Byoung-Hwa Kwon

Exciton Dissociation and Charge-Transport Enhancement in Organic Solar Cells with Quantum-Dot/N-doped CNT Hybrid Nanomaterials

The incorporation of InP quantum-dot/N-doped multiwalled carbon nanotube (QD:NCNT) nanohybrids in the active layer of poly(3-hexylthiophene)/indene-C60 bisadduct (P3HT/ICBA) bulk-heterojuction solar cells enhances V(OC) and J(SC) . The QDs encourage exciton dissociation by promoting electron transfer, while the NCNTs enhance the transport of the separated electrons and eventual charge collection. Such a synergistic effect successfully improves the power conversion efficiency (PCE) from 4.68% (reference cells) to 6.11%.

White-light emitting surface-functionalized ZnSe quantum dots: europium complex-capped hybrid nanocrystal

A color-tunable emitter comprising Eu complex-capped ZnSe quantum dot (QD) organic–inorganic hybrid nanocrystals (NCs) was simply synthesized by a hot-injection method, plus the addition of an Eu precursor. Hybrid NCs have the emission of both Eu complexes and ZnSe QDs, and they show bluish white light. In the case of composite NCs (Eu/Zn = 1.0), the emission increases up to 174% compared with that of pristine ZnSe QDs. It is due to the sensitization of the Eu complex acting as an antenna, so the energy obtained by the Eu complexes transfers to the ZnSe QDs. In addition, the NCs have a strong excitation band in the near-UV region, which gives them an advantage over wavelength converters for white light-emitting diodes (LEDs). The expected structure of the hybrid NCs was verified by TEM, XRD, and XPS. It features a zinc blende crystal structure identical to the ZnSe QD, along with Eu-based complexes that can be coordinated with the Se ion on the surface of ZnSe QDs. Therefore, new organic–inorganic hybrid luminescent material using the emission of both QDs and lanthanide (Ln) complexes can potentially serve as a light source in white LEDs.

Bright three-band white light generated from CdSe/ZnSe quantum dot-assisted Sr3SiO5:Ce3+,Li+-based white light-emitting diode with high color rendering index

In this study, bright three-band white light was generated from the CdSe/ZnSe quantum dot (QD)-assisted Sr3SiO5:Ce3+,Li+-based white light-emitting diode (WLED). The CdSe/ZnSe core/shell structure was confirmed by energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy. The CdSe/ZnSe QDs showed high quantum efficiency (79%) and contributed to the high luminous efficiency (ηL) of the fabricated WLED. The WLED showed bright natural white with excellent color rendering property (ηL=26.8 lm/W, color temperature=6140 K, and color rendering index=85) and high stability against the increase in forward bias currents from 20 to 70 mA.

Continuous In Situ Synthesis of ZnSe/ZnS Core/Shell Quantum Dots in a Microfluidic Reaction System and its Application for Light-Emitting Diodes

For the continuous production of quantum dots (QDs), continuous-fl ow microfl uidic reaction systems have been recognized as an effective and alternative strategy to the conventional batch systems due to precise controllability of reaction conditions, including high heat and mass transfer, temperature control, high surface-to-volume ratio, effi cient mixing, low reagent consumption, and continuous production. [ 1 , 2 ] In addition, the microfl uidic reaction system provides easy scale-up and reduces the reaction time, which is highly suitable for the large quantity production of monodisperse QDs for industrial applications in electronics and the life sciences. [ 3 ] Unfortunately, the most composition of previously synthesized QDs is mainly dedicated to Cd chalcogenide material, which is known to be a hazardous substance and to cause serious health problems and is therefore applied in limited applications. [ 4 ] In addition, research on blue-emission QDs has remained elusive because it is diffi cult to synthesize small sizes ( < 1.6 nm) of CdSe-based QDs that have a narrow size distribution and high quantum effi ciency. [ 5 ] Furthermore, several reported continuous reaction systems required more than two reactors or complex synthetic procedures, which may limit to produce core/shell QDs. [ 6 ]

Flexion bonding transfer of multilayered graphene as a top electrode in transparent organic light-emitting diodes

Graphene has attracted considerable attention as a next-generation transparent conducting electrode, because of its high electrical conductivity and optical transparency. Various optoelectronic devices comprising graphene as a bottom electrode, such as organic light-emitting diodes (OLEDs), organic photovoltaics, quantum-dot LEDs, and light-emitting electrochemical cells, have recently been reported. However, performance of optoelectronic devices using graphene as top electrodes is limited, because the lamination process through which graphene is positioned as the top layer of these conventional OLEDs is a lack of control in the surface roughness, the gapless contact, and the flexion bonding between graphene and organic layer of the device. Here, a multilayered graphene (MLG) as a top electrode is successfully implanted, via dry bonding, onto the top organic layer of transparent OLED (TOLED) with flexion patterns. The performance of the TOLED with MLG electrode is comparable to that of a conventional TOLED with a semi-transparent thin-Ag top electrode, because the MLG electrode makes a contact with the TOLED with no residue. In addition, we successfully fabricate a large-size transparent segment panel using the developed MLG electrode. Therefore, we believe that the flexion bonding technology presented in this work is applicable to various optoelectronic devices.

