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We report the design, fabrication, and characterization of high-performance, solution-processed hybrid (inorganic-organic) light emitting transistors (HLETs). The devices employ a high-mobility, solution-processed cadmium sulfide layer as the switching and transport layer, with a conjugated polymer Super Yellow as an emissive material in non-planar source/drain transistor geometry. We demonstrate HLETs with electron mobilities of up to 19.5 cm2/V s, current on/off ratios of >107, and external quantum efficiency of 10−2% at 2100 cd/m2. These combined optical and electrical performance exceed those reported to date for HLETs. Furthermore, we provide full analysis of charge injection, charge transport, and recombination mechanism of the HLETs. The high brightness coupled with a high on/off ratio and low-cost solution processing makes this type of hybrid device attractive from a manufacturing perspective.

This study reports on an organic solar cell structure utilizing K12 as a new low-tem- perature processable small-molecule acceptor material. Pentacene (PEN) and K12 were depos- ited onto indium tin oxide by means of organic vapor phase deposition (OVPD) as bilayer solar cells. The resulting solar cell was characterized electrically by current density-voltage (J − V) measurements and optically by photocurrent and reflectivity measurements. The J − V charac- teristic under AM 1.5 illumination indicates a short-circuit current of 0.45 mA∕cm 2 (Jsc), a fill factor of 38% (FF), and an open-circuit (Voc) voltage of 0.71 V. Current generation is found to predominantly occur in the K12 layer, although strong light absorption in the PEN layer is detected. We suggest that either a dipole shift between the layers or the fission of singlet excitons in the PEN layer leads to this observation. Although the efficiency of the device is low in combination with PEN, our experiment successfully demonstrates the use of K12 as a low- temperature acceptor material in OVPD processes. © 2014 Society of Photo-Optical

Photovoltaic performance in relation to charge transport is studied in efficient (7.6%) organic solar cells (PTB7:PC71BM). Both electron and hole mobilities are experimentally measured in efficient solar cells using the resistance dependent photovoltage technique, while the inapplicability of classical techniques, such as space charge limited current and photogenerated charge extraction by linearly increasing voltage is discussed. Limits in the short-circuit current originate from optical losses, while charge transport is shown not to be a limiting process. Efficient charge extraction without recombination can be achieved with a mobility of charge carriers much lower than previously expected. The presence of dispersive transport with strongly distributed mobilities in high efficiency solar cells is demonstrated. Reduced non-Langevin recombination is shown to be beneficial for solar cells with imbalanced, low, and dispersive electron and hole mobilities.

The synthesis of three pyrrolo[3,2-b]pyrrole-1,4-dione (isoDPP) derivatives is described, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 2, 1,4-diphenyl-3,6-di(thiophen-2-yl)pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 3, and 1,4-bis(4-(hexyloxy)phenyl)-3,6-di(thiophen-2-yl)pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 7 in which the molecular structures differ in the aromatic ring (phenyl or thiophene) attached to the nitrogen atom. Thin films of 2, 3, and 7 could be formed by evaporation under vacuum. In the case of 2 and 3 GIWAXS measurements showed that the film structural ordering was similar to that measured in single crystals. In contrast GIWAXS showed that 7 had features associated with liquid crystalline materials. Time dependent density functional theory (TDDFT) calculations predicted that the transition between the lowest energy singlet excitation (S1) and the ground state (S0) would be optically forbidden due to the centrosymmetric geometries of compounds. Photophysical measurements showed that the compounds were weakly luminescent, with low radiative rates in solution of order 106 s−1, which are consistent with the TDDFT predictions. Furthermore, photoinduced absorption (PIA) spectroscopy showed that there is a long-lived low energy state, which has been assigned as a triplet and provides a further non-radiative decay pathway for the excited state.

