Stability Of Organic Light-emitting Devices Research Articles

Low-temperature cross-linking of polyethyleneimine ethoxylated using silane coupling agents to obtain stable electron injection layers in solution-processed organic light-emitting devices

Cross-linking of polyethyleneimine ethoxylated using silane coupling agents improved the stability of organic light-emitting devices.

The electrochemical stability of organic materials and their influence on the operational stability of organic light-emitting devices

Organic light-emitting diodes (OLEDs) have drawn great attention because of their potential application in full-color displays and solid-state lighting. The efficiency of OLEDs has been significantly boosted over the last two decades; however, the device stability is still a challenge. As the charge-carrier transport in OLEDs is generally considered as a chain of redox processes between the neutral molecules and the corresponding radical ions, the electrochemical property of the materials should be an important factor associated with the operational stability of OLEDs. In this perspective, we reviewed the recent advances on the electrochemical property of OLEDs materials, focusing on revealing the correlation between the electrochemical properties of OLEDs materials and the operational stability of the corresponding devices. We try to summarize as follows: (1) The electrochemical instability of unipolar materials has been recognized as one of the intrinsic factors leading to the device degradation; (2) The high electrochemical stability of bipolar materials has proved to be conducive to significantly enhance the stability of the corresponding devices, but not guarantee the devices definitely with high stability; (3) The influence of the molecular stability on device stability, and the intrinsic degradations mechanism of OLEDs still remain to be further explored. It is expected that the in-depth knowledge of the materials stability and the degradation mechanisms of OLEDs can help to improve the stability of other organic optoelectronic materials and devices, thus promoting the development of organic electronics and related industries.

有机/有机界面污染对有机电致发光器件稳定性的影响

有机/有机界面污染对有机电致发光器件稳定性的影响

Stability of organic light-emitting device

Organic light-emitting device (OLED) has well-recognized advantages in simple structure, low-driving voltage, flexibility, large area and availablity. It shows tremendous commercial applications in optical communication, information display and solid-state lighting, and has been one of the most attractive projects in optoelectronic information field over the last decade. Since 1987, OLED has rapidly developed, its brightness and efficiency has reached the practical demands. However, one of the main challenges to the industrialization is the stability of the device. In this paper, some of the extrinsic and intrinsic degradation mechanisms in OLEDs are summarized and discussed, such as the dark-spot formation, morphological instability of organic thin film, metal-atom diffusion, Alq3 cationic and positive charge accumulation. After that, we summarize the approaches to obtaining the long lifetime OLED. Finally, some perspectives on the stability of OLED are proposed.

Space charge effects on the electroluminescence efficiency and stability of organic light-emitting devices with mixed emitting layers

In organic light-emitting devices (OLEDs), the decay rate of triplet state population in the electron/hole recombination zone is found to be highly sensitive to space charge densities, providing an avenue for inferring variations in their formation. In OLEDs containing mixtures of N,N′-Bis(naphthalen-1-yl)-N′-bis(phenyl)benzidine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ3) in the emitting layer, optimizing the NPB/AlQ3 is found to reduce hole space charges, and leads to an increase in electroluminescence stability. Conversely, electroluminescence efficiency is found to be only weakly dependent on the mixture composition, suggesting that hole space charges are not effective quenchers of AlQ3 singlet excitons in mixed emitting layer OLEDs.

Improving the stability of organic light-emitting devices using a solution-processed hole-injecting layer

The stability of organic light-emitting devices with a spin-coated film of 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) as hole-injection layer (HIL) was investigated. The lifetime of this device is increased to 40 900 h (with an initial luminance of 100 cd/m 2), which is 2.7 times as large as that of the control device with a vacuum-deposited film of m-MTDATA as HIL. A significant feature with this method is that the performance and the operational stability of the device with spin-coated HIL are little attenuated by the rough substrate coated by the indium-tin oxide film. The surface morphology of the solution-processed m-MTDATA thin film is quite even and uniform, and it acts as a smoothing layer in the device, which leads to the stability enhancement of the device.

Lifetime Improvement of Organic Light Emitting Diodes using LiF Thin Film and UV Glue Encapsulation

This work demonstrates the use of lithium fluoride (LiF) as a passivation layer and a newly developed UV glue for encapsulation on the LiF passivation layer to enhance the stability of organic light-emitting devices (OLEDs). Devices with double protective layers showed a 25-fold increase in operational lifetime compared to those without any packaging layers. LiF has a low melting point and insulating characteristics and it can be adapted as both a protective layer and pre-encapsulation film. The newly developed UV glue has a fast curing time of only 6 s and can be directly spin-coated onto the surface of the LiF passivation layer. The LiF thin film plus spin-coated UV glue is a simple packaging method that reduces the fabrication costs of OLEDs.

Improving the stability of organic light-emitting devices by using a hole-injection-tunable-anode-buffer-layer

Introducing a hole-injection-tunable-anode-buffer-layer (HITABL) at the indium tin oxide anode contact of an organic light-emitting device can finely tune hole injection to establish proper charge balance, thus remarkably improves its operational stability. The HITABL consists of two sublayers: (i) an ∼2.5nm thick metal (e.g., Ca, Mg, or Ag) sublayer and (ii) an ∼10nm thick tetrafluorotetracyanoquinodimethane (F4TCNQ) doped N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine sublayer. Hole injection can be tuned by changing (i) the metal in the first sublayer and/or (ii) the concentration of the F4TCNQ dopant in the second sublayer. The choice of the metal used in the first sublayer and/or the concentration of F4TCNQ in the second sublayer affect the hole-injection efficiency. Therefore, by using the HITABL, one can make the necessary diminutive adjustments to the hole injection of a device and achieve proper charge balance, resulting in a significant improvement in operational stability.

