In surface emissive type devices, spatial distribution of emissive patterns is quite important from two viewpoints: first, emissive colors are highly dependent on the location of emissive sites in thin film EL devices and careful design structures is needed for tuning emissive colors. Second, for the evaluations of absolute EL efficiency, the spatial distribution of emissive light are indispensable, although Lambert rule is usually accepted without enough experimental evidence. In this report, we evaluated both spatial distribution of emissive patterns and emission spectra at each emissive direction in the standard type TPD/Alq devices and related two-layer type devices with systematically varied layer thicknesses.

We report on a tunable laser based on a conjugated polymer blend system and investigate the underlying gain mechanism. A solid polymer blend consisting of the conjugated polymer poly(phenyl-p-phenylenevinyene) dispersed into an inert matrix of polymethylmetacrylat is examined. Emission line narrowing which can be attributed to amplified spontaneous emission (ASE) is observed at high excitation densities. Placing a block of the blend system into an external resonator yields true laser emission. Emission linewidth and peak intensity show a clear threshold behavior. The laser emission is highly collimated, coherent, and highly polarized. It can be tuned over a range of 300 meV. Gain spectra indicate that the gain mechanism can be explained within the molecular model for conjugated polymers in close analogy to the gain mechanism well known from dye lasers. Thin films of a methyl-substituted ladder-type poly(p- phenylene) are examined to check if the result obtained from a diluted system also hold for neat films. ASE is observed upon increasing the pump intensity. Additionally, quasi- resonant, spectrally very narrow emission lines can be observed. These emission lines are energetically offset from the excitation laser by energies corresponding to well known molecular vibrations. This confirms the previous assumption that the gain mechanism in conjugated polymers is linked to molecularly excited states.

We present data on the absolute photoluminescence quantum yield (phi) _PL, for a set of pure and molecularly doped organic solid films. The procedure uses an integrating sphere to provide accurate measure of the photoluminescence efficiency for solid, sub-micron thickness, films. Host materials include a common hole transport compounds, N,N- dipheny-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine, TPD, and two metal chelates used as electron transport and/or light emitting materials, tris (8- hydroxyquinolinolato) aluminum (III), Alq₃, and one of its methyl derivatives, tris (8- trimethylhydroxyquinolinolator) aluminum (III), Almq₃, Tetraphenylnapthacene, or rubrene, is used as the dopant. A substantial increase in (phi) _PL is measured with respect to the pure host. For example, the measured (phi) _PL increases from 0.25 and 0.40 for pure Alq₃ and Almq₃, respectively, to near unity upon doping with rubrene at approximately 1 mol percent. The data are discussed within the framework of Foerster energy transfer.

The high luminescence efficiencies and significant blue shift of the 2,3-disubstituted poly(1,4-phenylene vinylene) polymer 4 have prompted further investigation, and in this paper the synthesis and characteristics of the homopolymer 9 and copolymers 10 and 12 are described. Semiempirical calculations and single x-ray crystallography offer further insight into the explanation of the properties of this class of polymers. A promising organic semiconductor 15 has been prepared and used as the active layer in a thin film transistor. This material exhibited excellent device characteristics, including a field effect mobility of 0.02- 0.05 cm² V^-1s^-1 and a high On/Off ratio.

Homologous series of mixed oligoheterocycles based on end- capped oligothiophenes ECnT 1 were synthesized by introduction of electronegative heteroatoms like oxygen and nitrogen into the conjugated (pi) -system. This led to novel structures 2-11 in which thiophene units of the parent compounds are substituted by other heterocycles with more pronounced acceptor character. Other moieties like phenylsubstituted thiophenes, benzo(c)thiophene, and spiro- bithiophenes have also been implemented resulting in oligothiophenes 12-16. The characterization of the optical and electrochemical properties clearly reveals the influence of the heteroatoms on the electronic properties. Thus e.g., due to the electron withdrawing character of the central heterocycle oxidation of the oligomer is rendered more difficult while reduction is facilitated. In some cases, a hypsochromic shift of the longest wavelength absorption and emission is observed and additionally a significant enhancement of the fluorescence quantum yield in solution and in the solid state. The HOMO/LUMO energy differences determined from the optical measurement correspond qualitatively well with the values obtained from electrochemical data. First experiments on single layer organic light emitting diodes show that these mixed oliogoheterocycles can be used as emitting materials.

