The defense display science and technology (S&T) program must address problems facing warfighters that the commercial world will not. These problems require the creation of revolutionary display materials and devices, and the invention of visual system technologies. Breakthroughs needed in display technology for defense and security personnel may be organized into the following technical challenge areas: ultraresolution 25 megapixel devices and 300 megapixel systems (wall display systems at monitor pixel density), flexible plastic rollup displays (ultra-compact form factor when not in use), sparse data true-3D monitors (phosphors embedded in special polymer host matrices), virtual image and head mounted systems, wireless wearable video displays with ultra low weight and volume (including batteries), and intelligent displays with embedded chips providing integrated computing and communications functionalities. Organic photonics and electronics are central to the progress in these S&T challenge areas: significant materials progress is required to enable the display device capabilities required. These challenges and the results of a Department of Defense (DoD) Special Technology Area Review (STAR) on Displays are reviewed. A top-level roadmap is provided to summarize the defense and security S&T strategy.

This paper will discuss recent developments in making of passive and active matrix OLED devices on polyethylene napthalate (PEN) which has been improved for the use in display device applications. The improved film properties are meeting many of the desired needs and specifications for display and process applications as they are understood today. Improvements to the film's surface quality, and its inherent properties make PEN a good choice for OLED applications. Processing the plastic substrate through the OLED device making process can contribute to small changes in the substrate dimensions. Photolithography process is a critical part of the OLED device making process, knowing to compensate for substrate size does address technical challenges in overlay registration.

Organic light-emitting diodes (OLEDs) have recently attracted much interest among researchers as well as engineers as promising high quality self-emissive displays for all kinds of portable devices such as cellular phones, personal organisers, etc. While monochrome operation is sufficient for some applications, ultimately multi-color devices such as signs or even RGB (red, green, blue) matrix displays will be requested by the customer in the future. So far, this goal has been achieved with small-molecule devices fabricated by vacuum deposition. In contrast, electroluminescent (EL) polymers, which are commonly deposited by solution processing, seemed to be only poorly suited for this purpose owing to the lack of high-resolution patterning processes. Recent attempts, therefore, focus on the adaptation of common printing techniques such as screen printing and ink jetting, both having severe technical difficulties and drawbacks, such as limited resolution in the former and wetting issues in the latter case requiring extensive pre-treatment of the substrates. We demonstrate the use of a new class of EL polymers, which can be applied similar to a standard photoresist. Soluble polymers with oxetane sidegroups were crosslinked photochemically to yield insoluble polymer networks in the desired areas. The resolution of the process is sufficient to fabricate common pixelated matrix displays. Consecutive deposition of the three colors yielded a RGB device with efficiencies comparable to state-of-the-art EL polymers, even slightly reduced onset voltages, and improved efficiencies at high luminance levels. The improved thermal and morphological stability promises better performance in passive-matrix displays requiring high drive currents. The new method potentially allows efficient manufacturing of high-resolution multi-color polymer-based displays on large area using common lithography techniques.

Development of RGB material has already reached the level of commercial products for full color displays devices such as digital cameras. However, the key challenges that still remain are improving luminance efficiency for lower power consumption and better stability. Applying a co-dopant into the EML is an effective way to adjust device performance. A large proportion of power is consumed in red sub-pixels in full-color displays with RGB sub-pixels. Previously we found that rubrene doping of red emitting layer improved the luminance efficiency and operational stability by enhancing the energy transfer from host material to emitting dopant. To increase the efficiency, a phosphorescent material as a red emitter was studied. We review the requirements for and characteristics of RGB OLEDs and the impact of co-doping systems on the performance of full-color displays.

The status of the development of full-color polymer light emitting diodes will be presented. The focus of current materials research is on state-of-the-art red, green, and blue light-emitting polymers (LEP) with high efficiency, optimum color points, low driving voltages and long lifetimes in devices. A general overview of the progress of red, green and blue LEP lifetimes and efficiencies will be given and compared to requirements for both full-color passive and active matrix-displays for mobile display applications. Further, the status of ink-jet printing of LEPs for the industrialization of full-color displays will be discussed, and a comparison of the performance of spin coated and inkjet printed devices will be presented. In addition, two material-related topics studied recently will be discussed; namely, the lifetime of blue light-emitting devices correlated to processing, anodes, cathodes and the blue polymers themselves, and second, the consequences of pulsed-driving schemes on efficiency and lifetime.

