Recent studies of lasing and stimulated emission in luminescent (pi) -conjugated polymers performed by our group are presented. Optical properties of cylindrical, high-Q, polymer microcavities are discussed. The emission spectra of plastic microring and microdisk lasers were measured and analyzed. Cylindrical light emitting polymer microdiodes, as possible candidates for electrically-driven plastic lasers have been fabricated. Stimulated emission and lasing were also demonstrated for polymer solutions infiltrated in opal photonic crystals. In addition, two unusual regimes of stimulated emission characterized by narrow laser-like spectral lines were found in thin waveguiding polymer films. These regimes may be associated with random optical feedback introduced by light scattering inside the polymer films and amplified Raman scattering, respectively.

Using subpicosecond transient absorption spectroscopy, we have investigated the primary photoexcitations in thin films and solution of several phenylene-based conjugated polymers and an oligomer. We identify two features in the transient absorption spectra and dynamics that are common to all of the materials which we have studied from this family. The first spectral feature is a photoinduced absorption (PA) band peaking near 1 eV which has intensity-dependent dynamics which match the stimulated emission dynamics exactly over two orders of magnitude in excitation density. This band is associated with singlet intrachain excitons. The second spectral feature (observed only in thin films and aggregated solutions) is a PA band peaking near 1.8 eV, which is longer-lived than the 1 eV exciton PA band, and which has dynamics that are independent (or weakly-dependent) on excitation density. This feature is attributed to charge separated (interchain) excitations. These excitations are generated through a bimolecular process. By comparing to samples in which charged excitations are created deliberately by doping with C₆O, we assign these secondary species as bound polarons.

Para-phenylene type molecules are efficient photoluminescence emitters in the ultraviolet-blue-green spectral range. They are used in light emitting diodes (LEDs) and photopumped lasers. Photoexcited para-phenylene type molecules give rise to strong emission from singlet excitons, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons. Since the latter two processes represent absorption of the emitted light of singlet excitons, the presence of polarons and triplet excitons might be a fundamental problem for laser diodes made from para-phenylene type molecules. In our experiments we modify the molecular geometry by the application of hydrostatic pressures up to 80 kbar in a temperature range of 10 to 300 K. In particular we show how triplet and polaron states, which are present in LEDs under operation, react to the induced geometric changes. The spectra of ground state absorption, excited state emission, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons are significantly broadened and shifted in energy. In order to explain the observed behavior we have performed three-dimensional bandstructure calculations within density functional theory for the planar poly(para-phenylene). By varying the intermolecular distances and the length of the polymer repeat unit pressure effects can be simulated.

Sub-picosecond spectroscopy and ultrafast pump-probe experiments spectrally and temporally resolve the Forster energy transfer in blends from larger gap host to the smaller gap guest organic materials. The dynamics of the stimulated emission and photoinduced absorption of the polymer blends indicate that 10 - 20 ps are required for complete energy transfer. The Forster interaction ranges suggested by quantitative measurements of energy transfer rates are compared with the theoretical values as calculated from the spectral overlap. We discuss the effect of the excited state delocalization. The energy transfer dynamics in small organic molecule blends have a longer time scale (1 ns), corresponding to the much longer lifetimes of the organic dyes.

Highly luminescent poly(arylene vinylene)s can be prepared by a range of polycondensation methods. In this paper we report the synthesis of useful monomers and their application in the Gilch dehydrohalogenation and Wittig polycondensation methods to prepare highly luminescent poly(1,4-phenylene vinylene) (PPV) homo- and copolymers for use in light emitting devices.

An experimental set-up was used to optimize the layer thicknesses of hole transport materials and electron transport/emitter material in multi-layer light emitting diodes by combinatorial methods. The method is based on a movable mask/shutter technique and simultaneous evaporation of organic molecules resulting in linear gradients of layer thickness. This allows the preparation of different devices in one single experiment under identical conditions. In the first experiment, we studied the influence of the Alq₃ layer thickness on photometric and power efficiency in two layer devices using various TPD derivatives as hole transport material at a constant thickness. Some new low molecular weight TPDs and a polymeric TPD were utilized. Both photometric and power efficiencies depend considerably on the thickness of the Alq₃ layer. In a second experiment, the efficiency dependence on both the TPD and Alq₃ layer thickness was investigated simultaneously by preparing a landscape library with two orthogonal linear gradients of TPD and Alq₃. It was observed that the device efficiencies depend on both TPD and Alq₃ layer thicknesses and additionally on the total thickness of the organic layer.

