Lifetime Of Organic Light-emitting Diodes Research Articles

Duty ratio drive prediction model of lifetime degradation for organic light emitting diode-on-silicon microdisplay.

A duty ratio drive prediction (DRDP) model of luminance degradation for organic light emitting diodes (OLED) microdisplay is proposed in this paper. The traditional stretched exponential decay (SED) model is not applicable for OLED driven by duty ratio. The DRDP model introduces the duty ratio as the variables affecting the lifetime of OLED. By fitting the undetermined coefficients with the measured luminance data, the quantitative relationships among the initial luminance, duty ratio, and OLED lifetime are obtained. Meanwhile, the model quantifies the phenomenon of spontaneous luminance recovery, which occurs when OLED switches from bright to dark. Finally, the DRDP model is used to compensate the luminance degradation of OLED driven by duty ratio. The experimental results show that the average prediction accuracy of DRDP model for white, red, green, and blue (W/R/G/B) OLED degradation trend is 0.9623. The average prediction accuracy of W/R/G/B OLED lifetime is 0.6119, which is greater than that of SED model. The lifetime is extended by 89.83% after compensation.

Enhanced Operational Stability by Cavity Control of Single-Layer Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence.

Highly efficient organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have been realized in recent years, but the device lifetime needs further improvement for practical display or lighting applications. In this work, we present a device design principle by tuning the optical cavity of single-layer undoped devices, to realize efficient and long-lived TADF OLEDs. Extending the cavity length to the second-order interference maximum by increasing the emissive layer thickness broadens the recombination zone, while the optical outcoupling efficiency remains close to that of the thinner first-order devices. Such a device design leads to efficient and stable single-layer undoped OLEDs with a maximum EQE of 16%, a LT90 of 452h and LT50 of 3693h at an initial luminance of 1000cd m-2 , which is doubled compared to the first-order counterparts. We further demonstrate that the widely-used empirical relation between OLED lifetime and light-intensity originates from triplet-polaron annihilation, resulting in an extrapolated LT50 at 100cd m-2 of close to 90,000h, approaching the demands for practical backlight applications. This article is protected by copyright. All rights reserved.

Simulated and experimental analyses of the thermal dissipation of organic light-emitting diodes

Thermal management is important for organic light-emitting diodes (OLED). Thermal dissipation will significantly affect the electroluminescence and lifetime of OLEDs. In this work, we established a simple thermal resistance model to calculate the thermal distribution of OLEDs and theoretically simulated the results through COMSOL finite element simulation. We find that the maximum temperature for the circular OLED with a diameter of 400 µm is more than 20 ℃ higher than that of the OLED with a smaller diameter of 200 µm. Moreover, the maximum temperature of the rectangular device is 8 ℃ lower than that of the square device with the same active area. Most interestingly, the temperature at the center of the luminescence is about 12 ℃ higher than that of the edge under the same conditions. The simulation well matches the experiment without requiring the use of fitting parameters. The results represented an important advance in the thermal distribution of OLEDs and the potential design of alternative substrates for thermal management applications.

Lifetime measurement and aging mechanism analysis of OLED subpixels

The lifetime of organic light-emitting diodes (OLED) is of great importance for both display and lighting applications. In order to measure the brightness attenuation of each subpixel on the OLED screen at the same time, an experimental platform was built with a spectrometer and an automatic mechanical system. By testing the brightness decay rate of red, green, blue, and white light from the OLED screen under different initial brightness, combined with data fitting using the extended exponential decay model, the aging mechanism of each subpixel is compared and analyzed. The comparative analysis results show that the lifetime of the OLED obeys an exponential decay distribution, and which of the blue light has the fastest decay rate within a reasonable time range. These testing and analysis methods could provide reference and guidance for OLED manufacturers and users.

Through-Space Conjugated Electron Transport Materials for Improving Efficiency and Lifetime of Organic Light-Emitting Diodes.