This paper reports the synthesis as well as electrochemical and optical characterization of six highly luminescent TPA- based molecules, TPA-X-TPA, where X represent the vinylene and vinylene-arylene-vinylene units and examines their use as the light-emitting dopant of a composite emission layer in trilayer OLEDs. The device configuration ITO/HTL/EML/ETL/Ca/Al is based on a double heterostructure, where HTL is the hole-transport layer, ETL is the electron- transport layer, and EML is a composite film of the TPA-X- TPA dopant dispersed in an ET matrix. This composite EML is prepared by a vacuum codeposition technique. A variety of TPA-X-TPA doped OLEDs were fabricated and characterized by EL spectra and luminous efficiencies. We found that the EL- emission arise from the dopant molecule, blue EL was obtained with a luminous efficiency of 0.3 1m/W and a luminance of 50 cd/m² at 7V device voltage. In comparison with 1,4-bis(4'-diphenylamino-styryl)-2,5- dimethoxybenzene as the dopant molecule strong green EL was observed.

New structures for blue light emitting amorphous materials, based on an extended spiro-structural concept are reported. The chromophores are essentially based on p-oligophenyl chromophores, with further improved optical and morphological properties. The photoluminescence quantum efficiency of the blue emitter in amorphous solid state reach about 70 percent. New oxadiazone based electron transport materials are presented, where triptycenes have been used as new structural motif to increase the morphologic stability and preserve the electronic properties of the parent compound. Both, the emitting material and the electron transport materials can be processed into thin amorphous films with high morphologic stability by vapor deposition as well as by spin-coating from solutions in organic solvents.

A series of tris-chelated polypyridylruthenium (II) complex- containing polyurethanes has been synthesized and evaluated as emitters in solid-state thin film devices. These new materials were found to give rise to luminance levels of about 200 cd/m² with external quantum efficiencies in the 0.5-1 percent range. Transient capacitance and conductance measurements were made to elucidate the origin of the slow device response times of these materials. Much faster-responding devices were fabricated by depositing a ruthenium complex onto a hole-transported layer of poly(p- phenylene vinylene).

We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light emitting devices based on poly(phenylene) type materials as the hexaphenyl and the methyl substituted laddertype poly(para phenylene) (m-LPPP). The RGB-devices are fabricated with an external color conversion technique based on PHP, whereas the white light emission is generated by an internal excitation energy transfer from the blue m-LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present photophysical properties, like spectral line-shape site selectivity of photoluminescence (PL), and electroluminescence of bulk poly(para-phenylenevinylene) PPV films and isolated PPV chains incorporated into a self- assembled matrix material, which leads to the formation of a regular hexagonal array of channels with a diameter of about 15 angstrom, in which the conjugated polymer chains are contained. The structure of the nano-composite in organic- light-emitting-diodes. A suitably structured m-LPPP waveguide shows a spectrally very narrow high directional blue-green light output when optically pumped. The high optical gain of m-LPPP is a results of the spectral separation of stimulated emission and photoinduced absorption bands, thus spectral narrowing is even observable in below cut-off waveguides. Under resonant excitation conditions, we observe strong stimulated Raman scattering.

We present the first comprehensive study of the comparison between electroluminescence (EL) quantum efficiency and absolute photoluminescence (PL) quantum yield in various host-guest systems. We find that the maximum quantum yield of solid composite films primarily depends on the quantum yield of the guest molecule itself. In contrast, the maximum quantum efficiency of multilayered devices depends on both guests and hosts. Differences between the maximum quantum efficiency and quantum yield are discussed in terms of the carrier recombination process leading to the creation of the dopant exciton. We also find that in some cases doping prevents exciplex formation and leads to an increase in EL efficiency. This is attributed to rapid energy transfer form the host to the guest molecule followed by efficiency radiative decay.

