Kookheon Char

Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure

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.

Fabrication of Highly Ordered Multilayer Films Using a Spin Self‐Assembly Method

This work was supported by the Ministry of Education through the Brain Korea 21 Program at Seoul National University and by the National Program for Tera-level Nano-devices of the Ministry of Science and Technology as one of the 21st century Frontier Programs as well as by the Korean Ministry of Science and Technology (MOST) under Grant 99-07. X-ray reflectivity experiments performed at the Pohang Light Source (PLS) were supported in part by MOST and POSCO. We are very grateful to S.-H. Lee, H. Kang, J. Koo, and B. H. Seung for their assistance during the X-ray reflectivity experiments.

Multicolored Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films Using Layer-by-Layer Assembly Method

A systematic analysis of the exciton-recombination zone within all-quantum dot (QD) multilayer films prepared by a layer-by-layer assembly method was made, using sensing QD layers in QD-based light-emitting diodes (QLEDs). Large area practical multicolored colloidal QLEDs were also demonstrated by patterning and placing variously colored QDs (red, orange, yellow-green, and green) in the exciton-recombination zone.

Silicon-Cored Anthracene Derivatives as Host Materials for Highly Efficient Blue Organic Light-Emitting Devices**

Blue host materials for organic light-emitting diodes (OLEDs) based on silicon-cored (tetraphenylsilane) anthracene derivatives are synthesized. These compounds, with a non-coplanar molecular structure, have high glass-transition temperatures and good amorphous-film-forming capabilities. When doped with a blue-fluorescent dopant, blue emission with high color purity and high efficiency, up to 7.5 cd A(-1) and 6.3%, is achieved.

Highly efficient cadmium-free quantum dot light-emitting diodes enabled by the direct formation of excitons within InP@ZnSeS quantum dots.

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 and stable dye-sensitized solar cells based on SnO2 nanocrystals prepared by microwave-assisted synthesis

Highly efficient dye-sensitized solar cells (DSSCs) with excellent long-term stability were fabricated based on tin(IV) oxide (SnO2) nanocrystals with tunable morphologies and band energy levels. The nanocrystals were prepared by a facile, fast, and energy-saving microwave-assisted solvothermal reaction. Through variation of the precursor base used during nanocrystal synthesis control over morphology was achieved—precursor metal cations are known to have a strong influence on the growth process of SnO2 nanostructures. A simple and economic way to prepare semiconducting pastes for photoanodes was devised. The photovoltaic performance of dye-sensitized solar cells based on SnO2 photoanodes was investigated. A very high power conversion efficiency of up to 3.2%, based on very high Voc and comparable Jsc and FF [under 1 Sun condition (AM 1.5, 100 mW cm−2, with shading masks)] was achieved, reporting the highest efficiency value for the cells based on unmodified SnO2 nanocrystals so far. In order to elucidate the efficient cell behavior, electrochemical properties such as the charge transport in the photoanodes as well as SnO2/electrolyte interfacial properties were investigated. Uncharacteristically for DSSCs, all devices tested in the present study show an unusual long-term stability under ambient conditions over several weeks.

Thermal degradation behavior of polyaniline in polyaniline/Na+-montmorillonite nanocomposites

The thermal degradation behavior of polyaniline (PANI) in PANI/Na+-montmorillonite (Na+-MMT) nanocomposites prepared by in-situ intercalative polymerization of aniline into Na+-MMT has been investigated by thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The residual weight (TG curves) and its weight derivative (DTG curves) of the nanocomposites suggest that the PANI chains for PANI/Na+-MMT nanocomposites are more thermally stable than those for a simple PANI/Na+-MMT mixture. This improvement in the thermal stability for the nanocomposites is attributed to the presence of Na+-MMT nanolayers with a high aspect ratio acting as barriers, thus shielding the degradation of PANI in the nanogalleries and also hindering the diffusion of degraded PANI from the nanocomposites. The shielding effect of the nanolayers is found to be significant as the Na+-MMT content in the PANI/Na+-MMT nanocomposites is increased. The XRD patterns of the nanocomposites after TGA measurements indicate that the basal spacing (d001) of the PANI/Na+-MMT nanocomposites is almost intact, implying that the thermal decomposition of the PANI chains is believed to occur mainly outside the silicate layers.

