Effect of surface ligands on the performance of organic light-emitting diodes containing quantum dots

Abstract

Quantum dots (QDs) have numerous applications in optoelectronics due to their unique optical properties. Novel hybrid organic light-emitting diodes (OLEDs) containing QDs as an active emissive layer are being extensively developed. The performance of QD–OLED depends on the charge transport properties of the active layer and the degree of localization of electrons and holes in QDs. Therefore, the type and the density of the ligands on the QD surface are very important. We have fabricated OLEDs with a CdSe/ZnS QD active layer. These OLEDs contain hole and electron injection layers consisting of poly(9-vinyl carbazole) and ZnO nanoparticles, respectively. The energy levels of these materials ensure efficient injection of charge carriers into the QD emissive layer. In order to enhance the charge transfer to the active QD layer and thereby increase the OLED efficiency, the QD surface ligands (tri-n-octyl phosphine oxide, TOPO) were replaced with a series of aromatic amines and thiols. The substituents were expected to enhance the charge carrier mobility in the QD layer. Surprisingly, the devices based on the original TOPO-coated QDs were found to have the best performance, with a maximum brightness of 2400 Cd/m² at 10 V. We assume that this was due to a decrease in the charge localization within QDs when aromatic ligands are used. We conclude that the surface ligands considerably affect the performance of QD–OLEDs, efficient charge localization in QD cores being more important for good performance than a high charge transfer rate.