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1.
Small ; 16(40): e2002109, 2020 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-32930494

RESUMEN

Environmentally friendly ZnSe/ZnS core/shell quantum dots (QDs) as an alternative blue emission material to Cd-based QDs have shown great potential for use in next-generation displays. However, it remains still challenging to realize a high-efficiency quantum dot light-emitting diode (QLED) based on ZnSe/ZnS QDs due to their insufficient electrical characteristics, such as excessively high electron mobility (compared to the hole mobility) and the deep-lying valence band. In this work, the effects of QDs doped with hole transport materials (hybrid QDs) on the electrical characteristics of a QLED are investigated. These hybrid QDs show a p-type doping effect, which leads to a change in the density of the carriers. Specifically, the hybrid QDs can balance electrons and holes by suppressing the overflow of electrons and improving injection of holes, respectively. These electrical characteristics help to improve device performance. In detail, an external quantum efficiency (EQE) of 6.88% is achieved with the hybrid QDs. This is increased by 180% compared to a device with pure ZnSe/ZnS QDs (EQE of 2.46%). This record is the highest among deep-blue Cd-free QLED devices. These findings provide the importance of p-type doping effect in QD layers and guidance for the study of the electrical properties of QDs.

2.
Opt Express ; 27(20): 29178-29195, 2019 Sep 30.
Artículo en Inglés | MEDLINE | ID: mdl-31684656

RESUMEN

We herein propose a multiwalled carbon nanotube (MWCNT) doping method into liquid crystal (LC) alignment polyimides (PIs) with low resistivity for resolving both issues of voltage holding and image sticking in low-frequency-driven fringe-field switching (FFS) LC modes using negative dielectric LCs (n-LCs). By utilizing strong ion trapping ability of MWCNTs, the FFS n-LC cell aligned by low resistivity PIs with 0.05 wt% MWCNT doping exhibited an excellent voltage holding ratio of 99% under an extremely low operation frequency of 0.5 Hz and approximately 23.6 times better surface discharging property than that aligned by high resistivity PIs.

3.
Appl Opt ; 57(2): 119-129, 2018 Jan 10.
Artículo en Inglés | MEDLINE | ID: mdl-29328155

RESUMEN

We proposed and analyzed an optically isotropic nano-droplet liquid crystal (LC) doped with high k nanoparticles (NPs), exhibiting enhanced Kerr effects, which could be operated with reduced driving voltages. For enhancing the contrast ratio together with the light efficiencies, the LC droplet sizes were adjusted to be shorter than the wavelength of visible light to reduce depolarization effects by optical scattering of the LC droplets. Based on the optical analysis of the depolarization effects, the influence of the relationship between the LC droplet size and the NP doping ratio on the Kerr effect change was investigated.

4.
Opt Express ; 23(26): 34055-70, 2015 Dec 28.
Artículo en Inglés | MEDLINE | ID: mdl-26832062

RESUMEN

We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode that occurred at the moment of frame change even in the optimized DC offset condition for minimum image flicker. Based on the positional dynamic behaviors of LCs by using a high-speed camera, we found that the transient blink is highly related to the asymmetric responses of the splay-bend transitions caused by the flexoelectric (FE) effect. To remove the transient blink, the elastic property adjustment of LCs was an effective solution because the FE switching dynamics between the splay-enhanced and bend-enhanced deformations are highly dependent on the elastic constants of LCs, which is the cause of momentary brightness drop.

5.
Appl Opt ; 53(17): 3633-6, 2014 Jun 10.
Artículo en Inglés | MEDLINE | ID: mdl-24921125

RESUMEN

We present an optically isotropic switchable microlens array (MLA) based on liquid crystals (LCs) using the Joule heating electrode structure. The LC molecules were initially aligned vertically on the lens and electrode surfaces. By applying voltage to the transparent electrodes, the temperature of the LC layer could be changed. Above the clearing point temperature of LCs, the LC layer shows an averaged refractive index that differs from the nematic state refractive index. The MLA could have switching characteristics by index matching between the LC layer and polymer lens structure. The proposed switchable MLA shows high light efficiency with truly optically isotropic properties.