Conjugated dendrimers provide an excellent molecular architecture for tuning material properties for organic light emitting diodes. Here we demonstrate a modular approach allowing highly efficient fluorescent and phosphorescent emissive chromophores to be used to make red, green and blue solution-processed light emitting diodes. The choice of a common dendritic architecture ensures good solubility and film forming properties irrespective of the choice of core unit. In addition, this architecture allows blending of dendrimers with different cores without phase separation. We show that blending provides a simple but powerful way of tuning the color of dendrimer LEDs from deep blue to blue-green, and from green to red with little impact on the device properties.

Top-emitting organic light-emitting diodes (OLEDS) for next-generation active-matrix OLED-displays (AM-OLEDs) are discussed. The emission of light via the conductive transparent top-contact is considered necessary in terms of integrating OLED-technology to standard Si-based driver circuitry. The inverted OLED configuration (IOLED) in particular allows for the incorporation of more powerful n-channel field-effect transistors preferentially used for driver backplanes in AM-OLED displays. To obtain low series resistance the overlying transparent electrode was realized employing low-power radio-frequency magnetron sputter-deposition of indium-tin-oxide (ITO). The devices introduce a two-step sputtering sequence to reduce damage incurred by the sputtering process paired with the buffer and hole transporting material pentacene. Systematic optimization of the organic growth sequence focused on device performance characterized by current and luminous efficiencies is conducted. Apart from entirely small-molecule-based IOLED that yield 9.0 cd/A and 1.6 lm/W at 1.000 cd/m² a new approach involving highly conductive polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS) as anode buffers is presented. Such hybrid IOLEDs show luminance of 1.000 cd/m² around 10 V at efficiencies of 1.4 lm/W and 4.4 cd/A.

Large magnetic field effects (MFE) have been observed in organic light emitting diodes (OLED) based on a bilayer of tris (8-hydroxyquinoline) aluminum (Alq₃) and N,N’-Di(naphthalen-1-yl)-N,N’diphenyl-benzidine (NPB). They consist of an increase in electroluminescence (EL) of a few percent at low magnetic fields followed by a decrease in EL of 20+% at high fields. Associated with these two effects is a decrease in resistance of typically 1-3% as the magnetic field is increased. The magnitude of the high field effect (HFE) varies with temperature and current density, while the low field effect (LFE) survives even when the HFE is not present. The HFE is enhanced at low temperature and/or high current density. These effects are similar to those reported for anthracene single crystals suggesting a large triplet-triplet annihilation (TTA) component for the EL in Alq₃. However, transient EL studies fail to definitively identify a delayed luminescence component with a time scale appropriate for TTA in Alq₃. We discuss this and other questions concerning the origin of MFE in this system.

A new platform for luminescent chemical and biological sensors, integrating the excitation source, an OLED, with the sensing component, is described. The utility of the platform is demonstrated for an oxygen sensor and its potential is demonstrated for antibody-antigen immunoassays. The oxygen sensor is operable in two modes, i.e., photoluminescence (PL) intensity mode and lifetime mode, where changes in the PL intensity and lifetime, respectively, are correlated with the oxygen level. In the lifetime mode, the need for sensor calibration, which remains a challenge in real-world sensing applications, is eliminated. Attributes and issues related to sensor performance, including design and stability, are discussed.

We describe a novel method to measure permeation rates for oxidizing agents with very high sensitivity. The technique is based on monitoring the resistance of a degrading Ca sensor in situ, inside a climate chamber. A sensitivity limit below 10^-6 g/m² day is reported for accelerated measurement conditions of 38°C and 90% relative humidity. The benefits of the method are demonstrated for single- and double-sided barrier foils, and the temperature and humidity dependence of the transport through PET is analyzed in detail. The method is also applied to obtain permeation rates for a barrier-coated substrate after as well as during bending. Theoretical simulations are used to evaluate the influence of a defect-dominated transport mechanism on the experimental results and to model the time evolution of the concentration profile in a double-barrier stack. Implications for the development of barrier-enhanced substrates for flexible OLED applications are discussed.

