•A new roll-to-roll fabrication process of embedded AgNWs electrode is proposed.•The process can produce large-area and high-performance AgNWs electrode on plastic film.•Large-area flexible OLED is demonstrated using the embedded AgNWs electrode film.Silver-nanowires (AgNWs) transparent electrodes were successfully fabricated on a polyethylene terephthalate (PET) film substrate by roll-to-roll processes. The AgNWs were embedded in a polymer resin so that the surface was flat enough to fabricate organic light-emitting diodes (OLEDs) without an additional surface planarization. For the fabrication of the embedded AgNWs electrodes, a polyimide film was utilized as an AgNWs-carrying film to realize a roll-to-roll process for the first time. Large-area OLEDs were then deposited directly on the embedded AgNWs electrodes by a roll-to-roll vacuum deposition process. The OLEDs with the embedded AgNWs electrodes on a PET film showed 30–40% better performance than those with commercial indium-tin-oxide electrodes on glass substrates. The surface roughness, sheet resistance, and optical transmittance of the embedded AgNWs electrodes on a PET film were around 3 nm, 5 Ω/square, and 85%, respectively. The sheet-resistance uniformity of the embedded AgNWs electrode was higher than 90% in 15 cm × 45 cm area.

•New hybrid color-conversion layers are demonstrated to achieve white from blue emission.•High color rendering index of 93 is achieved using the hybrid color-conversion layer.•Utilization of a scattering layer improves the angular independence of the white emission.Hybrid color-conversion layers (CCLs) were developed to convert a blue emission from fluorescent organic light-emitting diodes (OLEDs) to obtain a white emission with a high color rendering index (CRI). The hybrid CCLs were composed of an inorganic phosphor, organic dye, and silicon dioxide (SiO2) scattering nanoparticles. The inorganic phosphors convert a part of blue emission from OLEDs to a green-yellow emission effectively. A part of the green-yellow emission was consecutively converted to a red emission with the organic dye. Using the hybrid CCLs, we obtained a balanced white emission with the highest CRI of 93 and the color temperature of 3500 K. The high CRI white OLED showed the power efficiency of 11 lm/W which was enhanced by 1.9 times from that of the blue fluorescent OLED. We showed that the utilization of the SiO2 nanoparticles did not only enhance the power efficiency but also significantly reduce the white color variation to the viewing angle.Download high-res image (191KB)Download full-size image

We developed a simple methodology for fabricating silver nanowire (AgNW) micropatterns on a plastic substrate using a photocurable polymer. The patterning method began with the lamination of a UV-curable prepolymer film onto the AgNW-coated rigid glass substrate. Selective UV exposure of the UV-curable prepolymer film through a photomask solidified the exposed regions, and the unexposed regions were simply removed by the solvent. AgNW micropatterns of various sizes and shapes could be readily formed across the entire plastic substrate. Importantly, this photopatterning process enabled the embedding of the AgNW structures into the polymer matrix, which dramatically reduced the surface roughness and enhanced the mechanical stability of the AgNW film. The AgNW structures served as transparent anode electrodes in organic light-emitting diodes (OLEDs) that performed well compared to OLEDs fabricated using conventional indium tin oxide (ITO) or conducting polymer electrodes. This simple, inexpensive, and scalable AgNW patterning technique provides a novel approach to realizing next-generation flexible electronics.

The effect of hole-transport-layer thickness on deep-blue emission was experimentally and theoretically elucidated for top-emitting micro-cavity organic light-emitting diodes. As a result of strong optical interference in the micro-cavity, the emission spectrum and intensity from the top-emitting organic light-emitting diodes was strongly dependent on the viewing angle. Optimizing hole-transport-layer thickness lead to deep-blue emissions at a wavelength of about 460 nm with a current efficiency of 5 cd/A at 10,000 cd/m2 luminance. Light out-coupling in the top-emitting organic light-emitting diodes was modeled and calculated to show the power dissipation, and the model explained the experimental results well.

We report thin-film moisture barriers based on Al2O3/ZrO2 nanolaminates grown by ALD for an encapsulation of OLEDs. In order to optimize the moisture-barrier performance of the nanolaminates, the most important factors affecting the performance were sought by measuring WVTR of the nanolaminates via an electrical Ca test. We found out that both the number of interfaces in the nanolaminates and the thickness of ZrO2 in a unit layer were responsible for the performance. By optimizing the nanolaminate structure, the moisture-barrier performance was enhanced up to 350% from a single layer of the same thickness. The WVTR of 30-nm-thick optimized nanolaminate barrier was 2 × 10−4 g/(m2 day) or less at ambient condition. A storage-lifetime measurement of an OLED with a 100-nm-thick encapsulation layer showed that it could exceed 70,000 h if stored at ambient condition.Graphical abstractHighlights► Al2O3/ZrO2 nanolaminate structure was optimized to show the lowest WVTR. ► Number of interfaces in the nanolaminate is very important for lowering WVTR. ► It is crucial for the ZrO2 thickness in nanolaminate to be kept <4 nm for lowering WVTR. ► The WVTR <2 × 10−4 g/(m2 day) is achieved at the total barrier thickness of 30 nm.