We have compared the optical and electronic properties of a series of porphyrin centered dendrimers containing stilbene dendrons. The first and second generation dendrimers could be spin-coated from solution to form good quality thin films. Incorporation into single layer light-emitting diodes gave red-light emission with maximum external quantum efficiencies of 0.02% and 0.04% for the first and second generation dendrimers respectively. We have determined by photoluminescence studies that energy can be transferred efficiently from the stilbene dendrons to the porphyrin core and that PL emission is from the core. Cyclic voltammetry studies on the dendrimers show that the reductions are porphyrin centered with the dendrons only affecting the rate of heterogeneous electron transfer between the electrode and the dendrimers. This suggests that charge mobility within a dendrimer film in an LED will be affected by the porphyrin edge to porphyrin edge distance. We have studied the hydrodynamic radii of the dendrimers by gel permeation chromatography and found as expected that the average porphyrin edge to dendron edge distance increases with generation. This is consistent with the slowing of heterogeneous electron transfer observed in the cyclic voltammetry on increasing the generation number and suggests that the dendrons are interleaved in the solid state to facilitate charge transport.

The DF and PLDMR of triplet exciton and polaron dynamics in (pi) -conjugated materials and devices is reviewed, and the significance of various processes involving these long-lived excitations is considered. These include the generation of singlet excitons by triplet-triplet annihilation, which leads to DF, and nonradiative quenching of such singlets by polarons or triplet excitons. The clear differences between the roles of these processes in (pi) -conjugated polymers vs small molecules and their implications for organic light emitting devices are discussed.

Face-to-face stacks of PTCDA molecules in films and multiple quantum wells are modeled using Frenkel and charge-transfer excitons that are coupled to a characteristic molecular vibration. The intensity and vibronic structure of film fluorescence are extensions of the mixed Frenkel-CT model for absorption and electroabsorption. Ab initio relaxation energies of PTCDA excited states and molecular ions provide independent estimates of model parameters, while the (pi) - (pi) * transition dipole gives the hopping integral J. PTCDA's close connections to conjugated hydrocarbons and its structural and electronic simplicity are emphasized.

The photophysics of a soluble, cynao-substituted poly (p- phenylene vinylene) and some related model compounds are compared using steady-state and time-resolved photoluminescence spectroscopy. The nature of the excited states in the model compounds are shown to differ markedly from the polymer as steric hindrance between the cyano group and an alkoxy group disrupts electron delocalization resulting in a twist in the molecular structure. In solution the polymer exhibits emission from both isolated chains, and from aggregates formed when the solution is allowed to equilibrate over a long period of time. The aggregate has its own distinct absorption and emission features and is assigned to a dimer- like species. When spun from the same solvent, thin films of the polymer exhibit the same emission features as the solution aggregates.

We have used pulsed radiolysis to elucidate the neutral triplet and charged excited states in soluble conjugated polymers, including poly(2-methoxy-5-(2'-ethyl-hexoxy)-p- phenylenevinylene), (MEH-PPV), poly(2,5-pyridinediyl) (PPY) and poly(4-hexyl-2,5-pyridinediyl) (HPPY) in various solvents. Using a range of triplet sensitizers we have determined the S₀ - T₁ energy separation in these polymers to be 1.27 plus or minus 0.07 eV, 2.4 plus or minus 0.1 eV and 2.5 plus or minus 0.1 eV respectively. Our experimental results confirm that photoinduced absorption features observed in the range 1 to 2 eV in these polymers, are due to triplet-triplet absorption. Triplet state lifetimes and molar absorption coefficients in solution have also been determined. This work not only demonstrates the versatility of the pulse radiolysis technique but also gives, for the first time, important and unambiguous assignment and detail on the various excited state species in conjugated polymers.

This paper describes the development of light emitting diode technology based on fluorene-containing polymers, prepared through the coupling of 9,9-disubstituted 2,7-bis-1,3,2- dioxaborolanyl-fluorene with a variety of aromatic dibromides. In these polymers the polyphenylene-like backbone provides the mechanical and chemical robustness and the C-9 of fluorene provides a site for physical property modifications without introducing significant torsional strain which would adversely affect conjugation. Polymer optical and electronic properties are tailored through selective incorporation of different aromatic unit into the AB alternating structure. LED devices emitting in blue, green, red and other colors are thus obtained.

Performance of organic electroluminescent (EL) devices has been found to depend critically on various processing parameters including the purity and deposition rate of organic materials and substrate temperature. The effects of these processing parameters were systematically investigated by using the time-of-flight measurement of carrier mobility, photoluminescence, Raman as well as photoemission spectroscopies. It was observed that carrier mobility in organic EL materials could be improved substantially by either increasing the material purity or decreasing the deposition rate. Concomitantly, the increase in carrier mobility also led to considerable enhancement in the efficiency of EL devices. By depositing organic EL materials at elevated substrate temperatures, significant improvement in EL efficiency was also obtained. The EL devices thus fabricated consisted of a crystalline hole-transporting layer (HTL). Such devices not only showed improved efficiency but also enhanced stability. The improvement in stability is attributed to the fact that the HTL was already crystalline in the fabricated device so that subsequent operation and storage of the device would not lead to further crystallization. Thus, the widely accepted degradation mechanism via operation-induced crystallization and interfacial diffusion appears not important in the devices fabricated with a crystalline HTL.

