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

Abstract— 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.     

The role of LiF buffer layer in tris‐(8‐hydroxyquinoline) aluminum‐based organic light‐emitting devices with Mg:Ag cathode

Abstract— The device characteristics of organic light‐emitting devices based on tris‐(8‐hydroxyqunoline) aluminum with a thin layer of LiF inserted at the ITO and organic interface or organic and Mg:Ag cathode interface were investigated. A thin layer of LiF can enhance the electron injection when it was inserted only between the organic electron‐transporting layer and the Mg:Ag alloy cathode, but can block hole injection when inserted between the ITO anode and the organic hole‐transport layer. By inserting both a 1.0‐nm LiF layer at side of the ITO anode and a 0.5‐nm LiF layer under the Mg:Ag cathode, the device, at a current injection of 10 mA/cm², exhibited the highest current efficiency of 8.2 cd/A and power efficiency of 1.93 lm/W for all the types of devices investigated in this study. Both the current efficiency and power efficiency of the device were improved by 1.2 times at a current injection of 10 mA/cm², compared to the standard device without any LiF buffer layer. This is due to the increased electron injection and decreased hole injection that off‐sets the imbalance of electron and hole injection and brings it towards the balanced injection of electrons and holes, thus reducing the non‐productive hole current.     

High‐efficiency red‐phosphorescent organic light‐emitting diode with the organic structure of 2‐TNATA/Bebq2:SFC‐411/SFC‐137

Abstract— High‐efficiency and simple‐structured red‐emitting phosphorescent devices based on the hole‐injection layer of 4,4′,4″‐tris(2‐naphthylphenyl‐phenylamino)‐triphenylamine [2‐TNATA] and the emissive layer of bis(10‐hydroxybenzo[h] quinolinato)beryllium complex [Bebq²] doped with SFC‐411 (proprietary red phosphorescent dye) have been researched. The fabricated devices are divided into three types depending on whether or not the hole‐transport layer of N,N′‐bis(1 ‐naphthyl)‐N, N'‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine [NPB] or the electron‐transport layer of SFC‐137 (proprietary electron transporting material) is included. Among the experimental devices, the best electroluminescent characteristics were obtained for the device with an emission structure of 2‐TNATA/Bebq²:SFC‐411/SFC‐137. In this device, current density and luminance were found to be 200 mA/cm² and 15,000 cd/m² at an applied voltage of 7 V, respectively. Current efficiencies were 15 and 11.6 cd/A under a luminance of 500 and 5000 cd/m². The peak wavelength in the electroluminescent spectral distribution and color coordinates on the Commission Internationale de I'Eclairage (CIE) chart were 628 nm and (0.67, 0.33), respectively.     

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.     

Improved performances in organic and polymer light‐emitting diodes using solution‐processed vanadium pentoxide as a hole injection layer

We report that a solution‐processed vanadium pentoxide (V₂O₅) layer can be utilized as an effective and stable hole injection layer in organic light‐emitting diodes and polymer light‐emitting diodes instead of polyethylene dioxythiophene : polystyrenesulfonate (PEDOT : PSS). The organic light‐emitting diode and polymer light‐emitting diode with the V₂O₅ layer have driving voltages that are 2.2 and 0.3 V lower for 1000 cd/m², respectively, than the devices with PEDOT : PSS. In addition, the devices with the V₂O₅ layer show improved operational stability compared with the devices with PEDOT : PSS. Therefore, a solution‐processed V₂O₅ layer can be utilized as an effective and stable hole injection layer instead of PEDOT : PSS.     

Ink‐jet‐printable phosphorescent organic light‐emitting‐diode devices

Abstract— A novel method for the fabrication of ink‐jet‐printed organic light‐emitting‐diode devices is discussed. Unlike previously reported solution‐processed OLED devices, the emissive layer of OLED devices reported here does not contain polymeric materials. The emission of the ink‐jet‐printed P²OLED (IJ‐P²OLED) device is demonstrated for the first time. It shows good color and uniform emission although it uses small‐molecule solution. Ink‐jet‐printed green P²OLED devices possess a high luminous efficiency of 22 cd/A at 2000 cd/m² and is based on phosphorescent emission. The latest solution‐processed phosphorescent OLED performance by spin‐coating is disclosed. The red P²OLED exhibits a projected LT50 of >53,000 hours with a luminous efficiency of 9 cd/A at 500 cd/m². The green P²OLED shows a projected LT50 of >52,000 hours with a luminous efficiency of 35 cd/A at 1000 cd/m². Also discussed is a newly developed sky‐blue P²OLED with a projected LT50 of >3000 hour and a luminous efficiency of 18 cd/A at 500 cd/m².     