PVK-based single-layer phosphorescent polymer OLEDs (organic light emitting diodes) with different rubrene concentrations were fabricated and examined for the Förster energy transfer from phosphorescent FIrpic dye to rubrene. We found out that at a certain rubrene concentration the energy transfer occurs abruptly and the transfer shows an abnormal evolution of electroluminescence (EL) spectrum due to the coincidence of peak wavelengths of bis[(4,6-difluorophenyl)-pyridinato-N,C2′](picolinate) iridium(III) (FIrpic) emission and 5,6,11,12-tetraphenylnaphthacene (rubrene) absorption. With the calculation of Förster radius and average distance between FIrpic molecules, we have related the calculated ratio between the number of FIrpic molecules within to that out of Förster radius with the degree of Förster energy transfer from EL spectra measured in the experiment. Experimental results were found to fit well with the predicted results especially at low rubrene concentrations.Highlights► Förster energy transfer between FIrpic and rubrene. ► Energy transfer shows an abnormal evolution of emission spectrum. ► Calculated Förster radius and degree of energy transfer by a simple model.

We prepared organic (self-assembled monolayer (SAM))–inorganic (TiO2) multilayer barrier films on polyethylene terephthalate substrate using atomic layer deposition and molecular layer deposition methods in the same deposition chamber. The water permeation was mainly blocked by the inorganic TiO2 layer. While the lag time was proportional to the thickness of the TiO2 layer, the steady-state permeation rate was relatively independent of the thickness. The multilayer approach was effective in extending the lag time due to both the tortuous path effect and the internal desiccant effect. Water permeation occurred sequentially in the organic–inorganic multilayer barriers by water accumulation in the organic SAM layers. The water vapor transmission rate was 7.0 × 10− 4 g/m2·day during the lag time of 155 h at 60 °C and a relative humidity of 85% with 5-dyad barrier film.Highlights► TiO2 and self-assembled monolayer (SAM) are grown by layer-by-layer deposition. ► We showed that the SAM layer is beneficial due to its internal desiccant effect. ► Water vapor transmission rate of the film is measured by electrical Ca test.

An ultrathin polydimethylsiloxane (PDMS) layer with a mean thickness of 1 nm was coated on soft magnetic carbonyl iron (CI) particles by using a simple thermal evaporation process, and then their physical characteristics were examined using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), thermal gravimetry analysis (TGA), and vibrating sample magnetometry (VSM). Magnetorheological (MR) fluid was prepared by using PDMS-coated CI powder, and its rheological behavior was investigated under different external magnetic field strengths using a rotational rheometer. The CI particles coated by a thin PDMS layer showed higher oxidation temperature than pristine CI particles and MR fluid consisting of PDMS-coated CI particles demonstrated better dispersion stability in a nonmagnetic carrier fluid.

Aluminum-doped ZnO (AZO) thin films are grown by ultrasonic-mist deposition method for the transparent conducting oxides (TCO) applications at low temperatures. The AZO films can be grown at a temperature as low as 200 °C with zinc acetylacetonate and aluminum acetylacetonate sources. The lowest resistivity of grown AZO films is 1.0×10−3 Ω·cm and the lowest sheet resistance of 1 μm thick films is 10 Ω/□, which is close to that of commercial indium tin oxide (ITO) or Asahi U-type SnO2: F glass. The highest carrier concentration and mobility are 5.6×1020 cm−3 and 15 cm2/V·sec, respectively. Optical transmittance of the AZO films is found over 75% for all growth conditions. We believe that the properties of grown AZO films in this study are the best among all reported previously elsewhere by solution processes.

We have investigated the effect of hole transporting materials in phosphorescent white polymer light-emitting diodes. The doping of a hole transporting material TCTA (4,4′,4″-tris(N-carbazolyl)-triphenylamine) resulted in enhanced luminance and efficiency while the doping of other hole transporting materials degraded the device performance. The effect of hole transporting materials on the performance of the device has been explained with the triplet energy level, HOMO (highest occupied molecular orbital)/LUMO (lowest unoccupied molecular orbital) level, and hole mobility of the materials. The doping of TCTA into an emissive layer increased the maximum current efficiency by 43%. Simultaneous utilization of TCTA both as an interlayer and a dopant in an emissive layer has been found to enhance the device performance further. The optimized phosphorescent white polymer light-emitting device showed the current efficiency of 20.5 cd/A with the luminance of 1800 cd/m2 at an operating voltage of 11.5 V.