The current-voltage characteristics of ITO/polymer film/Al or Au devices of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer can be fitted at high applied bias to a power law of the form J equals KV^m where m increases with decreasing temperature, log(K) is proportional to m, and K is proportional to d^-(alpha m) where d is the film thickness and (alpha) is a constant. (alpha) 2 and 1 for the Al and Au cathode devices respectively. Different single carrier space charge limited conduction (SCLC) theories, including either an exponential trap distribution or a hopping transport field and temperature dependent mobility, are used to try and explain this behavior. Both models are in good agreement with the general experimental results, but can also be criticized on a number of specific issues.Mixed SCLC models and the effect of dispersive transport are also explored. It is concluded that carrier mobility and trap measurements are required to distinguish between these models. To this end, initial trap measurements of ITO/PPV/Al devices using deep level transient spectroscopy (DLTS) are reported. Very deep positive carrier transport with emptying times > 4 minutes have been detected. The non-exponential DLTS transients have been successfully modeled on an isoelectronic trap level emptying to a Gaussian distribution of transport states, with a trap depth and density of 0.8eV and 4 by 10¹⁶ cm^-3 respectively.

Efficient conversion of electrical to optical energy in organic light-emitting diodes (OLEDs) depends on balancing the flux of holes injected at the anode with that of electrons at the cathode. In this paper, we discuss several concepts related to optimizing the power efficiency of OLEDs, and put them in the context of analytic and numerical models for OLED operation. A simple argument is used to relate the charge injection rate from each electrode to measurable properties of the organic layer, deriving he equivalent of the Richardson-Dushman equation for the metal- organic interface. We discuss the role of charge density in dictating the importance of both space charge effects and recombination. These ideas are illustrated with experimental data form device structures which exemplify the various types of behavior predicted.

The light emission from bi-layer organic devices (OLEDs) has been shown to be proportional to the current. Trap states have been speculated to contribute to the carrier transport in such devices. We will report on the bulk trap state properties of naphthyl-substituted benzidine derivative (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq₃) using thermally stimulated luminescence (TSL). Using a general order TSL expression, the four peaks in NPB were modeled with trap sates centered from 0.15 eV to 0.02 eV. The main Alq₃ peak is modeled as a distribution of trap states from 0.25 to 0.15 eV with two additional peaks observed at lower trap energies. For both materials, the trapping mechanism involves a combination of first and second order emission. Using TSL,the evolution of the trap states in Alq₃ has been studied as a function of coumarin 6 and NPB doping,k with doping levels from 0.1 percent to 2.0 percent. For Alq₃ doped with coumarin 6, we observe an almost 0.1 eV increase in the width of the trap states. Conversely, the Alq₃ samples doped with NPB do not show a change in the trap states. These trap depths are sufficient to support a trap charge limited model for the carrier transport in bilayer organic based light emitting diodes.

Charge transport in the glassy state of a variety of low molecular-weight organic compounds has ben studied. The hole drift mobility of the molecular glass was found to be of the order of 10^-6 approximately 10^-2 cm²V^-1s^-1 at an electric field of 1.0 by 10⁵ Vcm^-1 at room temperature, being higher than those of polymers and molecularly-doped polymer systems. The electric-field and temperature dependencies of charge- carrier drift mobility were analyzed in terms of the disorder formalism. The negative electric-field dependence of charge-carrier drift mobility was observed for the first time for the molecular glass. The relationship between molecular structure and charge-transport properties is discussed.

We have investigated the hole injection characteristics from indium tin oxide (ITO) into 4,4',4''-tris[N,-(3- methylphenyl)-N-phenylamino] triphenylamine (m-MTDATA) and have measured the hole carrier drift mobility of this compound in single-layer ITO/m-MTDATA/Au structures. We have found that ITO is able to provide trap-free space-charge- limited currents over a wide range of film thicknesses and have established unambiguously that the ITO/m-MTDATA is an ideal ohmic contact at high electric fields. The drift mobility of the m-MTDATA molecular glass was found to be electric field dependent and a negative field dependence was detected at fields lower than 1 by 10⁵ V/cm. Our observations clarify the role of m-MTDATA as a voltage- lowering hole-injecting buffer layer in organic light- emitting diodes.

