Degradation Of Organic Light-emitting Diodes Research Articles

Critical role of electrons in the short lifetime of blue OLEDs

Designing robust blue organic light-emitting diodes is a long-standing challenge in the display industry. The highly energetic states of blue emitters cause various degradation paths, leading to collective luminance drops in a competitive manner. However, a key mechanism of the operational degradation of organic light-emitting diodes has yet to be elucidated. Here, we show that electron-induced degradation reactions play a critical role in the short lifetime of blue organic light-emitting diodes. Our control experiments demonstrate that the operational lifetime of a whole device can only be explained when excitons and electrons exist together. We examine the atomistic mechanisms of the electron-induced degradation reactions by analyzing their energetic profiles using computational methods. Mass spectrometric analysis of aged devices further confirm the key mechanisms. These results provide new insight into rational design of robust blue organic light-emitting diodes.

Degradation Analysis of Organic Light-Emitting Diodes through Dispersive Magneto-Electroluminescence Response.

Understanding the stability and degradation of organic light-emitting diodes (OLEDs) under working conditions is a significant area of research for developing more effective OLEDs and further improving their performance. However, studies of degradation processes by in situ noninvasive methods have not been adequately developed. In this work, tris-(8-hydroxyquinolino) aluminum (Alq3)-based OLED degradation processes have been analyzed through the investigation of the device dispersive magneto-electroluminescence (MEL(B)) response measured at room temperature. By studying the change in the MEL(B) response during the device degradation under different external stimuli, such as exposing the device to the atmosphere and prolonged illumination by a strong visible light source, we have gained insight into the microscopic spin-dependent phenomena that control the recombination of e-h polaron pairs in the device. We found that the device degradation leads to a shorter e-h polaron lifetime, smaller dispersive parameter, and broader lifetime distribution function that shows increased disorder in the active layer. This study could offer a potential tool that may be beneficial for assessing the degradation of OLED devices based on various active layers.

Triplet-Polaron-Annihilation-Induced Degradation of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are alternatives to today's commercial phosphorescent OLEDs with heavy-metal dopants, but their limited stability remains a key issue. This work provides a quantitative insight into the degradation mechanisms. The authors reproduce the voltage and light output using a recently developed device model, and find that triplet-polaron annihilation is the driving force behind the formation of degradation traps. This understanding leads to the experimental demonstration of enhanced device lifetime, and will provide guidance towards improved stability of TADF OLEDs.

An Overlooked Charge‐Transfer Interaction in the Interfacial Triplet–Triplet Upconversion Process in Blue Organic Light‐Emitting Diodes

AbstractTwo low‐energy triplets can generate one singlet via triplet–triplet upconversion (TTU), and result in an exciton production yield exceeding 25% in conventional fluorescence‐based organic light‐emitting diodes (OLEDs). In most cases, since such low‐energy triplets induce no serious OLED degradation, TTU‐OLEDs are the only commercialized blue OLEDs so far. Herein, it is clarified that the charge‐transfer (CT) interaction at a hole‐transport/emitter‐layer interface is an overlooked pathway to enhance TTU yield significantly. First, a small energy offset at the interface enables the formation of a high‐energy CT exciton. Second, the lower energy triplet state originated from an anthracene moiety in the emitter layer collects the interfacial triplet CT. Third, due to the high‐density interfacial triplets formation, TTU at the interface contributes to the electroluminescence from the emitter layer or blue dopants even at low current density. This finding underlines the important role of the CT interface to exploit the full potential of TTU in pure‐blue OLEDs.