We have showed that the doping of an organic salt into a PVK-based polymer emissive layer could enhance the carrier balance greatly to result in higher luminance and luminous efficiency. It is found out that the salt-doped devices show the similar operating characteristics of frozen-junction light-emitting electrochemical cells (LECs). With the salt doping of 0.6 wt.% and an appropriate salt activation process, the fabricated PVK-based polymer light-emitting diodes (PLEDs) shows the luminous efficiency of 15 cd/A at the highest luminance of 55,000 cd/m2 even without an electron-injecting LiF layer. Due to the enhanced carrier balance, the luminous efficiency is found to be maintained from the turn-on voltage to the voltage for the maximum luminance, which means a linear relationship between luminance and current density.

PVK-based phosphorescent FIrpic-doped devices have been fabricated and the effect of carrier transporting materials on the device performance has been investigated. We have fabricated the single-layer and double-layer devices to investigate the role of electron- and hole-transporting materials both when they are mixed in the single- and separated from the emissive layer. We have studied NPB as a hole-transporting material, OXD-7 and PBD as electron-transporting materials. Even though PBD had been well known to be unsuitable as an electron-transporting material for FIrpic-doped PVK devices, it was found in this study that the separate utilization of PBD from PVK:FIrpic layer instead of mixing it with the emissive layer could enhance the device performance significantly. Nevertheless, the OXD-7 was found not only superior to PBD as the electron-transporting material for FIrpic-doped PVK devices but also suitable as an electron-transporting material for the single-layer devices.

The performance of direct fuel cells using dimethyl ether(DME)-based fuels is presented at a relatively low temperature of 80 °C. DME is supplied to the fuel cells either by gas phase or aqueous phase for the operation of direct fuel cells. In order to keep DME in liquid phase during operation, fuel cells were operated at higher pressure up to 5 bar. For further increase of the power density from direct DME fuel cells, DME was mixed with methanol solution and fed into the fuel cells by the vapor pressure of DME itself without a liquid pump. In this study, we have obtained the highest power density of 210 mW cm−2 at a temperature of 80 °C when the fuel cell is operated with the mixed fuel with 2 M methanol solution under 4 bar.

We have successfully showed the color mixing between fluorescence and phosphorescence by the incomplete Förster energy transfer. The emission color from fluorescent Rubrene and phosphorescent Ir(ppy)3 was found possible to be mixed for a single-layer device doped with both dyes, maintaining the high luminance efficiency of the phosphorescence. We also found out that the Förster energy transfer was very efficient and independent of the applied voltage with no loss of energy during the transfer.

The performance of fuel cells using dimethyl ether (DME) fuel is presented in this study at a relatively low temperature of 80 °C. DME is known to be less electrochemically active to Pt catalysts, compared with typical fuels for fuel cells such as hydrogen and methanol, especially at low temperatures. In order to compensate the poor performance of fuel cells using the less active DME fuel, the DME and methanol fuels are mixed and supplied to the fuel cells simultaneously. The methanol fuel is delivered to the fuel cells by the high vapor pressure of DME without a liquid pump. In this study, we investigated the effect of anode catalysts and the effect of mixing fuels of DME, water, and methanol on the performance of the fuel cell. For preparing a fuel mixture with DME, the solubility of DME in water and methanol solution has been also investigated.

Highly ordered palladium (Pd) nanowire arrays were successfully prepared by pulsed electrodeposition (PED) method using anodized aluminum oxide (AAO) nanotemplate. The Pd nanowire arrays are standing freely on a titanium-coated silicon wafer after removing the AAO template. The titanium film is used for providing conducting surface for the electrodeposition of palladium and also for removing alumina barrier layer formed as a result of the anodization of aluminum film. The diameter and length of the palladium nanowire in the arrays are about 80 nm and 0.8 μm, respectively. It has been shown that the well-ordered and free-standing Pd nanowire arrays can be grown uniformly in a relatively large area of about 5 cm2.

The performance of direct dimethyl ether fuel cells (DDMEFC) is presented in this study at the relatively low temperature of 80 °C. At temperatures lower than 100 °C, since water exists as liquid but DME as a gas, it is difficult for them to use simultaneously as fuel for DDMEFCs even with the low solubility of DME in water at atmospheric pressure. It has been found that the use of an interdigitated flow field enhances the performance of DDMEFC by facilitating the phase mixing of DME and water. Palladium (Pd) catalyst is not nearly electrochemically active to the anode reaction of DDMEFC and platinum–ruthenium (Pt–Ru) catalyst has been found to be effective anode catalysts for DDMEFC at low temperature.