The design of dual emitting cores for green thermally activated delayed fluorescent materials.

Dual emitting cores for thermally activated delayed fluorescent (TADF) emitters were developed. Relative to the corresponding TADF emitter with a single emitting core the TADF emitter with a dual emitting core, 3,3',5,5'-tetra(carbazol-9-yl)-[1,1'-biphenyl]-2,2',6,6'-tetracarbonitrile, showed enhanced light absorption accompanied by a high photoluminescence quantum yield. The quantum and power efficiencies of the TADF devices were enhanced by the dual emitting cores.

Two-dimensional micropatterns via crystal growth of Na2CO3 for fabrication of transparent electrodes.

The simple and versatile method to generate two-dimensional micropatterns by controlling precisely crystallization of sodium carbonate (Na2CO3) was investigated. Dense clusters of dendrites of salt crystals were homogeneously formed in a large area with an aqueous solution of Na2CO3 during evaporation of water. The dimensions and morphologies of dendritic salt crystals were tuned by changing the growth conditions such as salt concentration, relative humidity, and temperature. Then, 2D micropatterns of salt crystals were directly used as a mask for the deposition of a silver (Ag) layer to fabricate transparent electrodes. After salt crystals were completely dissolved in water, the network of an electrically conductive Ag layer, whose patterns were reversely produced from salt crystals, was generated on glass substrates. In addition, salt crystals were used as a master to prepare a replica mold of poly(dimethylsiloxane) (PDMS) for utilizing the imprinting technique. By imprinting a flexible PDMS mold with Ag inks, Ag micropatterns that were perfectly identical to dendrites of salt crystals were transferred to the other substrate.

Roles of interlayers in efficient organic photovoltaic devices.

This review discusses interfacial layers in organic photovoltaic devices. The first part of the review focuses on the hole extraction layer, which is located between a positive electrode and an organic photoactive material. Strategies to improve hole extraction from the photoactive layer include incorporation of several different types of hole extraction layers, such as conductive polymeric materials, self-assembled molecules and metal oxides, as well as surface treatment of the positive electrodes and the conductive polymeric layers. In the second part, we review recent research on interlayers that are located between a negative electrode and a photoactive layer to efficiently extract electrons from the active layer. These materials include titanium oxides, metal fluorides and other organic layers.

Charge carrier trapping and enhanced electroluminescent efficiency of blue light emitting polymer with gold nanoparticles.

We investigated the current injection, transport, and luminous efficiency behavior of organic light emitting diode (OLED) containing the 5-10 nm-sized gold particles mixed in the polyfluorene-type copolymer. This nanoparticle-conjugated polymer mixture layer was used as hole injection, transport, and light emitting layer for various structures of OLED based on the phosphorescent and fluorescent emitters. Due to the hole trapping at the nanopaticle, carrier injection is significantly reduced while the hole transport behavior is found to be barely affected. Hole trapping of nanoparticle in light emitting layer (at 4.7 approximately 9.4 x 10(-5) w/w fraction) resulted in an enhancement of efficiency (from 5.23 cd/A to 6.50 cd/A). The existence of the outcoupling effect also supports the carrier trapping behavior, which is amended mechanism of the improved efficiency compared to previously reported mechanism of enhanced photoluminescent stability by a hindrance of photo-oxidation.

Inkjet Printing of Mixed-Host Emitting Layer for Electrophosphorescent Organic Light-Emitting Diodes.

We have investigated the impact of the ink formulation on the properties of an inkjet-printed small molecular mixed host in a phosphorescent organic light-emitting diode (PhOLED). Host solubility, film roughness, and device efficiency improved by blending tris(4-carbazoyl-9-ylphenyl)amine (TCTA) with pyrido[3',2':4,5]furo[2,3- b]pyridine (3CzPFP). At a host ratio of 60:40 (TCTA/3CzPFP), the brightness increased by 33%, the efficiency roll-off at 1000 cd/m2 dropped to well below 10%, and the luminance half-lifetime (LT50) improved by 80% in comparison to the device with a single host (100% TCTA). When the optimized ink was deposited by inkjet printing, a maximum external quantum efficiency of 8.9% and a current efficiency of 28.8 cd/A were achieved at 1000 cd/m2 brightness. This amounted to around 84% of the efficiency of a spin-cast reference device. The obtained results provide a blueprint for designing enhanced PhOLEDs with inkjet-printed mixed hosts.

Correction: All-solution-processed, flexible thin-film transistor based on PANI/PETA as gate/gate insulator.

[This corrects the article DOI: 10.1039/C5RA23848J.].

Synthesis of a new polymeric host material for efficient organic electro-phosphorescent devices.

We have synthesized a new polymeric host material for phosphorescent dyes, which can be used in phosphorescent light-emitting layers. An alternating copolymer, composed of N-alkylcarbazole and tetramethylbenzene units was synthesized through the Suzuki coupling reaction. We fabricated electro-phosphorescent devices using the synthesized polymeric host doped with solution-processible green and red phosphorescent dyes. Light-emitting devices have an ITO/PEDOT/polymer + dopant/Balq3/Alq3/LiF/Al configuration. The device containing one of two studied green dopants (designated as green 1) in the polymeric host showed the best performance, with a maximum luminous efficiency of 29 cd/A. A thin film of this polymeric was successfully patterned by laser-induced thermal imaging (LITI), and an electro-phosphorescent device was fabricated using the patterned film. This patterned device showed performance characteristics similar to those of a spin-coated device.

