Heesun Yang

Over 40 cd/A Efficient Green Quantum Dot Electroluminescent Device Comprising Uniquely Large-Sized Quantum Dots

Green CdSe@ZnS quantum dots (QDs) of 9.5 nm size with a composition gradient shell are first prepared by a single-step synthetic approach, and then 12.7 nm CdSe@ZnS/ZnS QDs, the largest among ZnS-shelled visible-emitting QDs available to date, are obtained through the overcoating of an additional 1.6 nm thick ZnS shell. Two QDs of CdSe@ZnS and CdSe@ZnS/ZnS are incorporated into the solution-processed hybrid QD-based light-emitting diode (QLED) structure, where the QD emissive layer (EML) is sandwiched by poly(9-vinlycarbazole) and ZnO nanoparticles as hole and electron-transport layers, respectively. We find that the presence of an additional ZnS shell makes a profound impact on device performances such as luminance and efficiencies. Compared to CdSe@ZnS QD-based devices the efficiencies of CdSe@ZnS/ZnS QD-based devices are overwhelmingly higher, specifically showing unprecedented values of peak current efficiency of 46.4 cd/A and external quantum efficiency of 12.6%. Such excellent results are likely attributable to a unique structure in CdSe@ZnS/ZnS QDs with a relatively thick ZnS outer shell as well as a well-positioned intermediate alloyed shell, enabling the effective suppression of nonradiative energy transfer between closely packed EML QDs and Auger recombination at charged QDs.

Additive-free hollow-structured Co3O4 nanoparticle Li-ion battery: the origins of irreversible capacity loss.

Origins of the irreversible capacity loss were addressed through probing changes in the electronic and structural properties of hollow-structured Co3O4 nanoparticles (NPs) during lithiation and delithiation using electrochemical Co3O4 transistor devices that function as a Co3O4 Li-ion battery. Additive-free Co3O4 NPs were assembled into a Li-ion battery, allowing us to isolate and explore the effects of the Co and Li2O formation/decomposition conversion reactions on the electrical and structural degradation within Co3O4 NP films. NP films ranging between a single monolayer and multilayered film hundreds of nanometers thick prepared with blade-coating and electrophoretic deposition methods, respectively, were embedded in the transistor devices for in situ conduction measurements as a function of battery cycles. During battery operation, the electronic and structural properties of Co3O4 NP films in the bulk, Co3O4/electrolyte, and Co3O4/current collector interfaces were spatially mapped to address the origin of the initial irreversible capacity loss from the first lithiation process. Further, change in carrier injection/extraction between the current collector and the Co3O4 NPs was explored using a modified electrochemical transistor device with multiple voltage probes along the electrical channel.

Highly Bright Yellow-Green-Emitting CuInS2 Colloidal Quantum Dots with Core/Shell/Shell Architecture for White Light-Emitting Diodes

In this study, we report bright yellow-green-emitting CuInS2 (CIS)-based quantum dots (QDs) and two-band white light-emitting diodes (LEDs) using them. To achieve high quantum efficiency (QE) of yellow-green-emitting CIS QDs, core/shell/shell strategy was introduced to high quality CIS cores (QE = 31.7%) synthesized by using metal-oleate precursors and 1-dodecanethiol. The CIS/ZnS/ZnS QDs showed a high QE of 80.0% and a peak wavelength of 559 nm under the excitation of 450 nm, which is well matched with dominant wavelength of blue LEDs. The formation of core/shell/shell structure was confirmed by X-ray diffraction, transmission electron microscopy, and inductively coupled plasma-optical emission spectroscopy analyses. Intense and broad yellow-green emission band of the CIS/ZnS/ZnS is beneficial for bright two-band white light. When the CIS/ZnS/ZnS was coated on the blue LEDs, the fabricated white LED showed bright natural white light (luminous efficacy (η(L)) = 80.3 lm·W(-1), color rendering index (R(a)) = 73, correlated color temperature (T(c)) = 6140 K). The QD-white LED package showed a high light conversion efficiency of 72.6%. In addition, the CIS/ZnS/ZnS-converted white LED showed relatively stable white light against the variation of forward bias currents of 20-150 mA [color coordinates (x, y) = (0.3320-0.3207, 0.2997-0.2867), R(a) = 70-72, T(c) = 5497-6375 K].

Remote-type, high-color gamut white light-emitting diode based on InP quantum dot color converters

This work reported on synthesis of highly efficient, color-pure green- and red-emitting non-Cd InP/ZnS core/shell quantum dots (QDs) and their utilization as color converters for the fabrication of display backlighting QD-based white light-emitting diode (LED). Green and red QD emitters were first individually embedded into a transparent polymeric matrix of polyvinylpyrrolidone and the resulting two free-standing QD composite plates were then physically combined into a bilayered form. White QD-LED was fabricated by remotely loading the bilayered QD plate of a red-on-green configuration onto blue LED chip. This remote-type white device generated a spectrally well-resolved, tricolored electroluminescent spectrum, and exhibited luminous efficacies of 8.9−16.7 lm/W, depending on forward currents of 20−100 mA, and a high color gamut of 87%.

