In this paper, we present two techniques for fabricating efficient and bright organic light emitting devices. The first technique allows for an enhancement in the electron injection process. This is accomplished through inserting a layer of LiF with appropriate thickness between the cathode and a quinacridone doped organic layer. Devices with an Al/LiF cathode demonstrated a luminance in excess of 20,000 cd/m² and an external quantum efficiency of 3 percent, which is comparable to devices with a Mg/LiF cathode. These devices show maximum luminance of 45,000 cd/m² prior to failure in continuous bias operation. In the second technique, partially ionized beam deposition was utilized in the fabrication process of organic electroluminescent devices. Preliminary results indicate that devices fabricated with this technique are more efficient and brighter than similar devices fabricated with the traditional thermal evaporation process.

Oligomers and block copolymers structurally related to PPV were investigated under intense optical excitation. Their well-defined molecular structure allows a better control of emission properties than is usually feasible in semiconducting polymers. Stimulated emission is demonstrated in single crystals of PPV-type oligomers, and also in thin films obtained by spin casting of copolymers containing PPV- type blocks. Waveguiding is shown to provide the length of interaction required for mirrorless laser generation. Thin films of oligomers obtained through deposition from the vapor phase are polycrystalline, and the optical losses in the as-deposited films are too large for lasing to be achieved. These films show stimulated emission only after the domain size has been increased by annealing.Lasing occurs within individual crystalline domains with a threshold value comparable to that found for optically clear amorphous films of conjugated polymers.

High performance photonic and electronic devices fabricate from conjugated polymers have been demonstrated, including light emitting diodes, photovoltaic cells, photodiodes, optocouplers, and thin film transistors. In some cases, performance parameters have been improved to levels comparable to or better than their inorganic counterparts.Notably absent from this list of semiconducting polymer devices is the polymer laser diode. As the first important step in exploring the feasibility of such laser diodes, optically pumped stimulated emission, gain, and lasing have recently been observed in over a dozen different semiconducting polymers representing a variety of molecular structures with emission wavelengths spanning the visible spectrum. Because of their strong absorption, high density of chromophores, and Stokes-shifted luminescence, luminescent semiconducting polymers have potential as low- threshold laser media and as active media in InGaN/polymer hybrid light emitting devices. We give details on an ongoing effort on optically pumped lasers using microcavities and distributed feedback (DFB) and suggest two routes toward fabricating laser diodes using semiconducting polymers. Initial results show that the lasing threshold for DFB laser is one order of magnitude lower than that of a microcavity laser using the same polymer under similar optical pumping conditions.

We present high voltage pulsed electroluminescence (EL) measurements on light-emitting diodes (LED) based on thin films of poly(p-phenylenevinylene) (PPV) sandwiched between Indium-Tin-Oxide (ITO) and Aluminum electrodes. We observe two regimes in the LED operation depending on the driving pulsed current density. At low current densities, below 50A/cm², the pulsed EL follows its DC characteristics with yellow-green emission. Above some threshold current density we observe additional UV-violet emission; the amplitude of the pulsed UV EL increases exponentially with the applied voltage. When the amplitude of the voltage pulses is around 300 V, the current signal exhibits a sharp current peak followed by a dramatic increase in UV EL intensity but only moderate increase of the green emission. We propose a possible explanation for the appearance of the UV emission upon application of strong electrical pulse. It is due, we believe, to 'hot' carriers in strong fields which partially inhibit the formation of singlet excitons and enhance the probability for direct inter-band radiative transitions. We show that our very simple device can be operated at current density as high as 140 A/cm² and achieve a peak brightness of 10⁵ cd/m² without appreciable degradation.

Both directionality and intensity of emitted light from the organic electroluminescent (EL) device were strongly modified using a Fabry-Perot microcavity. The microcavity EL device exhibited a single mode emission that was significantly enhanced and the emission was sharply directed along the direction normal to the device surface. The luminous efficiency in the normal direction was much higher than that of the noncavity EL device. The sharply directed pure green emission exceeded 40000 cd/m² at a driving current of 800 mA/cm².

We report an experimental and theoretical study of the effects of interference in polymeric light-emitting diodes (LEDs). These effects are due to the complex optical structures of the devices, which include many layers of materials with different refractive indices, and are of considerable importance since they affect spectral distribution and intensity of the absorption and emission in a significant way. By way of comparison, they can also provide a flexible, non-invasive optical probe of the electroluminescent processes. In this paper we analyze single-layer diodes with indium-tin oxide (ITO) and Al electrodes, where poly (p-phenylene vinylene) (PPV) is the luminescent polymer. We find that photo-induced excitation of the radiative species produce different spectral shapes depending on the excitation energy which we can describe in terms of interference phenomena. The theoretical analysis is conducted by means of multilayer stack theory and transfer matrix calculations, and takes into account additional quenching effects due to In contaminations from the ITO electrode. The theoretical results are in good agreement with the experiment.

