Highly Efficient Blue–Green Quantum Dot Light-Emitting Diodes Using Stable Low-Cadmium Quaternary-Alloy ZnCdSSe/ZnS Core/Shell Nanocrystals

Abstract

High-quality blue-green emitting ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell quantum dots (QDs) have been synthesized by a phosphine-free method. The quantum yields of as-synthesized ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell QDs can reach 50-75% with emissions between 450 and 550 nm. The emissions of such core/shell QDs are not susceptible to ligand loss through the photostability test. Blue-green light-emitting diodes (LEDs) based on the low-cadmium ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell QDs have been successfully demonstrated. Composite films of poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) and ZnO nanoparticle layers were chosen as the hole-transporting and the electron-transporting layers, respectively. Highly bright blue-green QD-based light-emitting devices (QD-LEDs) showing maximum luminance up to 10000 cd/m(2), in particular, the blue QD-LEDs show an unprecedentedly high brightness over 4700 cd/m(2) and peak external quantum efficiency (EQE) of 0.8%, which is the highest value ever reported. These results signify a remarkable progress in QD-LEDs and offer a practicable platform for the realization of QD-based blue-green display and lighting.

The work mechanism and sub-bandgap-voltage electroluminescence in inverted quantum dot light-emitting diodes

“…Colloidal quantum dots (QDs) are a favored class of material, since they combine the properties of inorganic semiconductors, such as optical stability, narrow emission linewidth, and mechanical flexibility, with the functional physical properties of conventional inorganic semiconductors1234567, including solution processability and tunable bandgap. Owing to these unique features, QDs have been actively investigated to utilize in many optoelectronic devices, for example, down-converters in backlit displays89, light-emitting diodes (LEDs)1011121314, lasers15, photodetectors16 and solar cells171819.…”

Double-heterojunction nanorods

“…5d). These values represent significant improvements over the best reported C/S LEDs using similar device architecture and ARTICLE emission wavelength 9,45,46 (cf. maximum values of brightness ¼ 31,000 cd m À 2 , EQE ¼ 1.7%, current efficiency ¼ 3.9 cd A À 1 and power efficiency ¼ 3.8 lm W À 1 for orange/red emitting C/S LEDs in ref.…”

“…Having established the double-heterojunction structure that allows high PL quantum yields with narrow linewidths, we now discuss the impact of the staggered band offset of CdS and ZnSe by examining the electroluminescence (EL) performance of DHNRs. LEDs with DHNRs as emitting elements (DHNR LEDs) were fabricated with device design similar to reported core/shell nanocrystal LEDs (C/S LEDs) 9,45,46 . Figure 5a inset shows the schematic of our device structure.…”

“…7b) suggest that DHNRs are primarily orientated parallel to the device plane. The transport layers and electrodes used here have also been used in C/S LEDs with external quantum efficiencies (EQEs) reaching up to B1.7% for emission wavelength comparable to our DHNR LEDs 9,45,46 . Figure 5a shows the energy band diagram of our DHNR LEDs that should lead to efficient carrier injection to the DHNRs (especially for holes) owing to the presence of the double heterojunction.…”

CHALLENGES IN APPLICATION OF LUMINESCENT MATERIALS, A TUTORIAL OVERVIEW (Invited Review)

“…Quantum dots possess optical properties that are size dependent, both the absorption and the emission properties can be tuned to a large extent for particles with sizes in the nm regime The emission bands of individual quantum dots are relatively narrow in view of a rather small energy difference the energetically lowest absorption band and the emission band (delocalized optical transitions, resulting in a very limited lattice relaxation). Rather narrow size distributions are required to obtain macroscopic amounts of quantum dots that possess narrow emission spectra (FWHM can be smaller than some 40 nm, see also Figure 5 for CdTe), and a lot of research and development attention goes into this direction, see, a.o., [14][15][16][17][18][19][20]. The feature of tunable line emission distinguishes quantum dots from line emitters based on rare-earth ions, where the spectral position of the lines only varies over a few nm (beit that many rare-earth ions show emission in many lines and that the relative intensities of these lines is host-lattice and in many cases also concentration dependent).…”