Thin film barrier coatings for protecting Organic Light Emitting Diode (OLED) displays against the environment are extremely difficult to fabricate. The coatings must have extremely low water/oxygen permeability, no defects, cover several microns of topography, and be applied at temperatures below 100°C in a process that does not compromise the performance of the display. Vitex Systems has succeeded in depositing such coatings using an organic/inorganic, thin film multilayer structure termed Barix encapsulation. In this paper results on encapsulation of OLED test pixels and passive matrix displays will be shown. Lifetime and permeability tests conducted at high temperature and humidity demonstrate that this thin film coating can meet the necessary performance requirements for commercial OLED displays. Processing parameters, layer architecture and manufacturing techniques are analyzed and discussed. Thin film encapsulated displays are used to demonstrate the utility of the encapsulation technique.

High performance electrophosphorescent light emitting diodes (LEDs) were demonstrated by using conjugated polymers, poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), PFO end-capped with hole-transporting moieties (HTM), PFO-HTM, and PFO end-capped with electron-transporting moieties (ETM), PFO-ETM, as the hosts and the organometallic emitter, tris-[2,5-bis-2'-(9,9'-dihexylfluorene) iridium] [Ir(HFP)₃] as the guest. Electrophosphorescent LEDs fabricated from PFO, PFO-HTM, and PFO-ETM as the hosts emit red light with turn-on voltage around 5V, luminances (L) of 2040 cd/m², 1937 cd/m² and 2487 cd/m² at 290 mA/cm² (16 V), and luminance efficiencies (LE) of 1.40 cd/A, 1.38 cd/A and 1.80 cd/A at 4.5 mA/cm² for PFO, PFO-HTM, and PFO-ETM, respectively. The results demonstrate that high performance electrophosphorescence can be obtained from conjugated polymer-based LEDs that are fabricated by processing the active materials directly from solution.

Organic light emitting devices (OLEDs) are viewed as a potential next generation lighting source. Phosphorescent OLED (PHOLED) technology, with its inherently high efficiencies, represents the best opportunity to meet the challenging requirements of lighting. We discuss the requirements of OLEDs for lighting applications and present the state-of-the-art of white PHOLEDs, which have demonstrated the luminous efficiencies exceeding 30 cd/A at CIE coordinates of (0.35, 0.33).

This contribution focuses on triplet emitters. They represent attractive OLED materials, since their efficiencies can in principle be by a factor of four higher than of (small) singlet emitter molecules. On the basis of introductory models, it is discussed how the exciton formation process can be visualized, how the emitter states are populated, and why the excitation energy is finally harvested in the lowest triplet state. Further, it is shown that essential photophysical properties of organometallic emitters depend systematically on the metal participation in the triplet states and on the effective spin-orbit coupling that control the amount of zero-field splitting (ZFS) of the triplet state into substates. Increase of ZFS corresponds to more metal character in the triplet state. High metal character reduces the energy difference between excited singlet and triplet states, enhances the singlet-triplet inter-system crossing rate, lowers the emission decay time, changes the vibrational satellite structure, decreases the excited state reorganization energy, etc. These effects will be discussed by referring to well characterized compounds. Based on a new ordering scheme presented for triplet emitter materials a controlled development of compounds with pre-defined photophysical properties becomes possible.

Effects of the method of preparation of organic thin films and chemical doping on charge injection from electrodes were investigated. It was found that charge injection from electrodes and the performance of organic electroluminescent (EL) devices are affected by the method of preparation of organic thin films, depending on the kind of materials. Organic EL devices using spin-coated films of N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) or N,N'-di(p-biphenyl-4-yl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (p-BPD) as a hole-transport layer and tris(8-quinolinolato)aluminum (Alq₃) as an emitting layer exhibited higher injected current density and luminance than the corresponding devices using vacuum-deposited films of TPD or p-BPD. On the other hand, almost no difference was observed in the current density -- applied voltage and luminance -- applied voltage characteristics between Alq₃-based triple-layer organic EL devices fabricated with the vacuum-deposited and spin-coated films of 4,4',4"-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) or 4,4',4"-tris(2-naphthylphenylamino)triphenylamine (2-TNATA) as a hole-injection layer together with the vacuum-deposited thin film of TPD as a hole-transport layer. An organic EL device using iodine-doped m-MTDATA as a hole-injection layer, TPD as a hole-transport layer, and Alq₃ as an emitting layer operated at a lower drive voltage and exhibited higher external quantum efficiency relative to the corresponding device using undoped m-MTDATA. It is indicated that the use of iodine-doped m-MTDATA enhances not only hole injection from the ITO electrode but also electron injection from the cathode into Alq₃.