We have succeeded in fabricating submicrometer-sized organic light emitting diodes (OLED) with bright emission using a polymer blend. The cell structure was composed of ITO/hole transport layer(polymer blend layer)/emitting layer/MgAg. We used poly (methyl methacrylate) (PMMA) as an inert host material and N,N'-diphenyl-N,N'-bias(3- methylphenyl)-(1,1'-biphenyl)-4,4'-diamine- polycarbonate (PC-TPD) as a guest material. The polymer blend layers showed various morphology depending on the concentration of PC-TPD. With 2 wt percent of PC-TPD, and PC-TPD formed submicrometer-sized spherical domains, actually prolate spheroids, in the PMMA host layer. The domain diameters were around 200 nm according to AFM and SEM observations. The OLED including such a blend structure showed aggregation of bright submicrometer-sized electroluminescence spots. The number of working diodes exceeded four million per square centimeter.

The lowest singlet and triplet excited states in cyano- substituted phenylene vinylene oligomers are characterized by means of configuration interaction calculations. First, the vertical singlet-singlet, S₀ yields S₁, singlet- triplet, S₀ yields T₁, and triplet-triplet T₁ yields T_n, excitation energies are evaluated in oligomers ranging in size from two to five phenylene rings; the spatial extent of the S₁, T₁, and T_n excited states is estimated on the basis of a simple analysis of their wavefunctions. We then pay attention to the lattice distortions taking place in the lowest singlet and triplet excited states of these model oligomers. In each case, the results are compared to those obtained for the corresponding unsubstituted oligo(phenylene vinylene)s. Besides the bathochromic shift associated with the electroactive character of the substituents, an overall localization of the excited state wavefunction is found when going from unsubstituted to cyano-substituted oligomers.

In polymer light-emitting diodes (LEDs), a pre-requisite for efficient operation is the balance of injection and transport of electrons and holes. This has stimulated much research into suitable electron-injecting and transporting materials. We report the use of polypyridine as an efficient electron-transporting layer in polymer LEDs. We have achieved much improved LED performance by incorporating polypyridine as an electron-transporting polymer in LEDs using either poly(p-phenylenevinylene) or poly(2-methoxy, 5- (2' ethyl-hexyloxy)-p-phenylenevinylene) as the hole- transporting and emitting layer. the optimization of the layer thickness leads to increases in efficiency of up to a factor 60 over comparable single layer devices.

Electroluminescent devices from binary blends of conjugated polyquinolines are fabricated and used to systematically investigate the mechanisms of efficient electroluminescence (EL) in multicomponent conjugated polymer system. The roles of energy transfer, excited state complex formation, charge transport and trapping, miscibility and phase separation, and spatial confinement on EL efficiency can be directly probed by a judicious choice of binary blend components. Large enhancement of EL efficiency and device brightness was observed in some conjugated polymer blend systems compared to the component homopolymers. For example, binary blends of poly(2,2'-(2,5-thienylene)-6-6'-bis(4- phenylquinoline)) and poly(2,2'-(biphenylene)-6,6'- (4-phenylquinoline)) are found to exhibit EL quantum efficiency of up to a factor of 30 enhancement at 200 cd/m² luminance levels compared to the components. EL enhancement in this and other polymer blend system is shown by electric field-modulated photoluminescence spectroscopy and other experiments to be due to improved radiative electron-hole recombination efficiency facilitated by spatial confinement of excitons in the blends.

We report studies focusing on the nature of trap states present in single layer ITO/polymer/metal devices of poly(p- phenylene vinylene) and its soluble derivative poly(2,5- dialkoxy-p-phenylene vinylene). In the high applied bias regime the IV characteristics from 11 to 290K can be successfully modeled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density of between 10¹⁸ and 4 X 10¹⁷ cm^-3 and a characteristic energy E_t of 0.15 eV. Measured conductance transients of PPV are non-exponential and follow a power-law relationship with time whose decay rate decreases with decreasing temperature. This can be directly related to the emptying of the trap distribution deduced from the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form a Gaussian distribution of chain energy sites. The sites involved in carrier transport are those towards the center of the distribution. The deep sites in the tail of the distribution in the carrier energy gap are the observed traps for both positive and negative carriers. The same deep sites dominate the photo- and electroluminescence emission spectra. The model implies that the emissive material in organic light emitting diodes should be made as structurally disordered as possible.