Quantum Dot−Block Copolymer Hybrids with Improved Properties and Their Application to Quantum Dot Light-Emitting Devices

To combine the optical properties of CdSe@ZnS quantum dots (QDs) with the electrical properties of semiconducting polymers, we prepared QD/polymer hybrids by grafting a block copolymer (BCP) containing thiol-anchoring moieties (poly(para-methyl triphenylamine-b-cysteamine acrylamide)) onto the surfaces of QDs through the ligand exchange procedure. The prepared QD/polymer hybrids possess improved processability such as enhanced solubility in various organic solvents as well as the film formation properties along with the improved colloidal stability derived from the grafted polymer shells. We also demonstrated light-emitting diodes based on QD/polymer hybrids, exhibiting the improved device performance (i.e., 3-fold increase in the external quantum efficiency) compared with the devices prepared by pristine (unmodified) QDs.

Delivery of a Therapeutic Protein for Bone Regeneration from a Substrate Coated with Graphene Oxide

The therapeutic efficacy of drugs often depends on the drug delivery carrier. For efficient delivery of therapeutic proteins, delivery carriers should enable the loading of large doses, sustained release, and retention of the bioactivity of the therapeutic proteins. Here, it is demonstrated that graphene oxide (GO) is an efficient carrier for delivery of therapeutic proteins. Titanium (Ti) substrates are coated with GO through layer-by-layer assembly of positively (GO-NH₃⁺) and negatively (GO-COO⁻) charged GO sheets. Subsequently, a therapeutic protein (bone morphogenetic protein-2, BMP-2) is loaded on the GO-coated Ti substrate with the outermost coating layer of GO-COO⁻ (Ti/GO⁻). The GO coating on Ti substrate enables loading of large doses and the sustained release of BMP-2 with preservation of the structure and bioactivity of the drug. The extent of in vitro osteogenic differentiation of human bone marrow-derived mesenchymal stem cells is higher when they are cultured on Ti/GO- carrying BMP-2 than when they are cultured on Ti with BMP-2. Eight weeks after implantation in mouse models of calvarial defects, the Ti/GO-/BMP-2 implants show more robust new bone formation compared with Ti, Ti/GO-, or Ti/BMP-2 implants. Therefore, GO is an effective carrier for the controlled delivery of therapeutic proteins, such as BMP-2, which promotes osteointegration of orthopedic or dental Ti implants.

Structural changes of polyaniline/montmorillonite nanocomposites and their effects on physical properties

Polyaniline/montmorillonite (MMT) nanocomposites containing different PANI contents were prepared by the intercalation of aniline monomer into pristine MMT followed by the subsequent oxidative polymerization of the aniline in the interlayer spacings. The polyaniline/MMT nanocomposite structure intercalated with polyaniline (PANI) was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). From the full-width at half-maximum (FWHM) of the (001) reflection peaks in the XRD patterns, the PANI/MMT nanocomposite containing 12.3 wt% PANI (PMN12) was found to be in the most disordered state. The physical interaction between the intercalated PANI and the basal surfaces of MMT was monitored by FT-IR. The room-temperature conductivity (σRT) varied from 9.1 × 10−9 to 1.5 × 100 S cm−1 depending on the PANI content in the nanocomposites. The temperature dependence of dc conductivity (σdc(T)) of all the samples follows the quasi-1D variable range hopping (quasi-1D VRH) model (i.e., σdc(T) ∝ exp [−(T0/T)1/2]). The charge transport behavior of this system was interpreted from the slopes (T0) of the σdc curves and the highest T0 value was found for the PANI/MMT nanocomposite with 12.3 wt% PANI (PMN12). The FT-IR, σdc(T) and σRT results for the nanocomposites with varying content of PANI are consistently related to the structure of the PANI/MMT nanocomposites discussed in the XRD analysis. The structural argument was further supported by scanning electron microscopy (SEM) of all the samples. Thermogravimetric analysis (TGA) showed improved thermal stability for the intercalated nanocomposites in comparison with the pure PANI and a simple PANI/MMT mixture.