Light emitting electroluminescent devices have been studied in which the conjugated light emitting polymer is separated on both sides from the device electrodes by a film of nonconducting polyaniline. The devices operate under an AC applied potential.Aluminum, copper or gold serve as the metal electrodes. Flexible, completely organic polymer dispersed liquid crystal light valves have been fabricated from transparent plastic substrates on which a conducting film of polypyrrole has been deposited. A new concept, “microcontact printing”, is being investigated for patterning the polypyrrole.

New oxadiazole-based polymeric materials have been synthesized either as side chain copolymers with polymethacrylate or as main chain copolymers carrying solubilizing elements such as hexafluoropropylidene or meta-linked aromatic spacer units. Many of these materials exhibit blue luminescence in the solid state. The materials have been evaluated by photoluminescence and cyclic voltammetry studies, and as the electron transporting layer in a two-layer electroluminescent device with poly(p-phenylenevinylene) (PPV) as the emissive layer. The major conclusion is that these materials function by providing large heterojunction offsets at the Highest Occupied Molecular Orbital (HOMO) which block the passage of holes through the device.

Electroluminescent devices have been made from organo-soluble derivatives of poly(pphenylene). Solubility and processibility by the LB-technique is achieved by attaching alkoxy side groups to the backbone-p-phenylene units. These polymers are of the hairy-rod (HR) type. If transferred as monolayers from the air-water-interface, monodomain multilayers with large order parameters of chain orientation are obtained. A 130 nm thick LB-film of poly(2,5-diisopentoxy-pphenylene) shows blue photoluminescence at λmax = 3.08 eV (404 nm) with a tail extending to 2 eV. The anisotropy was (lII- l1)/ (l11, + l11)= 0.5. This LB-film between a transparent gold and an evaporated Al-electrode shows polarized light emission at E ≤ 6.107 V cm−1 with am=a λmax2.2 eV and an in-plane anisotropy of 0.54. Thin films obtained by spincoating of the same polymer show isotropic electroluminescence between ITO and Al-electrodes with an external quantum efficiency of about 0.03 %. Higher efficiencies up to 4 % were realized optimizing the device architecture and the electrodes. Photocrosslinkable sites are introduced as side groups to the poly(pphenylene) chain. This allows patterning of the LEDs. General features of the supramolecular architecture and typical defect structures occurring in films of polyconjugated macromolecules are discussed using prototypical polymers as examples. Important effects are chain segregation according to chain length and formation of disclinations.

We have identified excited-state singlet oxygen as a reactive intermediate in the solid state photo-oxidation of two poly(p-phenylene vinylene) (PPV) derivatives, poly(2,5-bis(5,6- cholestanoxy)-1,4-phenylene vinylene) (BCHA-PPV) and poly(2-methoxy,5-(2'-ethyl-hexoxy)- 1,4-phenylene vinylene) (MEH-PPV). Singlet oxygen is photosensitized via energy transfer from the polymer and undergoes 1,2-cycloaddition across the electron-rich vinyl double bond in the backbone of the polymer resulting in the formation of highly oxidized species such as esters. Volatile carbonyl species are also formed, and, consequently, the film thins. This degradation pathway occurs when the polymer film is exposed in air to light having energy above the bandgap of the polymer, from either a low-pressure UV mercury lamp or an Ar+ laser operating at 514 or 457 nm. Interestingly, the singlet oxygen reaction does not take place in model compounds with similar structures such as trans-stilbene. In an effort to understand which properties of the polymer make it susceptible to singlet oxygen attack, we have studied the effect of conjugation length and side groups on the reactivity of thin polymer films. Specifically, the role of electron withdrawing groups, steric protection of the vinyl double bond, and conjugation length effects have been investigated using both FrIR spectroscopy and semi-empirical computational chemistry calculations.

