Lifetime Of Organic Light-emitting Diodes Research Articles

A stepwise energy level doping structure for improving the lifetime of phosphorescent organic light-emitting diodes

A stepwise energy level doping structure for improving the lifetime of organic light-emitting diodes was developed by doping two emitters with different energy levels in the same host material as separated emitting layers.

Influence of material impurities in the hole-blocking layer on the lifetime of organic light-emitting diodes

We evaluated the influence of impurities in an organic material used for the fabrication of organic light-emitting diodes (OLEDs) on the lifetime of the fabricated devices. Despite no differences in the current-density–voltage characteristics and external quantum efficiencies of the devices, the lifetime was approximately nine times longer for devices with high-purity 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), which was used as a hole-block layer. Chlorine-containing impurities derived from T2T had the greatest influence on the lifetime of the OLEDs even though the amount of halogen in the source material was at most 0.9 ppm. On the other hand, the lifetime was not greatly influenced by other impurities even with concentrations up to 0.2%. Therefore, the purities of materials other than the emitter must also be closely controlled.

Influence of vacuum chamber impurities on the lifetime of organic light-emitting diodes.

We evaluated the influence of impurities in the vacuum chamber used for the fabrication of organic light-emitting diodes on the lifetime of the fabricated devices and found a correlation between lifetime and the device fabrication time. The contact angle of the ITO substrates stored the chamber under vacuum were used to evaluate chamber cleanliness. Liquid chromatography-mass spectrometry was performed on Si wafers stored in the vacuum chamber before device fabrication to examine the impurities in the chamber. Surprisingly, despite the chamber and evaporation sources being at room temperature, a variety of materials were detected, including previously deposited materials and plasticizers from the vacuum chamber components. We show that the impurities, and not differences in water content, in the chamber were the source of lifetime variations even when the duration of exposure to impurities only varied before and after deposition of the emitter layer. These results suggest that the impurities floating in the vacuum chamber significantly impact lifetime values and reproducibility.

Enhanced lifetime of organic light-emitting diodes using soluble tetraalkyl-substituted copper phthalocyanines as anode buffer layers

Soluble tetraalkyl-substituted copper phthalocyanines were employed as anodic buffer layers of OLEDs, achieving enhanced stability and durability compared with PEDOT:PSS.

Tetra-methyl substituted copper (II) phthalocyanine as a hole injection enhancer in organic light-emitting diodes

We have enhanced hole injection and lifetime in organic light-emitting diodes (OLEDs) by incorporating the isomeric metal phthalocyanine, CuMePc, as a hole injection enhancer. The OLED devices containing CuMePc as a hole injection layer (HIL) exhibited higher luminous efficiency and operational lifetime than those using a CuPc layer and without a HIL. The effect of CuMePc thickness on device performance was investigated. Atomic force microscope (AFM) studies revealed that the thin films were smooth and uniform because the mixture of CuMePc isomers depressed crystallization within the layer. This may have caused the observed enhanced hole injection, indicating that CuMePc is a promising HIL material for highly efficient OLEDs.

Phosphorescence lifetimes of organic light-emitting diodes from two-component time-dependent density functional theory.

"Spin-forbidden" transitions are calculated for an eight-membered set of iridium-containing candidate molecules for organic light-emitting diodes (OLEDs) using two-component time-dependent density functional theory. Phosphorescence lifetimes (obtained from averaging over relevant excitations) are compared to experimental data. Assessment of parameters like non-distorted and distorted geometric structures, density functionals, relativistic Hamiltonians, and basis sets was done by a thorough study for Ir(ppy)3 focussing not only on averaged phosphorescence lifetimes, but also on the agreement of the triplet substate structure with experimental data. The most favorable methods were applied to an eight-membered test set of OLED candidate molecules; Boltzmann-averaged phosphorescence lifetimes were investigated concerning the convergence with the number of excited states and the changes when including solvent effects. Finally, a simple model for sorting out molecules with long averaged phosphorescence lifetimes is developed by visual inspection of computationally easily achievable one-component frontier orbitals.

