Formation of dense CIGS layers from nanocrystal inks: the relation between surface chemistry and annealing behavior

Recent developments in wet-processing of the I-III-VI2 chalcopyrite compound Cu(In,Ga)(S,Se)2 (CIGS) has shown that this approach is catching up in efficiency with vacuum-based deposition methods (1). Of particular interest is the synthesis of colloidal nanocrystals (NCs) as solution-based CIGS precursors, where several synthesis procedures have been reported (2-4). At present, little is known about the surface chemistry of CIGS NCs, while it has been acknowledged that foreign elements in the final layer, such as carbon, might have detrimental consequences on the performance of the absorber layer. In this study, we present a route to exchange the as-synthesized ligand of CuInS2 NCs, oleylamine, for hexanethiol, which strongly reduces the amount of carbon present in the system. Using NMR spectroscopy, we are able to investigate the binding behavior of both ligands, and quantify the amount of surfactants. Moreover, we prove that CuInS2 NCs can be stabilized with (NH4)2S, based on a procedure recently developed by Nag et al (5), providing an all-inorganic precursor with an excess of chalcogen species. By comparing the annealing behavior of these different systems using in-situ XRD, we demonstrate the importance of ligand-exchange procedures with respect to sintering characteristics and are able to effectively reduce the carbon content in the final layer. We believe that these results can increase the feasibility of nanocrystal inks as solution-based precursor for CIGS thin-film solar cells. 1 Prog. Photovolt: Res. Appl., 2010 18, 434–452 2 J. Am. Chem. Soc., 2008 130, 16770-16777 3 Chem. Mater., 2008, 20, 6906–6910 4 Nano letters, 2008, 8, 2982 -2987 5 J. Am. Chem. Soc. 2011, 133, 10612–1062