Recently there have been reports on color variable light emitting devices. Here we present a new approach to such devices based on conjugated polymers. the device consists of a blend of pyridine-phenylene and thiophene-phenylene based copolymers sandwiched between two redox polymers: emeraldine base form of polyaniline and sulfonated polyaniline (SPAN). ITO and Al are used as electrodes. The devices work under either polarity of driving voltage with different colors of light being emitted from different locations, red from emitting polymer/SPAN interface under forward bias and green from bulk of the emitting polymers under reverse bias. Electroluminescence of the devices peak at 550 nm with a shoulder at 585 nm under reverse bias while they show a single peak at 665 nm under forward bias. The relative fast time response allows the rapid switch of colors and AC operation.

We have recently reported on the fabrication of organic light emitting devices based on sequentially adsorption layers of a polycationic poly(p-phenylene vinylene) (PPV) precursor and poly(methacrylic acid) (PMA). Here we have fabricated devices with PPV precursor and poly(acrylic acid) (PAA) in an effort to further improve device performance by controlling the nature of the polyanion with which the PPV precursor is assembled. We have seen dramatic differences in device performance by systematically varying the bilayer composition and the total film thickness by controlling the solution parameters and the total number of bilayers deposited. In addition, the conversion temperature has also been shown to strongly influence device characteristics. The current, best performing device has been for a system in which the bilayer thickness is around 60 angstrom, approximately half of which is due to the PPV. From this system we have been able to achieve luminance levels greater than 1000 cd/m² using an aluminum cathode and an ITO anode. Such high brightness levels from a PPV single slab device with an aluminum top electrode are quite unusual.

The synthesis and luminescent properties of the homopolymers (4,5) and copolymers (9,10) carrying ion-transporting side chains are reported. When fabricated as alight emitting electrochemical cell the copolymer 10 exhibited a significant reduction in turn-on voltage and improved luminous efficiency compared with a conventionally fabricated polymer light emitting device. Similar results were observed with the pyridine copolymer 13. The model meta-linked trifluoromethyl substituted distyrylbenzene derivative 16 has been synthesized and its crystal structure has been determined with a view to evaluating the related polymers as charge transporting materials.

In this paper our recent advances in the development of a novel class of highly efficient DSB-segmented copolymers 1-5 are reported. (-Y-C₆H₄-CH equals CH-C₆H₂(OR)₂-CH equals CH-C₆H₄)_n (1): Y equals NC₆H₅ (2): Y equals O (3): Y equals CO (4): Y equals CHOH (-Y-C₆H₄- CH equals CR'-C₆H₄-CR' equals CH-C₆H₄-)_n (5): Y equals NC₆H₅ R' equals CH₃OC₆H₄. 1,2,3 have successfully been synthesized using the HORNER/WITTIG carbonyl olefination of appropriate dialdehydes which are based on triphenylamine, diphenylether, and diphenylketone. 4 was prepared by polymer analogous carbonyl reduction of 3. This approach resulted in high molecular and soluble materials exhibit green (1,3) and blue (2,3) luminescence with an excellent photoluminescence efficiency. Our goal in this paper is to illustrate the changes in the electrooptical properties that have been caused by segmentation of the polyconjugated PPV backbone into distyrylbenzene (DSB) segments which are connected by mono-atomic Y groups. The chemical character of Y determines significantly the oxidation potentials. Due to its low and reversible oxidation potential 1 shows the most favorable properties for low voltage LED's: green electroluminescence with luminance in the range of 100-500 cd/m² at 7-10 V has been demonstrated. There has also been prepared an electron donating polymer 5 having additional phenyl substituents attached to the vinylenic unit. These phenyl groups are responsible for high glass transition temperature and helped to solubilize the chain.

Soluble light-emission alternating copolymers based on alkyfluorene and bis-phenylenevinylene were synthesized using Wittig coupling reaction. The PL and EL efficiency of emissive polymers increased in the blends diluted with non- emissive polymer such a PVK. The emission spectrum of the blends also became narrower than that of emissive polymer only due to the reduction of intermolecular excimer formation. The blends of emissive polymers with different band-gap energies were investigated in terms of energy transfer mechanisms. The characteristics of emissive polymer with lower band-gap energy dominates the PL and EL spectrum of blends. The quantum efficiency in the order of 0.025 percent was obtained the electrodes of ITO and aluminum.