Time-of-flight photocurrent (TOF) measurements on the blue emitting electroluminescent polymer poly(9,9-dioctylfluorene) (PFO) show that the room temperature hole mobility can vary from 10^-2 to 5x10^-5 cm²/Vs depending on how the polymer films are prepared. It also undergoes irreversible increases when the samples are annealed. These results can be related to PFOs complex phase behavior and show the importance of understanding and controlling the polymer film structure for device applications. We also present new TOF measurements on the green emitting electroluminescent polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT). Previous TOF measurements have shown that BT exhibits dispersive electron transport and that holes are very heavily trapped, no hole transport signal being measurable using this technique. The new TOF measurements on a recently synthesized batch of BT show less dispersive electron transport with a mobility of 10^-3 cm²/Vs as well as non-dispersive hole transport with a mobility of 2 x 10^-3 cm²/Vs. This new batch therefore exhibits the highly desirable property of both good electron and hole transport in the same electroluminescent polymer. It is proposed that this is due to a change in the molecular weight and/or polydispersity of the material and indicates the importance of further development of relatively well known materials. TOF measurements of the variation of the hole and electron mobility with temperature are examined within the framework of the Gaussian disorder model.

The operating voltage of organic light emitting diodes (OLEDs) is important for the power consumption of active or passive matrix displays since it influences both the power consumption of the OLED itself and the power consumption of the driver circuitry. We have shown that very low operating voltages can be achieved in small-molecule OLED by intentional electrical n- and p-type doping. Even more important than the reduction of the voltage is the fact that doping of the charge carrier transport layers improves charge injection, making it basically independent on the actual contact work-functions. Organic light emitting diodes (OLEDs) with electrically doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100cd/m² already at a voltage of 2.55V (based on a simple singlet emitter system), well below previous results for undoped small-molecule devices. With phosphorescent emitter dopants, high quantum and power efficiency of OLEDs with doped transport layers can be achieved: operating voltages and current efficiencies of 3.1V and 44cd/A (corresponding to approx. 44lm/Watt at 100cd/m²) are reported here. Inverted and fully transparent devices with parameters comparable to standard bottom-emitting OLED have been demonstrated as well.

We present high efficiency and high luminance molecular organic light-emitting diodes (MOLEDs) using a conducting polymer as a hole-injecting electrode (anode), a CsF/Al bilayer as a cathode, and silole derivatives as an emitter and/or an electron transporter. The conducting polymer films, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), were either spin-cast from aqueous dispersions or pre-coated on plastic substrates (courtesy Agfa Gevaert N.V. Belgium). The surface sheet resistance of the conducting polymer films is in the range of 150Ohms/sq ~ 1500 Ohms/sq. MOLEDs fabricated with a low sheet resistance (150 Ohms/sq) conducting polymer as an anode without using an ITO underlayer and CsF/Al as a cathode exhibit very low operating voltages (4.5V @ 100 cd/m² and 6.5V @ 1,000 cd/m²). This good device performance is attributable to the low sheet resistance of the conducting polymer anode and the high electron mobility of the silole derivative, namely 2,5-bis-(2',2"-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), used as an electron transporter. Efficient electron injection from the CSF/Al cathode to the PyPySPyPy electron injection/transport layer also contributes to better charge balance and improved device efficiency.

Polyaniline (PAni) dispersions can be efficiently used as hole injection layers (HIL) for passive and active matrix display applications. In earlier work the influence of conductivity and work function of HILs spin coated from water based PAni/PSS dispersions on device performance had already been presented. Recent investigations on hole transport mechanism in polyaniline systems now show the necessity of a minimum conductivity and an optimum work function for hole injection. Electrochemical Impedance Spectroscopy measurements combined with luminescence investigations showed that the lateral conductivity in the PAni films must be >10^-6 S/cm. Otherwise, a decrease in maximum efficiency and an increase in driving voltage in dependence on coating thickness occurs. Work function investigations on water-free, highly conductive polyaniline dispersions emphasize the theory of an optimum range for hole injection from the anode into the light emitting polymer. The work function of highly conductive, non-aqueous PAni dispersion (0.1-5 S/cm) was determined by Scanning Kelvin Probe method to be 4.5 - 4.7 eV, which is outside of the optimum range at about 4.95 - 5.05 eV for polymeric light emitting diodes, resulting in poor efficiency values (max. 30 - 50% compared to PAni/PSS standard).