The present communication proposes a mechanism for photoluminescence (PL) and electroluminescence (EL) quenching in aluminum(III) 8-hydroxyquinoline (Alq3) chelates. Our experiments indicate that in the presence of moisture, ligand exchange of 8-hydroxyquinoline with water can occur. At elevated temperatures, the liberated 8-hydroxyquinoline undergoes a condensation reaction in the presence of traces of oxygen, generating a dark, non-emissive polymeric compound. Spectroscopic and electrochemical methods were used to determine the band gap and energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) of this byproduct. Steady state PL experiments indicate that (0.5 to 1%) concentration of this byproduct, evenly dispersed in Alq3 films, results in dramatic PL quenching. Deliberate insertion of an approximately 50 Å thick film of this byproduct into the interface of a poly(p-phenylenevinylene)/Alq3 light emitting diode (LED) completely quenches the EL as well. Initial data suggest an energy transfer quenching mechanism.

PPV based light emitting thin film devices were fabricated using a layer-by-layer deposition technique involving the alternate spontaneous adsorption of a PPV precursor polymer and either poly(styrene-4-sulfonate) (SPS) or poly(methacrylic acid) (PMA). It was demonstrated that the polyanion used to self-assemble the PPV precursor strongly influences the characteristics and performance of the resulting LEDs. Devices fabricated with PPV created in the presence of SPS exhibited symmetric I–V curves, low luminance levels and very high current densities while PPV/PMA devices exhibited luminance levels in the range of 10–60 cd/m2 and classical rectifying behavior. These dramatic differences are primarily due to a low level of p-type doping activated during the thermal conversion of PPV and/or during device operation that confers excellent hole carrier transport capabilities to the PPV/SPS combination. Fabrication of a multi-slab type heterostructure device comprised of a PPV/SPS block (hole transporting block) and a PPV/PMA block (emitting block) resulted in improved performance with luminance levels significantly higher than previously obtained for a single slab PPV/PMA device (typically > 100 cd/m2). It was also demonstrated that the presence of very thin (about 20–30 Å thick) insulating layers at the Al/polymer interface improves device efficiency by a factor of 2–4.

We have investigated the performance of a class of heterocycles, 5,10-dihetera- 5,10-dihydroindeno[3,2b]indenes, as hole transport agents in simple double heterostructure organic light-emitting diodes with tris(8-hydroxyquinoline)aluminum (Alq). The best of these materials, 5,10-dihydroindolo[3,2b]indole, yields devices with luminance and lifetimes comparable to those obtained using N,N′-di-(3-methylphenyl)-N,N′- diphenyl-4,4′-diaminobiphenyl (TPD) as a hole transporting material.

We describe the principles of operation and device characteristics of novel organic light emitting diodes in which the emission originates in a number of optically active layers. The effective emission color can be controlled by adjusting the thicknesses of the individual layers, and in this manner white and other unsaturated color LEDs with external quantum efficiency > 0.5% have been fabricated. The maximum luminance that has been achieved is ∼4,700 Cd/m2.

Photogeneration of charge carriers and radiative excited states in pristine and photooxidized tris-(8-hydroxyquinoline) aluminum (Alq3) were studied by simultaneously measuring the photoconductive response and the kinetics of photoluminescence with subnanosecond resolution. 25 ps laser pulses at 355 nm were used for the excitation of vacuum-evaporated films of Alq3 2–4 μm thick. At equal excitation intensities the time dependence of the photoconductive response was very similar to that of the photoluminescence, independent of whether the sample was pristine or partially oxidized. Photooxidation of Alq3 substantially decreases the magnitude and lifetime of the photoconductivity and the photoluminescence. The decrease is shown to be a result of quenching of singlet molecular excitations by product(s) of photooxidation of Alq3. Photogeneration of charge carriers is controlled by the mutual annihilation of singlet excitons.

We present the optical and electronic properties of a new laddertype poly(paraphenylene) (LPPP), the m-LPPP, having the highest intramolecular order and highest environmental stability in the familiy of the LPPPs up to now. The m-LPPP is an excellent candidate for the usage as an active layer in electroluminescence (EL) devices, due to its well defined structure and high purity, as the spectral behaviour of the absorption coefficient cc confirms. cx is very low (αmin <12 cm−1) in the subgap region with a steep onset to the interband transition leading to welir"resolved vibrationally split maxima (αmax ≈ 1.1×105 cm−1). Moreover, the high quantum efficiency of fluorescence (up to 30 % in films) makes the m-LPPP a promising material for high efficiency-EL devices.