Recent results on the creation of novel emitting amorphous molecular materials and fabrication of blue or multi-color emitting organic light-emitting diodes (OLEDs) are described. It is shown that tri(p-terphenyl-4-yl)amine functions not only as a blue-emitting material with hole-transporting properties but also as a good host matrix for fluorescent dopants such as perylene. 5,5'-Bis(dimesitylboryl)-2,2'-bithiophene (BMB-2T) and bis{4-[bis(4-methylphenyl)amino]phenyl}- (alpha) -oligothiophene (BMA-nT) [n equals 1 to approximately 4)] are found to be a good blue-emitting amorphous molecular material with electron-transporting properties and good multi-color emitting amorphous molecular materials with hole-transporting properties, respectively, for OLEDs. Exciplex formation at the organic solid interface between the hole- and electron-transporting materials and its potential application for color tuning are also described.

Materials that can simultaneously act as liquid crystals, charge transport agents and emitters are of interest for potential applications in organic light emitting diodes. We report on the fabrication and characterization of single and multilayer diodes from a liquid crystalline oxadiazole derivative in a standard device configuration. Epitaxial deposition was used to produce oriented layers that led to devices with improved electrical properties.

Recent progress in the study of the energy level alignment and band bending at organic interfaces is reviewed, taking the examples mainly from the results of the group of the authors using ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and Kelvin probe method (KPM). As for the energy level alignment right at the interface, the formation of an electric dipole layer is observed for most of the organic/metal interfaces, even when no significant chemical interaction is observed. The origin of this dipole layer is examined by accumulating the data of various combinations of organics and metals, and the results indicate combined contribution from (1) charge transfer (CT) between the organic molecule and the metal, and (2) pushback of the electrons spilled out from metal surface, for the case of nonpolar organic molecule physisorbed on metals. Other factors such as chemical interaction and the orientation of polar molecules are also pointed out. As for the band bending, the careful examination of the existence/absence of band bending of purified TPD* molecule deposited on various metals in ultrahigh vacuum (UHV) revealed negligible band bending up to 100 nm thickness, and also the failure of the establishment of Fermi level alignment between organic layer and the metals. The implications of these findings are discussed, in relation to the future prospects of the studies in this field. (*:N,N'- diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine).

The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the organic luminescent semiconductor Gaq₃ relative to the Fermi level of Au was determined by depositing a Gaq₃ thin film in a multi-step growth procedure on an Au foil. Before growth and after each deposition step the sample was characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. Such measurements offer a direct way to determine the electronic structure of the interface. Our experiments demonstrate that this method allows distinguishing between band bending, charging and interface dipole related shifts in the UP-spectra. The additional XPS measurements allow the precise determination of the band bending occurring across the interface while comparison between XPS and UPS work function measurements allows one to pinpoint the organic film thickness dependent onset of charging phenomena. Our results show that the interface dipoles at Gaq₃ Schottky contacts with Au, Pt and Ag amount to 0.6 - 0.7 eV. Our experiments also show that final state screening shifts can be dismissed as insignificant in such orbital line-up measurements. This was shown at the chloroindium phthalocyanine (ClInPc)/highly oriented pyrolytic graphite (HOPG) interface where no such shifts were observed.

Three interfaces, anode interface, hole blocking layer and cathode interface were considered mainly from the viewpoint of materials. Vinyl polymers containing triphenylamine as a side group were investigated as an ITO buffer layer. When these polymers were doped with strong acceptor, they lowered operation voltage of OLED and also improved the thermal stability. Employment of high Tg hole transport material was also found effective for the thermally stable EL characteristics. Hole blocking material with a wider optical gap improved color purity of blue-emitting device. Various inorganic compounds were examined as a cathode interface layer to demonstrate that MgF2 was effective to improve operation lifetime of OLED.

One approach to increase the overall performance of organic light emitting devices is to separate the light-emitting volume from the ones which are assigned to charge injection or transport. We realized such polymer hetero-layer structures by combination of hole transporting materials like polyparaphenylenevinylene (PPV) with new electron transporting materials, i.e. heterocyclic polymers and heterocyclic low molecular compounds, especially phenyl quinoxalines (PQs). The electronic properties of these heterocyclic compounds have been investigated by various methods including ultraviolet photoelectron spectroscopy. PQs show electron affinities near 3.5 eV and ionization potentials below 6 eV. Measurements of thermally stimulated depolarization currents (TSDC) were carried out in order to study both dipolar relaxation and charge transport processes in single layer devices. The TSDC spectra revealed the prominence of both dipolar relaxation and of charge transport processes. The dipolar processes show activation energies between 0.4 eV and 1 eV which are typical values for small relaxing entities like polymer side groups. Current-voltage and current-luminance characteristics were used to study the prepared heterolayer devices. Double layers made of PPV and polyphenylquinoxaline (PPQ) are characterized by low onset voltages near 2 V and high luminous efficiency of more than 0.8 cd/A. The experimental findings show that PPQs are promising materials in the field of organic electroluminescence.