Highly efficient blue organic light‐emitting diode with high color purity using 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) doped with 1,4‐bis[2‐(3‐N‐ethylcarbazoryl) vinyl]benzene (BCzVB)

Abstract— An efficient pure blue multilayer organic light‐emitting diode employing 1,4‐bis[2‐(3‐N‐ethylcarbazoryl)vinyl]benzene (BCzVB) doped into 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) is reported. The device structure is ITO (indium tin oxide)/TPD (N,N′‐diphenyl‐N,N′‐bis (3‐methylphenyl)‐1,1′biphenyl‐4,4′diamine)/CBP:BCzVB/Alq₃ (tris‐(8‐hydroxy‐quinolinato) aluminum)/Liq (8‐hydroxy‐quinolinato lithium)/Al; here TPD was used as the hole‐transporting layer, CBP as the blue‐emitting host, BCzVB as the blue dopant, Alq₃ as the electron‐transporting layer, Liq as the electron‐injection layer, and Al as the cathode, respectively. A maximum luminance of 8500 cd/m² and a device efficiency of 3.5 cd/A were achieved. The CIE co‐ordinates were x = 0.15, y = 0.16. The electroluminescent spectra reveal a dominant peak at 448 nm and additional peaks at 476 nm with a full width at half maximum of 60 nm. The Föster energy transfer and, especially, carrier trapping models were considered to be the main mechanism for exciton formation on BCzVB molecules under electrical excitation.     

Efficiency enhancement of solution‐processed single‐layer blue‐phosphorescence organic light‐emitting devices having co‐host materials of polymer (PVK) and small‐molecule (SimCP2)

Abstract— A new type of single‐layer blue‐phosphorescence organic light‐emitting devices (OLEDs) containing poly(9‐vinylcarbazole) (PVK) and small‐molecule‐based amorphous ambipolar bis(3,5‐di(^9H‐carbazol‐9‐yl)phenyl) diphenylsilane (SimCP2) as the co‐host material have been demonstrated. All active materials [PVK, SimCP2, Flrpic (blue‐phosphorescence dopant), and OXD‐7 (electron transport)] were mixed in a single layer for solution processing in the fabrication of OLEDs. The SimCP2 small‐molecule host has adequate high electron and hole‐carrier mobiltieis of ～10^?4 cm²/V‐sec and a sufficiently large triplet state energy of ～2.70 eV in confining emission energy on FIrpic. Based on such an architecture for single‐layer devices, a maximum external quantum efficiency of 6.2%, luminous efficiency of 15.8 cd/A, luminous power efficiency of 11 lm/W, and Commision Internale de l'Eclairage (CIE^x,y) coordinates of (0.14,0.32) were achieved. Compared with those having PVK as the single‐host material, the improvement in the device performance is attributed to the balance of hole and electron mobilities of the co‐host material, efficient triplet‐state energy confinement on FIrpic, and the high homogeneity of the thin‐film active layer. Flexible blue‐phosphorescence OLEDs based on solution‐processed SimCP2 host material (withou PVK) have been demonstrated as well.     

Preparation and characterization of highly efficient blue TPBI:FIrpic organic light‐emitting devices

Abstract— Blue phosphorescent organic light‐emitting devices (PhOLEDs) using 1,3,5‐tris(N‐phenyl‐ benzimiazole‐2‐yl)benzene [TPBI] as the host and bis((4,6‐difluorophenyl)‐pyridinate‐N,C2′)picolinate [FIrpic] as the dopant in the emitter were fabricated with different treatments of the hole‐transport layers and doping levels. Among the experimental devices, the best electroluminescent characteristics were obtained in the device with the combined hole‐transport layer of N,N′‐diphenyl‐N,N′‐bis‐[4‐ (phenyl‐m‐tolylamino)‐phenyl]‐biphenyl‐4,4′‐diamine [DNTPD]/1, 1‐bis‐(di‐4‐polyaminophen yl)‐ cyclo‐hexane [TAPC] and a doping level of 10‐vol.% FIrpic. The device with a structure of DNTPD/TAPC/TPBI:Firpic (10%) showed a luminance of 1300 cd/m² at an applied voltage of 10 V, a maximum current efficiency of 18 cd/A, and color coordinates of (0.17, 0.43) on the Commission Internationale de I'Eclairage (CIE) chart.     

Hole‐rich host materials for blue‐phosphorescent OLEDs

Abstract— Stable and efficient organic light‐emitting devices (OLEDs) are an integral part of the future of lighting and displays. The hole accumulation at the hole‐transport/emissive‐layer interface in such devices is considered to be a major pathway for degradation and efficiency loss. Here, the design and synthesis of two charge‐transporting host materials, based on the phosphine oxide (PO) moiety, engineered to improve hole transport of the emissive layer, will be reported. The compounds are an extension of a molecular design strategy which incorporates a hole‐transporting moiety and an electron‐transporting moiety. These materials were designed with two hole‐transport moieties (HTms) to further improve hole transport, compared to the first‐generation host materials that were designed with one hole‐transport functional group. The triplet exciton energy was maintained at a level greater than that of FIrpic (2.7 eV) to prevent exciton quenching. The EHOMO and ELUMO of the two classes of molecules (i.e., 1 HTm vs. 2 HTms) were similar; however, their device performance varied greatly. Emission zone experiments were conducted to further characterize the difference in charge transport between the molecules.     