A series of 4-aminonaphthalimide functionalized polymers has been synthesized. They differ by the nature of their backbone: and by the nature of the substituent at the imide nitrogen atom. The absorption and emission properties of these polymers have been investigated. Photoluminescence quantum yields in the solid state up to 35 percent were observed. Cyclic voltammetry in conjunction with UV-visible spectrometry have been performed in order to determine the HOMO and LUMO energy levels of the different materials. Electroluminescent devices were fabricated with these polymers as emitting layers, and ITO and Ca as anode and cathode, respectively. Monolayer devices showed a limited performance. Efficient green light emission was obtained with a bilayer device based on PVK as a hole transport material and a polystyrene derivative as an emitting layer. A maximum luminance of 7100cd/m² was obtained under 16V. The device had a maximum external quantum efficiency of 1 percent and a maximum external energetic efficiency of 0.2 percent. Doping PST-NI-BuP with 20 percent DCM resulted in red-orange emission with a brightness as high as 1800 cd/m². Doping PST-NI-BuP with 20 percent DCM resulted in red-orange emission with a brightness as high as 1800 cd/m². Moreover, this study showed a strong influence of the chemical environment of the naphthalimide moiety on the photoluminescence and electroluminescent properties of the polymers.

The growth of insoluble and intractable thin organic films is presently reported by a novel self-assembly technique. Bifunctional 8,8'-dihydroxy-5,5'-biquinoline is reactively self-assembly with diethyl zinc to form a linear coordination polymer. These films were characterized by FTIR, UV/VIS spectroscopy, profilometry, and photoluminescence measurements. The film growth on glass or indium-tin oxide coated substrates was monitored by increasing absorbance at 396.6 nm using UV/VIS spectroscopy and profilometry. FTIR spectroscopy indicated that the self- assembled films are polymeric in nature. The reduction of end-group hydroxyls and a significant red-shift of the photoluminescence emission spectrum with respect to zinc bisquinoline powder was attributed to an increase in conjugation length. Single layer light emitting diodes fabricated int his fashion exhibited an orange electroluminescence, consistent with the corresponding photoluminescence spectrum.

We report investigations into a poly(1,20-distyrylbenzene- co-1,2-[4-(p-ethylphenyl)]triazole) (TRIDSB) electron transport material and its incorporation into single and multilayer LEDs. Multilayer devices have been investigated with poly(p-phenylenevinylene) (PPV) and poly(2-methoxy-5- (2'-ethylhexyloxy)-1,4-phenylenevinylene) as hole transport layers (HTLs). The incorporation of the polymer into an ITO/PPV/TRIDSB/Al LED facilitates electron injection into the hole transporting emissive layer and results in a ten fold increase in the external quantum efficiency for electroluminescence (EL) of the PPV layer from 0.008 percent to 0.08-0.1 percent. In an ITO/I-MEHPPV/TRIDSB/Al device the corresponding increase in the quantum efficiency for EL from the 1-MEHPPV is fifty fold, from 0.002 percent to between 0.06-0.11 percent. The polymer has been shown to be thermally stable with no glass transition temperature or melting point detected within the range 25-250 degrees C.

We report the fabrication of efficient green light-emitting diodes using a side-chain polymer based on a high-electron affinity (EA) naphtalimide moiety (PNI). The chromophore is attached to a polymethacrylate backbone through a spacer, and emits in the green with high efficiency. In single-layer light-emitting diodes (LEDs), we find that the electroluminescence (EL) efficiency is not limited by Al cathodes as for poly(p-phenylene-vinylene), PPV, and we attribute this to the increased EA. We report maximum internal efficiencies of about 1.7 percent for Ca and 0.9 percent for Al in double-layer devices where PPV serves as both Hole-injector and emitter. Compared to some oxadiazole based electron injection/transport layer, PNI gives higher efficiencies at high currents, and longer lifetimes. Tuning of emission in the red is possible by dye-doping the PNI and causing the emission to happen in this layer. We discuss the properties of the different device configurations with a view to the electronic structure of the materials and in particular to the influence of the thickness of the individual layers on efficiency and driving conditions.