The paper discusses the carrier transport mechanism and the electrical characteristics of light-emitting electrochemical cells (LECs) based on a methyl substituted laddertype poly(para phenylene) (m-LPPP). An intensive set of measurements, such as Current-Voltage (I-V), Capacitance- Voltage (C-V) and temperature investigations are used to track the formation of the highly conductive contacts, the initial diffusive carrier transport regime, the onset of a net voltage drop across the polymeric bulk and the final resistive behavior of the device in which carriers are driven by drift. In particular, the strong increase of the capacitance is shown to be a signature of the large carrier injection through the contacts and their diffusion through the active layer. Clearly visible hysterisis behavior in the I-V and C-V plots are also discussed to suggest polymer electrochemical doping at the interfaces.

Polymer light-emitting diodes (LED) with aligned conjugated molecules and polymers as the active layer can emit polarized light as shown recently in several reports. We have synthesized several liquid-crystalline polyfluorenes with different side chain patterns. The thin polymer films were aligned at elevated temperatures on a rubbed polyimide layer. For the most suitable substitution, a dichroic ratio of twelve was observed in absorption. This polymer was used to construct light-emitting devices with ITO and Ca electrodes. Suitable hole transport molecules were added to the polyimide in order to obtain layers with good alignment properties and large hole mobility. Polyimide and poly-fluorene layer thicknesses were varied to optimize the device performance. For those optimized devices, blue light with a dichroic ratio in emission of fifteen was measured and the luminance was approx. 100 Cd/m² at 18 V. Even though these values are still below the requirements in application, the high dichroic ratio combined with a reasonable brightness represents an important step towards the use of organic LEDs as illumination sources in LCD displays.

We report on the observation of switching or 'off-phase' electroluminescence (EL) spikes in thick devices (300 nm to 600 nm) based on poly-2-methoxy,5-(2'-ethylhexoxy)-1,4- phenylenevinylene (MEH-PPV) conjugated polymer sandwiched between Aluminum and Indium-Tin-Oxide (ITO) electrodes. We also report on impedance spectroscopy studies on the same samples. Transient EL measurements were carried out using voltage pulses with pulse width in the range between several microseconds and 10 nanoseconds and voltage pulse height up to 1000 V. The short pulses are obtained using a home-built voltage pulse source consisting of inorganic photo-switch governed by light pulses from Nd:YaG laser. The electrical pulse rise-time is determined by the laser pulse rise-time, which is around 3 nanoseconds. The off-phase EL is manifested by the appearance of two EL spikes at the turn-on and the turn-off edges of the voltage pulse; for very short voltage pulses the two spikes merge. We demonstrate that the EL emission spectrum associated with the off-phase EL is similar, although not identical, to the regular injection EL spectrum of MEH-PPV, suggesting that the off-phase EL is also due to singlet exciton recombination. We show, for the first time, transient EL as a function of the pulse height up to 200 V using pulse width as narrow as 15 nanoseconds. For such strong and short pulses the EL emission is dominated by the off-phase EL mechanism. We provide evidence from impedance spectroscopy that one of the relaxation processes has a characteristic time of the order of magnitude of the time scales observed in the off-phase EL. This may shed light on the relaxation processes in the switching EL.

Cyanine dye electroluminescence was observed in single layer light-emitting diodes based on electron-hole conducting polymers. The appearance of light emission in the visible range depended on the reduction/oxidation potentials of the dyes used. In polymer systems, the electroluminescence of cyanine dye J-aggregate nanocrystalline phase was detected for the first time. J-aggregate/polymer composites exhibiting intense absorption band with a maximum in the red range were developed on the base of polyimides and a copolymer coformed from phenylacetylene and 4-methylcoumarin-4-pentynoate doped with the dye luminophores. Efficient electrolumunescence was revealed in these systems. It was demonstrated that J- aggregates play an active role in charge carrier transport in the composite materials. In particular, anthracene-containing polyimides exhibiting electron-hole transport appeared to be appropriate media for the generation of J-aggregate electroluminescence.

We review our research activities in the field of organic light-emitting diodes (OLED's) aiming practical applications to full-color flat panel displays. The central issue of the research is to develop the suitable materials. The side-by- side patterning of discrete RGB sub-pixels without using dopants is straightforward and attractive from aspects of energy conversion efficiencies and productivity in comparison with the other methods proposed so far. We have been concerned in emitting material systems which emit R, G and B lights respectively using a common set of a hole-injecting layer, a hole-transporting layer (HTL), and electron transporting layer (ETL). Our research goal is to develop the good performance OLED matrices without using fluorescent dopants. Green light is obtained with an ordinary single hetero-structure. Blue light is achieved from the hole-transporting layer by inserting a hole-blocking layer between the HTL and the ETL. The maximum emission intensity was about 10,000 cd/m² at the applied voltage of 9.5 V and the color was as good as (0.15, 0.16) in the CIE chromaticity coordinates. Our current research focuses on new red materials, which are suitable for the layered structures. These materials systems would provide full-color display panels with the minimum number of materials used.

