Shorter Is Not Always Better: Analysis of a Ligand Exchange Procedure for CuInS2 Nanoparticles as the Photovoltaic Absorber Material

With nanoparticle-based inks, thin-film solar cells can be printed directly from solution, which promises cost reduction compared to conventional manufacturing methods. For synthesis of nanoparticles, long organic ligands are necessary to control the size and shape of the particles and stabilize them in the solvent of choice. However, these capping ligands act like an insulating barrier between the nanoparticles. Therefore, a common strategy to obtain nanoparticle-based solar cells is to replace the ligand shell originating from synthesis by other ligands that are as short as possible. Here, we show a successful and complete ligand exchange procedure for CuInS2 nanoparticles with 4-methylbenzenethiol, a relatively short ligand in comparison to the original ligand 1-dodecanethiol. The ligand exchange procedure was studied in detail by infrared spectroscopy and thermogravimetric analysis. Furthermore, solar cells were prepared in order to investigate the impact of the ligand exchange on the device properties. To get a deeper insight into the device physics, charge transport was studied in single-carrier devices.