Evolution of Morphology and Composition during Annealing and Selenization in Solution-Processed Cu2ZnSn(S,Se)4

Cu2ZnSn(S,Se)(4) absorbers deposited from a nontoxic, DMSO-based molecular ink have yielded the most efficient, hydrazine-free Cu2ZnSn(S,Se)(4) photovoltaic (PV) device made through solution-processing. Although this chemistry has been widely adopted, absorber morphologies with a device-limiting fine-grained bottom layer are often reported. Here we demonstrate how the annealing profile of coatings from this ink critically affect absorber morphologies. Calibrated glow discharge optical emission spectroscopy (GDOES) is used to determine depth-dependent elemental ratios and atomic concentrations of impurities before and after selenization. An annealing temperature of 400 degrees C is shown to exhibit the most pronounced fine-grained bottom layer, which contains 10 atom % carbon (C) and 5 atom % nitrogen (N). Raman analysis of the mechanically exfoliated, fine-grained layer reveals that it contains amorphous carbon nitride, which is attributed to the polymerization of thiourea decomposition products during annealing. An annealing temperature of 300 degrees C avoids this polymerization and allows C and N to escape during selenization, while an annealing temperature of 500 degrees C vaporizes C and N compounds before selenization. The annealing profile of 500 C 1.5 min/layer removes nearly all but similar to 0.25 atom % of C and N, which impedes the formation of a fine-grained layer and allows for a PV device efficiency of 10.7%. It is also shown how lithium-doping enhances sodium transport from the soda-lime glass substrate.