Hole-injecting Buffer Layer Research Articles

Dual Ag electrodes for semitransparent organic light-emitting diodes

Abstract Thick ReO3 film (>30 nm) is found to serve as an active hole-injecting layer as well as a passive buffer layer to suppress point discharge of Ag anode. This buffered Ag electrode enables fabrication of transparent organic light emitting diodes (TOLEDs). The excellent conductivity of ReO3, second only to metal amongst oxide semiconductors, is made possible to use a thick hole-injecting buffer layer. The use of thick ReO3 layer alleviates strong local electric field on a rough electrode surface. For device optical design, the transfer matrix method is used to provide theoretical guidance on electrode fabrication. Based on microcavity theory in TOLEDs, the optical path parameters are also optimized to the thicknesses of different functional organic films. To test the device performance, we fabricated normal fluorescent green Alq3 devices. The data show that TOLEDs with ReO3-buffered Ag electrodes have good photochromic uniformity on both sides of the devices.

Interface modified thermally stable hole transporting layer for efficient organic light emitting diodes

Electrical transport in thermally stable 2, 7-bis [N, N-bis (4-methoxy-phenyl) amino]-9, 9-spirobifluorene (MeO-Spiro-TPD) thin films has been investigated as a function of temperature and organic layer thickness. ITO/MeO-Spiro-TPD interface was found to be injection limited and has been studied in detail to find barrier height for hole injection. The thickness of tetra-fluoro-tetracyano-quinodimethane thin films were optimized to be used as hole injection buffer layer which resulted in switching of charge transport mechanism from injection limited to space charge limited conduction above a critical thickness of 3 nm. Hole mobility has been measured using transient space charge limited conduction (SCLC), field dependent SCLC, and top contact transistor characteristics. The charge carrier transport in interface modified hole only devices was analysed using Gaussian disorder model. The thermal stability of MeO-Spiro-TPD has been investigated by atomic force microscopy and X-ray diffraction studies. The study indicates a thermally stable and highly efficient hole transport material for application in organic semiconductor based devices.

Enhanced Lifetime of Organic Light-Emitting Diodes Using an Anthracene Derivative with High Glass Transition Temperature

Highly stable and efficient phosphorescent organic light-emitting diodes (OLEDs) were demonstrated by using anthracene-based hole injection buffer layer possessing high glass transition temperature. We synthesized a new anthracene derivative, 9,10-bis(3,3'-(N',N'-diphenyl-(N-naphthalene-2-yl)benzene-1,4-diamine) phenyl)anthracene (TANPA) and characterized its optical and thermal properties. It showed high glass transition temperature of 154 degrees C which could be attributed to the insertion of anthracene into the aromatic amino group with triphenylamine. We also utilized TANPA for the hole injection and transport layers in phosphorescent OLEDs. Since TANPA has high glass transition temperature, the OLEDs using this material exhibited higher operational stability compared to the device without TANPA. When we use TANPA as the hole injection layer in combination with a widely-used hole transporting material, N,N'-di(1-naphthyl)-N,N-diphenylbenzidine (NPB), the devices showed high enhancement in terms of the operational lifetime, driving voltage change, and device efficiency, originating from the electron-hole charge balance as well as good thermal stability of TANPA.

Improved Efficiency of Flexible Organic Light-Emitting Diodes by Insertion of Ultrathin SiO2Buffer Layers

An ultrathin hole-injection buffer layer (HBL) using silicon dioxide (SiO2) by electron beam evaporation in flexible organic light-emitting diode (FOLED) has been fabricated. While the current of the device at constant driving voltage decreases as increasing SiO2thickness. Compared to the different thicknesses of the buffer layer, the FOLED with the buffer layer of 4 nm showed the highest luminous efficiency. The atomic force microscopy (AFM) investigation of indium tin oxide (ITO)/SiO2topography reveals changes at the interface between SiO2and N,N′-bis-(1-naphthl)-diphenyl-1,1′-bipheny-4,4′-diamine (NPB), resulting in ultrathin SiO2layers being a clear advantage for a FOLED. However, the SiO2can be expected to be a good buffer layer material and thus enhance the emission performance of the FOLED.