Conjugated dendrimers are of interest as novel materials for light-emitting diodes. They consist of a luminescent chromophore at the core with highly branched conjugated dendron sidegroups. In these materials, light emission occurs from the core and is independent of generation. The dendron branching controls the separation between the chromophores. We present here a family of conjugated dendrimers and investigate the effect of dendron branching on light emission and charge transport. We apply a number of transport measurement techniques to thin films of a conjugated dendrimer in a light-emitting diode configuration to determine the effect of chromophore spacing on charge transport. We find that the mobility is reduced by two orders of magnitude as the size of the molecule doubles with increased branching or dendrimer generation. The degree of branching allows a unique control of mobility by molecular structure. An increase in chromophore separation also results in a reduction of intermolecular interactions, which reduces the red emission tail in film photoluminescence. We find that the steady-state charge transport is well described by a simple device model incorporating the effect of generation, and use the materials to shed light on the interpretation of transient electroluminescence data. We demonstrate the significance of the ability to tune the mobility in bilayer devices, where a more balanced charge transport can be achieved.

We have fabricated light‐emitting diodes with poly(p‐phenylenevinylene) as the emissive layer, and with an electron‐transporting layer formed from a solid state dispersion of 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low‐voltage regime and an eightfold improvement in the high‐voltage regime over devices without the electron‐transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron‐transport layer is to confine holes to the emissive layer.

ONE advantage of using conjugated polymers in semiconductor applications is that they can be processed using techniques well established for conventional polymers. We reported recently that poly(p-phenylenevinylene) could be used as the active layer in a light-emitting diode1, producing yellow/green emission. We have now found that related copolymers, comprising a combination of different arylene units, can be chemically tuned to provide a range of materials with considerably improved properties for this and other applications. By incorporating two different leaving groups into a precursor copolymer, we can selectively eliminate one of these, to give a conjugated/non-conjugated copolymer, or both, to give a fully conjugated copolymer. This allows us to induce local variations in the Π-Π* electronic energy gap at both the molecular and supramolecular level. Variations at the molecular level can act to trap excitons, hindering their migration to quenching sites, and we find that these materials give strongly enhanced quantum yields for electroluminescence (by a factor of up to 30). They also allow control of the colour of emission. Variations at the supramolecular level, by patterning the films to control the progress of conversion, allow the production of structures suitable for multicolour displays. The ability to pattern the film also allows for fabrication of optical waveguides, as regions with different energy gaps have different refractive indices.

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.

Organic light emitting devices (OLED) were operated using an AC voltage with DC offset (operating voltage). Electroabsorption (EA) signals as a function of a DC-offset were measured at 2f. Higher absolute DC offset voltages lead to a reduction of the EA signal. The effect observed can well be explained in terms of an extended electroabsorption model including Thomas-Fermi screening: The AC component of the electric field is shielded due to charge injection caused by the DC offset This new device characterization technique allows monitoring the charge carrier statistics in the device during operation. It was found that the EA signal indicates the threshold of monopolar and bipolar charge injection. The presence of negative charges enhances drastically the injection and presence of positive charges in the device

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 characterisation of an insoluble MEHPPV(I-MEHPPV)prepared via a chioro precursor route. Opticalabsorption and emission spectra are discussed with reference to those ofthe common soluble variant. PL quantum efficienciesare 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 800cd/m2 are found for the multilayer device and a peak ELexternal quantum efficiency ofO.1 1% (power conversion efficiency of 1.5x1O5W/W) is obtained.Keywords: I-MEHPPV, precursor route, EL, LED, quantum efficiency, power conversion efficiency. INTRODUCTION Traditionally MEHPPV has been synthesised to be a polymer that is soluble in its conjugated form-MEHPPV).MEHPPV has been shown to preferentially transport holes. In this paper we present the optical and electrical characterisationof an insoluble form of MEHPPV (I-MEHPPV) and its incorporation into multilayer polymer LEDs. The insolubility of theI-MEHPPV layer in common organic solvents lends itself to applications within multilayer device heterostructures sincesubsequent layers may be spin coated on top without dissolving the I-MEHPPV film.

A pentafluorophenol active-ester porphyrin has been synthesised and covalently attached to an aminopropylsilylated glass surface by amide bond formation. Zinc is inserted into the surface-bound porphyrins and their interaction with amine ligands is investigated by UV–VIS spectroscopy. The high affinity of the zinc porphyrin monolayers for bidentate ligands such as 1,4-diazabicyclo[2.2.2]octane, 4,4′-bipyridyl and 1,3-di(4-pyridyl)propane demonstrates that the porphyrins are in close proximity on the surface and yet are not so tightly packed as to prevent ligand intercalation. Effective molarities of up to 0.35 mol dm-3 are observed.

