PVK-based LEDs doped by cadmium sulphide nanocrystals coated by mixed thiophenol and 1-decanethiol

Nanoparticles capped with more than one ligand have great potential to improve the properties of optoelectronic devices. To discuss this possibility, we synthesized cadmium sulphide nanocrystals coated by two different ligands, 1-decanethiol and thiophenol. In addition, as a reference, cadmium sulphide nanocrystals coated with thiophenol were synthesized. Photoluminescence and UV–Vis absorption measurements indicated that the maximum emission peaks of both cadmium sulphide nanoparticles were located around 600 nm and the absorption edges showed the existence of two different types of crystalline structure, cubic zinc blende and wurzite-type hexagonal. But the nanoparticles with both ligands had higher emission intensity than nanocrystals coated only by thiophenol. Finally, hybrid light-emitting devices have been manufactured with the following structure: Indium Tin Oxide/Poly(3,4-ethylenedi-oxythiophene): poly(styrene sulfonate)/poly(9-vinylcarbazole): cadmium sulphide/Aluminum. These devices presented an active layer composed of poly(9-vinylcarbazole) doped by cadmium sulphide nanocrystals coated only by thiophenol in one case and with thiophenol and 1-decanethiol in the other case. The inclusion of nanocrystals in the active layer increased the current density of these devices. In addition, the solubility in chlorobenzene of nanoparticles with both ligands was improved, due to the presence of 1-decanethiol on their surface. Moreover, the presence of the second ligand intensified the influence of nanoparticles in electroluminescence measurements compared to cadmium sulfide nanocrystals without 1-decanethiol.