Effects of Postannealing Process on the Properties of RuO2 Films and Their Performance As Electrodes in Organic Thin Film Transistors or Solar Cells

RuO(2) films were deposited on SiO(2) (300 nm)/N++Si substrates using radio frequency magnetron sputtering at room temperature. As-deposited RuO(2) films were annealed at different temperatures (100, 300, and 500 °C) and ambients (Ar, O(2) and vacuum), and the resulting effects on the electrical and physical properties of RuO(2) films were characterized. The effect of annealing atmosphere was negligible, however the temperature highly influenced the resistivity and crystallinity of RuO(2) films. RuO(2) films annealed at high temperature exhibited lower resistivity and higher crystallinity than as-deposited RuO(2). To investigate the possibility to use RuO(2) film as alternative electrodes in flexible devices, as-deposited and annealed RuO(2) films were applied as the source/drain (S/D) electrodes in organic thin film transistor (OTFT), catalytic electrodes in dye sensitized solar cell (DSSC) and as the hole-injection buffer layer (HIL) in organic photovoltaic (OPV). Except for OTFTs (μ ≈ 0.45 cm(2)/(V s) and on/off ratio ≈ 5× 10(5)) with RuO(2) S/D electrodes, the DSSC and OPV (3.5% and 2.56%) incorporating annealed RuO(2) electrodes showed higher performance than those with as-deposited RuO(2) electrodes (3.0% and 1.61%, respectively).

Carbon nanotube modified electrodes for enhanced brightness in organic light emitting devices

Acid oxidised multiwall carbon nanotubes (COOH-MWCNT) have been introduced as a hole injection buffer layer in an organic light emitting diodes (OLED). We show that the OLED with COOH-MWCNT as a buffer layer provides higher brightness with lower operating voltages. The addition of a COOH-MWCNT buffer layer has enabled a further increase in the brightness of our OLED devices operating in excess of 20,000 cd/m 2 due to enhanced hole injection by several orders of magnitude. The increase in current injection and brightness does not alter the optical emission spectrum at different operating voltages in these devices. A model is proposed to explain this increase in current injection and brightness based on the modified electron energy band alignment.

Optical and Electrical Properties of OLED with the Structure of ITO/ m-MTDATA/Meo-TPD/Alq3/LiF/Al

In this paper,the organic light emitting device (OLED) with the structure of ITO/m-MTDATA/Meo-TPD/Alq3/LiF/Al has been fabricated using Alq3 as a light-emitting layer, m-MTDATA(4,4',4"-tri(3-methyl-phenylphenylamino) tripheny- lamine) as a hole injection buffer layer and Meo-TPD (N, N, N ', N'-tetrakis (4-methoxyphenyl)-benzidine)as a hole transport layer, as well as luminous and electrical properties were studied. The turn on voltage of device is about 6v. Its maximum brightness reaches 14300cd/m2 with applied voltage of 13v, sixteen times higher than that of the structure of ITO/Alq3/LiF/Al. When the applied voltage is 8V, the electroluminescence spectra peak is at 540nm, showing green.

Facile synthesis of organo-soluble surface-grafted all-single-layer graphene oxide as hole-injecting buffer material in organic light-emitting diodes

Graphene oxide (GO) was modified by surface grafting and the product P-GO exhibited a surprisingly good solubility in common solvents (as high as 1.8 mg mL−1 in DMF) without sonication and can be spin coated to form large single-layer sheets on a substrate. In contrast to GO, all-single-layer P-GO sheets are readily formed in solution, which are highly useful for separation from other multi-layer sheets and as precursors to all-single-layer graphene. The unique property of single-layer P-GO made it a promising functional material, such as hole-injecting buffer (HIB) material in organic light-emitting devices. The devices using P-GO as a hole-injecting buffer layer on the ITO electrode perform better than those with PEDOT:PSS.

Organic light emitting diodes using NaCl:N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine composite as a hole injection buffer layer

Composite buffer layers of N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and NaCl at the anode/organic interface were found to be very effective on the hole injection enhancement from an indium tin oxide anode to the hole-transport layer (HTL) of NPB. Two maxima of significant current injection with respect to compositional variation were observed, implying multiple injection mechanisms of the tunneling effect and other interfacial effects. From a longer operation lifetime, the enhanced device stability was also confirmed as compared with a standard device with copper phthalocyanine as the hole injection layer. Those results are partly attributed to the better mechanical contact between anode and HTL via the composite buffer, observed from atomic force microscopy measurement.

