Myeongjin Park

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.

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.

R/G/B/Natural White Light Thin Colloidal Quantum Dot‐Based Light‐Emitting Devices

Bright, low-voltage driven colloidal quantum dot (QD)-based white light-emitting devices (LEDs) with practicable device performances are enabled by the direct exciton formation within quantum-dot active layers in a hybrid device structure. Detailed device characterization reveals that white-QLEDs can be rationalized as a parallel circuit, in which different QDs are connected through the same set of electrically common organic and inorganic charge transport layers.

High-Power Genuine Ultraviolet Light-Emitting Diodes Based On Colloidal Nanocrystal Quantum Dots.

Thin-film ultraviolet (UV) light-emitting diodes (LEDs) with emission wavelengths below 400 nm are emerging as promising light sources for various purposes, from our daily lives to industrial applications. However, current thin-film UV-emitting devices radiate not only UV light but also visible light. Here, we introduce genuine UV-emitting colloidal nanocrystal quantum dot (NQD) LEDs (QLEDs) using precisely controlled NQDs consisting of a 2.5-nm-sized CdZnS ternary core and a ZnS shell. The effective core size is further reduced during the shell growth via the atomic diffusion of interior Cd atoms to the exterior ZnS shell, compensating for the photoluminescence red shift. This design enables us to develop CdZnS@ZnS UV QLEDs with pure UV emission and minimal parasitic peaks. The irradiance is as high as 2.0-13.9 mW cm(-2) at the peak wavelengths of 377-390 nm, several orders of magnitude higher than that of other thin-film UV LEDs.

Improved Efficiency of Inverted Organic Light-Emitting Diodes Using Tin Dioxide Nanoparticles as an Electron Injection Layer

We demonstrated highly efficient inverted bottom-emission organic light-emitting diodes (IBOLEDs) using tin dioxide (SnO2) nanoparticles (NPs) as an electron injection layer at the interface between the indium tin oxide (ITO) cathode and the organic electron transport layer. The SnO2 NP layer can facilitate the electron injection since the conduction band energy level of SnO2 NPs (-3.6 eV) is located between the work function of ITO (4.8 eV) and the lowest unoccupied molecular orbital (LUMO) energy level of typical electron transporting molecules (-2.5 to -3.5 eV). As a result, the IBOLEDs with the SnO2 NPs exhibited a decrease of the driving voltage by 7 V at 1000 cd/m(2) compared to the device without SnO2 NPs. They also showed a significantly enhanced luminous current efficiency of 51.1 cd/A (corresponds to the external quantum efficiency of 15.6%) at the same brightness, which is about two times higher values than that of the device without SnO2 NPs. We also measured the angular dependence of irradiance and electroluminescence (EL) spectra in the devices with SnO2 NPs and found that they had a nearly Lambertian emission profile and few shift in EL spectrum through the entire viewing angles, which are considered as remarkable and essential results for the application of OLEDs to display devices.

Study on the internal field and conduction mechanism of atomic layer deposited ferroelectric Hf0.5Zr0.5O2 thin films

The internal field (Eint) in ferroelectric films is an important factor which can affect the reliability of practical devices utilizing two memory states which results from the remanent polarizations of ferroelectric films. In the current work, the Eint in TiN/Hf0.5Zr0.5O2/TiN capacitors was controlled by changing the annealing atmosphere (N2, O2, and forming gas). The magnitude of negative Eint in O2-annealed samples was the largest, whereas that in the forming gas-annealed sample was the smallest. The magnitude of Eint can be understood based on the asymmetric distribution of oxygen vacancies near top and bottom TiN electrodes. Despite the large magnitude of Eint, the two remanent polarizations can be reliably retained due to the large coercive electric field of Hf0.5Zr0.5O2 films, and this is expected to be beneficial for application in semiconductor memory devices. During the repetitive electric field cycling for the wake-up process, the change in Eint in O2- and forming gas-annealed samples showed the opposite tendency: the magnitude of Eint in the O2-annealed Hf0.5Zr0.5O2 film decreased, whereas that in the forming gas-annealed film increased. This difference is believed to be due to the redistribution of oxygen vacancies with electric field high enough for the migration of oxygen vacancies. The conduction mechanism of electrons through Hf0.5Zr0.5O2 films was also examined, and the results fitted best with the Poole–Frenkel emission model with the shallow traps for all the samples with a reasonable optical dielectric constant value for Hf0.5Zr0.5O2.