Here we report the observation of ohmic hole injection from a conducting polymer anode into poly(9,9-dioctylfluorene) (PFO) in a polymer light-emitting diode (LED) structure. Although initially non ohmic, the contact can be made locally ohmic by electrically conditioning the device at voltages higher than the electroluminescence (EL) onset voltage. The ohmic nature of the contact in selected regions is confirmed by the appearance of dark injection space-charge-limited transient currents, which yield hole mobilities in good agreement with those measured by the time-of-flight method. The appearance of ohmic injection is discussed within a model that assumes the existence of electron traps near the anode interface. When the sample is conditioned electrons are injected from the cathode and are trapped near the anode inducing an interfacial dipole that reduces the barrier for hole injection.

The spectral characteristics of polyfluorene (PF) based light-emitting diodes (LEDs) are discussed. First, conditions that facilitate photo-oxidation of PF are investigated. We show that dense chain packing and addition of hole-trapping moieties lead to increased defect formation. Second, devices containing either a defined low concentration of keto-defects or of the polymer poly(9,9-octylfluorene-co-benzothiadiazole) (F8BT) are presented. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green and blue emission components. It is shown that either blending of hole-transporting molecules into the emission layer at low concentration or the incorporation of a suitable hole-transporting layer reduces the green emission in the PF:F8BT blend, similar to what is observed for the keto-containing PF layer. We conclude that photo-oxidation leads to the formation of keto-defects that mainly constitute weakly-emissive electron traps.

We report systematic measurements of the evolution of the emission characteristics of PFO whilst undergoing photo-oxidation. Pure PFO and highly diluted PFO/polystyrene blended films were prepared for the studies by spin-coating. Each film was oxidized by exposure to the 351 nm line of a cw Ar⁺ laser. Both the kinetics of the various spectral components and the photoluminescence intensity for each film was monitored as a function of oxidation time and their respective behaviors were compared. Our results demonstrate that there is a strong tendency for singlet intrachain excitons initially created on pristine PFO segments to migrate to the fluorenone moieties produced by photo-oxidation. However, we conclusively show that emission from states localized at these defect sites cannot account for the appearance of the broad green emission band (g-band) that is well-known to occur in degraded polyfluorenes. Instead, it is shown that the g-band must emanate from interchain states that are formed after energy has been transferred to the fluorenone moieties (either via energy transfer form non-defective PFO segments or by direct excitation).

A series of soluble arylamine-based hole transporting molecules with a fluorene core and with various ionization potentials have been synthesized. The transport properties of these molecules doped into polystyrene have been measured by time-of-flight experiments and compared to those of analogous compounds with a biphenyl core (TPD). Reorganization energies between the neutral molecules and their cations have been calculated using density functional theory. The effects of bond length and geometry relaxations on the overall reorganization energy in these two classes of molecules are discussed. Molecules from both classes have been doped into polystyrene and used as hole-transport layers (HTLs) in multi-layer light-emitting diodes with the structure ITO/HTL/AlQ₃/Mg:Ag [ITO = indium tin oxide, AlQ₃ = tris(8-hydroxyquinolinato)aluminum]. The electroluminescent properties and lifetime measurements at constant current have been evaluated. Significant variations in lifetime when using different substituents have been observed.

Eastman Kodak Company and SANYO Electric Co., Ltd. recently demonstrated a 15" full-color, organic light-emitting diode display (OLED) using a high-efficiency white emitter combined with a color-filter array. Although useful for display applications, white emission from organic structures is also under consideration for other applications, such as solid-state lighting, where high efficiency and good color rendition are important. By incorporating adjacent blue and orange emitting layers in a multi-layer structure, highly efficient, stable white emission has been attained. With suitable host and dopant combinations, a luminance yield of 20 cd/A and efficiency of 8 lm/W have been achieved at a drive voltage of less than 8 volts and luminance level of 1000 cd/m². The estimated external efficiency of this device is 6.3% and a high level of operational stability is observed. To our knowledge, this is the highest performance reported so far for white organic electroluminescent devices. We will review white OLED technology and discuss the fabrication and operating characteristics of these devices.

OLED technology has improved to the point where it is now possible to envision developing OLEDs as a low cost solid state light source. In order to realize this, significant advances have to be made in device efficiency, lifetime at high brightness, high throughput fabrication, and the generation of illumination quality white light. In this talk, the requirements for general lighting will be reviewed and various approaches to meeting them will be outlined. Emphasis will be placed on a new monolithic series-connected OLED design architecture that promises scalability without high fabrication cost or design complexity.

