Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers

Colloidal quantum dots, with their tunable luminescence properties, are uniquely suited for use as lumophores in lightemitting devices for display technologies and large-area planar lighting. In contrast to epitaxially grown quantum dots, colloidal quantum dots can be synthesized as highly monodisperse colloids and solution deposited over large areas into densely packed, solid-state multilayers, which have shown promise as efficient optical gain media11. To be a viable platform for colour-tunable electrically pumped lasers, the present-generation quantum-dot LEDs must be modified to withstand the extended, high-current-density operation needed to achieve population inversion. This requirement necessitates a quantum-dot LED design that incorporates robust charge transport layers. Here we report the use of sputtered, amorphous inorganic semiconductors as robust charge transport layers and demonstrate devices capable of operating at current densities exceeding 3.5 A cm with peak brightness of 1,950 Cd m and maximum external electroluminescence efficiency of nearly 0.1%, which represents a 100-fold improvement over previously reported structures. Previous efforts at building colloidal quantum-dot (QD) LEDs with inorganic charge transport layers have demonstrated only limited quantum dot (QD) electroluminescence (EL) efficiency. One study placed a multilayer of QDs between indium tin oxide (ITO) and silver electrodes. The low luminescence efficiency (10 Cd A) of these structures could be attributed to a number of sources, including luminescence quenching of the QDs by plasmon modes in the highly conductive electrodes, luminescence quenching by QD charging, or low efficiency due to imbalance in the polarity of charges injected into the device, which could lead to excess background current14. More recent work using doped GaN transport layers surrounding colloidal QDs reported external quantum efficiencies (EQEs) of 0.001–0.01%, although much of the observed EL arose from the GaN (ref. 8). These devices exhibited non-uniform emission across the pixel area, and fabrication required the specialized deposition technique of energy-neutral atomic-beam lithography or epitaxy. In contrast to this early attempt, in the present work we demonstrate the first general method that reproducibly fabricates patterned, all-inorganic optoelectronic devices with functional QD lumophores. We use amorphous, radiofrequency (RF)-sputtered metal oxides as QD-LED charge transport layers, deposited at room temperature to be broadly compatible with colloidal QDs and many other constituent films. With this advance, allinorganic, QD-containing devices can, for the first time, be systematically engineered, as exemplified by our demonstration of QD-LEDs that manifest 100-fold higher EQEs than previously reported all-inorganic colloidal QD-LED structures. We follow three main guidelines in the choice and preparation of the metal oxide charge transport layers. First, we chose mechanically smooth and compositionally amorphous films to prevent electrical shorts or the formation of preferred current channels through the device structure. Second, we grew semiconducting oxide films with low free-carrier concentrations to minimize quenching of the QD EL through free-carrier plasmon modes. Third, the hole transport layer (HTL) and the electron transport layer (ETL) were chosen to have similar freecarrier concentrations and energy-band offsets to the QDs so that electron and hole injection into the QD layer was balanced. An excess of one type of carrier in the QD region results in QD NiO