Red‐phosphorescent OLEDs employing bis(8‐quinolinolato)phenolato‐aluminum(III) complexes as emission‐layer hosts

Abstract— We have used bis(8‐quinolinolato)phenolato‐aluminum complexes as emission‐layer hosts in red‐phosphorescent OLED devices. This enabled high‐efficiency long‐lived OLED devices with a simple device structure that does not require a hole‐blocking layer. Devices with a red‐phosphorescent dopant introduced into a noble bis(8‐quinolinolato)phenolato‐aluminum complex exhibited a high efficiency of 12 cd/A at CIE color coordinates (0.65, 035) and a long operating lifetime of 30,000 hours or more at an initial luminance of 700 cd/m². Moreover, triplet‐triplet annihilation was reduced in the devices because of the wide emission zone enabled by the complex and the short phosphorescent lifetime of the red‐phosphorescent dopant. We have successfully incorporated these red‐phosphorescent devices into commercial OLED displays.     

A new wettability‐control technique for fabricating color OLED panels by an ink‐jet‐printing method

Abstract— The fabrication technique for color OLED panels by means of wettability‐controllable hole‐injection material (HIM) and a photocatalytic lithography method achieves both precise ink‐jet printing and long‐lifetime devices. The technique enables us to selectively change the non‐wetting surface of a hole‐injection layer (HIL) of metal‐oxide nanoparticles (MONPs) into a wetting surface without damage to the device performance. Wetting patterns formed by this method with photocatalyst‐coated photomasks made it possible to print emission material with patterns of precisely 98‐μm widths on the hole‐injection layer. A fluorescent green‐emitting device fabricated with an HIM of MONPs by the photocatalytic treatment exhibited a long lifetime of 365 hours at30,000 cd/m², which can be extrapolated to a lifetime of more than 110,000 hours at 1000 cd/m², assuming an acceleration coefficient of 1.7. A two‐color device and a monochrome passive‐matrix panel were also successfully fabricated. The two‐color device emitted light without the mixing of colors. The monochrome panel displayed alphabetical characters with good uniformity and no flaws.     

Printable phosphorescent organic light‐emitting devices

Abstract— A new approach to full‐color printable phosphorescent organic light‐emitting devices (P²OLEDs) is reported. Unlike conventional solution‐processed OLEDs that contain conjugated polymers in the emissive layer, the P²OLED's emissive layer consists of small‐molecule materials. A red P²OLED that exhibits a luminous efficiency of 11.6 cd/A and a projected lifetime of 100,000 hours from an initial luminance of 500 cd/m², a green P²OLED with a luminous efficiency of 34 cd/A and a projected lifetime of 63,000 hours from an initial luminance of 1000 cd/m², a light‐blue P²OLED with a luminous efficiency of 19 cd/A and a projected lifetime 6000 hours from an initial luminance of 500 cd/m², and a blue P²OLED with a luminous efficiency of 6.2 cd/A and a projected lifetime of 1000 hours from an initial luminance of 500 cd/m² is presented.     

Flexible inverted bottom‐emitting organic light‐emitting devices with a semi‐transparent metal‐assisted electron‐injection layer

Abstract— The development of highly efficient and color‐saturated green‐fluorescent C545T dye‐doped flexible inverted bottom‐emitting organic light‐emitting diode (IBOLED) is reported. This was enabled by the insertion of a silver (Ag) based semi‐transparent metal‐assisted electron‐injection layer between the ITO cathode and n‐doped electron‐transporting layer on a flexible polyethersulphone substrate. This flexible IBOLED with an ITO/Ag bilayer cathode with its synergistic microcavity effect achieved luminous efficiencies of 20.4 cd/A and 12 lm/W and a saturated CIE^x,y of (0.28, 0.68) at 20 mA/cm², which are 1.5 times higher than those of a conventional OLED.     

Highly efficient green polymer light‐emitting diodes through interface engineering

Abstract— By interface engineering, we have improved the quantum efficiency for green polyfluorene polymer light‐emitting diodes by three fold: the efficiency improved from 9 to 28 cd/A. This interface engineering was achieved by inserting a thin layer of calcium (2) acetylacetonate, denoted as Ca(acac)², at the polymer/metal cathode interface. The Ca(acac)² layer behaves in a multifunctional way. It assists the electron injection by lowering the electron‐injection barrier. In the meanwhile, hole injection is enhanced by the accumulated electrons in the polymer layer. This effect is believed to lower the barrier height for hole‐injection through the Schottky effect. Finally, the Ca(acac)² layer works as a hole block layer to block the holes. As a result of the charge balance and charge confinement, the device quantum efficiency increases dramatically.     