In recent years there has been a considerable interest int he photoluminescence (PL) and electroluminescence (EL) properties of conjugated polymers, because of their potential application as the emitting layer in light- emitting devices. While poly(p-phenylenevinylene) (PPV) and its derivatives have been investigated most intensively among Al conjugated polymers, only few studies have been made on poly(p-phenyleneethynylene)s (PPEs), which feature a triple rather than a double bond in the conjugated backbone. Here we present out recent experiments on polymer light- emitting diodes (LEDs) based on poly(2,5-dialkoxy-p- phenyleneethynylene)s. The devices under investigation consist of the PPE emitting layer which is sandwiched between an indium tin oxide (ITO) and an aluminium, calcium or chromium electrode. Yellow-green electroluminescence with a brightness of up to 20 cd/m² was observed for different PPEs. The EL intensity follows the applied bias and current as expected, and the EL emission essentially matches the PL spectrum. Interestingly, no significant difference in device performance was observed with respect to internal quantum efficiencies and onset-voltage, when comparing Al and Ca electrodes. These results suggest, that in the case of PPEs the hole injection barrier seems to be a limiting factor, while the injection of electrons from the low work function electrode is facilitated. Consequently, these devices favorably comprise an electron injecting contact with moderate work function which, as expected, was also found to lead to an improved device stability.

We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5-tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4-ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between -4.97 and -5.16 eV. By using a polymeric TPD as hole transport layer and tris(8-quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cs?m² were obtained under ambient conditions, using airstable Al-electrode as cathode and ITO as anode.

Electroluminescent devices consisting of triphenylene derivatives as hole transport layer (HTL) and 8- hydroxyquinoline aluminium complex (Alq₃) as emitting layer with ITO as anode and aluminium as cathode are presented. Triphenylene compounds were evaporated or spin coated from solution. In addition side chain polymers with triphenylenes are used. Finally we prepared a polymeric HTL by photopolymerization of an acrylate functionalized triphenylene monomer. Some of the devices, based on ITO/Triphenylene/Alq₃/Al showed brightness values up to 1000 cd/m² and low turn on voltages from 4-5 V.

Three copolymers containing alternating rigid and flexible blocks and five PPVs oligomers have been designed and synthesized for the best understanding the relationship of molecular structure and properties of as LED. The results showed that the solubility was improved and the peak positions of absorption and emission spectra were shifted to longer wavelength with different groups substituting.

Multicolored electroluminescent (EL) devices has been realized utilizing poly((1-dodecyloxy-4-methyl-1,3- phenylene)(2,5"-terthienylene)) (mPTTh) as an emitting layer and tris(8-hydroxyquinoline) aluminum (Alq3) as an electron transport layer. A single layer EL device of mPTTh polymer emits orange-colored light. EL intensity increase as the thickness of Alq3 layer increases up to 30 nm, but the emission color becomes diversified when the Alq3 layer thickness is greater than 30 nm since the relative peak intensity of green EL from Alq3 layer grows. EL color is changed form orange to greenish orange depending on the thickness of Alq3 layer. EL efficiency of the double layer device was greatly enhanced by 3000 times in compared to that of single layer device. Alq3 layer in device acts as a hole blocking electron transporting layer and an emitting layer as a function of the thickness of Alq3 layer.

In this abstract a novel EL device has been presented, in which a SiO₂ layer is inserted between emission layer and cathode. Comparing with the same thick single layer device, power conversion efficiency and quantum efficiency of the device are improved owing to inserting a layer SiO₂ film.

In organic thin film electroluminescence device with ITO/PPV/TPD:CuPc/Alq:DCMl/Al structure, the TPD:CuPc Langmuir-Blodgett (LB) film as electron-blocking layer was inserted between PPV and Alq:DCMl layer. A device without TPD:CuPc layer or a device with monolayer of TPD:CuPc which was applied at higher DC voltage exhibited EL emission from both the PPV and Alq:DCMl layers. Whereas a device with monolayer of TPD:CuPc layer which was applied at lower voltage or a device with double layers of TPD:CuPc had an EL emission only from Alq:DCMl layer implying that the TPD:CuPc layer can effectively block the electrons and the Alq:DCMl electron transport layer worked as the emitting layer. Hence, we can spatially control the recombination zone and obtain different EL emission by adjusting the thickness of carrier blocking layer.

We report the characterization of an insoluble MEHPPV prepared via a chloro precursor route. Optical absorption and emission spectra are discussed with reference to those of the common soluble variant, PL quantum efficiencies are also reported. Results obtained for single ITO/I-MEHPPV/Al and double layer ITO/I-MEHPPV/electron transport layer (ETL)/Al LED structures are discussed. Peak luminances of 800 cd/m² are found for the multilayer device and a peak EL external quantum efficiency of 0.11 percent is obtained.

