Woo-Seuk Song

Highly Efficient, Color-Pure, Color-Stable Blue Quantum Dot Light-Emitting Devices

For colloidal quantum dot light-emitting diodes (QD-LEDs), blue emissive device has always been inferior to green and red counterparts with respect to device efficiency, primarily because blue QDs possess inherently unfavorable energy levels relative to green and red ones, rendering hole injection to blue QDs from neighboring hole transport layer (HTL) inefficient. Herein, unprecedented synthesis of blue CdZnS/ZnS core/shell QDs that exhibit an exceptional photoluminescence (PL) quantum yield of 98%, extraordinarily large size of 11.5 nm with a shell thickness of 2.6 nm, and high stability against a repeated purification process is reported. All-solution-processed, multilayered blue QD-LEDs, consisting of an HTL of poly(9-vinlycarbazole), emissive layer of CdZnS/ZnS QDs, and electron transport layer of ZnO nanoparticles, are fabricated. Our best device displays not only a maximum luminance of 2624 cd/m(2), luminous efficiency of 2.2 cd/A, and external quantum efficiency of 7.1%, but also no red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as a spectral match between PL and EL.

Noninjection, one-pot synthesis of Cu-deficient CuInS2/ZnS core/shell quantum dots and their fluorescent properties

Non-toxic, environment-benign colloidal CuInS(2) (CIS) quantum dots (QDs) were synthesized through a facile noninjection, one-pot approach by reacting Cu and In precursors with dodecanethiol dissolved in 1-octadecence at 220 °C. The Cu:In precursor molar ratio was varied from 1:1 to 1:4 to intentionally generate Cu-deficient CIS QDs. Depending on the stoichiometry of the QDs, their emission peak wavelengths were tuned in red-deep red region. More Cu-deficient CIS QDs (Cu:In=1:4) were found to be more efficient than ones with Cu:In=1:1. After successive ZnS shell was overgrown on the surface of core QDs with Cu:In=1:4, the resulting core/shell QDs exhibited a highly efficient yellow emission with a quantum yield of ~50%. A substantially blue-shifted emission from the core/shell QDs versus core ones was described by suggesting an alternative recombination pathway that may be induced by the ZnS shell coating.

Facile, air-insensitive solvothermal synthesis of emission-tunable CuInS2/ZnS quantum dots with high quantum yields

Yellow-to-red emitting CuInS2/ZnS (CIS/ZnS) core/shell quantum dots (QDs) with a maximum photoluminescence quantum yield (QY) of ∼65% were facilely synthesized via a stepwise, consecutive solvothermal approach. The size-sorting experiments on CIS/ZnS QDs indicated that the ZnS shell was formed with a relatively uniform thickness even though the size of pre-existing CIS core QDs was not monodisperse. CIS core QDs solvothermally grown for different reaction times of 5–6 h exhibited poor QYs of <8.8% with deep red emissions as a result of donor–acceptor pair (DAP) recombination. Upon shell overcoating, the band gap energies of CIS/ZnS QDs increased by 0.14–0.23 eV, depending on the CIS core growth time. Such increased band gaps were attributed presumably to the reduction of actual CIS core size, originating from the formation of the alloyed interfacial layer at CIS/ZnS during a long shell growth. Compared to CIS core QDs, the emission of CIS/ZnS QDs was markedly blue-shifted by 0.10–0.18 eV. This blue-shift was discussed based on the shift of QD band gap-dependent donor/acceptor energy levels and the donor–acceptor distance-dependent DAP recombination process.

Synthesis of color-tunable Cu–In–Ga–S solid solution quantum dots with high quantum yields for application to white light-emitting diodes

Synthesis and application of I–III–VI type Cu–In–Ga–S (CIGS) quantum dots (QDs) with varied In : Ga ratios to a white light-emitting diode (LED) are reported. The band gap and emission energies of CIGS QDs are found to be systematically higher with increasing Ga content as a result of an appropriate formation of CIS–CGS solid solution. ZnS shell-capped QDs of the CIGS/ZnS core–shell emit quite tunable wavelengths of 520–578 nm with exceptionally high photoluminescence quantum yield of 72–83%, depending on the composition. By combining highly fluorescent CIGS/ZnS QDs as color converters with a blue-emitting LED, efficient white QD-based LEDs with high luminous efficacies of 69.1–75.0 lm W−1 at a forward current of 20 mA are fabricated. A white QD-LED with an enhanced color rendering property is further developed by choosing and blending two kinds of CIGS/ZnS QDs, and its electroluminescent properties are characterized in detail as a function of applied current.

Preparation of a photo-degradation- resistant quantum dot-polymer composite plate for use in the fabrication of a high-stability white-light-emitting diode.

We report on the synthesis of highly fluorescent double-ZnS-shell-capped, yellow-emitting Cu-In-S quantum dots (QDs) with a surprisingly high quantum yield of 92%, the preparation of a free-standing QD-polymethylmethacrylate composite plate, and the application of the QD plate in the fabrication of QD-based white-light-emitting diodes (WLEDs). A free-standing QD plate with QDs embedded uniformly inside a polymeric matrix is used to fabricate a remote-type, resin-free WLED. The QD plate-based WLED displays a high luminous efficiency; however, it suffers from a significantly unstable device performance due to QD degradation upon prolonged photo-excitation. An exceptional operational stability of the QD plate-based WLED is realized by generating hybrid double layers of an organic adhesion layer and a gas barrier layer of sol-gel-derived silica, rendering the QD plate impermeable to oxygen. Our success in achieving a color converter robust against photo-degradation and applying it in the fabrication of a reliable QD-based LED is greatly encouraging as regards the development of next-generation QD-based LED lighting sources.