Thermally stable electron transport (ET) materials with high electron mobility and high triplet state energy level are highly desired for the fabrication of efficient and stable organic light‐emitting diodes (OLEDs). Herein, a new design strategy of constructing through‐space conjugated folded configuration is proposed to explore robust ET materials, opposite to the widely used planar configuration. By bonding two quinolines to the 9,10‐positions of phenanthrene, two novel folded molecules with high thermal and morphological stabilities and high triplet state energy levels (>2.7 eV) are created. These folded molecules possess excellent ET ability with electron mobilities of three orders of magnitude higher than those of linear and planar counterparts. Theoretical calculation and crystallography analysis demonstrate the through‐space conjugated folded configuration has not only reduced reorganization energy but also enlarged charge transfer integral at various dimensions, bringing about efficient multi‐dimensional ET, independent of molecular orientation. By adopting the folded molecule as ET layers, OLEDs with no matter delayed fluorescence or phosphorescence emitters can achieve high external quantum efficiencies and long operational lifetimes simultaneously. This work paves a new avenue towards robust ET materials to improve efficiency and stability of OLEDs.

Effect of arylamino-carbazole containing hole transport materials on the device performance and lifetime of OLED

We synthesized four hyper-conjugated aromatic hole transporting materials with different molecular geometry and energy levels by attaching arylamino moiety attached to the carbazole core. A brominated carbazole moiety reacted with an arylamino moiety using a Buchwald-Hartwig reaction. The characteristics of these hole transporting materials were investigated using TGA, DSC, UV–Vis and luminescence spectroscopy. The energy levels of all materials used in this study were estimated from cyclic voltammograms and absorption spectra. The hole transporting properties of the synthesized molecules were measured using single-carrier devices. All four hole transporting materials showed similar hole mobility. The effectiveness of hole transporting materials was compared by fabricating green-emitting organic light emitting diode (OLED) devices. It turned out that the device performances were critically dependent on the relative energy levels of the hole transporting layer and emission layer. However, the molecular geometry greatly influenced the device lifetime, determining thermally induced crystallization by the heat produced during device operation. • Hole transporting materials with different molecular symmetry were synthesized. • Hole transporting materials profoundly affect device performances and lifetime. • Device performances were affected by the energy level relative to the emission layer. • Device lifetime is affected by the molecular geometry of hole transporting materials.

Thermally stable inverted organic light-emitting diodes using Ag-doped 4,7-diphenyl-1,10-phenanthroline as an electron injection layer

The thermal stability of organic functional materials affects the performance and lifetime of organic light-emitting diodes (OLEDs). We have developed a thermally stable inverted OLEDs (IOLEDs) by employing silver (Ag) doped into 4,7-diphenyl-1,10-phenanthroline (Bphen) as an n -type doped electron injection layer (EIL). We found that the formation of Ag complexes by coordination reaction could enhance the thermal stability and produce an asymmetric diffraction pattern based on an analysis of grazing incidence small angle X-ray scattering. Interestingly, with the annealing temperature increasing to 100 °C, the electrical properties of electron-only cells show differentiated phenomenon that the current density based on Ag dopant remains basically unchanged, which is opposite to Cs 2 CO 3 dopant. In addition, at the high temperature of 100 °C, the IOLEDs with Cs 2 CO 3 doped Bphen as an EIL was damaged completely, while the Ag dopant-based devices still maintained good photoelectrical characteristics. Finally, we have demonstrated that the optimized IOLEDs achieved a 40.3% enhancement in current efficiency compared to the conventional device. This work provides a new strategy to increase the thermal stability and performance for the application of IOLEDs operated under high temperature. • Ag complexes could enhance the thermal stability and produce an asymmetric diffraction pattern. • At 100 °C, the Ag dopant based IOLEDs devices still maintained excellent photoelectrical characteristics. • The IOLEDs achieved a 40.3% enhancement in current efficiency compared to the conventional device.