A unified OLED aging model combining three modeling approaches for extending AMOLED lifetime

AbstractAging is still the most challenging issue for organic light‐emitting diodes (OLEDs), which causes the image‐sticking artifacts on active‐matrix organic light‐emitting diode (AMOLED) displays and limits their lifetime. To overcome this demerit, an aging model is necessary to compensate for aging artifacts. In this paper, we present a unified OLED aging model, which combines three feasible modeling approaches of OLED degradation, namely, data‐counting, electro‐optical, and correlation methods. The model can be used to predict the efficiency decay of OLED pixels during operation. It mitigates weaknesses and limitations of each of these three models and deploys their strengths, respectively. In the first aging stage, the data‐counting model is prioritized, and in the later stages, it is calibrated using the correlation model. The dependency of the efficiency decay on the operation point of OLED is covered by the electro‐optical model. The unified model is based on both phenomenal and physical effects. It delivers more reliability to determine an OLED's degradation over a long‐term operation and a wide operation range like current amplitude and/or temperature range. The unified aging model applies to either an analog or a digital driving scheme. A corresponding compensation based on the aging model can be applied for extending the AMOLED lifetime.

Approaches for Long Lifetime Organic Light Emitting Diodes.

Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in‐depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better‐quality OLED materials and, accordingly, longer lifespan devices.

Comprehensive Model of the Degradation of Organic Light-Emitting Diodes and Application for Efficient, Stable Blue Phosphorescent Devices with Reduced Influence of Polarons

We present a comprehensive model to analyze, quantitatively, and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polaron, exciton, exciton-polaron interactions, exciton-exciton interactions, and a newly proposed impurity effect. The loss of efficiency during degradation is presented as a function of quencher density, the density and generation mechanisms of which were extracted using a voltage rise model. The comprehensive model was applied to stable blue phosphorescent OLEDs (PhOLEDs), and the results showed that the model described the voltage rise and external quantum efficiency (EQE) loss very well, and that the quenchers in emitting layer (EML) were mainly generated by dopant polarons. Quencher formation was confirmed from a mass spectrometry. The polaron density per dopant molecule in EML was reduced by controlling the emitter doping ratio, resulting in the highest reported LT50 of 431 hours at an initial brightness of 500 cd/m2 with CIEy<0.25 and high external quantum efficiency (EQE) >18%.

Understanding degradation of organic light-emitting diodes from magnetic field effects

The impact of magnetic field effects on the electroluminescence of organic light-emitting diodes is commonly used to characterize exciton dynamics such as generation, annihilation, and performance degradation. However, interpreting these effects is challenging. Here, we show that magnetic field effects in organic light-emitting diodes can be understood in terms of the magnetic response of device characteristics derived from polaron-pair and triplet exciton quenching processes, such as triplet-polaron interactions and triplet-triplet annihilation. Device degradation shows a clear relationship with the amplitude of the magnetic field effects, enabling non-destructive measurement of the degradation. The results and proposed mechanism provide a better understanding of magnetic field effects on organic light-emitting diodes and device degradation phenomena.

Exciton-triggered luminance degradation of organic light-emitting diodes

Abstract Organic light-emitting diodes (OLEDs) are attractive for display and lighting applications. However, the demand for high stability in outdoor display and lighting requires further material and device developments and theoretical modeling. Here an analytical function for predicting time-dependent luminance loss in OLEDs is derived based on excitonic defect generation and interaction. Despite the simplicity, this function fits extremely well vast amount of experimental data reported by numerous industrial and academic laboratories for a variety of devices including simple bilayer devices and multilayered devices using singlet, triplet, multiple emitters in variety of host:dopant combinations. The uniqueness of the derived function is expected to allow it to be broadly used for understanding root-cause in degrading devices and for modeling OLED lifetime.

Investigation of Luminance Degradation in Organic Light-Emitting Diodes by Impedance Spectroscopy

We characterize the degradation of organic light-emitting diodes (OLEDs) by employing the impedance spectroscopy. An integrated OLED is compared with the hole-only device (HOD) and the electron-only device to understand the degradation mechanism under voltage stress for 12 h in ambient conditions. Changes in impedance observed from the Cole-Cole plot of the HOD reveal that the degradation mechanism of the OLED is attributed to unbalanced recombination resulting from the deterioration of hole injection into the emissive layer.

