We measured electric-field-induced fluorescence quenching (EFIFQ) under various temperatures in both undoped and
fluorescent dye-doped tris(8-hydroxyquinoline )aluminum (AlQ3) layers of organic light-emitting devices (OLEDs).
Results show that for a given temperature doped AlQ3 layers demonstrate smaller EFIFQ than undoped ones. The
phenomenon is attributed to the narrower energy band-gap of the guest molecule relative to that of the host material,
which makes it less prone to electric-field-induced dissociation of the excited state. Results also show that for a given
doping condition increasing the temperature leads to an increase in EFIFQ, indicating that the EFIFQ is a thermally
assisted process.
We measured delayed electroluminescence in abrupt heterojunction undoped and doped small molecule organic light emitting diodes (OLEDs) based on NPB and AlQ3 hole and electron transport and emitter molecules, after the excitation currents are switched off and reverse bias applied to the sample. The experiments indicate that delayed light emission is a result of two distinct processes: emissive excited singlet state generation by either triplet-triplet annihilation or recombination of trapped positive and negative charges in the device. Under reverse device bias these two mechanisms have distinctly different signatures. Undoped devices show dominant light emission contribution from triplet-triplet annihilation, while in rubrene and coumarine doped devices delayed light emission comes predominantly from recombination of trapped charge. Therefore these molecules act as recombination centers when doped into AlQ3.
Cathodes of Organic Light Emitting Devices (OLEDs) are typically made of low work function metals, usually resulting in highly reflective back electrodes. In high ambient illumination, the reflective back electrodes reflect the incident ambient light, resulting in a decrease in contrast of the displayed image. We developed a reduced reflectance cathode utilizing a conductive light-absorbing layer made of a mixture of metals and organic materials. Devices utilizing the reduced reflectance cathode, named Black Cathode OLEDs, demonstrate enhanced contrast even in high ambient illumination. In this work, peformance of devices with cathodes containing a mixture of tris(8-hydroxyquinoline)aluminum (AlQ3), Mg and Ag, or a mixture of AlQ3 and Ag is addressed. The studied cathodes demonstrate ~ 9 - 12% sun/eye-integrated reflectance (SEIR), ~8X lower than that of coventional metal cathodes, while device turn-on voltage and stability are comparable. In modified Black Cathode OLEDs, ~1.8% cathode SEIR has been recently realized.
Doping the hole transport layer (HTL) of organic light emitting devices (OLEDs) was found to increase device operational stability. To this effect, the role of 5,6,11,12-tetraphenylnaphthacene (rubrene), a widely dopant for HTLs, in increasing OLED stability has been widely investigated. However, significant disagreements between various explanations for the increased stability, ranging from rubrene being a charge injection promoter, to its being a charge trap, still exist. We conducted an in-depth study on the influence of rubrene doping of HTL on device stability. The study was carried out on OLEDs of structure: indium-tin-oxide (ITO) anode/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) HTL / tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer / Mg:Ag cathode, in which different portions of the HTL were doped with rubrene. Compared to undoped devices, 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, 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 where the whole HTL was doped. In addition, for a fixed thickness of the doped portion, device stability was found to be essentially independent of the thickness of the undoped portion. The results demonstrate that increasing OLEDs stability by means of doping the HTL is associated with changes in bulk HTL hole transport properties rather than interfacial properties, and is consistent with OLED degradation mechanism based on instability of cationic AlQ3 species.
Temperature dependence of electroluminescence degradation is studied in organic light emitting devices containing an emitting layer composed of a mixture of different hole transport molecules and tris(8-hydroxyquinoline)aluminum (ALQ3) electron transport and emitter molecule. The emitting layer is sandwiched between hole and electron transport layers. Devices containing the hole transport molecule N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB), doped with quinacridone (DMQ) green emitter showed remarkable temperature stability. For these devices, a half-life of about 78,500 hours, 18,700 hours, and 8,600 hours can be projected for operating temperatures of 22°C, 70°C and 100°C, respectively, at an initial device luminance of 100 cd/m2. Activation energies for device degradation were determined for devices with different hole transport molecules and it was found that devices with higher activation energy show better high temperature stability. These results are consistent with the recently proposed degradation mechanism based on the unstable cationic AlQ3 species.
