Lifetime Of Organic Light-emitting Diodes Research Articles

Poly(3,4‐ethylenedioxythiophene) Derived from Poly(ionic liquid) for the Use as Hole‐Injecting Material in Organic Light‐Emitting Diodes

We report that poly(3,4-ethylenedioxythiophene) derived from poly(ionic liquid) (PEDOT:PIL) constitutes a unique polymeric hole-injecting material capable of improving device lifetime in organic light-emitting diodes (OLEDs). Imidazolium-based poly(ionic liquid)s were engineered to impart non-acidic and non-aqueous properties to PEDOT without compromising any other properties of PEDOT. A fluorescent OLED was fabricated using PEDOT:PIL as a hole-injection layer and subjected to a performance evaluation test. In comparison with a control device using a conventional PEDOT-based material, the device with PEDOT:PIL was found to achieve a significant improvement in terms of device lifetime. This improvement was attributed to a lower indium content in the PEDOT:PIL layer, which can be also interpreted as the effective protection characteristics of PEDOT:PIL for indium extraction from the electrodes.

Residual Halide Groups in Gilch-Polymerized Poly(p-phenylene-vinylene) and Their Impact on Performance and Lifetime of Organic Light-Emitting Diodes

On the basis of new insights into the reaction mechanism of the so-called Gilch route leading to poly(p-phenylene-vinylene)s (PPVs), the importance of vinyl halide defects for the performance of organic light-emitting diodes (OLEDs) is stressed in the present contribution. It is found that the current density, the luminance, and luminance efficiency are superior for PPVs that were subject to a long-term dehydrohalogenation. In particular, the device lifetime improves by a factor of 200 as long as the halide content is reduced from 0.4 to 0.05 wt %. The results imply that rather the mentioned vinyl halide defect than the often discussed tolane-bisbenzyl (TBB) defect has to be considered when investigating lifetime and performance of OLEDs. The device behavior is analyzed in view of a detailed study of the charge-carrier transport properties. We suggest that the penetration of electrons from the cathode in the PPV leads to a separation of halogen and thus to free halogen anions. The anions can move in the e...

Improved Efficiency and Lifetime of Organic Light-Emitting Diode with Lithium-Quinolate-Doped Electron Transport Layer

In this letter, we analyze the device performance of organic light-emitting diodes (OLEDs) with a lithium-quinolate (Liq)-doped electron transport layer (ETL). When the ETL:Liq doping ratio is 1:3, the luminous efficiency of the OLED is 7.1 cd/A; that of an OLED with a nondoped ETL is 5.3 cd/A. The corresponding OLED lifetime is improved by threefold that with a nondoped ETL, owing to the improved hole-electron charge balance and electrical stability of the Liq-doped ETL film. Additionally, when the Liq-doped device was combined with a Mg/Al cathode, the OLED produced had a longer lifetime than the other tested devices.

A study of ultraviolet-curable organic/inorganic hybrid nanocomposites and their encapsulating applications for organic light-emitting diodes

UV-curable organic/inorganic hybrid nanocomposites have been successfully synthesized with polymer monomers (tetramethoxysilane (TMOS)/3-glycidoxypropyl-trimethoxysilane (GPTMS)) nano-fillers (silica/alumina), curing agents (bisphenol A (BPA)), and photoinitiators (triaryl sulfonium hexafluoroantimonate (TSH)) by in situ polymerization. Their thermal stability, coefficient of thermal expansion (CTE), adhesion strength, and gas barrier capability have also been investigated with thermogravimetry analysis (TGA), thermomechanical analysis (TMA), micro-computer universal testing machine and moisture penetration tests. Experimental results indicate that nano-fillers are completely homogeneously dispersed in the polymer matrices and the increase of nano-fillers in the nanocomposites causes the raise of decomposition temperature ( T d) as well as gas resistance and the reduction of CTE as well as adhesion strength. Because lab-made organic/inorganic hybrid nanocomposites have been discovered to exhibit excellent gas barrier properties, we have also applied them for the encapsulation of organic light-emitting diodes (OLEDs) and flexible OLEDs. With lab-made nanocomposite d, the lifetimes of OLEDs and flexible OLEDs can be successfully lengthened to 92 and 36 h, respectively, whereas those of OLEDs and flexible OLEDs without encapsulation are 11 and 7 h, respectively.