We describe transient photocurrents across organic interfaces. Two well known hole transporting molecular solids are used in this study. The energetics are evaluated by electrochemical measurements in solution and photoelectron spectroscopy of solid films. Charge injection is complete and without apparent delay when favored energetically. When injection is energetically unfavorable, some carries are injected very rapidly, and the rest very slowly, relative to a time scale of a few microsecond(s) . This dichotomy may be connected with a tendency to hop along the interface repeatedly before crossing it. Roughness of the interface or mixing of the two materials there may exacerbate the delay of injection for carriers that make many such 'lateral' hops.

We present a microscopic theory of charge transport in conjugated polymers based on polaron drift along polymer segment and on polaron hopping between segments. We show that this model is able to reproduce the current-voltage characteristics of 'good' hole-only devices based on PPV in the whole range of fields. We also present a microscopic theory of electrode injection into conjugated polymers. We show that this model is able to reproduce the current voltage characteristics of contact limited devices based on PPV.

The electronic structure and chemical properties of organic/organic and organic/metal interfaces involving molecular semiconductors are investigated via photoemission spectroscopy. The alignment of electronic levels, electron and hole injection barriers, and interface dipoles are measured for each interface. Chemical reactions and interdiffusion dominate metal-on-organic contacts, whereas organic-on-metal and organic/organic interfaces are more abrupt. The rule of vacuum level alignment, expected to hold for organic molecular interfaces, breaks down for all metal/organic and several organic/organic interfaces, showing that electronic gap states and other interface effects cannot be neglected at these interfaces.

Bilayer organic light emitting diodes (OLEDs), based upon vacuum deposited molecules, or single layer OLEDs, based upon spin-cast polymeric materials, doped with these same molecules, produce light from emissive states of the lumophores which are created through annihilation reactions of radical species, which can be modeled through solution electrochemistry. Difference seen in solution reduction and oxidation potentials of molecular components of OLEDs are a lower limit estimate to the differences in energy of these same radical species in the condensed phase environmental. The light emitted from an aluminum quinolate (Alq₃)/triarylamine (TPD)-based OLED, or an Alq₃/PVK single layers OLED, can be reproduce from solution cross reactions of Alq₃/TPD⁺. The efficiency of this process increases as the oxidation potential of the TPD increases, due to added substituents. Radical cations and anions of solubilized version of quinacridone dopants (DIQA) which have been used to enhance efficiencies in these OLEDs, are shown to be electrochemically more stable than Alq₃ and Alq₃, and DIQA radical annihilation reactions produce the same emissive state as in the quinacridone-doped OLEDs. Electrochemical studies demonstrate the ways in which other dopants might enhance the efficiency and shift the color output of OLEDs, across the entire visible and near-IR spectrum. Chemical degradation pathways of these same molecular components, which they may undergo during OLED operation, are also revealed by these electrochemical studies.

Efficiency of organic light emitting devices can be increased by separating the zones of exciton recombination from the electrodes by the insertion of additional organic layers which act as protecting - and as hole or electron blocking layers. We realized such polymer hetero-layer structures by combination of hole transporting and emitting materials like polyparaphenylenevinylene (PPV) or its derivatives with new electron transporting materials, i.e. heterocyclic polymers and heterocyclic low molecular compounds, especially phenyl quinoxalines. Current-voltage molecular compounds, especially phenyl quinoxalines. Current-voltage and current-luminance characteristics were used to study the prepared heterolayer devices. Optical spectroscopy as well as UV photoelectron spectroscopy were used to characterize the electronic structure of the individual materials. Quantum chemical calculations completed the spectroscopic studies and supported the interpretation of experimental findings. Double layers made of PPV and polyphenylquinoxaline (PPQ) are characterized by low onset voltages of about 2.2 V, high efficiency, and high brightness reaching values of more than 2000 cd/m² at a driving voltage of 10 V. The experimental findings show that PPQs are promising materials for organic electroluminescence applications.

