Abstract
In this paper, we study the DMSO/thiourea/chloride salt system for synthesis of pure-sulfide hbox {Cu}_2hbox {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}. However, chloride and sulfoxide species are released continually until 500,^{circ }hbox {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} 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 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.
Highlights
In this paper, we study the dimethyl sulfoxide (DMSO)/thiourea/chloride salt system for synthesis of pure-sulfide Cu2ZnSnS4 (CZTS) thin-film solar cells under ambient conditions
Similar mixtures have been investigated by Collard et al and Xin et al who found that the black and yellow colours stem from the existence of Cu(II)S nanoparticles and Cu(II)Cl2 or Cu(II)DMSO2Cl2, respectively[14,26]
Once CuCl2 × 2H2O is added to the DMSO/thiourea solution, we find that the intensity of the Raman band at 749 cm−1 (C=S stretching mode of TU) is significantly lowered, but it remains unaffected by the addition of Zn and Sn salts to the ink (Supplementary Fig. S2 and Fig. 2d)
Summary
We study the DMSO/thiourea/chloride salt system for synthesis of pure-sulfide Cu2ZnSnS4 (CZTS) thin-film solar cells under ambient conditions. Chloride and sulfoxide species are released continually until 500 ◦C , suggesting the advantage of a higher pre-annealing temperature, which is commonly observed in the spin-coating routines in literature. The pure-sulfide Cu2ZnSnS4 (CZTS) contains neither toxic (Cd or Se) nor rare (In and Ga) elements It adapts a similar device architecture and comparable manufacturing facilities as the commercially available CIGS. Efficiencies exceeding 10% for solution-processed CZTSSe have been demonstrated using aprotic molecular inks[7]. This was achieved using the solvent dimethyl sulfoxide (DMSO) and salts of thiourea (TU) and metal chlorides, and without any additional elements—a method first described by Ki and Hillhouse[8].
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