The improvement of thin film barrier performances of organic–inorganic hybrid nanolaminates employing a low-temperature MLD/ALD method

The investigations reported in this study were carried out to determine the feasibility and properties of alucone/Al2O3 hybrid nanolaminate films for thin film encapsulation (TFE) at low temperature, using an integrated molecular layer deposition (MLD) and atomic layer deposition (ALD) process. The combination of alucone (MLD) and Al2O3 (ALD), with O3 serving as the oxidant in place of conventional H2O, has been evaluated experimentally. In our studies, O3-based encapsulation layers were observed to be smoother, displaying an average roughness of 0.342 ± 0.016 nm with three laminate layers. However, H2O-based laminates showed a much higher roughness of 0.843 ± 0.024 nm under identical conditions. Estimates of water vapor transmission rate (WVTR) yielded significantly better results for O3-based laminates, with values decreasing linearly from 3.22 × 10−3 g m−2 per day to 2.37 × 10−5 g m−2 per day as the number of laminate layers increased from one to three, while a gentle decline trend from 1.83 × 10−3 g m−2 per day to 5.92 × 10−4 g m−2 per day was obtained for the H2O-based laminate. This indicates that the hybrid nanolaminates exhibited improved water barrier properties when O3 was used as the oxidant instead of H2O. In particular, the O3-based films did not decrease the performance of organic light-emitting diodes (OLEDs). In fact, the lifetime of OLEDs with O3-based encapsulation was approximately two-fold longer than the H2O-based encapsulation. Thus, we believe that the alucone/Al2O3 hybrid encapsulation film, in which O3 serves as the oxidant, is a promising candidate for use in future OLED applications.

Bismuth Trifluoride as a low-temperature-evaporable insulating dopant for efficient and stable organic light-emitting diodes

Bismuth Trifluoride (BiF3), with a high thermal stability and a low deposition temperature, has been studied as a novel dopant for the conventional hole transporting material of N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB). The efficiency and lifetime of organic light-emitting diodes (OLEDs) have been remarkably improved by using BiF3 doped NPB. For an optimized green device, a current efficiency of 21.6cd/A is reached, 40% higher than the control device without BiF3. And the lifetime is increased from 115h to 222h at room temperature. The enhanced efficiency and lifetime are attributed to the improved balance of holes and electrons in the emissive layer. Most importantly, the thermal stability at an elevated temperature of the OLEDs with BiF3 doped NPB is largely improved, showing an order of magnitude longer lifetime than the control device at 80°C.

Novel approaches for energy efficient solid state lighting by RGB organic light emitting diodes – A review

Abstract This paper emphasizes on the novel approaches for energy efficient and eco-friendly solid state lighting. Limitations and global haphazards of currently used lighting systems such as Incandescent lamps, compact fluorescent lamps can be overcome by replacing the present lighting system by green technology called solid state lighting, which is possible only with organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs). This paper also explains important characterization techniques used to evaluate the performance, efficiency, life time; colour rendering index (CRI), Internationale de l’Eclairage (CIE) coordinates and correlated color temperature (CCT) of OLEDs. Review of literature on red, blue, green (RBG) light emitting materials and OLED devices is illustrated since the very first synthesized complex and a range of device architectures are presented and appraised. Measures to increase the efficiency and the life time of OLEDs and handling the degradation issues of the organic materials for OLEDs are also discussed. With these measures if we succeed in improving the efficiency, performance and life time, the present lighting system can be replaced by eco-friendly, energy efficient green technology called Solid state lighting, which would play a significant role in reducing global energy consumption.

The hydrogen/deuterium isotope effect of the host material on the lifetime of organic light-emitting diodes

The hydrogen/deuterium primary kinetic isotope effect provides useful information about the degradation mechanism of OLED host materials. Thus, replacement of labile C-H bonds in the host with C-D bonds increases the device lifetime by a factor of five without loss of efficiency, and replacement with C-C bonds by a factor of 22.5.

Exciton quenching by diffusion of 2,3,5,6-tetrafluoro-7,7’,8,8’-tetra cyano quino dimethane and its consequences on joule heating and lifetime of organic light-emitting diodes

In this Letter, the effect of F(4)-TCNQ insertion at the anode/hole transport layer (HTL) interface was studied on joule heating and the lifetime of organic light-emitting diodes (OLEDs). Joule heating was found to reduce significantly (pixel temperature decrease by about 10 K at a current density of 40 mA/cm(2)) by this insertion. However, the lifetime was found to reduce significantly with a 1 nm thick F(4)-TCNQ layer, and it improved by increasing the thickness of this layer. Thermal diffusion of F(4)-TCNQ into HTL leads to F(4)-TCNQ ionization by charge transfer, and drift of these molecules into the emissive layer caused faster degradation of the OLEDs. This drift was found to reduce with an increase in the thickness of F(4)-TCNQ.