An organic light-emitting diode (OLED) microdisplay is described. The OLED display has a top-emitting structure integrated on single-crystal active-matrix driver chips and is sealed in a fully hermetic package. All processing and fabrication is done on 8-in. wafers until final dicing and assembly. The basic structure of the displays, the key manufacturing and processing steps, and initial performance of a monochrome green active-matrix OLED device structure are presented. Key advantages of the chosen approach as well as remaining challenges and requirements for improvements are discussed.

In this paper the fundamental properties underlying the transfer of organic fluorescent dyes to local areas in polymer thin films by both liquid phase (ink-jet printing) and evaporation/diffusion transport methods are examined, with the goal of achieving full color displays based on organic light emitting diodes made from such polymers. Ink-jet printing offers a simple non-contact method for forming patterns, but a critical issue is the redistribution of dyes and other molecules in the liquid droplet before it dries. Masked large area evaporations allows one to rapidly pattern large areas, but its rate depends on the ability of dyes to diffuse through polymer films.

Ultra-violet and X-ray photoemission spectroscopy and current- voltage measurements were used to investigate the fundamental mechanisms responsible for the improvement of hole injection between modified indium-tin-oxide (ITO) surfaces and the hole- transport layer (HTL) of an organic light emitting diode. Two ITO surface modification techniques were investigated: oxygen- plasma treatment and deposition of an ultra-thin organic interlayer between the ITO and the HTL. We demonstrate that the improvement in injection is due to an increase in surface work function of ITO mediated by the presence of an oxygen radical in the first case, and to the presence of an intermediate energy level between the ITO Fermi level and the HTL highest occupied molecular orbital in the second.

A buffer layer is often placed between an ITO (indium-tin- oxide) electrode and a hole transport layer (HTL) of organic EL devices made of low-molecular-weight materials. Cu- phthalocyanine is the representative material of the buffer layer. Form the analysis of the current-voltage properties of the devices, we found that buffer layer hinders the current when it is very thin, 60 nm or less. On the other hand, when it is thick, it enhances the current. In order to clarify these effects of the buffer layer, we studied the hole injection process from the ITO electrode into the HTL in devices with and without the buffer layer. The results indicate that the holes are accumulated at the buffer layer/HTL interface because of the energy gap. These holes are injected into the HTL across the barrier. However, some of the holes are back transferred to the ITO. This rate is dependent on the thickness of the buffer layer and the voltage applied to the devices. The competition between forward and backward movements of the accumulated holes determines the current- voltage characteristics of the organic EL devices.

Two transparent conducting oxides (TCO) thin films including tin-doped indium oxide (ITO) and aluminum-doped zinc oxide (AZO) were grown on glass substrates by pulsed laser deposition (PLD). The structural, electrical and optical properties of these films were investigated as a function of target composition and film growth temperature. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm² at growth temperatures ranging from 25 degrees Celsius to 400 degrees Celsius in oxygen pressures ranging from 1 to 100 mTorr. For a 300 nm thick ITO film deposited at 300 degrees Celsius in oxygen pressure of 10 mTorr, the resistivity was 2 X 10^-4 (Omega) -cm and the average transmission in visible range (400 - 700 nm) was 85%. The Hall mobility and carrier density for a 150 nm thick ITO film deposited at 300 degrees Celsius were 27 cm²/V-s and 1.4 X 10²¹ cm^-3, respectively. For a 100 nm thick AZO film deposited at 200 degrees Celsius in an oxygen pressure of 5 mTorr, the resistivity was 3.8 X 10^-4 (Omega) -cm and the average transmission in visible range (400 - 700 nm) was 90%. The Hall mobility and carrier density for the same AZO film were 18 cm²/V-s and 9.1 X 10²⁰ cm^-3, respectively. AFM measurements indicated that the RMS surface roughness of the ITO films (approximately 5 angstrom) was slightly lower than that of the AZO films (approximately 7 angstrom). XPS measurements showed that the work function of ITO films grown at 250 degrees Celsius was 4.51 plus or minus 0.05 eV, which is higher than that (4.05 plus or minus 0.05 eV) of the AZO films grown at 200 degrees Celsius. The PLD ITO films were used to fabricate organic light-emitting diodes (OLEDs). The electroluminescent (EL) performance was measured and the luminous power efficiency was calculated to be 0.6 lm/W, which is comparable to that measured with commercially available sputter-deposited ITO anodes.

