We report highly bright and efficient inverted structure quantum dot (QD) based light-emitting diodes (QLEDs) by using solution-processed ZnO nanoparticles as the electron injection/transport layer and by optimizing energy levels with the organic hole transport layer. We have successfully demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23,040, 218,800, and 2250 cd/m(2), and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. It is also noticeable that they showed turn-on voltages as low as the bandgap energy of each QD and long operational lifetime, mainly attributed to the direct exciton recombination within QDs through the inverted device structure. These results signify a remarkable progress in QLEDs and offer a practicable platform for the realization of QD-based full-color displays and lightings.

We demonstrate a single-step synthetic method for highly luminescent (i.e., quantum yield up to 80%) and stable quantum dots (QDs) by using the reactivity difference between Cd and Zn precursors and that between Se and S precursors. A wide range of emission wavelengths (500-610 nm) with a narrow fwhm (<35 nm) is obtained by changing the ratios of the precursors. Under the reaction conditions selected, Cd-and Se (with a bit of S)-based cores are formed first and Zn-and S-based shells are formed successively; therefore, the QDs have a core/shell structure with composition gradients, which relieve the lattice mismatch between core and shells. The QDs are characterized using the combined techniques of HR-TEM, UV-vis, PL spectroscopy, and ICP-AES. The QDs also have energy gradients depending on their compositions in a radial direction, which energetically confine carriers (electrons and holes) to the cores. This leads to the stability of QDs during their surface passivation from oleic acid to mercaptopropionic acid and ensures their processibility for further purposes such as optoelectronic and biological applications.

We demonstrate bright, efficient, and environmentally benign InP quantum dot (QD)-based light-emitting diodes (QLEDs) through the direct charge carrier injection into QDs and the efficient radiative exciton recombination within QDs. The direct exciton formation within QDs is facilitated by an adoption of a solution-processed, thin conjugated polyelectrolyte layer, which reduces the electron injection barrier between cathode and QDs via vacuum level shift and promotes the charge carrier balance within QDs. The efficient radiative recombination of these excitons is enabled in structurally engineered InP@ZnSeS heterostructured QDs, in which excitons in the InP domain are effectively passivated by thick ZnSeS composition-gradient shells. The resulting QLEDs record 3.46% of external quantum efficiency and 3900 cd m(-2) of maximum brightness, which represent 10-fold increase in device efficiency and 5-fold increase in brightness compared with previous reports. We believe that such a comprehensive scheme in designing device architecture and the structural formulation of QDs provides a reasonable guideline for practical realization of environmentally benign, high-performance QLEDs in the future.

Highly efficient green‐light‐emitting diodes (LEDs) based on CdSe@ZnS quantum dots (QDs) with a chemical‐composition gradient are demonstrated. Through the moderate control of QD coverage in multilayered devices, excellent device performance has been achieved. The color‐saturated green‐light emission (see figure for Commission Internationale de l'Eclairage (CIE) co‐ordinates) is mainly from the QD layers (more than 99% of total emission).

Utilizing the reactivity difference between TOPSe and TOPS, we synthesized InP@ZnSeS QDs with the composition gradient in a radial direction where ZnSe alleviated lattice strain and ZnS protected QDs from degradation so that we achieved QDs with high QE and photo/chemical stability. In terms of systematic investigation on the relationship between the shell nanostructure and QD stability, we demonstrated that QDs with thick gradient shells exhibited high QE and much enhanced stability against the shell degradation under UV irradiation, ligand exchange, or rigorous purification. This enhanced stability of InP@ZnSeS QDs is attributed to the improved uniformity of composition gradient shells, the efficient confinement of exciton wavefunctions, and the minimized surface oxidation and non-radiative decay via surface states generated by photo-oxidation or ligand exchange. Using InP@ZnSeS QDs with enhanced stability, we were able to demonstrate InP-based colloidal green-emitting QD-LEDs. Although the current status of InP@ZnSeS QDs is not fully optimized to realize practical optoelectronic devices, the approach taken in the present study (i.e., the composition gradient shell structure naturally made from reactivity difference in precursors) will give clues to facilitate the synthesis of InP QDs with advanced nanostructures.