Inkjet printing of blue phosphorescent light-emitting layer based on bis(3,5-di(9H-carbazol-9-yl))diphenylsilane.

In this study, micropatterning of a blue light emitting, tetraphenylsilane-based phosphorescent material by inkjet printing was investigated. Bis(3,5-di(9H-carbazol-9-yl))diphenylsilane (SimCP2) doped with iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) was dissolved in a solvent mixture, and various conditions for the solvent composition and drying of films were examined. Homogeneous dot and line patterns with controllable thickness and smooth surface were obtained from a mixture of chlorobenzene and cyclohexanone at a moderate printing speed of 3 mm s-1 and a droplet ejection frequency of 70 Hz. An inkjet-printed device was designed and fabricated in [ITO/PEDOT:PSS /PVK/SimCP2:Flrpic/TSPO1/TPBi/LiF/Al] configuration, from which sky-blue light (0.14, 0.25) was obtained with a luminous efficiency of 10.73 cd A-1 and a power efficiency of 6.13 lm W-1. This amounted to 68% of the performance of an identical device where the emitting layer was spin coated. These results show the potential of inkjet printing as a low-cost patterning method for low molecular weight emitters in blue light emitting devices.

Room temperature processed high mobility W-doped In2O3 electrodes coated via in-line arc plasma ion plating for flexible OLEDs and quantum dots LEDs.

We fabricated W-doped In2O3 (IWO) films at room temperature on a flexible PET substrate using an in-line arc plasma ion plating system for application as flexible transparent conducting electrodes (FTCEs) in flexible organic light emitting diodes (OLEDs) and quantum dots light emitting diodes (QDLEDs). Due to the high-energy flux of the sublimated ions generated from the plasma region, the IWO films showed a well-developed crystalline structure with a low sheet resistance of 36.39 Ohm/square and an optical transmittance of 94.6% even though they were prepared at room temperature. The low sheet resistance of the IWO film processed at room temperature is attributed to the high mobility (59 cm2/V-s) in the well-developed crystalline structure of the ion-plated IWO film and screening effect of W dopants. In addition, the better adhesion of the ion-plated IWO film on the PET substrate led to small critical outer and inner bending radii of 6 and 3 mm, respectively, against substrate bending. Due to the low sheet resistance, high optical transmittance, better crystallinity, better adhesion, and outstanding flexibility of the ion-plated IWO films, the flexible OLEDs and QDLEDs with the IWO electrodes showed better performances than flexible OLEDs and QDLEDs with sputtered flexible ITO anodes. This indicates that in-line arc plasma ion plating is a promising large area coating technique to realize room temperature processed high-quality FTCEs for flexible OLEDs and QDLEDs.

Foldable graphene electronic circuits based on paper substrates.

Graphene electronic circuits are prepared on paper substrates by using graphene nanoplates and applied to foldable paper-based electronics. The graphene circuits show a small change in conductance under various folding angles and maintain an electronic path on paper substrates after repetition of folding and unfolding. Foldable paper-based applications with graphene circuits exhibit excellent folding stability.

Dispersion Stability and Electrorheological Properties of Polyaniline Particle Suspensions Stabilized by Poly(vinyl methyl ether).

In the present article, a novel method of stabilization of a semi conductive polyaniline particle nonaqueous suspension of electrorheological (ER) materials was introduced. Using as the steric stabilizer poly(vinyl methyl ether) (PVME), a dispersion polymerization of aniline was performed and stable aqueous dispersions of the polyaniline were obtained. However, a stable colloidal suspension of polyaniline in silicone oil medium can be obtained only with a low concentration of PVME (0.75%), although the average size of the individual, redispersed particles in oil generally decreased with increasing concentration of PVME. At higher concentrations of PVME, formation of highly packed particle aggregates was investigated, as seen in the scanning electron microscopy images. The polyaniline suspension stabilized by 0.75% PVME content exhibited the best dispersing state and, therefore, showed the highest zero-field viscosity. Such a well-stabilized polyaniline suspension also showed the maximum ER property, namely the largest normalized yield stress in an electric field. Copyright 2001 Academic Press.

Rheological Properties and Stabilization of Magnetorheological Fluids in a Water-in-Oil Emulsion.

The rheological properties and dispersion stability of magnetorheological (MR) fluids consisting of hydrophilic treated carbonyl iron particles dispersed in a water-in-oil emulsion were studied for the first time by the use of a stress-controlled rheometer and sedimentation test. In order to improve the stability of the MR fluids, carbonyl iron magnetic particles were chemisorbed by Tween 80 and a water-in-oil emulsion was employed as a continuous phase for MR fluids. Attraction between hydrophilic-treated carbonyl iron and water emulsion in continuous phase plays a critical role in greatly improving stability of dense carbonyl iron particles against sedimentation without restricting rheological properties. On application of magnetic fields, the suspensions show a striking increase in viscosity. Since constant stress is generated within the limit of zero shear rate, the plateau in the flow curve corresponds to the Bingham yield stress. Under a low external magnetic field, the yield stress varied as B(3/2), indicating that local magnetization saturation occurs between the neighboring magnetized particles. The yield stress has an approximately linear relation to the particle volume fraction. Copyright 2001 Academic Press.