6.
Opt Express ; 21(1): 129-34, 2013 Jan 14.
Artículo en Inglés | MEDLINE | ID: mdl-23388903

RESUMEN

We propose a polarizer-free liquid crystal display (LCD) with an electrically switchable microlens array. The incident lights are controlled to focused or defocused states by index matching of the lens polymer and LC layer. By adopting two light-blocking masks that have a circular stop pattern and the complementary open pattern, the LCD was able to realize the entire gray scale. Additionally, to achieve fast response time characteristics, we introduce polymerized RMs within the alignment layers.

7.
Polymers (Basel) ; 14(3)2022 Jan 31.
Artículo en Inglés | MEDLINE | ID: mdl-35160574

RESUMEN

In general, optical properties of a top-emitting organic light-emitting diode (OLED) are dependent on the cavity effect of the OLED structure. Therefore, the optical path length of the many thin solid films in the OLED, which is strongly affected by the refractive index and thickness of each material, controls the cavity effect of the cell. In previous research, a parameter space method for optimizing the inorganic layer thickness of a red OLED structure was introduced to achieve the required bandwidth and peak wavelength. This is a simple method with high accuracy and can also be applied to red, green, and blue OLED structures. To design an OLED cell with a practical approach, however, the RGB OLED device requires the thickness of each inorganic layer and organic layer in all three R, G, and B OLED structures to be same. In this study, we applied the parameter space method to an RGB OLED device to find out and optimize the thickness of three inorganic parameters: Indium Tin Oxide (ITO), cathode, and capping layer (CPL) using the finite-difference time-domain (FDTD) method. The parameters ITO, cathode, and CPL were scanned from 18 to 21 nm, 5 to 100 nm, and 10 to 200 nm, respectively. The peak wavelength and bandwidth lines of the three spectral colors were placed on a map of the three inorganic layer thickness parameters to find the optimized points that can provide the desired optical characteristics with the same film thickness in the cell.

8.
RSC Adv ; 9(72): 42561-42568, 2019 Dec 18.
Artículo en Inglés | MEDLINE | ID: mdl-35542867

RESUMEN

Organic light-emitting diode (OLED) displays are highly susceptible to the harsh environmental conditions found outdoors, like exposure to direct sunlight as well as UV radiation and storage temperature, resulting in a loss of luminance and lifespan, pixel shrinkage, and permanent damage and/or malfunction of the panel. Here, we fabricated top emission OLEDs (TEOLEDs) using Yb : LiF (1 : 1, 2 nm)/Ag : Mg (10 : 1, 16 nm) and Mg : LiF (1 : 1, 2 nm)/Ag : Mg (10 : 1, 16 nm) cathode units and the performances of the devices were investigated by subjecting them to UV radiation. A fabricated red TEOLED (control device), employing a standard Mg : LiF (1 : 1, 2 nm) electron injection layer (EIL) and an Ag : Mg (16 nm) cathode, showed a rapid decrease in luminance and a fast increase in driving voltage at 10 mA cm-2 over time after UV irradiation for 300 h. However, a cathode unit comprising a Yb : LiF (1 : 1, 2 nm) EIL and an Ag : Mg (10 : 1, 16 nm) cathode showed no loss of luminance or increase in driving voltage at 10 mA cm-2 over time after UV irradiation for 300 h. Therefore, we investigated the changes occurring in both cathode units due to UV irradiation using the lift-out FIB-TEM technique and EDS mapping. With UV irradiation for 300 h, Ag atoms migrated toward the center of the cathode, Mg atoms migrated toward the CPL, and no Mg atoms were observed in the EIL area. In contrast, we observed (i) no substantial migration of Ag atoms and they were located at the center of the cathode, (ii) no migration of Mg atoms toward the CPL layer, and (iii) no movement of Yb atoms after UV irradiation. Furthermore, the UV irradiated red TEOLED with an Mg : LiF (1 : 1, 2 nm) EIL showed (i) deterioration in electron injection into the emissive layer (EML) and an increase in the EIL/metal interface resistance, and (ii) a remarkable shift of the J-V curve to the higher voltage side, while almost no such changes were observed in the TEOLD with a Yb : LiF (1 : 1, 2 nm) EIL. Also, an almost identical RGB pixel emitting area was noticed in the Yb : LiF (1 : 1, 2 nm) based devices after UV irradiation for 300 h. These results suggest that Yb could become a good candidate for the cathode unit, providing better device stability against harsh environmental conditions as well as excellent electron injection properties.

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