Positive isotope effect in thermally activated delayed fluorescence emitters based on deuterium-substituted donor units

Isotope effect (IE) has been widely applied to probe the kinetics and mechanisms of organic reactions, and hole/electron transport behaviors of semiconductors. The subtle isotope substitution on fluorophores could intrinsically influence their luminescence efficiency and the operational lifetime of organic light-emitting diodes (OLEDs). So far, the IE on the photophysical properties, thermal stabilities and electroluminescence performance of thermally activated delayed fluorescence (TADF) emitters has not been reported. In this work, two deuterium-substituted TADF emitters, TPAAP-D and TPAAQ-D, have been designed and synthesized to investigate the IE in TADF. The comparison study showed that the deuterium substitution led to a decrease in the non-radiative decay rate of S1 (KnrS) and a decrease in the intersystem crossing rate constant (KISC), thus enhancing the luminescence efficiency of deuterated TADF materials. The origin of this positive IE was correlated with the increase in normal-mode displacement (ΔQ) and Huang-Rhys factor (Sj) of deuterated TADF. Furthermore, TPAAP-D and TPAAQ-D exhibited remarkably higher decomposition temperatures than their native counterparts. Importantly, TPAAP-D can sensitize a near infrared (NIR) dye (BF) with the electroluminescence (EL) peak at 760 nm (FWHM = 45 nm) and EQE of 2.8%, which was the best result among the reported fluorophore emitters.

Improving lifetime of organic light-emitting diodes using hydrophobic films as passivation layers

In this work, we have optimized the passivation process to improve the lifetime and reliability of organic light-emitting diode (OLED) devices. A hydrophobic polymer (HPP) was used as the passivation layer and characteristics of the optimized OLED devices with the film were investigated. The HPP films exhibited excellent permeating barrier properties toward water because they were hydrophobic and had low surface energy. In addition, in OLEDs, the HPP films prevented darkspot growth on the emitting area.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

Improved Efficiency Roll-Off and Operational Lifetime of Organic Light-Emitting Diodes with a Tetradentate Platinum(II) Complex by Using an n-Doped Electron-Transporting Layer.

The efficiency roll-off and operational lifetime of organic light-emitting diodes (OLEDs) with a tetradentate Pt(II) emitter is improved by engaging an n-doped electron-transporting layer (ETL). Compared to those devices with non-doped ETL, the driving voltage is lowered, the charged carrier is balanced, and the exciton density in the emissive layer (EML) is decreased in the device with n-doped ETL with 8-hydroxyquinolinolatolithium (Liq). High luminance of almost 70,000 cd m−2 and high current efficiency of 40.5 cd A−1 at high luminance of 10,000 cd m−2 is achieved in the device with 50 wt%-Liq-doped ETL. More importantly, the extended operational lifetime of 1945 h is recorded at the initial luminance of 1000 cd m−2 in the 50 wt%-Liq-doped device, which is longer than that of the device with non-doped ETL by almost 10 times. This result manifests the potential application of tetradentate Pt(II) complexes in the OLED industry.

Comprehensive study on operational lifetime of organic light-emitting diodes: effects of molecular structure and energy transfer

Organic light-emitting diodes (OLEDs) have been studied as a key technology for next-generation displays. Although OLEDs with internal quantum efficiencies of approximately 100% have been achieved using phosphorescent or thermally activated delayed fluorescent (TADF) emitters, little is known about the correlation between the molecular structure of emitters and the lifetime of OLEDs. Here we observed the effect of the molecular structure on the lifetime of OLEDs by utilizing the efficient energy transfer from TADF materials to phosphorescent emitters. We found that specific molecular structures are effective in extending the stability of TADF materials and the OLEDs fabricated using these materials.

Controlling Structure and Properties of Vapor-Deposited Glasses of Organic Semiconductors: Recent Advances and Challenges.

The past decade has seen great progress in manipulating the structure of vapor-deposited glasses of organic semiconductors. Upon varying the substrate temperature during deposition, glasses with a wide range of density and molecular orientation can be prepared from a given molecule. We review recent studies that show the structure of vapor-deposited glasses can be tuned to significantly improve the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs). We highlight the ability of molecular simulations to reproduce experimentally observed structures, setting the stage for in silico design of vapor-deposited glasses in the coming decade. Finally, we identify research opportunities for improving the properties of organic semiconductors by controlling the structure of vapor-deposited glasses.