The accepted model for light emission and propagation in organic LEDs (OLED) which consists of several optically thin functional layers deposited on a thick substrate is a classical dipole located in the emitting layer. The propagation of the emitted light is commonly described by a Fourier expansion of the dipole field into plane waves which represent the various radiating and bound modes of the layered structure in k-space. To calculate the electric and magnetic fields inside and outside the LED an integration over the individual plane waves has to be performed. This entails numerical difficulties which can be overcome elegantly with the so-called Green’s tensor approach for stratified media recently developed by the second author. In our contribution we demonstrate the applicability of this method to the computation of electromagnetic field distributions in organic LED structures. Visualizations of typical field distributions arising from individual dipoles are presented and discussed thus allowing a more intuitive understanding of effects relating to dipole location and orientation and material absorption. Furthermore it is shown that scattering of bound modes by particle like inhomogeneities of the layer stucture can be effectively modelled with the Green’s tensor approach. Visualizations are presented and discussed with regard to increased light extraction.

We have demonstrated a new approach to enhance the light extraction efficiency for organic light emitting diode (OLED). Self-aligned, nanoporous alumina film was used as a quasi-2D photonic crystal to modify the optical wave propagation and coupling of light from the OLED. Waveguiding modes are turned into leaky modes. The experimental results showed an increase of over 50% in the coupling efficiency of the nanoporous device, without affecting the electrical properties of the OLED. We used the effective medium theory to model the optical properties of the nanoporous media and simulated the optical characteristics of the devices.

One of the limitations on OLED performance is the optical extraction efficiency, η_ex, which is the ratio of light generated within the device to light emitted into the ambient. Ideally η_ex is equal to unity. Typical estimates for this efficiency factor in OLEDs range between 0.17-0.5. We present a simple radiative transport model that quantifies the effect of volumetric light scattering on light output in terms of a small set of readily measured parameters. Our methodology is sufficiently general to parameterize and describe many of the light extraction schemes found in the literature. We will present a set of model calculations using parameters typical of many OLEDs, and show that the introduction of light scattering sites within the otherwise transparent substrate can increase light extraction efficiencies by at least a factor of 1.4. We also present experimental data to validate our analysis and demonstrate a high level of agreement between model and experiment.

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.

Tris (8-hydroxyquinoline) aluminum (Alq₃) is a commonly used electron transporting and/or light emitting material in organic light emitting diodes (OLEDs). However, it is well known that Alq₃ is very sensitive to atmosphere exposure and that photoluminescence of Alq₃ films decreases with the time of atmosphere exposure. Degradation is also a serious problem in Alq₃ based OLEDs. Several degradation mechanisms have been identified in these devices, including formation of unstable cationic species due to passage of holes. Therefore, there is lots of interest in improving the stability of Alq₃. We have synthesized Tris (8-hydroxyquinoline-5 sulphonic acid) aluminum [Al(qS)₃] in order to improve the stability. We performed electron spin resonance measurements on Alq₃ and Al(qS)₃ powders. Unlike Alq₃ which exhibited strong ESR signal, Al(qS)₃ produced no detectable ESR signal indicating absence of free radicals in this material. To test the environmental stability of Al(qS)₃ films, we have performed photoluminescence (PL) measurements in humid air at different temperatures and found that Al(qS)₃ exhibits improved stability. After comparing the stability of Alq₃ and Al(qS)₃ thin films, fabrication of the light emitting diodes with Al(qS)₃ emitting layer was attempted in order to compare the performance with Alq₃ based devices.