Ferroelectricity in undoped-HfO2 thin films induced by deposition temperature control during atomic layer deposition

HfO2 thin films, extensively studied as high-k gate dielectric layers in metal-oxide-semiconductor field effect transistors, have attracted interest of late due to their newly discovered ferroelectricity in doped HfO2. The appearance of the ferroelectric orthorhombic phase of HfO2 was previously examined in variously doped and undoped systems, but the effects of process-variable changes on the physical and chemical characteristics of a thin film and the resulting ferroelectricity have not been studied systematically. Here, the evolution of ferroelectricity in HfO2 thin films through deposition temperature control during atomic layer deposition was systematically examined without the intentional doping of metallic elements other than Hf. The lower-temperature-deposited HfO2 showed an increased impurity concentration, which was mainly carbon, and the involvement of these impurities suppressed the lateral grain growth during the crystallization thermal treatment. The grain size reduction could stabilize the metastable orthorhombic phase, whose surface and grain boundary energies are lower than those of the room-temperature-stable monoclinic phase, by increasing the grain boundary areas. The 9 nm-thick HfO2 thin film deposited at 220 °C exhibited a remanent polarization value of 10.4 μC cm−2 and endured up to 108 switching cycles, which is a 102-fold improvement compared to the previously reported undoped 6 nm-thick HfO2. This can be ascribed to the decrease in the relative portion of defective interfacial layers by increasing the total film thickness. The strategy of using deposition temperature control is a feasible method for the fabrication of these new lead-free binary ferroelectric thin films.

HLA-A, -B and -DRB1 polymorphism in Koreans defined by sequence-based typing of 4128 cord blood units

Human leucocyte antigen (HLA) alleles and haplotypes differ significantly among different ethnic groups, and high‐resolution typing methods allow for the detection of a wider spectrum of HLA variations. In this study, HLA‐A, ‐B and ‐DRB1 genotypes were analysed in 4128 cord blood units obtained from Korean women using the sequence‐based typing method. A total of 44 HLA‐A, 67 HLA‐B and 48 HLA‐DRB1 most probable alleles were identified. Of these, high‐frequency alleles found at a frequency of ≥5% were 6 HLA‐A (A*02:01, A*02:06, A*11:01, A*24:02, A*31:01, A*33:03), 5 HLA‐B (B*15:01, B*44:03, B*51:01, B*54:01, B*58:01) and 7 HLA‐DRB1 (DRB1*01:01, DRB1*04:05, DRB1*07:01, DRB1*08:03, DRB1*09:01, DRB1*13:02, DRB1*15:01) alleles. At each locus, A*02, B*15 and DRB1*04 generic groups were most diverse at allelic level, consisting of 8, 11 and 10 different alleles, respectively. Two‐ and three‐locus haplotypes estimated by the maximum likelihood method revealed 73 A‐B, 74 B‐DRB1 and 42 A‐B‐DRB1 haplotypes with frequencies of ≥0.3%. A total of 193 A‐B‐DRB1 haplotypes found at a frequency of ≥0.1% were presented, and the six most common haplotypes were A*33:03‐B*44:03‐DRB1*13:02 (4.6%), A*33:03‐B*58:01‐DRB1*13:02 (3.0%), A*24:02‐B*07:02‐DRB1*01:01 (2.7%), A*33:03‐B*44:03‐DRB1*07:01 (2.5%), A*30:01‐B*13:02‐DRB1*07:01 (2.2%) and A*24:02‐B*52:01‐DRB1*15:02 (2.1%). Compared with previous smaller scale studies, this study further delineated the allelic and haplotypic diversity in Koreans including low‐frequency alleles and haplotypes. Information obtained in this study will be useful for the search for unrelated bone marrow donors and for anthropologic and disease association studies.

The effect of band gap alignment on the hole transport from semiconducting block copolymers to quantum dots

Semiconducting hole transporting block copolymers were chemically modified to adjust their energy levels to that of CdSe/CdS/CdZnS red quantum dots. Hybrids with optimized energy levels could be used to build strongly improved quantum dot based LEDs (QLEDs).

HLA-A, -B, -DRB1 allele frequencies and haplotypic association from DNA typing data of 7096 Korean cord blood units.

Human leukocyte antigen (HLA) distribution in 7096 Korean cord blood (CB) units preserved at the public CB bank was analyzed by using the polymerase chain reaction with sequence-specific oligonucleotide probe (PCR-SSOP) method. A total of 14 HLA-A, 33 HLA-B, 13 HLA-DRB1 alleles and 2470 three-locus haplotypes were identified. The results are generally similar to those from the previous Korean studies, but the frequencies of less frequent haplotypes < 0.1% are more relevant and infrequent haplotypes with strong linkage disequilibrium were newly found because of the large sample size. Our results showed some similarities to those of other Asians but also some differences, suggesting a rationale for an Asian network for a hematopoietic stem-cell donor registry. Results from this large-scale analysis will be useful in Korean and Asian registry planning.