Fully transparent quantum dot light-emitting diode integrated with graphene anode and cathode.

A fully transparent quantum dot light-emitting diode (QD-LED) was fabricated by incorporating two types (anode and cathode) of graphene-based electrodes, which were controlled in their work functions and sheet resistances. Either gold nanoparticles or silver nanowires were inserted between layers of graphene to control the work function, whereas the sheet resistance was determined by the number of graphene layers. The inserted gold nanoparticles or silver nanowires in graphene films caused a charge transfer and changed the work function to 4.9 and 4.3 eV, respectively, from the original work function (4.5 eV) of pristine graphene. Moreover the sheet resistance values for the anode and cathode electrodes were improved from ∼63,000 to ∼110 Ω/sq and from ∼100,000 to ∼741 Ω/sq as the number of graphene layers increased from 1 to 12 and from 1 to 8, respectively. The main peak wavelength, luminance, current efficiency, and optical transmittance of the fully transparent QD-LED integrated with graphene anode and cathode were 535 nm, ∼358 cd/m2, ∼0.45 cd/A, and 70-80%, respectively. The findings of the study are expected to lay a foundation for the production of high-efficiency, fully transparent, and flexible displays using graphene-based electrodes.

Color-by-blue display using blue quantum dot light-emitting diodes and green/red color converting phosphors

We report a novel full-color display based on the generation of full-color by a highly efficient blue QD-LED light approach, or so called color-by-blue QD-LED display. This newly proposed color-by-blue QD-LED display combines a blue CdZnS/ZnS QD-LED blue subpixel and excitation source with front-emitting green/red phosphor subpixels. It is carefully estimated that the detailed display characteristics as well as full color-conversion and reasonable device efficiency of blue, green, and red satisfy the minimum requirements for display application. Also, we would like to emphasize that the proposed blue, green, and red device shows maximum luminance of 1570, 12920, and 3120 cd/m², respectively, luminous efficiency of 1.5, 12.1, and 2.5 cd/A, respectively, and external quantum efficiency of 6.8, 2.8, and 2.0%, respectively. It is expected that full color generation by color-by-blue QD-LED will lead to further technological advancements in the area of efficient and facile display applications.

Cu−In−Ga−S quantum dot composition-dependent device performance of electrically driven light-emitting diodes

Colloidal synthesis of ternary and quaternary quantum dots (QDs) of In/Ga ratio-varied Cu−In1−x−Gax−S (CIGS) with nominal xu2009=u20090, 0.5, 0.7, and 1 and their application for the fabrication of quantum dot-light-emitting diodes (QLEDs) are reported. Four QLEDs having CIGS QDs with different compositions are all solution-processed in the framework of multilayered structure, where QD emitting layer is sandwiched by hybrid charge transport layers of poly(9-vinlycarbazole) and ZnO nanoparticles. The device performance such as luminance and efficiency is found to be strongly dependent on the composition of CIGS QDs, and well interpreted by the device energy level diagram proposed through the determination of QD valence band minima by photoelectron emission spectroscopic measurement.

Simple process of hybrid white quantum dot/organic light-emitting diodes by using quantum dot plate and fluorescence

In this work, the simple process of hybrid quantum dot (QD)/organic light-emitting diode (OLED) was proposed to apply a white illumination light by using QD plate and organic fluorescence. Conventional blue fluorescent OLEDs were firstly fabricated and then QD plates of various concentrations, which can be controlled of UV–vis absorption and photoluminescence spectrum, were attached under glass substrate of completed blue devices. The suggested process indicates that we could fabricate the white device through very simple process without any deposition of orange or red organic emitters. Therefore, this work would be demonstrated that the potential simple process for white applications can be applied and also can be extended to additional research on light applications.

Quantum-dot-based white lighting planar source through downconversion by blue electroluminescence

We report the unprecedented fabrication of a planar white lighting quantum dot light-emitting diode (QD-LED) through integrating a CdZnS QD-based blue electroluminescence (EL) device with a free-standing polymethyl methacrylate (PMMA) composite film embedded with orange-emitting Cu-In-S (CIS) green-greenish yellow-emitting Cu-In-Ga-S (CIGS) QDs. The hybrid device successfully generates bicolored white emission that comprises blue EL and downconverted QD photoluminescence. The hybrid QD-LEDs loaded with the composite film embedded with one type of QDs exhibit a limited white spectral coverage, consequently producing low values (<65) in color rendering index (CRI). Thus, the QD-PMMA film consisting of a blend of green CIGS and orange CIS QD downconverters is applied for obtaining a higher-CRI white light through the spectral extension, resulting in a much improved CRI of 75-77. Various EL performances of the hybrid planar white device versus the reference blue QD-LED are also characterized in details.