High efficiency and long lifetime in organic light-emitting diodes using bilayer electron injection structure

A lithium quinolate-based electron injection structure was developed to improve luminance efficiency and lifetime of organic light-emitting diodes (OLEDs). A electron injection material based on Li complex, 8-hydroxyquinolinato lithium (Liq), was introduced as an electron injection material for OLEDs and the efficiency and lifetime of OLEDs were investigated according to the structure of the electron injection layer. A bilayer electron injection structure, a mixed layer of tris(8-hydroxyquinoline) aluminium (Alq 3) and Liq and a Liq layer, showed high efficiency of 11.6 cd/A compared with 9.8 cd/A for lithium fluoride (LiF). In addition, the extrapolated lifetime of OLED with the bilayer electron injection structure was improved by 40% at 1000 cd/m 2.

Lifetime of organic light emitting diodes on polymer anodes

We report on the use of a thin layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as anode for bottom emission organic light emitting diodes (OLEDs). The combination of polymer anodes with OLEDs having either electrically doped or undoped hole transport layers in direct contact with the polymer is shown. We discuss the impact of the annealing conditions of the polymer on the OLED lifetime in comparison to indium tin oxide anodes. Supported by a differential thermal analysis of PEDOT:PSS, a strong influence of residual water in the polymer on the device lifetime is found. Additional heating of the polymer anode in a dry ambient prior to OLED deposition is necessary to achieve high device lifetimes. At a luminance of 260 cd/m2, pin-OLEDs on a PEDOT:PSS anode show no measurable device degradation during 5200 h of operation.

Operational degradation of organic light-emitting diodes: Mechanism and identification of chemical products

Despite the importance of the operational lifetime of organic light-emitting diodes (OLEDs) in practical applications, little is known about the nature of chemical reactions associated with efficiency losses during operation. To gain an insight into a chemical mechanism of operational degradation, we studied operation-induced changes in chemical compositions of fluorescent and phosphorescent OLEDs utilizing carbazole derivatives in emissive layers. We detected substantial losses of the emissive components, including the carbazole-derived host 4,4′-bis(N-carbazolyl)biphenyl (CBP) and, if present, phosphorescent dopant. Several different materials were found only in the degraded OLEDs, and some of them were isolated and identified by nuclear magnetic resonance and mass spectrometry. A similar set of products was found upon UV irradiation of CBP films. Structures of degradation products suggest that the key step in operational degradation of OLEDs is homolytic cleavage of weaker bonds, e.g., an exocyclic C–N bond in CBP, in the excited state, followed by radical addition reactions to yield stabilized π radicals. Overall, OLED operation leads to the accumulation of the neutral radical species and their reduced or oxidized forms, acting as nonradiative recombination centers and luminescence quenchers.

Dissociation of iridium(III) phosphorescent emitters upon adsorption on Cu(110) revealed by scanning tunneling microscopy

The phosphorescent emitters used in organic light emitting diodes (OLEDs) play a crucial role for tuning the color and the luminescence intensity. The authors have investigated by scanning tunneling microscopy the adsorption of iridium(III) phosphorescent emitter molecules used in OLEDs on a partly oxidized Cu(110) surface. Surprisingly they find that 50% of the emitters have dissociated upon adsorption at the substrate. The findings suggest that the decrease in the lifetime of OLEDs, which are manufactured by vacuum vaporization technique, is due to the dissociation of emitter molecules present in the device.

Improving operating lifetime of organic light-emitting diodes with polycyclic aromatic hydrocarbons as aggregating light-emitting-layer additives

It is common in organic light-emitting diode technology to construct a light-emitting-layer (LEL) host with materials that resist luminescence-reducing aggregation, which is one of the common reasons behind a phenomenon widely referred to as concentration quenching. However, if a host material in its aggregated state has a substantial quantum yield of fluorescence (e.g., at least several percents), it may yet be useful. We describe a group of aggregating flat and rigid polycyclic aromatic hydrocarbons (PAHs) as LEL additives. These molecules readily form emissive aggregates when added to the LEL. In the resulting devices, the aggregates show low-to-moderate external quantum efficiencies (EQE) of 0.2%–1.3%. Significantly, the addition of these PAHs increases device half-life (t50) 4–200 times, depending on the additive, up to 100000h upon operation at 40mA∕cm2. The lifetime increase occurs with many diverse classes of PAHs. The EQE can be improved to 3.7% by further adding a proper dopant while maintaining the increased lifetime. A possible link between the ability to aggregate and the lifetime increase is illustrated by comparing aggregation-prone perylene and aggregation-resistant 2,5,8,11-tetra-t-butylperylene (TBP). Despite the similarity between the two additives with respect to their initial device performance, perylene’s stronger ability to aggregate correlates with the eight times longer half-life versus that for TBP.