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.

A photochemical investigation into operational degradation of arylamines in organic light-emitting diodes

In this study, we investigated the operational degradation of 4,4′-bis(N-carbazolyl)biphenyl (CBP), an arylamine commonly used as phosphorescent host or hole transport material in organic light-emitting diodes (OLEDs), which have been subject to a recent surge in demand for use in the optical display market. In view of the important roles of organic components in the stability and lifetime of OLEDs, we initiated an investigation of the operational degradation of CBP in OLEDs in an effort to elucidate the degradation mechanism. Our experimental approach to this task is by performing photochemistry on CBP in solution, thereby avoiding actual operation of CBP-based OLEDs. Prior to the experiments, we calculated the C–N homolytic bond dissociation energy in CBP, and synthesized two CBP derivatives based on molecular engineering considerations. Furthermore, we performed TiO2 photocatalytic decomposition of arylamines as a feasibility test for another operational degradation pathway. Based on both the photochemical and photocatalytic experiments, multiple operational degradation pathways of arylamines emerge.

평판 유리로 봉인된 유-무기 보호 박막을 갖는 OLED 봉지 방법

To study encapsulation method for large-area organic light emitting diodes (OLEDs), red emitting OLEDs were fabricated, on which <TEX>$Alq_3$</TEX> as organic buffer layer and LiF and Al as inorganic protective layers were deposited to protect the damage of OLED by epoxy. And then the OLEDs were attached to flat glass by printing method using epoxy. The basic structure of OLED doped with rubrene of 1 vol.% as emitting layer is ITO(150 nm) / 2-TNATA(50 nm) / <TEX>${\alpha}$</TEX>-NPD(30 nm) / <TEX>$Alq_3$</TEX>:Rubrene(30 nm) / <TEX>$Alq_3$</TEX>(30 nm) / LiF(0.7 nm) / Al(100 nm). In case of depositing <TEX>$Alq_3$</TEX>, LiF and Al and then attaching of flat glass onto OLED, current density, luminance, efficiency and driving voltage were not changed and lifetime was increased according to thickness of Al as inorganic protective layers. The lifetime of OLED/<TEX>$Alq_3$</TEX>/LiF/Al_4/glass structure was 139 hours increased by 15.8 times more than bare OLED of 8.8 hours and 1.6 times more than edge sealed OLED of 54.5 hours.

Effect of encapsulation technology on organic light emitting diode lifetime

A kind of green organic light-emitting diodes (OLED) was prepared via vacuum thermal evaporation, of which the multilayer structure was indium-tin oxide (ITO)/copper-phthalocyanine (CuPc) (200 A)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) (600 A)/N′-diphenyl-N,N′-tris(8-hydroxyquinoline) aluminium (Alq3) (400 A):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-(l)benzopyropyrano(6,7,8-i, j)quinolizin-11-one (C545T) (2%)/Alq3 (200 A)/LiF (10 A)/Al (1000 A). And we used both traditional glass encapsulation and thin film encapsulation (TFE) technologies to protect the device, reducing impact of vapor and oxygen. Organic film offered an excellent surface morphology, while inorganic film was nearly a perfect barrier to vapor and oxygen. Both of them constituted the encapsulation unit of TFE. According to the results of acceleration life test, the operation lifetime of device using TFE was 22% less than that of device using traditional glass cap encapsulation. So, the technology of TFE should be optimized further, and the quality of TFE needs a great improvement. There is a long way to go and a lot of hard work before realizing flexible display with OLED, but the dream will be true one day.

Properties of Interface between Organic Hole-Transporting Layer and Indium Tin Oxide Anode Modified by Fluorinated Self-Assembled Monolayer

The electronic structure and chemical properties of the interface between indium tin oxide (ITO) modified by a fluorinated self-assembled monolayer (F-SAM) and a N,N '-bis(1-naphthyl)-N,N '-diphenyl-1,1'-diphenyl-1,4'-diamine (α-NPD) layer were investigated in order to clarify the effects of the F-SAM modification of ITO anodes on the driving voltage and lifetime of organic light-emitting diodes (OLEDs). Ultraviolet and X-ray photoelectron spectroscopy revealed that the F-SAM modification of ITO led to a shallower highest occupied molecular orbital level in the α-NPD layer near the interface than in conventionally treated ITO, a chemical reaction between F-SAM and α-NPD, and the migration of adsorbed fluorine into the α-NPD layer. These results indicate that high conductance, the suppression of crystallization, and the inhibition of oxidation in the hole-transporting layer along with a small hole-injection barrier height at the anode/HTL interface contribute to the excellent properties of OLEDs having ITO anodes modified by F-SAM.