The polymer-light emitting diodes (PLEDs), which were fabricated in air stable sandwich structures ITO/m-LPPP/Al, are characterized by high quantum efficiencies up to 4% and cwoperation lifetimes around 170 h. The PLEDs produced homogeneous bright light in the blue and blue-yellow spectral range, with the dominant emission maximum located at 461 nm. The onset field significantly depends on the thickness of the m-LPPP layers (Emin ≈ 0.5 MV/cm; Emax ≈ 2.6 MV/cm) due to ordering effects in the cast polymer. In the blue-green spectral range, the shape of the EL spectra and hence the emission colour can be controlled, by varying the applied electric field.

In an effort to better understand the relationship between molecular structure and photophysical properties, we have prepared and investigated a series of novel poly(2,5-dialkoxy-p-phenyleneethynylene)s. Wide angle X-ray diffraction measurements show that the supramolecular structure can be easily and significantly influenced by the nature of substituents covalently linked to the rigid-rod polymer main chains. Polymers which have sterically hindered side chains are essentially amorphous, while those with only linear side chains can form lamellar structures with a significant degree of long-range order. High photoluminescence quantum yields, up to 0.86 in solution and 0.36 in the solid state, have been measured. While the solution quantum yields are independent of the functionalization, solid state quantum efficiencies were found to be related to the degree of long-range order in the samples. In samples with a high degree of long-range order, the close proximity of the coplanar oriented polymer backbones is assumed to lead to the formation of eximer complexes which provide non emissive decay channels and, hence, result in comparable low photoluminescence quantum yields. In samples that adopt only a small extent of long-range order, the rigid-rod conjugated polymer backbones behave as if they were ‘dissolved’ in a hydrocarbon solvent and consequently high quantum efficiencies are obtained. Preliminary results indicate the suitability of these polymers as the emitting layer in electroluminescent devices.

Electroluminescent diodes utilizing poly(3-alkylthiophene) (PAT) containing fluorescent dyes have been fabricated and their unique enhancement of emission efficiency has been studied for PAT diodes doped with different fluorescent dye materials. Remarkable enhancement of electroluminescent efficiency has been observed, and the mechanism of emission enhancement in PAT diode by doping of fluorescent dye has been discussed in PAT with different aLkyl-side-chain length.

The relationships between emission quantum efficiency and emissive regions in organic thin-film electroluminescent (EL) devices were studied. As an emissive layer (EML) and an electron transport layer (ETL) material, 9, 10-bis(4-diphenylaminostyryl)anthracene and 1,3- bis(4-tert-butylphenyl-1,3,4-oxadiazolyl)phenylene, respectively, were used. A zone doped with 2,4-bis(4-diethylamino-2-hydroxyphenyl)-1,3-dihydroxycycrobutenediylium dihydroxide was formed in an EML. The relationships between the emission intensities from the dopant and the positions of doped zones gave information on the emissive regions in each EL device. The emissive region in the single-layer (SL) device consisting only of an EML extended over the EML. That in the two-layer device (DL-E) in which an EML was combined with an ETL was located within 10 nm-wide region near the EML/ETL boundary. Moreover, the emission efficiency of the DL-E device was found to be about 20 times as high as that of the SL device. Therefore, it was found that the carrier recombination within the narrow region sufficiently apart from electrodes gave high emission efficiency.

Excited state intramolecular proton transfer (ESIPT) has been demonstrated in new intramoleculary hydrogen bonded (IHB) polymers of interest as photostabilizers, triplet quenchers, photochromic materials, laser dyes and electroluminescent materials. The new IHB polymers containing the 2-(2-hydroxyphenyl)benzoxazole moiety in the main chain were used to explore the effects of polymer structure, extended conjugation and competition with excimer formation on the ESIPT process. It was found that polymer structure, and particularly extent of conjugation, affects an IHB polymer's ability to exhibit ESIPT.

