solar cells under ambient conditions. We map out the ink constituents and determine the effect of mixing time and filtering. The thermal behavior of the ink is analyzed, and we find that more than 90% of the solvent has evaporated at $$250\,^{\circ }\hbox {C}$$ 250 ∘ C . However, chloride and sulfoxide species are released continually until $$500\,^{\circ }\hbox {C}$$ 500 ∘ C , suggesting the advantage of a higher pre-annealing temperature, which is also commonly observed in the spin-coating routines in literature. Another advantage of a higher pre-annealing temperature is that the worm-like pattern in the spin-coated film can be avoided. We hypothesize that this pattern forms as a result of hydrodynamics within the film as it dries, and it causes micro-inhomogeneities in film morphology. Devices were completed in order to finally evaluate the effect of varying thermal exposure during pre-annealing. Contrary to the previous observations, a lower pre-annealing temperature of $$250\,^{\circ }\hbox {C}$$ 250 ∘ C results in the best device efficiency of 4.65%, which to the best of our knowledge is the highest efficiency obtained for a pure-sulfide kesterite made with DMSO. Lower thermal exposure during pre-annealing results in larger grains and a thicker $$\hbox {MoS}_2$$ MoS 2 layer at the CZTS/Mo interface. Devices completed at higher pre-annealing temperatures display the existence of either a Cu-S secondary phase or an incomplete sulfurization with smaller grains and a fine-grain layer at the back interface.