Light-adaptable display for the future advertising service

ABSTRACT In this paper, a new light-adaptable display (LAD) structure with minimum power consumption is proposed for the future advertising service, and the demonstrated results are reported. An organic light-emitting diode with color reflection (colored OLED) was applied for the reflective- and emissive-mode device, and a guest-host liquid-crystal device (GH-LC) was adopted for the light shutter device. The current efficiency and reflectance of the colored OLED were 35.15 cd/A at 457 cd/m2 luminance and 63% for the yellow color, respectively. The measured contrast ratio of GH-LC was 15.5:1 at dark-room conditions, respectively. Transparent oxide thin-film transistors were used for the backplane, and their average mobility was 9.08 cm2/V s, with a 0.5 standard deviation. Through the optimization of the fabrication process and the structures of each device, the LAD adaptively operating according to the environmental illuminance from dark to 10,000 nits was successfully demonstrated. Moreover, a new LAD driving method was proposed for minimizing the power consumption.

Organic wavelength‐converting‐film‐based hybrid planar white light‐emitting diodes

— In this study, organic wavelength-converting films (WCFs) applied to InGaN blue LED-based hybrid planar WLED has been fabricated. The organic dye layer in the WCF was formed between the upper and bottom polymer sheets by using a simple roll-laminating technique. Subsequently, the hybrid planar WLEDs have been fabricated based upon these films. The luminous efficiency of green WCF-based hybrid planar WLEDs with a single blue LED chip was 34.6 lm/W and that of red-WCF-assisted green WCF-based hybrid planar WLEDs was 27.3 lm/W under 20 mA. The use of WCF to fabricate hybrid planar WLEDs showed better stability than that of directly coating organic color-convergence materials (CCMs) on the LED chips. It only decreased to about 10% of the initial wavelength-converting intensity after 1 hour of continual operation at 20 mA.

Overcoming the efficiency limit of organic light-emitting diodes using ultra-thin and transparent graphene electrodes

We propose an effective way to enhance the out-coupling efficiencies of organic light-emitting diodes (OLEDs) using graphene as a transparent electrode. In this study, we investigated the detrimental adsorption and internal optics occurring in OLEDs with graphene anodes. The optical out-coupling efficiencies of previous OLEDs with transparent graphene electrodes barely exceeded those of OLEDs with conventional transparent electrodes because of the weak microcavity effect. To overcome this issue, we introduced an internal random scattering layer for light extraction and reduced the optical absorption of the graphene by reducing the number of layers in the multilayered graphene film. The efficiencies of the graphene-OLEDs increased significantly with decreasing the number of graphene layers, strongly indicating absorption reduction. The maximum light extraction efficiency was obtained by using a single-layer graphene electrode together with a scattering layer. As a result, a widened angular luminance distribution with a remarkable external quantum efficiency and a luminous efficacy enhancement of 52.8% and 48.5%, respectively, was achieved. Our approach provides a demonstration of graphene-OLED having a performance comparable to that of conventional OLEDs.

Light- and space-adaptable display

ABSTRACT Presented is a flexible dual-mode display operable in reflective and emissive mode according to ambient light for optimal visibility on a nonplanar surface. Flexible-backplane-embedding organic light-emitting diodes (OLEDs), thin-film transistors (TFTs), and control electrodes for liquid crystal (LC) shutter are realized by the laser lift-off (LLO) method and the newly developed polyimide (PI) delamination technique. A novel color-filterless LC shutter with color dyes is merged with this flexible backplane for operation in reflective mode. This work opens up a promising approach to building displays that are adaptive to the surrounding environment for better usability.

Mechanistic Understanding of Improved Performance of Graphene Cathode Inverted Organic Light-Emitting Diodes by Photoemission and Impedance Spectroscopy

Modification of multilayer graphene films was investigated for a cathode of organic light-emitting diodes (OLEDs). By doping the graphene/electron transport layer (ETL) interface with Li, the driving voltage of the OLED was reduced dramatically from 24.5 to 3.2 V at a luminance of 1000 cd/m2. The external quantum efficiency was also enhanced from 3.4 to 12.9%. Surface analyses showed that the Li doping significantly lowers the lowest unoccupied molecular orbital level of the ETL, thereby reducing the electron injection barrier and facilitating electron injection from the cathode. Impedance spectroscopy analyses performed on electron-only devices (EODs) revealed the existence of distributed trap states with a well-defined activation energy, which is successfully described by the Havriliak-Negami capacitance functions and the temperature-independent frequency dispersion parameters. In particular, the graphene EOD showed a unique high-frequency feature as compared to the indium tin oxide one, which could be explained by an additional parallel capacitance element.