Several issues related to the organic electroluminescence devices are reviewed. Numerical simulation results are presented for the trap-charge limited conduction processes. Detailed band and charge profiles reveal the physics governing the carrier conduction processes. The numerical model is also applied to the case of doping and various possible effects are studied.

We present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using a hybrid inkjet printing technique. In this demonstration, the polymer buffer layer is a wide bandgap, blue emitting semiconducting polymer (PPP-NRt₃⁺), prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconductor polymer, (MPS-PPV) which was printed onto the buffer layer.When a proper solvent was selected, MPS-PPV diffused into the buffer layer and efficient energy transfer took place from the PPP-NEt₃⁺ to the MPS-PPV generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for fabricating polymer multicolor displays with high-resolution, lateral patterning capability.

In this paper we will focus on the various issues which reduce the power efficiency of a complete display system vs. that of a single isolated organic LED, and then discuss the impact of these issues on display integration and design. Critical issues are the necessity of an active matrix design for high definition displays, and the desire for a power- efficient approach for full color. Both dry-etching and ink jet printing will be described as options for achieving patterned films.

Several groups have been investigating the possibility of using 1D photonic band gap (PBG) structures to make better reflectors. It was found that by using alternating layers of metal and dielectric, the reflectivity could be enhanced over the bulk metal for a narrow range of frequencies inside the gap. Alternately, we have recently demonstrated that these periodic metal/dielectric PBGs (MD-PBGs) have an unusually high transmittance in the pass band. It is possible to have a MD-PBG containing a total amount of metal of more than five optical skin depths thick, and yet maintain a transmittance of > 60 percent over the entire visible range of wavelengths. Due to the high conductivity of metals compared with semiconducting indium tin oxide films, the overall figure of merit for MD-PBGs can be much larger. In addition, the transmission band for the MD-PBGs can be designed to filter unwanted wavelengths and provide optimum color control.

Multicolor organic electroluminescent QVGA display device shave been developed through application of patterned lateral three-color electroluminescent layers which are driven by the single matrix method. The devices have achieved almost practical level in their performance. The organic electroluminescent layers and the metal cathodes have been fabricated using metal shadow masks. Further examination of the panel fabrication process and the organic materials has been made, and the results have been applied to the display devices. The organic electroluminescence device structure of the display panel is a four-layer stack consisting of a hole injection layer, hole transport layer, light emitting layer, and electron transport layer. The display pixel consists of three color elements which are green, orange, and blue. The green and blue emitting materials and the electron transport material have been newly developed. The display panel has been encapsulated by a glass cap and driven by the single matrix method. At the Japan Electronics Show in October 1997, the early stage sample of this display device has been demonstrated at video rate moving pictures. After then several improvement have been achieved on the driving technology and on the panel fabrication technology and the display succeeded to perform at 80 cd/m² of maximum luminance. In this paper,the fabrication and operating method of the electroluminescent panel are reported.

One of the themes that has emerged in the field of organic light emitting devices (OLEDs) is the importance of developing materials that form robust films, capable of undergoing wet and dry processing conditions. It is through such characteristics that polymers may have unique opportunities in the effective and inexpensive study of novel single and double layer thermosetting and solvent soluble polymer light emitting devices based on triarylamines for hole transport layer and compounds possessing the blue emitting 9,9-dialkylfluorene unit for the emitting and electron transport layer. These devices possess high thermal stability, high quantum efficiency, and high bandgap emission. We fabricated dot matrix displays based on analogs of these materials where cathode separation was achieved 'in situ' by a resist structure with reentrant wall profile that was built on top of the EL layers. This is a demonstration that polyfluorenes are robust and compatible with standard resist processing.

