Semiconductor quantum dot light-emitting diodes (QD-LEDs) allow easy color tuning by changing the QD size. [1] Similar to organic LEDs (OLEDs) based on phosphorescent molecules, QD-LEDs do not suffer from the spin statistics that limit the internal quantum efficiency of fluorescent OLEDs. Nevertheless, the efficiency of current QD-LEDs is far less compared to state-of-the-art phosphorescent OLEDs, like those based on iridium emitters. [2] Early QD-LEDs used CdSe as the emitter in a poly(N-vinylcarbazole)/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3, 4-oxadiazole (PVK/PBD) matrix, but these devices showed very low external quantum efficiency (h ext % 0.0005%). A somewhat higher efficiency (h ext % 0.001-0.01%) was obtained in a layered device, where a layer of CdSe QDs was spin cast on a layer of poly( p-phenylene vinylene) (PPV). The efficiency of these layered devices was further enhanced to h ext ¼ 0.22%, by using QDs with a CdSe core and a CdS shell. Additional progress in the efficiency of QD-LEDs was achieved by changing the polymer layer, introducing an electron injection layer, and optimizing the thickness of the QD layer, resulting in efficiencies of h ext ¼ 2%. Recently an h ext of $2.1% was obtained by annealing the QD layer at temperatures up to 180 8C, which removes the organic ligands from the surface of the QD, leading to improved charge injection and higher efficiency. The efficiencies of these devices remain limited, mainly because of ineffective charge injection into the QDs. Several factors contribute to the poor carrier injection. First, QDs generally have an inorganic shell of a wide bandgap material (e.g., CdS or ZnS) to increase photostability and improve emission quantum yields by passivating surface defects. Second, they are covered by a layer of organic ligands, which is needed during their growth and provides solubility in organic solvents to allow processing. However, these organic and inorganic layers form a tunneling barrier for charge injection. Another complication arises from the fact that the valence bands of the QDs are generally shifted to lower energy compared to the highest occupied molecular orbital (HOMO) levels of commonly used organic hole-injection layers. This introduces significant energy barriers to hole injection.
In this work, we demonstrate that Cu-doped CdS (CdS:Cu) QDs do not suffer from these limitations. Charge recombination occurs readily on CdS:Cu QDs when blended into the polymer layer. As a result, QD-LEDs with unprecedented high efficiencies are obtained using QD cores that lack an inorganic passivating shell.