Life time extension method for simultaneous emission driving AMOLED displays

A life time extension method for simultaneous emission driving active matrix organic light emitting diode (AMOLED) displays is proposed. The proposed driving method extends emission time by driving even and odd row lines separately. The extended emission time reduces organic light emitting diode (OLED) degradation because the current density of the OLED decreases as emission time increases. The proposed driving method increases the emission time by 50% compared with the conventional simultaneous emission driving method. Measurement results show the proposed driving method increases the OLED life time by 70.07% when the target luminance of the OLED is 300 cd/m 2 and the life time is assumed that the time until the OLED luminance becomes 75% of the initial luminance. The proposed pixel structure using the even-odd separation driving scheme compensates the electrical characteristic variation of the polycrystalline-silicon thin film transistors and the maximum luminance error of pixel to pixel is 0.26% in a 3.8-inch AMOLED panel.

평판 유리로 봉인된 다층 무기 박막을 갖는 OLED 봉지 방법

To study encapsulation method for large-area organic light emitting diodes (OLEDs), red emitting OLEDs were fabricated, on which LiF and Al were deposited as inorganic protective films. And then the OLED was attached to flat glass by printing method using epoxy. In case of direct coating of epoxy onto OLED by printing method, luminance and current efficiency were remarkably decreased because of the damage to the OLED by epoxy. In case of depositing LiF and Al as inorganic protective films and then coating of epoxy onto OLED, luminance and current efficiency were not changed. OLED lifetime was more increased through inorganic protective films between OLED and flat glass than that without any encapsulation (8.8 h), i.e., 47 (LiF/Al/epoxy/glass), 62 (LiF/Al/LiF/epoxy/glass), and 84 h (LiF/Al/Al/epoxy/glass). The characteristics of OLED encapsulated with inorganic protective films (attached to flat glass) showed the possibility of application of protective films.

Lifetime Amelioration for an AMOLED Pixel Circuit by Using a Novel AC Driving Scheme

This paper describes how a reversed-bias voltage affects organic light-emitting diodes (OLEDs) with green emitting Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :C545T (40 nm). The luminance efficiency and the active area of the test OLED devices are 8-11 cd/A and 2.5 mm × 2.5 mm, respectively. The OLED lifetime under an ac driving scheme can be apparently improved, in contrast with that under a dc driving scheme. Experimental results indicate that adjusting the duty ratio of the ac driving scheme can ameliorate the brightness uniformity; meanwhile, the normalized luminance of a 20% duty ratio after a 3600-s stress is 94.3%, and that of dc stress is 76.4%. Therefore, a novel pixel circuit with three p-type thin-film transistors (TFTs) for an active-matrix OLED application is proposed to ameliorate the brightness uniformity. Variations of the threshold voltage of the driving TFT are compensated for, and the performance and lifetime of the OLED are improved by using an ac driving method. Simulation results indicate that the output current is almost constant over the data range. Furthermore, the current error rate of the proposed pixel circuit is less than 6.5%.

Anode‐voltage‐control circuit for compensation of luminance deterioration

Abstract— An external driving circuit that has realized long lifetime, power‐consumption control, and peak luminance for organic light‐emitting diode (OLED) displays have been developed. This circuit realizes an effective method for constant‐anode‐voltage (CV) driving refered to as clamped inverter (CI) driving. The feature of CV driving is to achieve low‐power consumption compared with constant‐anode‐current (CC) driving and to control the power consumption and peak luminance according to the image because display luminance can be easily changed by controlling the anode voltage. On the other hand, CV driving has the problem that luminance deterioration appears to be serious compared with that of CC driving because the current of the OLED element decreases according to usage time. To cope with this, a lifetime compensation circuit that has increased the anode voltage so that it compensates for the luminance deterioration has been developed. This circuit can compensate not only the decrease in current but also the decrease in luminance at a constant current that CC driving cannot. However, increasing the anode voltage causes an increase in stress on the OLED element. The influence of stress on OLED lifetime was verified. As a result, it was confirmed that this circuit can extend the lifetime by 32% even if the anode voltage is increased, causing stress on the OLED structure.