Sang-Youp Yim

Highly Efficient, Color-Reproducible Full-Color Electroluminescent Devices Based on Red/Green/Blue Quantum Dot-Mixed Multilayer

Over the past few years the performance of colloidal quantum dot-light-emitting diode (QLED) has been progressively improved. However, most of QLED work has been fulfilled in the form of monochromatic device, while full-color-enabling white QLED still remains nearly unexplored. Using red, green, and blue quantum dots (QDs), herein, we fabricate bichromatic and trichromatic QLEDs through sequential solution-processed deposition of poly(9-vinlycarbazole) (PVK) hole transport layer, two or three types of QDs-mixed multilayer, and ZnO nanoparticle electron transport layer. The relative electroluminescent (EL) spectral ratios of constituent QDs in the above multicolored devices are found to inevitably vary with applied bias, leading to the common observation of an increasing contribution of a higher-band gap QD EL over low-band gap one at a higher voltage. The white EL from a trichromatic device is resolved into its primary colors through combining with color filters, producing an exceptional color gamut of 126% relative to National Television Systems Committee (NTSC) color space that a state-of-the-art full-color organic LED counterpart cannot attain. Our trichromatic white QLED also displays the record-high EL performance such as the peak values of 23,352 cd/m(2) in luminance, 21.8 cd/A in current efficiency, and 10.9% in external quantum efficiency.

Synthesis of Si Nanosheets by a Chemical Vapor Deposition Process and Their Blue Emissions

We synthesized free-standing Si nanosheets (NSs) with a thickness of about <2 nm using a chemical vapor deposition process and studied their optical properties. The Si NSs were formed by the formation of frameworks first along six different <110> directions normal to [111], its zone axis, and then by filling the spaces between the frameworks along the <112> directions under high flow rate of processing gas. The Si NSs showed blue emission at 435 nm, and absorbance and photoluminescence (PL) excitation measurements indicate that enhanced direct band transition attributes to the emission. Time-resolved PL measurement, which showed PL emission at 435 nm and a radiative lifetime of 1.346 ns, also indicates the enhanced direct band gap transition in these Si NSs. These outcomes indicate that dimensionality of Si nanostructures may affect the band gap transition and, in turn, the optical properties.

Localized surface plasmon-enhanced near-ultraviolet emission from InGaN/GaN light-emitting diodes using silver and platinum nanoparticles.

We demonstrate localized surface plasmon (LSP)-enhanced near-ultraviolet light-emitting diodes (NUV-LEDs) using silver (Ag) and platinum (Pt) nanoparticles (NPs). The optical output power of NUV-LEDs with metal NPs is higher by 20.1% for NUV-LEDs with Ag NPs and 57.9% for NUV-LEDs with Pt NPs at 20 mA than that of NUV-LEDs without metal NPs. The time-resolved photoluminescence (TR-PL) spectra shows that the decay times of NUV-LEDs with Ag and Pt NPs are faster than that of NUV-LEDs without metal NPs. The TR-PL and absorbance spectra of metal NPs indicate that the spontaneous emission rate is increased by resonance coupling between excitons in the multiple quantum wells and LSPs in the metal NPs.

Localized surface plasmon-enhanced green quantum dot light-emitting diodes using gold nanoparticles

We develop a localized surface plasmon (LSP)-enhanced CdSe/ZnS green quantum dot (QD) light-emitting diode (LED) containing Au nanoparticles (NPs) embedded in a ZnO electron transport layer. Au NPs blended in ZnO solution are directly spin coated onto the QD emissive layer to provide strong coupling between LSPs in Au NPs and excitons in QDs, greatly enhancing the electroluminescence (EL). Photoluminescence (PL) and EL intensities are greatly enhanced by 4.12 and 4.33-fold, respectively. Maximum PL and EL enhancement ratios of 4.47 and 4.54 are observed at 535 and 532 nm, respectively, and these are similar to the LSP resonance wavelength of 536 nm for Au NPs in ZnO films. The results indicate that the EL enhancement of the QD-LED is attributed to strong resonance coupling between excitons in the QDs and LSPs in the Au NPs in ZnO films.