In the past several years, many research groups have been working on the engineering of organic light-emitting devices (OLEDs) into emissive displays. One of the major manufacturing challenges is that vacuum-deposited, low- molecular-weight organic materials are not very resistant to thermal and chemical processing, and it is therefore problematic to pattern them using standard masking and etching techniques. This has resulted in a substantial amount of the display design being manufactured into the substrate prior to the substrate being coated with the multilayer film of organic materials. In this paper an analysis of the various anode-on-substrate configurations which may be employed for OLED displays will be described and several high-work-function anodes as substitutes for the standard OLED anode of indium-tin oxide discussed.

A novel method for fluorescence anisotropy using incoherent laser light is presented. The incoherent upconversion technique to measure picosecond molecular motion of rhodamine 6G is investigated experimentally.

We report a series of polymers with linear, macrocyclic and hyperbranched structures for electroluminescent (EL) applications. The polymers are polycarbazoles (PC) containing different substituents. The polymers are amorphous and are soluble in common organic solvents such as chloroform, and tetrahydrofuran (THF). High optical quality films were obtained by spin-coating from the polymer solutions of chloroform or THF. All these polymers show strong photoluminescence under a UV-lamp illumination. Single and bilayer EL devices consisting of anode/hole transfer layer/electron transfer layer/cathode have been fabricated and characterized. The effects of polymer structures on the energy levels and EL properties are discussed. The results indicated that macrocyclic oligomers and hyperbranched polymers are new candidates for EL devices.

CdSe nanoparticles were synthesized using microemulsion route, which surfaces were modified in-situ with ambiphylic species. The structural defects could be removed relatively and the broad particle size distribution could be narrowed efficiently by refluxing the samples a mixture of toluene/methanol. Several methods, such as optical absorption, luminescence, and transmission electron microscopic were used to characterize these nanoparticles. Single-layer electroluminescence devices of the nanoparticles were fabricated by spin coating techniques. Electroluminescence from CdSe nanoparticles shows obvious difference with photoluminescence, indicating the different emitting states.

Bicarbazyle's are stable and small organic molecules with external quantum yields of 0.1 percent in single layer devices. Devices from (EtCz)₂ emit in the violet-blue spectral region. Multilayer configuration with transport layers show a significant higher brightness and quantum efficiency and addition of (OcCz COOH)₂, improves the device stability. We find that the current voltage characteristics of (EtCz)₂ is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. Although the applicability of the polaron concept to small molecules is somewhat questionable, the excellent result obtained with the model reveal the importance of molecular relaxation processes during the jump of the carrier.

Dye TPB has been successfully introduced into polymeric multilayer films by means of LB technique without any chemical modification. X-ray diffraction and optical absorption data indicate that the films have ordered structure with a period of about 5.8 nm that is similar to a superlattice. The TPB doped polymeric LB films have also been used to fabricate an electroluminenscence (EL) device that emits in the blue region at room temperature. Compared with cast films, the photoluminescence and electroluminescence spectra of the TPB-doped LB films show that the exciton shifts to higher energy and that the full width at half maximum of the emission peak becomes narrower.

Electroluminescence (EL) from heterostructure devices made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals have been investigated, along with cathodoluminescence (CL) from thin films of ZnS doped with CdSe-ZnS core-shell nanocrystals. In the EL devices, the organic PPV structure, built next to the anode using the technique of molecular layer-by-layer sequential adsorption, serves as the hole transport layer. The inorganic layer, adjacent to the electrode and made of spin cast CdSe nanocrystals passivated with either organic groups or with a thin layer of ZnS, is the emitting layer. The ZnS host film in the CL devices, built using chemical vapor deposition, serves as the support medium for the dispersed nanocrystals, but also provides additional passivation to the surface of those nanocrystals. We find that the EL and CL signals almost exclusively originate from the inorganic nanocrystal in both cases, i.e., EL comes from the nanocrystal layer in the heterostructure device while CL is generated from the dispersed particles in the composite film. The external EL quantum efficiency, (eta) _EL, is not enhanced by the presence of ZnS overcoating, opposed to the observed increase in the photoluminescence (PL) quantum yield. However, we find that the CL emission and its stability are substantially improved by the presence of ZnS around the emitting nanocrystal cores. These observations reflect a difference in the effects of overcoating ont he various luminescence processes. On the one hand, a ZnS overlayer is associated with an additional energetic barrier that reduces the efficiency of charge injection into the nanocrystals for EL. On the other hand, PL and CL processes only benefit from the surface passivation with ZnS.