Grancini et al. reported that “hot [charge transfer state] dissociation produces an overall increase in the charge generation yield” in organic solar cells (OSCs). If true, this is an important result as the process could dramatically improve efficiencies. Hot excitons are a 'hot' topic but was the increase in internal quantum efficiency (IQE) seen by Grancini et al. really due to hot excitons? In this Correspondence we demonstrate that when interference and other optical effects are correctly accounted for, the apparent strong contribution from hot excitons is absent...

The effect of a zinc oxide optical spacer layer in broad-band polymer-fullerene solar cells is presented. The complimentary absorption in the donor and acceptor components allows photocurrent generation through photoinduced electron and hole-transfer mechanisms. Simulations of the optical-field distribution reveal that an optical spacer can be used to tune the spectral response to favor one photocurrent generation pathway via enhanced absorption in either the acceptor or donor component. Experimental results confirm these simulations, and the spacer is shown to enhance overall photocurrent in devices with thin active layers ( 90 nm).

We present a study on three generations of fluorescent carbazole dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of vapors of the (deuterated) TNT analogue 4-nitrotoluene (pNT) from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was fully quenched and could not be recovered with flowing nitrogen at room temperature but only upon heating to 40-80 °C. Although the majority of the absorbed pNT could be removed with this method the recovered films were found to still contain a residual pNT concentration of ~0.1 molecules per cubic nanometer. However, the proportion of the PL recovered increased with generation with the third generation dendrimer exhibiting close to full recovery despite the presence of residual pNT. This result is attributed to a combination of two effects. First, the dendrimer films present a range of binding sites for nitroaromatic molecules with the stronger binding sites surviving the thermal recovery process. Second, there is a large decrease of the exciton diffusion coefficient with dendrimer generation, preventing migration of the excitation to the remaining bound pNT.

Accurate colour reproduction by image sensors requires four narrow-band absorbing photodetectors (blue, green, yellow and red), and this is generally achieved by using a broadband photodetector and colour filters. We have developed an organic photodetector in which colour selection is achieved by the light-absorbing layer of the detector. The photoactive layer is comprised of a green absorbing ketocyanine dye-cored dendrimer with the dendrons providing solution processability and prevention of aggregation in the solid-state, which can otherwise lead to the broadening of the absorption spectrum. The properties of the photoactive dendrimer were found to be dependent on the attachment point and number of dendrons. The best photodetector contained a bulk heterojunction (BHJ) light-absorbing layer comprised of the dendrimer with two first generation biphenyl-based dendrons attached to the nitrogen atoms of the ketocyanine dye blended with [6,6]-phenyl-C61-butryic acid methyl ester (PC60BM). The photodetector had a Full Width at Half Maximum (FWHM) of the response of 130 nm centred around 500 nm, an External Quantum Efficiency (EQE) of 8.2% at −1 V and the largest rectification ratio (2.7 × 104 at 0 V) so far reported for a green-absorbing organic photodetector. The performance of the photodetector was found to be approaching that required for current machine vision systems.

We present measurements of the optical absorption and emission properties of poly(p-phenylene vinylene) (PPV) -related materials focusing on the differences between molecules isolated by dispersion in an inert host and concentrated molecular films. Optical absorption spectra, photoluminescence (PL) spectra, PL efficiency, and time-resolved PL spectra of dilute blends of PPV oligomers with 2--5 phenylene-phenyl rings are compared with those of dense oligomer and polymer films. In dilute oligomer-poly(methyl methacrylate) (PMMA) blends with high PL efficiency, the PL decay is exponential, independent of both temperature and oligomer length. This implies that the fundamental radiative lifetime of PPV oligomers is essentially independent of oligomer length. Concentrated spin-cast oligomer films and polymers have a faster and strongly temperature-dependent PL decay that approaches that of the dilute oligomer results at low temperature. The differences in PL decay correspond to changes in PL efficiency. The efficiency of the oligomer-PMMA blend is high and only weakly temperature dependent, whereas that of concentrated films is lower and strongly temperature dependent, decreasing by more than a factor of 3 from 10 to 350 K. The quenching of the PL efficiency in concentrated films is due to migration to extrinsic, impurity related centers as opposed to an intrinsic intermolecular recombination process. The PL spectrum of a dilute oligomer blend redshifts substantially, both as the excitation energy is decreased and as the emission time increases. This spectral redshift is due to disorder-induced site-to-site variation and not to diffusion to lower-energy sites. In contrast, no spectral shift with excitation energy or emission time was observed for dense oligomer films. \textcopyright{} 1996 The American Physical Society.