Evolution of electromagnetic interference through nano-metallic double-slit

We investigated the characteristics of the near- and far-field regions of the interference for nano-metallic double-slits using a two-dimensional finite-difference time-domain (FDTD) method. We have found that the patterns in the near-field region have a phase difference of pi with respect to those in the far-field region. A boundary, which separates the interference patterns of the two regions exists as a half circle and grows as the distance between the two slits increase. It is also found that evanescent waves can be enhanced and confined by coating the double-slit with a dielectric cladding.

Finite-difference time-domain analysis of self-focusing in a nonlinear Kerr film.

By using a finite-difference time-domain method, we analyze self-focusing effects in a nonlinear Kerr film and demonstrate that the near-field intensity distribution at the film surface can reach a stable state at only a few hundred femtoseconds after the incidence of the beam. Our simulations also show that the formation of multiple filamentations in the near-field is quite sensitive to the thickness of the nonlinear film and the power of the laser beam, strongly indicating the existence of nonlinear Fabry-Perot interference effects of the linearly polarized incident light.

Dual optical functionality of local surface plasmon resonance for RuO2 nanoparticle–ZnO nanorod hybrids grown by atomic layer deposition

We demonstrate that a hybrid nanostructure consisting of a RuO2 nanoparticle (NP)–ZnO nanorod confers dual local surface plasmon resonance (LSPR) enhancement of ultraviolet (UV) light emission and increase of visible light absorption. A RuO2 NP–ZnO nanorod hybrid was fabricated by an atomic layer deposition method. The size and compositional control of the RuO2 NP allowed (i) visible light absorption increase by the LSPR effect and (ii) proper interfacial electronic alignment of the RuO2–ZnO nanojunction, leading to LSPR coupling with UV light emission enhancement. Based on the combined LSPR effect factor, the dual functionality of LSPR was maximized with 10–20 nm sized RuO2 NPs. These results suggest that a sophisticated design of the nanostructure material heterointerface enables LSPR enhancement of both light harvesting and emission.

Enhanced Emission Efficiency of GaN-Based Flip-Chip Light-Emitting Diodes by Surface Plasmons in Silver Disks

We demonstrate the surface plasmon (SP)-enhanced flip-chip blue light-emitting diodes (LEDs) having silver (Ag) disks in the p-GaN layer. The optical output power of an SP-enhanced flip-chip LED with Ag disks is increased by 45% at 20 mA without showing any degradation of electrical characteristics, compared with that of a conventional flip-chip LED. The increase in optical output power is attributed to the improved internal quantum efficiency of LEDs because of the increase in the spontaneous emission rate by the resonance coupling between the excitons in multiple quantum wells and the SPs in the Ag disks.

Light‐Emitting Diodes with Hierarchical and Multifunctional Surface Structures for High Light Extraction and an Antifouling Effect

Bioinspired hierarchical structures on the surface of vertical light-emitting diodes (VLEDs) are demonstrated by combining a self-assembled dip-coating process and nanopatterning transfer method using thermal release tape. This versatile surface structure can efficiently reduce the total internal reflection and add functions, such as superhydrophobicity and high oleophobicity, to achieve an antifouling effect for VLEDs.

Blending of n-type Semiconducting Polymer and PC61BM for an Efficient Electron-Selective Material to Boost the Performance of the Planar Perovskite Solar Cell

The highly efficient CH3NH3PbI3 perovskite solar cell (PeSC) is simply achieved by employing a blended electron-transport layer (ETL) consisting of PC61BM and P(NDI2OD-T2). The high molecular weight of P(NDI2OD-T2) allows for a thinned ETL with a uniform morphology that optimizes the PC61BM ETL more effectively. As a result of this enhancement, the power conversion efficiency of a PC61BM:P(NDI2OD-T2)-based PeSC is 25% greater than that of the conventional PC61BM based-PeSC; additionally, the incorporation of P(NDI2OD-T2) into PC61BM attenuates the dependence of the PeSC on the ETL-processing conditions regarding its performance. It is revealed that, in addition to the desirable n-type semiconducting characteristics of PC61BM:P(NDI2OD-T2)-including a higher electron-mobility and a more-effective electron selectivity of a blended ETL for an efficient electron extraction-the superior performance of a PC61BM:P(NDI2OD-T2) device is the result of a thinned and uniformly covered ETL on the perovskite layer.