Indium tin oxide (ITO) is commonly used as a hole-injecting electrode in organic electroluminescent devices due to its transparency and good electrical conductivity. Various solutions of acid and base have been used to modify the work function of ITO. Depending on the solution, a negative or positive work function shift with respect to that of the standard ITO has been obtained. We have investigated the interface formation between treated ITO and N,N'-bis-(1- naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB), an organic materials often used as hole transport layer in OLEDs, using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The barrier for hole injection, which is defined as the energy difference between the Fermi level and the highest occupied molecular orbital of NPB, decreases from base-, to standard, to acid-treated ITO. No significant reactions is observed for NPB deposited on standard ITO. For the acid- treated ITO substrate, the XPS results point to the reaction between the proton of the adsorbed acid layer and the NPB nitrogen. XPS results also suggest that at low NPB coverage, islanding is occurred at the standard ITO surface.

The intrinsic degradation of hydroxyquinoline aluminum (AlQ₃)-based organic light emitting devices, that leads to the long-term decrease in the electroluminescence efficiency of the devices operated under constant current conditions, has been studied. The role of stabilizing agents, such as introducing a copper phthalocyanine buffer layer at the hole injection contact, doping of the hole transport layer, and using mixed layers of hole and electron transport materials has been investigated. Devices, which allow predominantly holes to be transported through the AlQ₃ layer, showed significant decrease in photoluminescence after prolonged current flow. These results lead to the conclusion that the degradation of AlQ₃ cations is the major cause of intrinsic long-term device degradation. This mechanism also explains some new results on the degradation of devices containing dual layer and doped hole transport layers as well as the increase in lifetime of devices containing more efficient electron injecting contacts.

The temperature stability of OLEDs was investigated by observing the I-V and EL-V characteristics of various devices stored at elevated temperature (up to 140 degrees Celsius). Results reported in this paper concern the standard KODAK structure for a green OLED (i.e. anode/CuPc/NPB/AlQ₃/cathode), the standard IDEMITSU structure for a blue OLED (i.e. anode/CuPc/NPB/DPVBi/AlQ₃/cathode) and variants of those using high T_g materials consisting of a spiro- bifluorene core. Use of Spiro-TAD as a hole transport material (HTM) and of Spiro-DPVBi as an emitting material (EM) resulted in considerable improvements. While the initial performance of the virgin devices is considerably unchanged, the temperature stability increases dramatically: for the green OLED no significant deterioration up to 140 degrees Celsius is found, compared to the standard device including NPB already starting to degrade slightly above 100 degrees Celsius; the blue OLED is stable up to approximately 120 degrees Celsius (particularly the color coordinates of the emitted light) whereas the standard device using DPVBi already deteriorates at around 80 degrees Celsius.

Charge trapping will have a strong effect on the performance of organic light emitting diodes. Here different models for such trapping in disordered organic semiconductors are presented. The benefits of different transient experimental techniques are explored. Results are presented for electroluminescent polymer diodes which are fully depleted or contain a depletion region type Schottky barrier. The transient behavior can be explained by a single energy trap site emptying to a Gaussian distribution of transport sites.

We present a temperature-dependent single carrier device model for polymer light-emitting diodes. The model includes both the injection of charge carriers over a barrier and the transport of charge across the device. To test the model, the temperature dependence of an LED based on the conjugated polymer poly[2-methoxy, 5-(2'ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with indium tin oxide (ITO) and aluminum contacts was studied. Good agreement with experiment is found using a strongly field and temperature dependent mobility. Current-voltage characteristics were fitted over a temperature range from 100 K to 300 K using three parameters: the barrier to injection, the zero-field mobility, and the field dependence of the mobility. The resulting mobility parameters have an activation energy type form and are found to vary with temperature according to previously reported results. The barrier height to injection is found to decrease strongly between 300 K and 220 K, but decreases more slowly below 220 K. This reduction with temperature is found to relate to the red-shift of the absorption peak of MEH-PPV. The model is used to fit current-voltage characteristics of aged devices. The effect of photo-oxidation is well described by the model through a reduction of mobility at constant barrier height, giving insight into the effects of the creation of charge trapping carbonyl groups in the bulk polymer on injection and transport.

