SnS thin films realized from colloidal nanocrystal inks

Abstract

Tin sulfide (SnS), having a direct band gap of 1.3eV, is a promising absorber material for solar energy conversion. We synthesized colloidal SnS nanocrystals with a size tuneable from 5 to 20nm and low size dispersion. These nanocrystals can be processed as thin films using low-cost solution phase methods. They also offer the possibility of controlling the crystalline phase before deposition. With the goal to obtain dense and crack-free films of high conductivity, we used a layer-by-layer deposition technique. In the first step, the substrate was dipped in the nanocrystal colloidal solution (“ink”). Next, exchange of the nanocrystal surface ligands (oleylamine, trioctylphosphine, oleic acid) was carried out by dipping the substrate into a solution of small cross-linking molecules (1,4-benzenedithiol). This exchange enhances the electronic coupling and charge carrier mobilities by reducing the interparticle distance. At the same time it assures the immobilization of the nanocrystals to avoid their removal during subsequent depositions. The thickness of the nanocrystal thin films was controlled in a range of 100–250nm by varying the number of the alternating nanocrystal deposition and ligand exchange steps. Scanning electron microscopy and atomic force microscopy investigations show that the obtained films are dense and homogeneous with a surface roughness as low as 3 to 4nm root mean square. Using an inverted structure, the heterojunction of a SnS nanocrystals film with n-type ZnO nanocrystals shows a strongly increased current density under white light irradiation with respect to the dark.