Degradation Behaviors of NPB Molecules upon Prolonged Exposure to Various Thermal Stresses under High Vacuum below 10-4 Pa.

Understanding of the long-term thermal stabilities of organic light-emitting diode (OLED) materials during film deposition is important to accurately identifying their processing windows. The thermal stresses imposed on OLED materials in the evaporation source during the deposition process may cause phase transition and/or degradation of the source materials, which results in variations in their purity and thermal properties, such as the vapor pressure and, ultimately, the device degradation. In this work, we designed a simple and efficient apparatus to determine the long-term thermal stability of OLED materials, which allows prolonged heating of a minimal amount of the sample (∼2 g) for 50 h even under high vacuum below 10–4 Pa where the organic powder samples easily and rapidly vaporized because of exposure to temperature above their deposition temperature. We used this apparatus to evaluate the thermal degradation behaviors of N,N′-bis(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), which is a widely used hole-transporting material in OLEDs, upon prolonged exposure to various thermal stresses. After prolonged heating at 330 °C (380 °C) for 25 h (50 h), the change in purity, mass, vapor pressure, and phase of the heated NPB were analyzed by high-performance liquid chromatography, liquid chromatography–mass spectrometry, thermogravimetric analysis, and X-ray diffraction. The lifetime of OLEDs using the heated NPB was measured to study how the thermally induced material degradation affects the device characteristics. The results showed that the NPB degradation caused by prolonged exposure to 330 °C accelerated over time. In addition, it was confirmed that the degradation products with high molecular weight that form due to exposure to 380 °C was the main cause of device degradation.

Killer impurities in vacuum chamber that affect the lifetime of organic light-emitting diodes

We have evaluated a method to investigate killer impurities in vacuum chambers that affect the lifetimes of organic light-emitting diodes (OLEDs) processed in these chambers. In addition to chambers for the deposition of organic and metal films, an exposure chamber was installed to expose the OLEDs to impurities and residual water in a vacuum chamber during device fabrication. We studied a method to investigate the effects of these vacuum chamber impurities after establishing the reproducibility of the device lifetimes. These device lifetimes were affected by the cleanliness of the exposure chamber. Increased exposure times led to shorter device lifetimes, even if the contact angle in the exposure chamber was reduced to less than 5° using plasma cleaning. Furthermore, the device lifetime did not degrade when the partial pressure of water within the exposure chamber was reduced using a cryotrap. We were also able to evaluate the Kapton tape and vacuum greases that were used and determined whether they affected the device lifetime. These results suggest that the influence of residual water and impurities can be separated and it would then be possible to evaluate the influence of the impurities alone on the device lifetime.

Recent Progress of the Lifetime of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescent Material.

Recently, the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs) have been dramatically upgraded due to development of organic materials and device structure. In particular, an intramolecular or intermolecular complex based on thermally activated delayed fluorescent (TADF) materials has greatly contributed to improving OLED device performance. Although high external quantum efficiency has been the main objective of the development of TADF materials as hosts and emitters, recent interest has been directed towards the lifetime of TADF-material-based OLEDs. For the past several years, remarkable advances in the lifetime of phosphorescent and TADF OLEDs have been made using TADF materials as hosts or emitters in the emitting layer. Therefore, since TADF materials are useful as both hosts and emitters for a long lifetime, this work discusses the recent progress made in developing TADF materials for long-lifetime OLEDs.

Enhancing the Kinetic Stability and Lifetime of Organic Light-Emitting Diodes based on Bipolar Hosts by using Spiroconjugation.