We report electroluminescence (EL) of polyimides prepared from 9,10-bis(m-aminophenylthio)-anthracene (BPTA) and 1,3-bis(3,4- dicarboxyphenoxy)benzene or 2,2-bis[4-(3,4- dicarboxyphenoxy)phenyl]-propane dianhydrides. The new polyimide-type of polymers, aromatic polyimides with and without sulphur atoms in the backbone, were investigated as electron-hole transporting and light-emitting materials for use in single- and multilayer electroluminescent diodes. These polymers belong to a new class of donor-acceptor compounds and have some advantages over their counterparts. They are efficient electron and hole conductors and known as photoluminophores exhibiting intense photoluminescence of exciplex origin. Some of them were used earlier as hole conducting layers together with tris(8-quinolinolato)aluminum complex as an electron conducting layer in bilayer LEDs of high brightness. Donor-acceptor interactions determine optical, electrical and photoelectrical properties of these polymers. A direct correlation was revealed between transporting characteristics and electroluminescent properties for these electroactive materials. At room temperature, the electron and hole drift mobilities directly measured by conventional TOF techniques indicated effective bipolar transport. The simplicity of synthesis, high thermal stability, organic solvent solubility together with excellent film-forming properties make the polymers potentially of interest for technological applications. Properties of the uni- and bilayer LEDs based on these materials are discussed in terms of the band structure, bipolar transport and electron donor-acceptor interactions.

Incoherent fluorescence optical kerr gate can be used in principle to measure fluorescence anisotropies and determine molecular reorientational times. In this work a novel method for fluorescence anisotropy using Optical Kerr Gating with incoherent laser light is presented. We have obtained incoherent optical kerr signals for parallel and perpendicular fluorescence polarization for a 10^-3 M solution of rhodamine 6G in ethanol.

The electronic structures of 8-hydroxyquinoline aluminum (Alq₃)/electron injection layer/Al interfaces, used in organic electroluminescent devices, were measured by ultraviolet photoelectron spectroscopy (UPS). LiF and alkaline earth fluorides (CaF₂, SrF₂ and BaF₂) were used as an electron injection layer. Shifts of the highest occupied molecular orbital (HOMO) level and the vacuum level of Alq₃ layer due to the insertion of the fluorides were observed. These shifts indicate that the alkaline earth fluoride layers as well as the LiF layer at the Alq₃/Al interface reduce the barrier height for electron injection from the Al to Alq₃. The reduction of the barrier height is consistent with the driving voltage in the organic EL device in which these fluorides are used as the electron injection layers. We believe that lowering in the driving voltage in organic EL devices with the thin insulator layers, such as LiF and alkaline earth fluorides, is attributable to the reduction of the barrier height.

The starburst molecules derived from triphenylamine have attracted much attention in relation to the application to organic electroluminescent devices since they have good thermal stability and hole-transporting nature due to their amorphous character. We performed UV photoemission spectroscopic (UPS) study of electronic structures of starburst molecules and their interfaces with indium tin oxide (ITO). The sample molecules studied were 1,3,5-tris(2- methylphenylphenylamino)benzene (o-MTDA), 4,4',4'-tris(3- methylpheyl phenylamino) triphey-lamine (m-MTDATA), 1,3,5- tris[4-(3-methylphenylphenyl amino)phenyl[benzene (m- MTDAPB), and 1,3,5-tris [N-(4- dipheylaminophenyl)phenylamino] benzene (p-DPA-TDAB). The observed ionization potentials were 5.4 plus or minus 0.1 eV, 5.0 plus or minus 0.1 eV, 5.45 plus or minus 0.05 eV, and 5.15 plus or minus 0.05 eV, for o-MTDA, m-MTDATA, m-MTDAPB, and p- DPA-TDAB, respectively. The whole valence region of UPS spectra were measured by using synchrotron radiation. The difference in bulk electronic structure among these molecules was discussed in comparison with MOPAC molecular orbital calculation. At ITO interfaces, the vacuum level shift was observed for these materials, indicating that the traditional model with an assumption of a common vacuum level at organic/metal interface is not valid even in the case of ITO electrode. The direction of the shifts was negative, i.e. the vacuum level of the starburst molecules is below that of the ITO electrode. The magnitude of the shift was dependent on the surface cleanliness of ITO substrate.

We report on the fabrication and properties of single layer blue light-emitting diodes (LEDs) based on conjugated polymer blends. We have used poly(9,9-dioctylfluorene) (PFO) as the host and a hole transport triarylamine/fluorene copolymer as the guest. Despite the fact that the photoluminescence quantum efficiency of the blend is lower compared than that of the host and guest polymers on their own, an enhancement in both the electroluminescence quantum and power efficiency is seen for the blend. This observation indicates that the hole transport material leads to a significant improvement in hole injection and transport and a greatly improved charge carrier balance factor. A careful comparison of the photoluminescence and the electroluminescence spectra reveals that more emission originates from the guest polymer for electroluminescence than for photoluminescence. This can be rationalized by the expectation that both Forster transfer and charge transfer from the host to the guest occur under electrical operation of the device. Only Forster transfer is expected for optical excitation. A much higher brightness and a lower turn on and operating voltage is achieved for the blend. The emission from our optimized blue single layer LED reaches a maximum brightness of 1550 cd/m² and has a maximum external quantum efficiency of .4% and a maximum power efficiency of 0.3 lm/W.