The improvement of near-ultraviolet electroluminescence of ZnO nanorods/MEH-PPV heterostructure by using a ZnS buffer layer

A near-ultraviolet light emitting diode (LED) consisting of ZnO nanorods/MEH-PPV heterostructure and a layer of ZnS as the modification buffer layer is reported. Specially, the ZnO nanorods are grown on the ZnS hole-injecting buffer layer by hydrothermal method. Then the devices of ITO/ZnS/ZnO/ZnOnanorods/MEH-PPV/Al are prepared. Under forward bias, not only strong near-ultraviolet (N-UV) electroluminescence (EL) at 380 nm of ZnO band edge emission and weak defect-related emissions of ZnO nanorods at 410, 510 and 760 nm are observed, but also the exciton emission of MEH-PPV at 580 nm is detected. Along with increasing the thickness of ZnS film, the emission at 580 nm strengthens. Compared with the device ITO/ZnO/ZnOnanorods/MEH-PPV/Al we have reported in the past, the experimental results show that the suitable insertion of a ZnS thin layer can enhance the N-UV EL. Under the same voltage, the light intensity can be increased up to 10 times more than that of the device without a ZnS layer. Moreover, the turn-on voltage is reduced remarkably. The effect of ZnS as the hole-injecting buffer layer with different thickness on the EL of ZnO nanorods/MEH-PPV heterostructure is investigated.

Effects of different buffer layers on the electro-luminescence performances in white organic light-emitting diodes

Abstract The effects of different hole injection materials as the buffer layer on the electro-luminescence (EL) performances of white organic light-emitting diodes (WOLEDs) are investigated in detail. It is found that the EL performances and electric properties were strongly dependent on the structure of the used hole injection materials with different thicknesses, which directly affected the injection and transport properties in devices, and thus the EL efficiency and lifetime. It can be seen that a hybrid buffer layer of 5 nm aluminum fluoride (AlF 3 )/15 nm 4,4′,4″-tris(3-methylphenylphenylamino) (m-MTDATA) as the hole injection buffer layer shows the best EL performances in efficiency and lifetime, showing a promising hole injection material in WOLEDs. The mechanisms behind the enhanced performance of the hybrid buffer layer in WOLEDs are discussed based on X-ray photoelectron spectroscopy (XPS) measurement.

Reduced efficiency roll-off in highly efficient and color-stable hybrid WOLEDs: The influence of triplet transfer and charge-transport behavior on enhancing device performance

By wisely utilizing combinations of exciton-managed orange-phosphorescence/sky blue-fluorescence, we report on high-efficiency white organic light-emitting diodes (WOLEDs) with high color-stability together with reduced efficiency roll-off. Two WOLEDs with the peak current efficiencies of 34.5 and 42.5 cd A −1 which slightly shift to 34.2 and 40.0 cd A −1 at 1000 cd m −2 are demonstrated. Systematic investigations focused on the effects of hole-injection buffer layer, thickness of emitting-layer (EML) and the stepwise-doping profile of orange phosphorescent EML on the device performance have been performed. It is evidently shown that careful management of the triplet/singlet excitons and charge-carriers is key to realizing such high performance of the devices. The results will beyond doubt be helpful to the design of both material and device structure for high-performance WOLEDs and next-generation solid-state lighting sources.

Improvement of brightness and efficiency in organic light-emitting diodes using 1,3-bis(4- tert-butylphenyl-1,3,4-oxadiazoyl) phenylene as the hole buffer layer

We have reported the efficient Poly (2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEHPPV) organic light-emitting diodes based on 3-bis(4- tert-butylphenyl-1,3,4-oxadiazoyl) phenylene (OXD-7) and ITO bilayer anodes. OXD-7 was inserted between ITO anode and MEHPPV and acted as a hole buffer layer. The results demonstrated that the efficiency and brightness of the devices with ITO/OXD-7/MEHPPV/LiF/Al were four factors higher than that of devices with ITO/MEHPPV/LiF/Al. However, the efficiency and brightness of devices with ITO/OXD-7/MEHPPV/Al were worse than that of devices with ITO/MEHPPV/Al. Our results suggested that the better performance of devices should have not only the balance of hole and electron injection, but also higher hole and electron density in the emissive layer. OXD-7, as an electron-transporting material, was demonstrated to be a good candidate for hole injection buffer layer.

Electrically Conducting Amorphous Molecular Material: Iodine Doped m-MTDATA as a Hole Injection Buffer Layer in Organic Electroluminescent Devices

An electrically conducting amorphous molecular material was prepared by iodine doping of 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA). The use of iodine-doped m-MTDATA, which was identified as m-MTDATA radical cation species with counter anions, as a hole-injection buffer layer in tris(8-quinolinolato)aluminum (Alq3)-based organic electroluminescent devices led to the reduction of drive voltage and the enhancement of external quantum efficiency as compared with the corresponding device using undoped m-MTDATA. It is indicated that the use of iodine-doped m-MTDATA facilitates not only hole injection from the ITO electrode but also electron injection from the cathode into Alq3.