Plasma preparation on indium-tin-oxide anode surface for organic light emitting diodes

A simple bilayer structure of organic light emitting diode (OLED) was used to study the characteristics of various anode preparations. Systematic investigation of indium tin oxide (ITO) anode surface treatment of OLEDs was performed to get the optimum condition for the ITO anode. The ITO surface was treated by Ar or O 2 plasma with different RF power, chamber pressure and exposure time. Atomic force microscopy was used in measuring the surface morphology and roughness analyses. Sheet resistance of ITO anodes reduces to less than 7 Ω/□ and surface roughness of ITO anode decreases more than 20%. Hence, the electric characteristics of OLED were improved. The operating voltage of OLED devices with Ar or O 2 plasma treated anodes decreased from 7.5 to 6.5 V and 6.0 V, respectively. We also show the presence of less dark spots in the OLEDs with plasma-treated ITO anode. As a result, the stability and lifetime of OLEDs can be improved obviously.

Improved performances of the organic light-emitting devices by doping in the mixed layer

The brightness, efficiency and lifetime of organic light-emitting diodes (OLEDs) were remarkably improved by doping in a mixed layer. In this device, the emitting layer consists of a mixture of α-naphthylphenybiphenyl amine (NPB), tris (8-hydroxyquinolinolate) aluminum (Alq3) (referred to as the mixed layer) and an emissive dopant 5,6,11,12-petraphenylnaphthacene (rubrene), where the concentration of NPB declined while the concentration of Alq3 was increased gradually in the deposition process. The device exhibited a maximum emission of 49,300 cd/m2 at a destroy voltage of 35 V and a maximum efficiency of 7.96 cd/A at 10 V, respectively. The efficiency improves twofold in comparison with the conventional doped devices. Meanwhile, the device exhibited superior operational stability with a half-life time of 1000 h at a starting luminance of 1000 cd/m2 by a constant current driver.

Praseodymium oxide coated anode for organic light-emitting diode

Capping an indium–tin–oxide anode with a thin layer of praseodymium oxide (Pr2O3) has been found to enhance not only hole-injection, quantum, and power efficiencies but also the lifetime of organic light-emitting diode made using copper (II) phthalocyanine as the anode buffer layer, N, N′-diphenyl-N,N′ bis(3-methylphenyl-1,1′-biphenyl-4,4′-diamine as the hole-transport layer and tris-8-hydroxyquinoline aluminum as the electron-transport/emission layer. The best results have been obtained on diodes with ∼1 nm thick Pr2O3. A possible mechanism behind the improvement is discussed.

Effects of Passivation and Desiccant on Organic Light-Emitting Diodes.

The lifetime of organic light-emitting diodes (OLEDs) strongly depends onremoving H2O from the passivation atmosphere. Therefore, practical OLEDs require excellent passivation to block invading moisture and desiccant to remove moisture in a metal can or a glass cap. We observed the emitting area in OLEDs passivated by a glass cap. Dark spots in OLEDs passivated without a desiccant and grew more rapidly than those in OLEDs passivated with a desiccant. We speculated that the difference between both samples is caused not only by the imperfection of our passivation, but also occurrence of H2O from the OLED device.

Photoluminescence of Alq[sub 3] Stabilized by a Phenolic Compound

Over the last 10 years, several organic compounds have been used to realize efficient electroluminescent devices, organic light-emitting diodes (OLED), which have now become competitive with the well-known and older inorganic semiconductor diodes, LED. Among these new luminescent compounds, the metallorganic molecule has an efficient emission band in the green region of the electromagnetic spectrum, and is easily handled. However, OLEDs realized with and other organic compounds display a lifetime defined as the time required by the emission to reduce by half its initial value, equal to about 5000 h, which is considered to be too short for practical applications. Several studies have been performed on organic compounds to understand their degradation, which has been ascribed to both intrinsic and extrinsic effects; among the latter, exposition to atmospheric agents is considered to be very important. Simple experiments have been made to verify the oxidation processes and the possibility to halt or retard them by using appropriate chemical compounds. Butylated hydroxytoluene, a molecule belonging to the big family of phenols, proved to be effective in preserving the photoluminescence of films. Our results support the idea that phenols, which are known to be strong antioxidant products, can increase the lifetime of OLEDs realized with small organic molecules and polymers. © 2002 The Electrochemical Society. All rights reserved.

Effect of Polymer Layer Morphology by Thermal Treatment on I-V Characteristics of Electroluminescence Device

The morphology change of polymer HTL by thermal treatment is investigated, which affects the performance of organic light emitting diodes (OLEDs). OLEDs are fabricated with the structure of ITO/Polymer HTL/Emitting layer (Alq3)/Electrode (Al). The polymer HTL layer was treated by heat, which causes the morphology change like the aggregation of polymer HTL. The aggregation of polymer HTL due to the heat affects the turn-on voltage and lifetime of OLED.