The preparation and optical characterization of a novel PPV derivative displaying reversible tunable photophysical and electrophysical properties, poly-(5-vinylene-5'- (vinylene-1,4-phenylene)-2,2'-bipyridine), p-BVP, (1), is reported and its application in the preparation of tunable organic electroluminescent devices is described. The photophysical properties of the new polymer, such as its absorption, emission and electroluminescence are sensitive to the present of even minute traces of vapors of different acids and bases such as ammonia, formic acid and haloacids. The acid/base vapor induced optical changes are reversible and can be repeated many times without any significant degradation of the optical and mechanical properties of the films. Intermediate spectra can be generated simply by controlling the exposure time of the films to acidic or basic vapors. Similar effects were observed for two other polymers, poly-(5-vinylene-5'-tri(vinylene-1,4- phenylene)-2,2'-bipyridine), p-BTVP, (2), and its random analog p-BRTVP, (3). The tunability of the photo- and electrophysical properties of the polymers originates probably from structures changes associated with protonation-deprotonation processes and aggregation phenomena.

We have investigated the lasing properties of several luminescent conducting polymers, i.e. DOO-PPV and the bi- substituted polyacetylenes PDPA-nBu, and PH_xPA, dissolved in various polar and non-polar solvents. PPV polymers emit with high quantum efficiencies in broad emission bands cantered in the orange/red region of the spectrum, depending on the solvent, and the PDPA polymers emit in the blue/green region. Our tested laser resonators include polymer solutions excited with 100 ps pulses from a regeneratively amplified mode-locked Nd:YAG laser. We obtain pulsed, low-threshold laser operation with repetition rate of up to 1 kHz. Resulting mainly from recent reported originally in the literature. The dependencies of threshold pump energy and output versus input power characteristics on material parameters are investigated for a fixed optical gain length. The results are compared with the standard Rhodamine 590 organic dye system used in the same wavelength regions. We have observed that the well know phenomenon of 'concentration quenching' in dye molecules does not happen in polymers. Spectral narrowing in PDPA-nBu solution, emitting near 500 nm, is also obtained for the first time.

ZnS, ZnS:Cu and ZnS/CdS nanocrystals were synthesized in polymer matrices with a variety of methods. TEM, absorption spectra and small-angle x-ray scattering studies showed that the particle sizes of ZnS, ZnS:Cu and ZnS/CdS nanocrystals were 3.0, 3.2 and 2.0 nm respectively. Electron diffraction results showed that the ZnS and CdS nanocrystals have the hexagonal structures and Cu ions existed in ZnS with the form of Cu₂S. A hole transport material tetraphenylbenzidine (TPB) were also doped into ZnS nanocrystals/polymer to improve the electroluminescent (EL) property of the ZnS nanocrystals. ZnS:Cu, ZnS/CdS and TPB/ZnS nanoparticles/polymer composite as emitters were used to fabricate a single layer structure light-emitting diode between ITO and Al electrodes respectively. The photoluminescence and EL properties of these doped systems were studied. These EL devices had low turn on voltage and the blue electroluminescence from Cu:ZnS and ZnS/CdS and a violet-blue electroluminescence from TPB:ZnS were observed respectively at room temperature under ambient air. The EL mechanism is also discussed.

A novel light emitting poly(arylene vinylene) derivative comprising TPD as the arylene unit, has been developed as a high molecular film forming material. This alternating copolymer has been synthesized by polycondensation using PO- activated carbonyl olefination of dialkoxysubstituted xylylene-bisphosphonate and the appropriate TPD-dialdehyde. Photoluminescence, cyclovoltammetric oxidation and LED- measurements are reported and discussed. In a single layer device with the structure ITO/TPD-PPV/Ca green electroluminescence at low drive voltage is demonstrated.

Several novel families of amorphous molecular materials with high glass-transition temperatures (Tg) that function as charge-transport or emitting materials for organic LEDs have been deigned and synthesized. Double-layer and multilayer devices using these novel amorphous molecular materials were fabricated and their performance investigated. The use of the novel amorphous molecular materials with high Tgs enabled the fabrication of thermally stable organic LEDs; one of the devices was found to operate even at 170 degrees C. The multilayer device consisting of double hole-transport layers and an emitting layer was found to enhance significantly the durability of the device. Exciplex formation at the organic/organic solid interface in organic LEDs has also been investigated.

Various fluoro-functionalized aromatic 1,3,5-triazine monomers were prepared. A series low molar mass and poly- (1,3,5-triazine)-ethers were synthesized by a condensation reaction. The polymers as well as the low molar mass compounds have excellent thermal stability and are amorphous. In order to examine the potential to apply these compounds in organic electroluminescent devices, the redox properties were studied by cyclic voltammetry. It was found that the monomers have high electron affinity and reach LUMO values in the range of -2.7 to -3.1 eV. Additionally high oxidation stability with HOMO values lower than -6.4 eV follows hole blocking capabilities. This opens the possibility to utilize 1,3,5-triazine containing materials as electron injection/hole blocking layer in LEDs. First LED results are in agreement to these high electron affinities.

