Solvent-mediated surface ligand exchange to enhance the performance of quantum-dot light-emitting diodes

Colloidal quantum dots (QDs) with core-shell structure and long-chain organic ligands are widely used as emitting-material layers (EMLs) for quantum-dot light-emitting diodes (QLEDs). However, the poor conductivity of long-chain organic ligands severely affects charge injection into QD-EMLs and charge transport among QDs, usually leading to poor device performance. To address the issue, we partially replaced the original long-chain oleic acid (OA) ligands capping on the QD surface with short-chain 1,2-ethanedithiol (EDT) ligands via solvent-mediated solid-state ligand exchange to fabricate high-performance QLEDs. The effects of solvent (acetonitrile and ethanol)-mediated EDT ligand exchange on the properties of QDs and device performance were comparatively investigated. Our results show that the ligand exchange is more sufficient when using ethanol instead of acetonitrile as the solvent for EDT, which is not only capable of improving charge transport among QDs but also capable of enhancing hole-electron injection balance in QLEDs. As a result, the QLED device based on QDs after EDT (in ethanol) ligand exchange exhibits a maximum luminescence of 190,200 cd/m2, a maximum current efficiency of 18.34 cd/A, a maximum power efficiency of 9.60 l m/W and a peak external quantum efficiency of 12.26%, appreciably outperforming those based on pristine QDs and QDs after EDT (in acetonitrile) ligand exchange. After aging, its maximum current efficiency, maximum power efficiency and peak external quantum efficiency can reach up to 35.91 cd/A, 23.30 l m/W and 24.00%, respectively. Our results highlight the importance of surface chemistry control via solvent-mediated ligand exchange to fabricate high-performance QLEDs, and suggest that ethanol may be a more suitable solvent for EDT to fulfill solid-state ligand exchange of core/shell QDs.