In order to apply polymer light-emitting diodes in commercial products a number of lifetime specifications have to be met. In this paper we report on the performance and stability of polymer light-emitting diodes based on dialkoxy-substituted fully conjugated PPV. Lifetime measurements have been performed on small and large area devices under different conditions, including variations in temperature, luminescence intensity and humidity. It will be shown that polymer LEDs can withstand extreme lifetime tests successfully. The result are compared with lifetime specifications for applications in consumer applications and are discussed in terms of the stability of the emissive polymer. Spectral measurements as a function of the operational lifetime are presented.

Organic hole transport materials are used in organic LEDs, where they substantially improve device performance if placed as a hole transport layer (HTL) between the anode and the electroluminescent layer (EL). Soluble polymeric hole transport materials with high glass transition temperatures are of particular interest, because they allow for efficient device fabrication through spin casting of the HTL, and high glass transition temperatures have been found to improve thermal and long-term stability of the device. The redox potential of the hole transport material determines the facility of charge injection at the anode/HTL and the HTL/EL interfaces, thus affecting the overall device efficiency. We have synthesized a series of soluble hole-transporting polymers with glass transition temperatures in the range of 130 degrees C to 150 degrees C. The synthetic method allows facile substitution of the hole transport functionality with electron-withdrawing and electron-donating groups, which permits tuning of the redox potential of the polymer. These polymers have been used as HTL in tow-layer devices ITO/HTL/Alq/Mg. The maximum external quantum efficiency increase, if the redox potential is changed to facilitate reduction of the hole transport material at the HTL/EL interface. Electron-deficient derivatives show higher external quantum efficiencies. The device stability, however, follows the opposite trend.

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.

We present photo- and electro-luminescence, and hole mobility measurements of carbazole (Cz) substituted polyacetylene (PA-Cz) and poly(diphenylacetylene) (PDPA-Cz). The photoluminescence (PL) of the interband transition in PA-Cz thin film is quenched. PDPA-Cz shows a green-yellow emission with a PL efficiency about 30 percent of the interband transition. The hole mobility of PDPA-Cz is determined to be approximately 10 ⁷ cm²/Vs and the ionization energy is 5.3 eV. PDPA-Cz forms robust thin films and is thermally stable up to 470 degrees C. For a structure of ITO/PDPA-Cz/Alq(tris(8-quinolinolato) aluminum)/MgAg EL quantum efficiency over 1 percent is achieved.

We have elaborated for the first time organic-inorganic hybrid light-emitting diodes. These devices emitting in the orange and in the green are formed of one, two or three hybrid thin layers exhibiting different functionalities and sandwiched between indium-tin oxide and metallic electrodes. These layers have been prepared by the sol-gel technique from silane precursors modified with hole or electron transporting units and with light-emitting DCM or naphthalimide moieties.

The fabrication of organic light emitting diodes with vacuum sublimed molecules is emerging as a competitive flat panel display alternative because of brightness, efficiency and operating lifetimes of these devices. The requirement for a low work function metal as a cathode limits the choice of materials to reactive elements such as magnesium, calcium, lithium that can be alloyed or co-deposited with silver or aluminum for greater stability and lifetime of the device. From the scandium-subgroup of elements, we have investigated lanthanum as a potential cathode because preliminary studies indicated that the electroluminescence onset could occur as low as 4 volts. Current-voltage, Auger spectroscopy and spectro-photometric data will be presented on standard devices using indium tin oxide, triphenyl diamine derivative and aluminum quinolinate and lanthanum-based cathodes.