Bayesian degradation modeling for reliability prediction of organic light-emitting diodes

Simpler degradation models are generally preferred to simplify analytical procedure of failure-time estimation which follows the degradation modeling. However, the luminosity degradation of organic light-emitting diode (OLED) tends to exhibit an initial unstable period followed by stable and more gradual degradation. The degradation mechanisms of OLED luminosity are illustrated via a stochastic two-compartment model. Conjoining the data with prior information accumulated from field testing, we propose two hierarchical Bayesian models to characterize the nonlinear degradation path of OLED: Bayesian change-point regression model and Bayesian bi-exponential model. The hierarchical Bayesian models effectively fit the nonlinear degradation paths of OLEDs. Analytical results of OLED degradation indicate that reliability estimation from the hierarchical Bayesian models can be substantially improved over the log-linear model which has been widely accepted as a degradation model of light displays.

Degradation Mechanisms of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Molecules

Degradation of organic light-emitting diodes (OLEDs) operated continuously at a constant current density is investigated using photoluminescence techniques. The OLEDs contained the thermally activated delayed fluorescence emitting dopant (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). OLED degradation proceeds mainly on the basis of excited-state instability of host molecules rather than processes related to 4CzIPN. Additionally, the electrochemical instability of radical cations and anions influences long-term OLED degradation. The formation of exciton quenchers and nonradiative carrier recombination centers acts to reduce OLED luminance. These findings highlight the need for new host material development to fabricate more stable TADF-OLEDs.

Elucidating the Crucial Role of Hole Injection Layer in Degradation of Organic Light-Emitting Diodes

Although the luminous efficiency has been significantly improved in multilayered organic light-emitting diodes (OLEDs), understanding the major factors that influence degradation of OLEDs remains a major challenge due to their complex device structure. In this regard, we elucidate the crucial role of hole injection layer (HIL) in degradation of OLEDs by using systematically controlled hole injection interfaces. To analyze charge injection dependent degradation mechanism of OLEDs, we fabricate multilayered small-molecule OLEDs with molecularly controlled HILs. Although a reduced hole injection energy barrier greatly improves both a luminous efficiency and an operational lifetime (>10 times) of the OLEDs at the same time, large hole injection energy barrier increasingly aggravates its charge injection and transport during device operation. By using various kinds of nondestructive analyses at gradual stages of degradation, we demonstrate that accumulated charges at interfaces due to inefficient charge injection accelerates rate of device degradation.

Origin of external quantum efficiency degradation in organic light-emitting diodes with a DC magnetron sputtered cathode

This paper characterizes changes in organic light-emitting diodes to better understand the origin of the decrease in external quantum efficiency (ηext) when switching the cathode deposition method from evaporation to DC magnetron sputtering. An increase of driving voltage and a hole-barrier-dependent decrease of ηext when the Al cathode is sputtered suggest that disruption of carrier balance and penetration of holes from the emissive layer (EML) into the electron transport layer (ETL) are significant sources of the device degradation. When the ETL was doped with Li, degradation was suppressed and the increase in driving voltage was drastically reduced although ηext still decreased by 5%–7%. Analysis of the films by time-of-flight secondary ion mass spectrometry indicates that Li diffuses into the EML when Al is sputtered, and Li is shown to act as an exciton quencher that can decrease ηext. Doping of the ETL is also used to significantly suppress the performance reduction with sputtered cathodes even when using a phosphorescent emitter having high ηext.

Physical mechanism responsible for degradation of organic light-emitting diodes

Degradation of organic light-emitting diodes (OLEDs) is primarily due to the formation of the non-emissive centers, and their formation rate is dependent on the temperature. We observed the linear dependence of total luminance on the temperature, and formulated a novel degradation model to present an analytic function for the OLED degradation with physically meaningful fitting parameters. It was found that the stretched exponential decay (SED) behavior of the degradation is mainly due to the temperature dependence of the luminance.