Transparent conductive indium tin oxide (ITO) films with thin Ni-doped surface layers were prepared for organic light emitting diode (OLED) application. The top Ni-doped ITO surface layer were synthesized using Ni (RF) and ITO (DC) co-sputtering method at 120°C and annealed at 300°C for 10 minutes in vacuum to form a modulated work function layer in contact with the subsequently deposited light emitting organic layers. OLED devices with an Al/Alq3/NPB/Ni-doped ITO/ITO/glass structure were fabricated to investigate the effect of the Ni-doped ITO layer on the characteristics of the luminescence efficiency. The depositions of the Al/Alq3/NPB stacked films on top of the Ni-doped ITO/ITO/glass sample were conducted using thermal evaporation in a cluster tool without breaking the vacuum. Initial results show that the device turn-on voltage decreases from 10 volts to 6 volts and the luminescence efficiency was improved by 36% due to the existence of the Ni-doped ITO layer. It was also found that the optical transmittance of the ITO film decreased with the Ni concentration, resulting in external quantum efficiency deterioration by 3%. It was suspected that the presence of Ni (Φ~5.2eV compared to that of ITO ~4.2eV) on ITO surface decreases the heterojunction barrier height at the ITO/NPB interface, allowing more effective transportation of hole-carriers and hence an enhancement on the external quantum efficiency. However the optical impurity scattering of the Ni atoms in the ITO matrix caused the deterioration of the optical transparency and negative effect on the external quantum efficiency.

Changes in the external quantum efficiency of bilayer organic light emitting devices with layer length have been measured for devices of configuration ITO\TPD\Alq\Mg:Ag with the Alq length varying between 25-200nm. It has been independently concluded for similar devices that the thickness of the Alq layer can be optimised with regard to the external quantum efficiency. However, our simulations of the internal quantum efficiency of this structure with an electrical transport model predict that the internal quantum efficiency is invariant with respect to the Alq layer thickness. We deduce that optical microcavity effects cause the variation in external quantum efficiency. These microcavity effects alter the external efficiency through optical interference and through altering the singlet exciton density profile. A combined electrical-optical model based on our electrical transport model and an optical model has been used to calculate the external efficiency for these devices. We find a clear variation in efficiency with layer thickness, matching the experimentally observed trends.

Optically pumped organic semiconductor thin-films have been processed on first and second order distributed feedback gratings. The organic thin-films were made by co-evaporation of tris-(8-hydroxy quinoline)aluminium (Alq₃) and the laser dye 4-(Dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2). The DFB laser wavelength varied depending on the grating period between 647.8 nm and 668.6 nm for first order operation and between 626.7 nm and 640.1 nm for second order operation. By evaporating the same organic film on both resonator designs we could compare first and second order laser parameters. We measured laser output characteristics and determined threshold energy values for different wavelengths and for first and second order of the Bragg grating. The laser threshold energy of the first order organic DFB laser was reduced by a factor 8 compared to the second order laser. Minimum threshold energy density was measured for a first order sample with 13.8 μJ/cm². Reducing the laser threshold value is especially important for future applications like electrically driven organic solid-state lasers, where it will be more difficult to reach the laser threshold excitation.

We report the design, syntheses, structure-property relationships, and applications of alternating copolymers containing heteroaromatic rings. Polymers with different structures were synthesized by Suzuki couplings of 1,4-phenyldiboronic acid with various dibromoarenes. The effect of heteroatoms was studied by comparing polymers containing biphenyl, bipyridine, bithiazole, or thienyl moieties. The changes in the polymer structure led to changes in singlet and triplet energies, absorption and emission properties, as well as energy transfer parameters. The effect of polymer structure on absorption maxima, emission properties, excimer formation, aggregate formation, as well as singlet and triplet energies were studied. Alternating copolymers of bipyridyl moieties gave rise to blue-emitting polymers with emission maxima at 410nm, where as the use of bithiazole moieties red-shifted the emission. All of these polymers can be used in polymer light-emitting devices as the emitting layers, where thienyl containing polymers can be used as hole transport layers. The polymers transferred energy efficiently to Coumarins and europium complexes, allowing for their potential use in longer wavelength emitting applications. The ability of bipyridine containing polymers for use as metal ion sensors was studied.

Electrophosphorescence tuned from the green to red (522 nm - 650 nm) was achieved from double-layer light emittng devices using osmium (Os) complexes doped blend of either poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD), or poly(vinyl naphthalene) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVN:PBD) as the emitting layer. Blending PVN with PBD greatly suppresses the electromer emission of PVN. The PVN:PBD blend emanates a short wavelength EL emission peaking at around 375 nm, which well overlaps with the absorption spectra of the Os complexes and ensures very efficient energy transfer to the Os complex dopants. PVK:PBD has an EL emission around 450 nm which does not overlap the absorption bands of the osmium complexes and also produces devices of lower efficiency, but PVK is a better transport layer and therefore produces brighter devices. The best external quantum efficiency, of the double-layer devices was 2.2%, with a photometric efficiency of 1.9 cd/A. The brightest device achieved was 1,600 cd/m².