Improving the stability of organic light-emitting devices by using a thin Mg anode buffer layer

Introducing a thin Mg layer at the hole injection contact of organic light-emitting devices remarkably improves their operational stability. Devices in which a ∼2.5nm thick Mg layer is inserted between the indium tin oxide anode and a tetrafluoro-tetracyanoquinodimethane-doped hole transport material layer exhibit a significantly longer lifetime compared to similar devices without the Mg layer. After 600h of operation at a current density of 62.5mA∕cm2 with a 50% duty cycle, the luminance of devices containing the Mg layer decreases by only ∼10% of the initial value. The stability enhancement resulting from using the Mg layer is attributed to improved balance in charge injection at the anode and cathode contacts.

Enhanced stability of organic light-emitting devices fabricated under ultra-high vacuum condition

Studies on the intrinsic degradation of organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinolinolato)aluminum (III) (Alq 3) revealed that the operation stability of the OLEDs depends on the process pressure during device fabrication. Lowering of the pressure resulted in stable devices. In sharp contrast, differences in the initial device characteristics were marginal in all devices. Analyses with a quadrupole mass spectrometer indicated that the primary difference in the pressure during device fabrication was attributable to the amount of residual water. The results show that the degradation of OLEDs is associated with the electrochemical reaction of Alq 3 with water.

Fluorocarbon film as cathode protective coating in organic light-emitting devices

The present work demonstrates the use of fluorocarbon (CFx) film as a cathode protective layer to enhance the stability of organic light-emitting devices (OLEDs). Devices with the CFx cathode protective layer showed a threefold increase in operational lifetime, about 10000h, comparing to those without the CFx layer. Growth of dark spots was significantly suppressed. The superior moisture-resistant capability of the CFx is attributed to its hydrophobic nature and low moisture permeation. As most OLED production lines are already equipped with CFx deposition facilities, the present cathode protective coating can be easily adopted for further stability enhancement without additional capital investment.

Improved performance and stability of organic light-emitting devices with Al–Cu alloy cathode

We have developed a stable green organic electroluminescent (EL) device by using an Al–Cu alloy as a cathode, which has better performance and reliability than the device with an Al cathode. The device with an Al–Cu alloy cathode achieved an EL efficiency of 3.78cd∕A(2.18lm∕W) at 20mA∕cm2 and the operating voltage was 5.46 V, while device with Al cathode had an EL efficiency of 3.1cd∕A(1.34lm∕W) and 7.3 V at the same drive condition. The Al–Cu device achieved a 20% decay lifetime (t80) of 1234 h at an initial brightness of 756cd∕m2, which is twice the lifetime of conventional device with the state-of-the-art LiF∕Al cathode.

Improved performance and stability of organic light-emitting devices with silicon oxy-nitride buffer layer

The use of silicon oxy-nitride (SiOxNy) as an anode buffer layer in organic light-emitting devices (OLEDs) with a configuration of indium tin oxide (ITO)/SiOxNy/α-naphtylphenyliphenyl diamine (NPB)/8-hydroxyquinoline aluminum/Mg:Ag has been studied. With a SiOxNy buffer layer several angstroms thick, the device efficiency increased from 3.0 to 3.8 cd/A. The buffer layer also protected the ITO surface from contamination due to air exposure. Upon exposing the cleaned ITO substrate to air for one day before device fabrication, the device current efficiency and turn-on voltage degraded to 2.1 cd/A and 4.3 V, respectively, from 3 cd/A and 3.3 V for the device fabricated on an as-cleaned ITO surface. In contrast, devices prepared on air-exposed SiOxNy/ITO surface had almost the same current efficiency (3.85 cd/A) and turn on voltage (3.7 V) comparing to devices (3.8 cd/A and 3.7 V) fabricated on freshly prepared SiOxNy/ITO surface. The results suggested that SiOxNy is a promising anode buffer layer for OLEDs, for both efficiency and stability enhancements.

Study of organic light emitting devices with a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped hole transport layer

We investigated the stability of an organic light emitting device (OLED) with structure of indium–tin–oxide (ITO) anode/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine hole transport layer (HTL)/tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer/Mg:Ag cathode, in which different portions of the HTL were doped with 5,6,11,12-tetraphenylnaphthacene (rubrene). Compared to undoped devices, the stability of OLEDs in which HTL doping was limited to only a thin interfacial layer at either the ITO or AlQ3 interface was essentially the same, whereas the stability of OLEDs in which a substantial portion of the HTL was doped was about an order of magnitude higher, and approached that of devices in which the whole HTL was doped. The color of the emission depended only on the material in immediate contact with AlQ3. The results demonstrate that increasing the OLED’s stability by means of doping the HTL is associated with changes in bulk HTL properties rather than interfacial properties, and is consistent with the OLED degradation mechanism based on the instability of AlQ3 cationic species.