Hole transporting properties of tris-(8-hydroxyquinolinato) aluminum (Alq) were investigated using time-of-flight (TOF) photocurrent transients and photoinduced discharge techniques. A thin layer of Alq was inserted between two hole transporting layers. Within the time domain of the photocurrent transients,the fraction of holes escaping from one side of the Alq layer to the other side is strongly field, temperature, and Alq thickness dependent. The results of photoinduced discharge experiments indicate that eventually the holes escaped the trilayer samples. The injection barrier is estimated to be approximately 0.12 eV. The hole penetration range is estimated to be 10-30 angstrom in the range of field strengths studied, therefore suggesting the recombination zone of electrons and holes in Alq-containing electroluminescent (EL) devices is confined close to the interface of Alq and the hole transporting layer. The results demonstrate the importance of the hole injection barriers and hole trapping to the performance of organic EL devices using Alq as the electron transporting and emissive layer.

Conduction in pristine conjugated polymers is by polaron hopping between sites corresponding to conjugation lengths. The strong increase of current I with voltage V observed for both emission-limited and ohmic contacts is due in large part to mobility increase as increasing field makes it more possible to overcome internal barriers, such as energy differences between sites. For emission-limited contacts an additional source of nonlinear increase of I with increasing V is greater ability to escape return to the injecting electrode due to the image force. For ohmic contacts additional nonlinearity comes from space charge effects. We are able to fit I vs. V for electron or hole conduction is some poly(p-phenylene vinylene), PPV, derivatives with ohmic contacts for reasonable values of the parameters involved.

The behavior of polymeric electroluminescent devices made using ZrC cathodes, a soluble phenylene vinylene polymer, and Au anodes is reported. ZrC is a highly air-stable metal with a work function of about 3.6 eV. Polycrystalline thin films can be formed on sapphire with electron beam evaporation and subsequent annealing at ca. 500 deg C. The devices exhibit a lower turn-on voltage than corresponding devices with Al cathodes, but very low efficiency; their lifetime is not longer than with other cathodes. The current-luminance-voltage-time characteristics suggest that the recombination zone is close to the cathode, that the metal is not exidized and efficiently quenches the excitons, and that the negative charge carriers are effectively trapped by the polymer.

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). At model interfaces of an Al/Alq3/TPD/Au device aluminum, TPD: N,N'-diphenyl-N- N'-(3-methylphenyl)-1,1'-biphenyl-4,4PRM-diamine) and DP-NTCI/metal interfaces , the vacuum level shifts were observed, indicating the invalidity of the traditional energy level alignment model where a common vacuum level is assumed at organic/metal interface. On the other hand, the results for Alq3/TPD interface showed little vacuum level shift, leading to apparent applicability of the traditional model. The observed interfacial electronic structures corresponds well to the injecting nature of the interface. The effect of air exposure to the interfacial electronic structure work function were discussed, together with the results for porphyrin/metal interfaces.

The effects of (i) aquaregia-treatment of the indium tin oxide (ITO) substrate anode and (ii) a controlled Al₂O₃ buffer between the emitting layer (EML) and the Al cathode layer on the behavior of efficient vacuum evaporated multilayer organic light-emitting diodes (LEDs) is described. The organic layers include hole transporting layers (HTLs) such as '3-armed star' 4,4',4"- tris(N-(3-methoxyphenyl)-N-phenylamine-triphenylamine) and triphenyl diamine, and the EMLs consist either of green- emitting 8-tris-hydroxyquinoline aluminum (Alq₃) or blue-emitting amino oxadiazole fluorene. While the aquaregia treatment enhances the performance, the optimized treatment depends on the HTL. This observation implies that the enhancement is due not only to contact area effects. Similarly, an Al₂O₃ layer of suitable thickness also enhances the current injection and EL output significantly. This enhancement is believed to be due to increased charge carrier density near the HTL/EML interface which results from removal of the intrinsic organic/Al interface gap states which trap injected carriers and quench singlet excitons nonradiatively.

We review recent results form two types of small molecule organic light emitting devices (OLEDs). For flat panel display applications, we have developed a novel OLED pixel in which the R, G and B emission layers are vertically stacked to provide a simple fabrication process, minimum pixel size, and maximum fill factor. In separate experiments, we have worked towards achieving electrically- pumped organic lasers by demonstrating low-threshold lasing in an optically-pumped thin film double heterostructure consisting entirely of organic semiconducting materials.