There is a considerable interest in the use of metal centered materials as a light source in the growing field of organic light emitting devices (OLED's). In these devices, a polymeric host matrix containing either a carbazole type polymer or polyfluorene derivatives is used to help facilitate energy transfer to the luminophore. We have shown that by using a gadolinium complex that consist of three equivalents of a chelated dibenzoylmethane b-diketone ligand and one equivalent of a phenanthroline type ligand as a component in the host matrix, the performance of a double layer type OLED is improved. We have studied OLED systems that contain tris chelated europium compounds that contain three equivalents of partially fluorinated β-diketone type ligands and an equivalent of a phenanthroline type ligand. In these devices, the external efficiency has shown a 30-fold increase. We have also shown there is an increase for Osmium based OLED's that use the gadolinium complex as part of the polymer matrix. In these devices, the maximum quantum efficiency increased from 2.1% to a value of 3.8%.

Flexibility is one of the most frequently mentioned advantages if organic light emitting diodes are compared to other display technologies. In this contribution we show how the different functional layers respond to applied mechanical stress. To characterize the intrinsic flexibility of the stacked layers in an organic light emitting diode separately, samples with anode and cathode layers on flexible plastic substrates are investigated separately first. We observe that the ITO can withstand more than 30 000 bending cycles, concave as well as convex, down to a radius of curvature of 8 mm without apparent damage. Furthermore, the operational characteristics of completed flexible organic light-emitting devices built on indium-tin oxide coated poly(ether sulfone) under single bending cycles are investigated. Performance data taken at 15 mm radius of curvature show no influence compared to the non-planar conditions.

Emission properties of Ir(ppy)₃ and of Ir(ppy)₂(CO)(Cl) (ppy^- = 2-phenylpyridinate) are investigated between 1.2K and 300K. Distinct differences are found for the emission spectra, decay behavior, and relaxation dynamics. For both compounds, the emission spectra are broad or only moderately resolved, nevertheless, from time-resolved investigations individual properties of the triplet substates can be deduced. For Ir(ppy)₃, the emission stems from three well separated triplet substates I, II, and III with ΔE_II,I = 13.5 cm^-1 and ΔE_III,I = 83cm^-1. The decay times are τ_I = 145 μs, τ_II = 11 μs, and τ_III = 750 ns. At ambient temperature, all three substates contribute to the emission process, while at 1.5K, the emission results only from substate I. This is due to fast relaxation processes. For Ir(ppy)₂(CO)(Cl), also three substates are identified, but they are only separated by less than 1 cm^-1. Therefore at T = 1.2K, all three substates emit independently with three decay times (τ_I = 330 μs, τ_II = 100 μs, τ_III = 9 μs) due to long spin-lattice relaxation (SLR) times. With increasing temperature to T ≥ 30K and thus growing SLR rates the emission decay becomes monoexponential. In particular, from the amount of splitting (zero-field splitting, ZFS) of the emissive triplet, it is concluded that the emission of Ir(ppy)₃ stems from ³MLCT substates, which result from metal-to-ligand charge transfer (Ir5dppyπ*) states, while Ir(ppy)₂(CO)(Cl) emits from ligand centered triplet substates (³LC) of ppyππ* character. Cyclovoltammetric data are also given and discussed in relation to the spectroscopic data.

The electronic structure of the interface formed by Mg deposition onto 2,5-bis(6’-(2’,2"-bipyridyl))-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) was investigated using ultraviolet, inverse, and X-ray photoemission spectroscopies. PyPySPyPy is of interest for use as an electron injection/transport layer in high efficiency organic light-emitting diodes. Upon deposition of Mg onto PyPySPyPy there is a shift of the occupied energy level structure to higher binding energy, away from the Fermi level, and appearance of two energy levels within the energy gap of PyPySPyPy. The lowest unoccupied molecular orbital is also shifted to higher binding energy.

Electromodulation (EM) spectroscopy has been used to probe the electric field distribution in polymer light-emitting diodes. Below the turn-on bias, the EM spectrum is dominated by electroabsorption of the emissive layer. The electroabsorption signal vanishes at the turn-on bias. Under operation, the EM spectrum is due to excited state absorption from injected charge and bleaching of the ground state absorption of the emissive layer. We conclude that the internal electric field is effectively screened by accumulation of electrons at the anode.