This work reports on color controlled light converters fabricated from organic dyes (coumarin, DCM) and luminescent organic (Alq₃) and inorganic (SrGa₂S₄:Eu⁺⁺) molecules dispersed in a transparent polymer host matrix. These blends were pumped with a commercial GaN blue LED having a peak emission at 430 nm. First are reported the photoluminescence spectra of Alq₃- DCM and coumarin-SrGa₂S₄:Eu⁺⁺-DCM blends. By photoexcitation of these blends with GaN blue LEDs (which is partially absorbed by the dispersed dyes), it has been observed that the color can be tuned from blue to red depending on the composition. In particular, very pure white emitting hybridLEDs could have been obtained with organic- inorganic blends. Observed luminescence is due to cascade light absorption and emission from the various dyes in the blend. The influence of the polymer host matrix on the emitted color was also evidenced. Our results show how multicolor and white emitters could be fabricated with commercially available organic materials and rare earth doped inorganic dyes.

In this paper we report on Organic Light Emitting Diodes (OLEDs) based on heterojunctions between a polymer, Poly(N- vinylcarbazole) (PVK) as hole transport layer (HTL) and an evaporated layer of Tris (8-Hydroxy)quinoline) Aluminum (Alq₃) as electron transport layer (ETL) and emissive layer (EM). The electrical properties of ITO/PVK/Alq₃/Mg-Ag were thoroughly investigated and Trapped Charges Limited (TCL) currents were shown to be the main transport process in these devices. In order to assess the pertinence of PVK as HTL, a comparison with other HTL deposited by vacuum evaporation was carried out. The color of the emitted light then could have been tuned by adding DCM in PVK. Light emitted by Alq₃ was partly absorbed by DCM thus exciting the photoluminescence of DCM. Exiting light was thus a combination of green-yellow characteristic of Alq₃ and of red-orange due to DCM. Wavelengths spanning the range 530-620 nm were obtained for DCM concentrations in PVK up to 20% wt. without significant degradation of the external quantum yield.

We demonstrate ultrafast (100 fs) carrier generation in poly(phenylene vinylene), PPV, and poly[2-methoxy-5-(2- ethyl-hexyloxy)-(phenylene vinylene)], MEH-PPV, by using femtosecond transient spectroscopy in the mid-IR in order to probe the infrared active vibrational active (IRAV) modes. The 10% carrier generation quantum efficiency in MEH-PPV with no electric field applied, implies primary photogeneration of charge carriers. The recombination dynamics in PPV and MEH-PPV indicate that the carrier lifetime is sensitive to the strength of the interchain interaction.

A combinatorial approach combining vapor deposition of organic molecules and a mask technique was used to prepare on one substrate a matrix of 49 organic light emitting diodes (OLEDs) with different configuration and layer thickness. A landscape library with two orthogonal, linear gradients of an emitter and a hole blocking electron transport material on top of a hole transport layer of constant thickness was prepared. The aim of this experiment was to investigate the influence of an additional electron transport material on the efficiency. Using a semi-automated measurement set-up, the device parameters for each of the 49 OLEDs were evaluated. The existence of an optimum Alq₃ layer thickness for two-layer devices ITO/TPD/Alq₃/Al is confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq₃ layer. But an improvement of photometric efficiency can be obtained by replacing the optimum Alq₃ layer thickness by certain combinations of Alq₃/spiro-Quinoxaline layers.

We present a study that focuses on a comparison of the absolute photoluminescence quantum efficiency ((phi) _pl) with the electroluminescence quantum efficiency ((eta) _el) using a guest-host active layer. We also report the luminous power efficiency of devices based on this same emissive layer. The active layer consists of a series of metal quinolates used as the host for the dopant rubrene. We find that rubrene doping enhances the (phi) _pl of the metal quinolate host materials, tris(8-hydroxyquinolinato) gallium III (Gaq₃), tris(8-hydroxyquinolinato) aluminum III (Alq₃), and tris(4-methyl-8-hydroxyquinolinato) aluminum III (Almq₃), from 0.13, 0.25, and 0.42, respectively, to approximately 1.0 for all metal quinolates. This is achieved by efficient Forster energy transfer from host to guest molecules. We also find that doping enhances the (eta) _el of devices using Gaq₃ or Alq₃ as the active layer from 0.6% and 1.0%, respectively, to 2.2% for both hosts when measured at a current density of 100 A/m². The (eta) _el of devices based on Almq₃ increases from 1.6% to 2.9% upon doping with rubrene. At a brightness of 100 cd/m², the luminous power efficiency of devices based on the metal quinolates increases from 1.0 lm/W, 0.99 lm/W, and 2.1 lm/W to 3.6 lm/W, 4.0 lm/W, and 3.8 lm/W for Gaq₃, Alq₃, and Almq₃, respectively, when doped with an optimized concentration of rubrene. These enhancements are attributed to carrier trapping followed by direct recombination on the rubrene dopant as well as efficient energy transfer from the host to rubrene.