Poor device stability has been a major concern for organic light emitting devices (OLEDs). The relatively short operational lifetime of the OLEDs is predominantly attributed to an intrinsic degradation behavior, which leads to a decrease in the electroluminescence quantum efficiency of the devices in time. Recently, we found that the injection of holes in tris(8-hydroxyquinoline) aluminum (AlQ3), the most widely used organic electroluminescent material, is the main factor responsible for the intrinsic degradation behavior in OLEDs. The photoluminescence quantum efficiency of AlQ3 has been found to decrease as a result of predominantly hole current flow. Further studies using time-resolved fluorescence measurements reveal that degradation is also associated with a decrease in the lifetime of the AlQ3 excited states, thus revealing the nature of the degradation products as luminescence quenchers. Various phenomena pertaining to device degradation will be discussed.
The intrinsic degradation of hydroxyquinoline aluminum (AlQ3)-based organic light emitting devices, that leads to the long-term decrease in the electroluminescence efficiency of the devices operated under constant current conditions, has been studied. The role of stabilizing agents, such as introducing a copper phthalocyanine buffer layer at the hole injection contact, doping of the hole transport layer, and using mixed layers of hole and electron transport materials has been investigated. Devices, which allow predominantly holes to be transported through the AlQ3 layer, showed significant decrease in photoluminescence after prolonged current flow. These results lead to the conclusion that the degradation of AlQ3 cations is the major cause of intrinsic long-term device degradation. This mechanism also explains some new results on the degradation of devices containing dual layer and doped hole transport layers as well as the increase in lifetime of devices containing more efficient electron injecting contacts.
We studied photoconductivity in particle dispersions and vacuum evaporated films of benzimidazole perylene (BZP) pigment in contact with a tetraphenyldiamine (TPD) hole transport layer. Xerographic photoreceptors based on evaporated BZP photogenerator layers showed about two times higher carried generation efficiency than photoreceptors containing generator layer in the form of a particle dispersion. Time resolved fluorescence measurements were performed on these two types of structures. Fluorescence time evolution was compared in samples with and without TPD in a polycarbonate overcoat layer. Significantly larger decrease in average fluorescence lifetime was observed in structures containing thin evaporated films compared to the structures with the pigment in the form of particle dispersion. This indicates that reduced carrier generation efficiency in particle dispersion sin a consequence od exciton bulk trapping which significantly reduces the number of excitons reaching BZP/TPD interface. As carrier generation in BZP/TPD system is a result of exciton dissociation at the interface to produce electron-hole pairs. Thus a smaller number of excitons reaching the interface leads to reduction of carrier generation efficiency.
We studied an organic light emitting device (OLED) involving electroluminescence from a mixed layer consisting of a hole transport material (HTM) and an emitting electron transport material (ETM) and including thin electron and hole injection contacts. A naphthyl-substituted benzidine derivative (NPB) and tris (8-hydroxyquinoline) aluminum (ALQ3) are used as the HTM and the emitting ETM, respectively. Following a control-experiment approach, the efficiency and the operational lifetime of OLEDs adopting the new structure are compared to those of conventional bilayer devices made of the same materials and fabricated under the same conditions. Efficiency is calculated from the luminance-current density-voltage characteristics. Lifetime tests are carried at constant current density in dry air. Photoluminescence is used to detect changes in the quantum efficiency of the ALQ3 on mixing with the HTL. Compared to a conventional bilayer device, the new device structure leads to approximately 50 percent higher efficiency and an order of magnitude increase in the operational lifetime. The higher efficiency is attributed to (i) reduced leakage of charge carrier to the electrodes, (ii) exciton confinement away from the metal cathode, and (iii) higher quantum efficiency of the emitting electron transport material due to mixing with the hole transport material. Possible reasons for the higher stability are also discussed.
A recently described scanning stylus instrument1 has been used to investigate the
electrical properties of individual microscopic electrical defects in organic xerographic
photoreceptors. Using a shielded scanning stylus with an effective diameter of 85 pm we
have mapped the shape of individual microscopic electrical defects with 5 im resolution
and measu red thei r cu rrent-voltage characteristics. H ig h resolution ma ps reprod uced the
shape of the stylus indicating that the underlying defect is much smaller than the stylus
size. The observed current-voltage characteristics were approximately quadratic in the
applied voltage and the currents were basically constant on the time scale of tens of
seconds. This observation is an indication of charge injection from the substrate and space
charge limited (SCL) currentflow. The magnitude ofthe current enables an estimate of the
upper limit of the size of the injecting spot of about 4 pm. This conclusion is derived on the
basis of one dimensional SCL current flow and the argument is presented which indicates
thatthe defects may be much smaller, possibly point-like in nature.
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