We report the electroluminescence behaviour of the devices made with the polymer blend of two emissible polymers, poly(2-methoxy,5-(2′ethyl-hexoxy)-1,4-phenylene vinylene)(MEHPPV) and poly[ 1,3-propanedioxy- 1,4-phenylene- 1,2-ethenylene-(2,5-bis(trimethylsilyl)- 1,4- phenylene)-l,2-ethenylene-l,4-phenylene](B-polymer) luminescing at 580 nm and 480 nm respectively. The quantum efficiency increases with the composition ratio of B-polymer by two order of magnitude of that of MEH-PPV. Luminescence behaviour of the blend system was explained by hole blocking, exciton transfer behavior of the matrix polymer (B-polymer) and efficient confinement of the injected carriers in MEH-PPV.

The influence of laser irradiation on an electroluminescent polymer, poly(2,5-di-n-hexyloxy-l,4- phenylene vinylene) is studied. Photoluminescence, infrared spectroscopy, and photothermal deflection spectroscopy were used to identify changes in the optical properties of the polymer after laser irradiation. It is found that optical irradiation leads to the opening of the vinyl double bond and decrease in conjugation length. Photo-oxidation of the polymer is believed to be responsible for the optical degradation of the polymer.

Most conjugated polymer-based light-emitting devices have been shown to be tunnel diodes which can only operate under forward DC driving field. Recently we have reported the fabrication of symmetrically configured AC light-emitting (SCALE) devices based on heterocyclic aromatic conjugated polymers. By adding an “insulating” layer (e.g. emeraldine base (EB) form of polyaniline) on both sides of the emitting layer, the SCALE devices emit light under both forward and reverse DC bias as well as AC driving voltage. The SCALE device structure ITO/J/emitterFl/M, has been shown to be quite general, and can be applied to a variety of electroluminescent polymers (emitter), insulating polymers (I) and electrode materials (M). Here we summarize and compare the performance of SCALE devices fabricated with different emitter, insulator, and electrode materials. The role of the insulating layer in the SCALE device operation is examined and a model that emphasizing the interface states is proposed to account for the device operation.

Recently we have reported, significant second harmonic generation from spin coated and bulk films of a soluble polydiacetylene (PDA), namely poly{8-[[[butoxy carbonyl] methyl]urethanyl]-1-(5-pyrimidyl)-octa-l,3,-diyne}} (BPOD) without recourse to poling. Poly(BPOD) is an asymmetrically substituted PDA in which a polarizable heteroaromatic pyrimidyl group is attached as one of the side groups. In addition to reviewing the earlier results, we report a series of new soluble PDAs of the same class with further functionalization, elaborating on the already defined design principle.

A second class of polymers with azo dye based NLO chromophores incorporated into the structure has been processed optically to fabricate holographic elements. Trans-cis-trans isomerization driven reorientation and macromolecular motion leads to the formation of surface relief gratings upon exposure to an interference pattern. Fabrication of optical elements based on these polymers will be elaborated.

Three approaches to the construction of high-Tg polyimides and polyureas as second-order NLO materials are discussed. In the first approach, copolymerization of bismaleimides with o,o‚-diallylbisphenol-A followed by functionalization with high-β NLO chromophores using a Mitsunobu procedure yields, after poling and curing, a series of heavily crosslinked chromophoric polyimides with Tg values as high as 320°C and partially resonant χ(2) responses as high as 1.0×10-7 esu (42 pm/V at 1064 nm, 1.17 eV). In the second two approaches, copolymerization of the chromophore 4,5-bis(4‚-aminophenyl)-2-(4„- nitrophenyl)imidazole with bismaleimides or diisocyanates yields polyimides and polyureas with Tg values as high as 292°C and partially resonant χ(2)values as high as 0.62× 10-7 esu(25 pm/V at 1064 nm). It is found that careful attention to the details of curing and crosslinking during poling results in NLO- active matrices exhibiting negligible decay in ( on aging in air at 100°C for periods of 1,000-4,000 h. The imidazole based materials exhibit only about 10% χ(2) decay on aging for 100 h at 200°C under N2.

This review gives a summary of our research in the applications of EO polymers for very broad band infrared modulators, in solving some of the initial problems one faces in integrating semiconductor electronics and EO polymers, and in the fabrication of low loss infrared waveguides.