Efficient white organic light‐emitting diodes based on a balanced split of the exciton‐recombination zone using a graded mixed layer as an electron‐blocking layer

Abstract— Efficient white organic light‐emitting diodes with both a graded mixed layer as the blue‐emitting layer and an electron‐blocking layer, and a DPVBi:Rubrene layer as a yellow‐emitting layer have been demonstrated. The mixing of the two colors occurs due to a balanced split of the exciton‐recombination zone by the graded mixed layer serving as the electron‐blocking layer. The white organic light‐emitting diode with an ITO/2‐TNATA 30 nm/NPB 30 nm/DPVBi:Rubrene (1.0 wt.%) 5 nm/NPB:DPVBi (9:1) 150 nm/NPB:DPVBi (5:5) 75 nm/NPB:DPVBi (3:7) 75 nm/NPB:DPVBi (2:8) 75 nm/NPB:DPVBi (0.5:9.5) 75 nm/BCP 5 nm/Alq³ 30 nm/LiF 0.5 nm/Al 100 nm structure is chosen as a device with an optimal configuration among devices investigated in this study. The employment of the graded mixed layer in the device is effective in suppressing the color shift at different voltages. The white light, with a Commission Internationale d'Eclairage chromaticity coordinates of (0.33, 0.34), is obtained with an applied voltage of 10.5 V for the device. At the applied voltage, the luminance is 4882 cd/m² and the current efficiency is 5.03 cd/A.     

Highly efficient fluorescent blue materials and their applications for top‐emission OLEDs

We developed new fluorescent blue dopants (BDs) for achieving high‐efficient blue organic light‐emitting diode. A new BD showed both high photoluminescent quantum yield >0.9 and highly horizontal orientation (S′ > 0.9) in doped film with keeping a chemical stability by introducing suitable substituents. We developed hole transporting materials and optimized the combination of hole transporting layers to decrease a carrier accumulation at the interface between electron blocking layer and emission layer. We found that the external quantum efficiency dependency from low to high current density was turned flat by promoting hole injection into emission layer. The top‐emission organic light‐emitting diode using the new BD and the optimized device architecture exhibited high efficiency of L/J/CIEy around 200 at CIEy = 0.043.     

Highly efficient deep‐blue organic light‐emitting devices

Abstract— A highly efficient deep‐blue organic light‐emitting device (OLED) incorporating a novel composite hole‐transport layer (c‐HTL) and an emitter based on the new non‐symmetrical mono(styryl)amine fluorescent dopant in the stable host MADN, which achieved a luminance efficiency of 5.4 cd/A with a Commission Internationale d'Eclairage (CIEx,y) of (0.14, 0.13) and an external quantum efficiency of 5.1% at 20 mA/cm² and 6.8 V, is reported. The increased device efficiency is attributed to an improved balance between hole and electron currents in the recombination zone.     

Organic‐polymer thin‐film transistors for active‐matrix flat‐panel displays?

Abstract— Organic‐polymer‐based thin‐film transistors (OP‐TFTs) look very promising for flexible, large‐area, and low‐cost organic electronics. In this paper, we describe devices based on spin‐coated organic polymer that reproducibly exhibit field‐effect mobility values around 5 × 10^?3 cm²/V‐sec. We also address fabrication, performance, and stability issues that are critical for the use of such devices in active‐matrix flat‐panel displays.     

All solution‐processed white quantum‐dot light‐emitting diodes with three‐unit tandem structure

Quantum‐dot light‐emitting diodes (QLEDs) are promising candidates for next generation displays. White QLEDs which can emit red, green and blue colors are particularly important; this is because the combination of white QLEDs and color filters offers a practical solution for high‐resolution full‐color displays. In this work, we demonstrate all‐solution processed three‐unit (red/green/blue) white tandem QLEDs for the first time. The white tandem devices are achieved by serially connecting the red bottom sub‐QLED, the green middle sub‐QLED and the blue top sub‐QLED using the inter‐connecting layer (ICL) based on ZnMgO/PEDOT:PSS heterojunction. With the proposed ICL, the two‐unit tandem QLEDs exhibit a high current efficiency of 22.22 cd/A, while the three‐unit white QLEDs exhibit evenly separated red, green and blue emission with a CIE coordinate of (0.30, 0.44), a peak current efficiency of 4.75 cd/A and a high luminance of 4206 cd/m². Displays based on the developed white QLEDs exhibit a wide color gamut of 114% NTSC. This work confirms the effectiveness of the proposed ZnMgO/PEDOT:PSS ICL and the feasibility of making all‐solution processed tandem white QLEDs by using the proposed ICL.