We examined how to enhance the lifetime of organic light-emitting diodes (OLEDs) based on bipolar host molecules ET-HT, where ET and HT refer to electron- and hole-transporting units, respectively, by analyzing their thermodynamic and kinetic stabilities. Our DFT calculations reveal that the thermodynamic stability of ET-HT is determined by that of its anion, which is difficult to improve by chemical modifications of ET and HT. The kinetic stability of ET-HT can be enhanced by the spiroconjugation between ET and HT, which occurs when their π-frameworks are extended and have an orthogonal arrangement. Green OLED devices were fabricated by using ET-HTs with and without spiroconjugation, to find that the device with spiroconjugation has a lifetime that is approximately 6 times longer than the one without spiroconjugation.

The Role of Charge Balance and Excited State Levels on Device Performance of Exciplex-based Phosphorescent Organic Light Emitting Diodes

The design of novel exciplex-forming co-host materials provides new opportunities to achieve high device performance of organic light emitting diodes (OLEDs), including high efficiency, low driving voltage and low efficiency roll-off. Here, we report a comprehensive study of exciplex-forming co-host system in OLEDs including the change of co-host materials, mixing composition of exciplex in the device to improve the performance. We investigate various exciplex systems using 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl-3,9-diphenyl-9H-carbazole, 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9-phenyl-9H-3,9′-bicarbazole, and 2-(3-(6,9-diphenyl-9H-carbazol-4-yl)phenyl)-4-phenylbenzo[4,5]thieno[3,2-d]pyrimidine, as electron transporting (ET: electron acceptor) hosts and 9,9′-dipenyl-9H, 9′H-3,3′-bicarbazole and 9-([1,1′-biphenyl]-4-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole as hole transporting (HT: electron donor) hosts. As a result, a very high current efficiency of 105.1 cd/A at 103 cd/m2 and an extremely long device lifetime of 739 hrs (t95: time after 5% decrease of luminance) are achieved which is one of the best performance in OLEDs. Systematic approach, controlling mixing ratio of HT to ET host materials is suggested to select the component of two host system using energy band matching and charge balance optimization method. Furthermore, our analysis on exciton stability also reveal that lifetime of OLEDs have close relationship with two parameters; singlet energy level difference of HT and ET host and difference of singlet and triplet energy level in exciplex.

Operational lifetimes of organic light-emitting diodes dominated by F\xf6rster resonance energy transfer

Organic light-emitting diodes are a key technology for next-generation information displays because of their low power consumption and potentially long operational lifetimes. Although devices with internal quantum efficiencies of approximately 100% have been achieved using phosphorescent or thermally activated delayed fluorescent emitters, a systematic understanding of materials suitable for operationally stable devices is lacking. Here we demonstrate that the operational stability of phosphorescent devices is nearly proportional to the Förster resonance energy transfer rate from the host to the emitter when thermally activated delayed fluorescence molecules are used as the hosts. We find that a small molecular size is a requirement for thermally activated delayed fluorescence molecules employed as phosphorescent hosts; in contrast, an extremely small energy gap between the singlet and triplet excited states, which is essential for an efficient thermally activated delayed fluorescent emitter, is unnecessary in the phosphorescent host.

Degradation Mechanism and Lifetime Improvement Strategy for Blue Phosphorescent Organic Light‐Emitting Diodes

Providing adequate lifetimes for organic light‐emitting diodes (OLEDs) has been a challenging issue for a long time because of the naturally weak chemical bonds of organic materials that can be damaged during electrical processes that drive light emission. The lifetime of OLEDs has been dramatically extended to the point where commercialization is feasible due to the development of stable materials and device structures that lessen the damage of the organic materials. However, the lifetime of high efficiency OLEDs represented by phosphorescent OLEDs is still short, and this prevents full utilization in red, green and blue colors. The lifetime of the blue phosphorescent OLEDs is particularly short, i.e., less than one tenth of the lifetime of conventional blue fluorescent OLEDs. Therefore, a large increase in the lifetime of the blue phosphorescent OLEDs is essential. In this work, recent results on the degradation mechanism and operational stability of blue phosphorescent OLEDs are reviewed by classifying them into material and device related approaches. In addition, ideal material and device plans to reach the lifetimes required for commercialization in blue phosphorescent OLEDs are proposed.