Stability improvement of organic light emitting diodes by the insertion of hole injection materials on the indium tin oxide substrate

The degradation of organic light-emitting diodes (OLEDs) is a very complex issue, which might include interfacial charge accumulation, material diffusion, and electrical-induced chemical reaction during the operation. In this study, the origins of improvement in device stability from inserting a hole injection layer (HIL) at the indium tin oxide (ITO) anode are investigated. The results from aging single-layer devices show that leakage current increases in the case of ITO/hole transport layer contact, but this phenomenon can be prevented by inserting molybdenum oxide (MoO3) or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN6) as an HIL. Moreover, X-ray photoemission spectroscopy suggests that the diffusion of indium atoms and active oxygen species can be impeded by introducing MoO3 or HAT-CN6 as an HIL. These results reveal that the degradation of OLEDs is related to indium and oxygen out-diffusion from the ITO substrates, and that the stability of OLEDs can be improved by impeding this diffusion with HILs.

Analysis of thermal degradation of organic light-emitting diodes with infrared imaging and impedance spectroscopy

We propose a route to examine the thermal degradation of organic light-emitting diodes (OLEDs) with infrared (IR) imaging and impedance spectroscopy. Four different OLEDs with tris (8-hydroxyquinolinato) aluminum are prepared in this study for the analysis of thermal degradation. Our comparison of the thermal and electrical characteristics of these OLEDs reveals that the real-time temperatures of these OLEDs obtained from the IR images clearly correlate with the electrical properties and lifetimes. The OLED with poor electrical properties shows a fairly high temperature during the operation and a considerably short lifetime. Based on the correlation of the real-time temperature and the performance of the OLEDs, the impedance results suggest different thermal degradation mechanisms for each of the OLEDs. The analysis method suggested in this study will be helpful in developing OLEDs with higher efficiency and longer lifetime.

Hole injection polymer effect on degradation of organic light-emitting diodes

Polythienothiophene:poly(perfluoroethylene-perfluoroethersulfonic acid) (PTT:PFFSA) has been used to enhance hole injection into small molecule OLEDs. Compared to devices with polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS) as the hole injection layer (HIL), the OLED using PTT:PFFSA as a HIL gives enhanced efficiency and a slower luminance decay as well as a slower rise in operating voltage. Further studies of capacitance–voltage characteristics reveal that positive trapped charges accumulate in the hole transporting layer during operation. These results thus highlight the significance of hole injection layer to OLED operational stability.

Reducing OLED Degradation Using Self-Compensated Circuit for AMOLED Displays

In this letter, the electric characteristics of the fabricated organic light-emitting diode (OLED) devices are analyzed after a bias stress. Experimental results demonstrate that when a constant voltage is applied to the OLED device, the OLED threshold voltage gradually increases, resulting in the driving current to decay. Therefore, a novel voltage driving scheme for active-matrix OLEDs using low-temperature polysilicon thin-film transistors (poly-Si TFTs) is proposed. This circuit uses three TFTs to increase the aperture ratio of panels and the driving current of the OLED device to ameliorate the luminance drop that is caused by OLED degradation. Simulation results indicate that the proposed pixel circuit has high immunity to VTH variations of the poly-Si TFTs and provides an extra compensation current against OLED degradation.

QM/MM investigation of the degradation mechanism of the electron-transporting layer

The mechanism of charge transfer among tris(8-hydroxyquinolinate)aluminum (Alq3) molecules in the electron-transporting layer (ETL) under amorphous conditions was theoretically investigated using both quantum mechanical/molecular mechanical (QM/MM) calculations and molecular dynamics (MD) simulations. The rate constant of the electron transfer was estimated for the equilibrated structure taken from the QM/MM MD simulations, based on the hopping model and Marcus theory. It was found that the coordination of a (LiF)4 cluster in ETL drastically lowers the energy of the lowest unoccupied molecular orbital in the Alq3 molecule. The small rate constant, namely the slow charge mobility, in ETL is believed to be causally related to the low-lying delocalized unoccupied molecular orbital of Alq3 coordinated by the (LiF)4 cluster. The results suggest that their interaction has a considerable influence on efficiency and is attributed in part to ETL degradation in organic light-emitting diodes.