Organic light-emitting devices with a 2-(4-biphenyl)-5-(4-butylphenyl)-1,3,4-oxadiazole layer between the α-naphtylphenyliphenyl diamine and 8-hydroxyquinoline aluminum

Organic light-emitting devices (OLEDs) with a 2-(4-biphenyl)-5-(4-butylphenyl)-1,3,4-oxadiazole layer between the α-naphtylphenyliphenyl diamine and 8-hydroxyquinoline aluminum were fabricated using a vacuum evaporation method. Compared to the different thickness of the buffer layer, the OLEDs with the 1.0nm buffer layer showed the maximum power efficiency. The enhancements in power efficiency result from an improved balance of hole and electron injections. After comparing among different density buffer layer, PBD are good candidates for hole-injecting buffer layer, and 1.0nm PBD buffer layer shows better operational durability and life.

Improved performance of organic light-emitting devices with 2-(4-biphenyl)-5-(4-butylphenyl)-1,3,4-oxadiazole

The use of 2-(4-biphenyl)-5-(4-butylphenyl)-1,3,4-oxadiazole (PBD) as an anode buffer layer in organic light-emitting devices (OLEDs) with a configuration of indum tin oxide/α-naphtylphenyliphenyl diamine/8-hydroxyquinoline aluminum/LiF/Al has been studied. With PBD buffer layer several angstroms thickness, the current efficiency increased and the stability of the device was improved. The results suggested that PBD is a promising anode buffer layer for OLEDs. After Compared it with the TiO 2 buffer layer, TiO 2 and PBD are good candidates for hole-injecting buffer layer. And PBD buffer layer shows better operational durability and life.

Effects of PEDOT:PSS Buffer Layer in a Device Structure of ITO/PEDOT:PSS/TPD/Alq3/Cathode

We have investigated the effects of hole-injection buffer layer in organic light-emitting diodes using poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)(PEDOT:PSS) in a device structure of <TEX>$ITO/PEDOT:PSS/TPD/Alq_{3}/cathode$</TEX>. Polymer PEDOT:PSS buffer layer was made by spin casting method. Current-voltage, luminance-voltage characteristics and efficiency of device were measured at room temperature with a variation of cathode materials; Al, LiF/Al, LiAl, and Ca/Al. The device with LiF/Al cathode shows an improvement of external quantum efficiency approximately by a factor of ten compared to that of Al cathode only device. Our observation shows that cathode is important in improving the efficiency of the organic light-emitting diodes.

Improving the Performance of Organic Light-Emitting Devices by Adding the Starburst Amine as Buffer Layer

We have fabricated an organic light-emitting devices with high efficiency and brightness utilizing thick layer of a starburst amine compound 4,4’, 4’’-tris{N,-(3-methylphenyl)-Nphenylamin} triphenylamine (m-MTDATA) as hole injection buffer layer. The brightness and the current efficiency were comparatively improved compared with those of the device without the buffer layer. The device with 10nm m-MTDATA act as buffer layer has max brightness was 18590cd/m2 at 17 V, and the highest luminous efficiency was achieved 4.30cd/A, at 7V, which is nearly three times than that of the device without it.

Enhanced brightness and efficiency in organic electroluminescent device using TiO 2 self-assembled layers

A new kind of TiO 2 self-assembled nanometer material has been fabricated and is used as a hole-injecting buffer layer in the Organic electroluminescent devices. The enhancements in brightness and efficiency are attributed to an improved balance of hole and electron injections due to blocking of the injected holes by the buffer layer and a more homogeneous adhesion of the hole transporting layer to the anode.

Enhanced emission in organic light-emitting diodes using Ta 2O 5 buffer layers

An ultra-thin hole-injection buffer layer using Ta 2O 5 by RF magnetron sputtering in organic light-emitting diodes has been fabricated. With the property of high dielectric constant, Ta 2O 5 can be expected to be a good buffer layer material and thus enhance the emission performance of the OLED. The Ta 2O 5 film blocks part of the injected holes from the ITO anode to the organic layer (TPD) and improves the balance of hole and electron injections. The power efficiency of the device with the Ta 2O 5 film can be increased by approximately 20%.