In this paper on polymer LEDs we discuss the formation of black spots, surface treatments of the anode, and photochemical degradation of the emissive polymer. We find that small pinholes in the cathode layer are the origin of the black spots. The black spots form when H₂O or O₂ diffuse through the pinholes and react with the cathode at the polymer-cathode interface.A model is presented that describes the growth of the spots. We find that for both indium-tin-oxide (ITO) and Au anodes, an UV/O₃ or an O₂ plasma cleaning treatment increases the work function by 0.8-0.9 eV. A higher work function may lead to a better hole injection and a reduction in the operating voltage. We present a method to measure the quantum yield for bleaching, (gamma) equals 1.6^$MN4 and (gamma) equals 1.7 10^-7 for bleaching of dialkoxy-PPV in air and vacuum, respectively, indicating that the polymer is 1000 times more stable in vacuum than in air.

The importance of the interfacial properties in organic light emitting devices (OLED) is well recognized. We have studied the formation of interfaces that occurs in OLEDs using mainly surface/interface analytical techniques in a well controlled ultra high vacuum environment. The results have shown that microscopic surface and interface properties are intimately related to the device characteristics and performance. Specifically, the metal electrode material is observed to quench strongly the luminescence of the organic material in the interface region. Proper treatments of the interface may at least partially recover the quenched luminescence. The implications of these results in the design and operation of organic light-emitting devices are discussed.

We present experimental evidence that orientation of the polymer chains in semiconducting polymer films leads to lower thresholds for gain narrowing. Two different materials have been investigated: neat films of poly(p-phenylene vinylene) (PPV) and blends of poly(2-methoxy-5-(2'- ethyl-hexyloxy)-1-4-phenylenevinylene) (MEH-PPV) in polyethylene (PE). Gain narrowing is not observed in non- oriented films in either of the two materials. However, free-standing films of PPV drawn to a ratio of (7:1) showed gain narrowing at threshold one order of magnitude lower than typically found for non-oriented films of soluble PPV derivatives. For the MEH-PPV blends, gain narrowing is observed in diluted films with concentrations of the active polymer of approximately 1 percent. The thresholds for these dilute, but chain extended and highly oriented free standing films are comparable to those obtained in neat thin film waveguides of the same material. These result are correlated with the microstructure of the films, as investigated by x- ray diffraction.

Enhanced performance has been observed for molecular organic light emitting diodes (MOLEDs) consisting of two to four organic layers sequentially vacuum vapor deposited onto patterned indium-tin oxide (ITO) on polyester (PET) films. For the device structures studied, the performance of diodes fabricated on polyester is comparable to or better than their analogs on glass substrates. For example, at 100 A/m², a luminous power efficiency of 4.4 lm/W and external quantum yield of 2.7 percent is observed for a device structure consisting of two hole transport layers, a doped emitting layer and an electron transport layer on a polyester substrate. The same device made on a glass substrate has a luminous power efficiency of 3.5 lm/W and external quantum yield of 2.3 percent. The enhanced performance of the plastic MOLEDs is attributed to increased optical output coupling. Electrical and optical performance for comparative device structures has been characterized by current-voltage-luminance measurements and electroluminescence spectra, and ITO surface morphology has been studied by Atomic Force Microscopy.

The emission mechanisms of rubrene doped molecular organic light-emitting diodes (MOLEDs) is discussed in terms of energy transfer and direct carrier recombination at the dopant. The emission mechanism is investigate by using single layer devices composed of 5,6,11,12- tetraphenylnapthacene (rubrene) as the dopant and N,N'- diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl- 4,4'-diamine (TPD) as the host. Efficient energy transfer form TPD to rubrene is suggested by current-voltage and drift mobility measurements of the doped and undoped TPD films in single layered devices. It is found that electrons can be injected into the hole transporter, TPD. The spatial distribution of the creation of rubrene excitons is studied by change in the thickness sand location of the doped TPD layer in multilayered devices. In rubrene doped TPD, the width of the emission zone extends about 20 nm form the Alq₃ interface. In undoped TPD, the diffusion length of TPD exciton is found to be no wider than 5 nm. The penetration depth of the electron injection into undoped TPD is found to be <EQ 5 nm from the Alq₃ interface. By rubrene doping, the penetration depth of electron injection seems to be extended beyond 5 nm. The dominant emission mechanism for rubrene-doped TPD is attributed to the electron-hole recombination on rubrene molecules.