Yoon Ho Huh

Solution processable single layer organic light-emitting devices with a single small molecular ionic iridium compound

We herein report on the occurrence of bright and efficient electrophosphorescence from a simple organic light-emitting diode (OLED) with a single organic layer comprised of a small molecular ionic iridium compound, formed using a solution process. The studied small molecular ionic iridium compound is [Ir(dfppy)2(bpy)]+PF6−, which exhibits excellent film-forming properties, bright green photoluminescence, and efficient bipolar carrier transport with balanced electron and hole mobilities of about 10−5 cm2/(V s). A high performance of the device was achieved by using a phosphorescent OLED (PHOLED) that was fabricated using the [Ir(dfppy)2(bpy)]+PF6− compound, with a peak brightness of about 18 000 cd/m2 and a peak current efficiency of 12 cd/A. A peak power efficiency of 2.5 lm/W was measured at 2800 cd/m2. These results suggest that the small molecular ionic iridium compound is a promising material for bright and efficient PHOLEDs manufactured using a simple solution process.

Improved homogeneity and surface coverage of graphene oxide layers fabricated by horizontal-dip-coating for solution-processable organic semiconducting devices

We herein report an investigation of graphene oxide (GO) thin layers fabricated by simple horizontal-dip (H-dip) coating on an indium-tin-oxide (ITO) anode as used in solution-processable organic semiconducting devices. Homogeneous and smooth GO thin films were successfully deposited via an aqueous dispersion of GO on an ITO electrode, with a high surface coverage and low surface roughness, and a thickness controlled by H-dip coating. The use of an H-dip-coated GO film as a hole-injecting interfacial layer (IFL) in organic light-emitting diodes (OLEDs) resulted in a remarkable improvement in device performance (17 cd A−1), better than that (12 cd A−1) of a reference OLED with a spin-coated GO IFL. Stacked bi-IFLs of GO/poly(ethylenedioxythiophene):poly(styrene sulfonate) (GO/PEDOT:PSS) fabricated by H-dip coating were also investigated as hole-injecting IFLs in OLEDs, and these showed an even better device performance (23 cd A−1). Furthermore, it was also shown that polymer solar cells with H-dip-coated GO/PEDOT:PSS hole-collecting bi-IFLs exhibited a remarkable improvement in power conversion efficiency (6.9%), which was also higher than that (4.8%) obtained with spin-coated bi-IFLs. These results clearly indicate that the H-dip-coating of GO(/PEDOT:PSS) can effectively modify the ITO interface to yield efficient hole-selective IFLs, representing considerable promise for use as an alternative to spin-coated IFLs in the mass production of solution-processable organic semiconducting devices.

Surfactant additives for improved photovoltaic effect of polymer solar cells

We herein report on the photovoltaic (PV) effect of a polymer bulk-heterojunction (BHJ) layer consisting of poly(3-hexylthiophene) : methanofullerene mixed with a surfactant additive. The mixed surfactant is poly(oxyethylene tridecyl ether) (PTE). Excellent performance was achieved by using the surfactant as an additive in the PV layer, thereby increasing the short-circuit current density and fill factor. These increases led to a power conversion efficiency of 4.51 ± 0.16%, which was higher than that (3.89 ± 0.08%) of the reference device without the surfactant. This improvement can be attributed to the increased dissociation efficiency of the bound e–h pairs, which may be due to the oriented PTE surfactant molecules at interfaces between the phase-separated BHJ domains. These results suggest that a PV layer mixed with a PTE surfactant additive is a promising functional composite material for use in polymer PV cells.

Organic semiconducting layers fabricated by self-metered slot-die coating for solution-processable organic light-emitting devices

We present the results of a study of flat, uniform, and stripe-patternable organic semiconducting layers produced by a slot-die coating method using a self-metered coating mode with blended solutions for the fabrication of bright, efficient, and large-area organic light emitting devices (OLEDs). It is shown that the self-metered slot-die coating process can produce high quality, homogeneous, and stripe-patterned thin films using the downstream meniscus of a blended solution, which can be controlled by adjusting the coating parameters of the capillary number of the coating solution by varying the gap height and coating speed. It is shown that very bright and efficient OLEDs (peak brightness ∼50 000 cd m−2 with maximum efficiencies of ∼29 cd A−1 and ∼14 lm W−1) were successfully demonstrated by manipulating the slot-die coated hole-injecting and electroluminescent layers that contained the phosphorescent Ir complex. In view of these results, we believe that the fabrication of organic semiconducting layers using the self-metered slot-die coating process is a promising new technique for high-throughput manufacturing such as via the roll-to-roll process.

Triple-stacked hole-selective layers for efficient solution-processable organic semiconducting devices

UNLABELLED We report on an investigation of water-processable triple-stacked hole-selective layers for solution-processable organic semiconducting devices using a simple horizontal-dip (H-dip) coating technique. Homogeneous layers were successfully deposited via H-dip-coating using aqueous solutions of graphene oxide (GO), molybdenum oxide (MoO3), and poly(ethylenedioxy thiophene):poly(styrene sulfonate) ( PEDOT PSS). The use of the triple-stacked GO/MoO3/ PEDOT PSS layers as hole-injecting layers (HILs) in solution-processable organic light-emitting diodes (OLEDs) resulted in a considerable improvement of device performance in terms of brightness (maximum brightness: 47,000 cd/m2) as well as efficiency (peak efficiency: 31.5 cd/A), exceeding those of an OLED with a conventional single PEDOT PSS HIL. Furthermore, polymer solar cells (PSCs) with these triple-stacked layers used as hole-collecting layers (HCLs) showed a considerable improvement in power conversion efficiency (6.62%), which was also higher than that (5.65%) obtained using the single PEDOT PSS HCL. These results clearly indicate the benefits of using triple-stacked GO/MoO3/ PEDOT PSS layers, which provide better hole-injection/collection, electron-blocking, and improved stability for high performance solution-processable OLEDs and PSCs.

