Spin-coated Cu₂ZnSnS₄ 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 Cu₂ZnSnS₄ (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 25°C. However, chlorine and sulfoxide species are released continually until 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 so-called "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°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 MoS₂ layer at the CZTS/Mo interface. Devices completed at higher pre-annealing temperatures display the existence of either Cu-S secondary phase or incomplete sulfurizations with smaller grains and the existence of a fine-grain layer at the back interface.