Fabrication of white light-emitting diodes based on solvothermally synthesized copper indium sulfide quantum dots as color converters

A facile, large-scalable solvothermal synthesis of copper indium sulfide (CIS) quantum dots (QDs) and their application to the fabrication of QD-based white light-emitting diodes (LEDs) are reported. Depending on CIS QD growth time of 2 versus 5 h, the core/shell structured QDs of CIS/ZnS exhibit tunable emissions of yellow-orange with excellent quantum yields of 55%-91%. A white QD-LED is realized by applying CIS (2 h)/ZnS QD as a blue-to-yellow color converter. Furthermore, a white QD-LED having a blend of yellow and orange QDs is fabricated to improve a color rendering property through spectral extension, and its electroluminescent properties are evaluated.A facile, large-scalable solvothermal synthesis of copper indium sulfide (CIS) quantum dots (QDs) and their application to the fabrication of QD-based white light-emitting diodes (LEDs) are reported. Depending on CIS QD growth time of 2 versus 5 h, the core/shell structured QDs of CIS/ZnS exhibit tunable emissions of yellow-orange with excellent quantum yields of 55%-91%. A white QD-LED is realized by applying CIS (2 h)/ZnS QD as a blue-to-yellow color converter. Furthermore, a white QD-LED having a blend of yellow and orange QDs is fabricated to improve a color rendering property through spectral extension, and its electroluminescent properties are evaluated.

Solvothermally grown Ce3+-doped Y3Al5O12 colloidal nanocrystals: spectral variations and white LED characteristics

Alcoholic media dispersible YAG : Ce colloidal nanocrystals were prepared by a solvothermal route. To obtain highly efficient nanocrystals, the Ce concentration and the reaction time were optimized. The surface of the nanocrystals was further modified with oleylamine to make them dispersible in a non-polar solvent. The luminescent characteristics such as luminescent efficiency, band width and peak wavelength of Ce3+ d → f transition of the YAG : Ce nanocrystals were compared with those of a micrometre-sized phosphor. The host of nanocrystals was compositionally modified by partially or completely substituting Y3+ with Tb3+ or Gd3+ ions, through which the Ce3+ emission peaks were red-shifted due to an increased crystal field strength around Ce3+ luminescent centres. White light-emitting diodes (LEDs) were fabricated by coating these YAG : Ce nanocrystals on blue LED chips and their electroluminescent properties were compared with those of a bulk YAG : Ce-based white LED.

Formation of green-emitting LaPO4:Ce,Tb nanophosphor layer and its application to highly transparent plasma displays

Uniformly sized LaPO4:Ce,Tb nanophosphors were produced by using lanthanide nitrates and phosphoric acid along with citric acid and polyvinylpyrrolidone (PVP). The most efficient nanophosphors were obtained by varying their post-annealing temperature and chemical composition. The optimized nanophosphors were subsequently dispersed in 2-methoxyethanol and a highly transparent green-emitting nanophosphor layer was formed on a glass substrate via spin-coating of the dispersion. The spin-coating was repeated up to 7 times to increase the thickness of the nanophosphor layer. Despite multiple coatings, all nanophosphor layers maintained an excellent visible transparency. Ultimately, using the nanophosphor layer deposited on glass substrate, mini-sized transparent plasma display panels (PDPs) were fabricated and their luminance characteristics were described.

Fabrication of warm, high CRI white LED using non-cadmium quantum dots

We reported on the synthesis of two bright non-cadmium quantum dots (QDs) of green (512 nm)-emitting InP/ZnS and orange (583 nm)-emitting CuInS2 (CIS)/ZnS core/shell and their application for the fabrication of solid-state lighting device. The spectral overlap between absorptions from both QDs and emission from InGaN-based blue light-emitting diode (LED) chip was excellent. Thus, the efficient down-conversion of blue-to-QD emission for the generation of white light was accomplished by sequentially dispensing InP/ZnS QDs on top of CIS/ZnS ones within epoxy resin. The white QD-LED generated a high-quality white light with a high color rendering index of 90 and a warm color temperature of 3803K under a drive current of 20 mA. Drive current-dependent variations of its primary electroluminescent properties were investigated in details.

Color-converting bilayered composite plate of quantum-dot–polymer for high-color rendering white light-emitting diode

Highly bright CuInS₂ quantum dots (QDs), whose emission is easily tuned in orange and greenish-yellow colors through manipulating the ZnS shelling procedure, are integrated with polymethyl methacrylate (PMMA) to obtain a free-standing composite plate of QD-polymer. The composite plate embedded with orange or greenish-yellow QDs is combined in a remote type with a blue light-emitting diode (LED). No color rendering index (CRI) value results when orange QDs are applied, whereas use of greenish-yellow QDs leads to a limited CRI of 72. A higher-color rendering QD-LED is demonstrated through a spectral extension by devising a unique bilayer-structured QD plate, where two types of QD are separately incorporated into each PMMA matrix, with a buffer layer of polymer blend inserted between them. The bilayered QD plate LED exhibits an improved CRI of 81, a high luminous efficacy of 71.2 lm/W at an input current of 20 mA, and exceptional high-stability luminescent characteristics against the variation of applied current.