Reflective type Solar-LCDs by using polarizing polymer solar cells

We present herein the results of a study of the reflective polarizing photovoltaic (PV) effects in an aligned polymer bulk-heterojunction PV layer. The PV layer consisted of a composite of regioregular poly(3-hexylthiophene) and methanofullerene (P3HT:PCBM) and the fairly uniform in-plane alignment of the P3HT:PCBM PV layer was achieved by means of a simple rubbing technique. The macroscopic axial orientation of the P3HT polymer in the aligned PV layer was observed to be significantly increased in the direction of rubbing with an axial orientational order parameter of 0.40. Moreover, it was also found that the reflective polarizing polymer solar cells (PSCs) that contained the aligned P3HT:PCBM layers exhibited a greater degree of anisotropy of 1.60 for the PV efficiencies under polarized illumination along the two principal axes. These reflective polarizing PSCs were applied to new reflective type solar cell-liquid crystal displays (Solar-LCDs), which exhibited a contrast ratio of 1.7. These results form a promising foundation for various energy-harvesting polarization-dependent opto-electrical Solar-LCD device applications.

Polarized electroluminescence from organic light-emitting devices using photon recycling

We present results that show highly polarized electroluminescence (EL) from an organic light-emitting device (OLED) by using a quarter-wave (λ/4) retardation plate (QWP) film and a giant birefringent optical (GBO) photonic reflective polarizer. Polarized EL light of 13,400 cd/m(2) with high peak efficiencies (greater than 10 cd/A and 3.5 lm/W) was obtained from an OLED in this way. These values are almost double those of a polarized OLED that only uses a polarizer. The direction of polarization of the emitted EL light from the polarized OLED corresponded to the passing axis of the GBO reflective polarizer. Furthermore, the degree of linear polarization obtained, i.e. the ratio between the brightness of two linearly polarized EL emissions parallel and perpendicular to the passing axis, is greater than 40 over the whole range of emitted luminance.

The Role of Free Radicals in Hemolytic Toxicity Induced by Atmospheric-Pressure Plasma Jet

Atmospheric-pressure plasma (APP) has received attention due to its generation of various kinds of reactive oxygen/nitrogen species (ROS/RNS). The controllability, as well as the complexity, is one of the strong points of APP in various applications. For biological applications of this novel method, the cytotoxicity should be estimated at various levels. Herein, we suggest red blood cell (RBC) as a good cell model that is simpler than nucleated cells but much more complex than other lipid model systems. Air and N2 gases were compared to verify the main ROS/RNS in cytotoxicity, and microscopic and spectroscopic analyses were performed to estimate the damages induced on RBCs. The results shown here will provide basic information on APP-induced cytotoxicity at cellular and molecular levels.

Improved impedance characteristics of all-water-processable triple-stacked hole-selective layers in solution-processed OLEDs

UNLABELLED We herein report an investigation of the device performance capabilities and impedance characteristics of solution-processed organic light-emitting devices (OLEDs) with all-water-processable triple-stacked hole-selective layers (HSLs) on an indium-tin-oxide (ITO) anode, fabricated using a simple coating technique. Highly smooth and homogeneous triple-stacked layers were deposited via horizontal-dip- (H-dip-) coating using aqueous dispersions of graphene oxide (GO), molybdenum oxide (MoO3), and poly(ethylenedioxy thiophene):poly(styrene sulfonate) ( PEDOT PSS). From the triple-stacked GO/MoO3/ PEDOT PSS HSLs used as hole-injection layers (HILs) in the OLEDs, which outperform a conventional single HIL of PEDOT PSS, it was found that OLEDs with triple-stacked HILs exhibited characteristic impedance properties, including low parallel resistance with trap-free space-charge-limited conductivity. Furthermore, it was shown that the relaxation frequency of a sample OLED with triple-stacked GO/MoO3/ PEDOT PSS HILs was much higher than that of a reference device with a single PEDOT PSS HIL. These impedance behaviors indicate that carrier (hole) injection in the sample OLED is more efficient than that in any of the other devices tested here. The results presented here clarify that the triple-stacked GO/MoO3/ PEDOT PSS layers can act as efficient HILs on an ITO anode, representing a remarkable advance in relation to the mass production of high-performance solution-processable OLEDs.

Solution‐processable double‐layered ionic p‐i‐n organic light‐emitting diodes

— Solution-processed double-layered ionic p-i-n organic light-emitting diodes (OLEDs), comprised of an emitting material layer doped with an organometallic green phosphor and a photo-cross-linked hole-transporting layer doped with photo-initiator is reported. The fabricated OLEDs were annealed using simultaneous thermal and electrical treatments to form a double-layered ionic p-i-n structure. As a result, an annealed double-layered OLED with a peak brightness over 20,000 cd/m2 (20 V, 390 mA/cm2) and a peak efficiency of 15 cd/A (6 V, 210 cd/m2) was achieved.