Photoluminescence and electroluminescence from a new conjugated dendrimer consisting of three distyrylbenzene units linked by a central nitrogen atom as core and meta-linked biphenyl units as dendrons were investigated. Bright electroluminescence was realized by using bilayer devices with blurred interface, which were fabricated by sequentially spin-coating a neat dendrimer and a dendrimer doped with 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). By optimizing the concentration of PBD, the maximum brightness and EL quantum efficiency reach 4100 cd/m(2) and 0.10%, respectively.

We show that it is possible to produce an efficient solution-processable phosphorescent poly(dendrimer) OLED with a 32 lm/W power efficiency at 100 cd/m2 without using a charge transporting host or any improvements in light extraction. This is achieved by using the dendrimer architecture to control inter-chromophore interactions. The effects of using 4,4',4″-tris(N-carbazolyl)triphenylamine (TCTA) as a charge transporting host and using a double dendron structure to further reduce inter-chromophore interactions are also reported.

We report on the charge transport properties of a simple solution processable small-molecule electron acceptor, 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12), designed for use in bulk-heterojunction organic photovoltaic cells. It was found that the molecular order in as-cast films can be dramatically improved by annealing at moderate temperatures (60 °C), which leads to a greatly enhanced electron mobility. Using the photoinduced charge extraction in linearly increasing voltage technique we measured bulk electron mobilities to be as high as 10−4 cm2 V−1 s−1, comparable with some of the best nonfullerene acceptor materials reported to date.

Three generations of fluorescent carbazole dendrimers with spirobifluorene cores are studied as model chemosensor systems for the detection of nitroaromatic explosives via fluorescence quenching. Stern–Volmer measurements in solution with a series of nitrated analytes including the 2,4,6-trinitrotoluene (TNT) byproduct 2,4-dinitrotoluene (DNT) and the plastic explosives taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB) showed an increase in affinity and hence quenching efficiency between the first and second generation dendrimers. In spite of the differences in the solution Stern–Volmer constants the solid state quenching response to the analytes was found to be independent of generation with the exception of 1,4-dinitrobenzene (DNB), where the quenching decreases with increasing generation. It was found that it was necessary to heat the films to release the analytes with the temperature required dependent on the analyte and/or dendrimer generation. These two results show that a simple solution Stern–Volmer analysis is not always sufficient for qualifying film sensing performance and that the drive to develop sensing materials with high solution Stern–Volmer constants for real applications needs to be reconsidered.

The major component of the matrix of many bacterial biofilms is a linear polymer of β-1,6-linked units of N-acetylglucosamine (polysaccharide intercellular adhesin or poly-N-acetyl-β-1,6-d-glucosamine). In order to facilitate synthetic vaccine construction we have developed a direct, inexpensive, and biologically compatible synthesis of the minimal building block of this polymer, a β-1,6-linked GlcNAc disaccharide, namely 6-O-[2-acetamido-2-deoxy-β-d-glucopyranosyl]-2-acetamido-2-deoxy-d-glucopyranose.

In this paper we discuss the principles of operation of polymer electroluminescent devices, and identify the factors which limit device efficiency. We identify how efficiencies can be improved by careful control of the polymer system, and by the use of multilayer structures to confine holes within the device. Using these techniques we can achieve efficiencies of better than 1% photons per electron in devices based on poly(p-phenylenevinylene). We also describe the use of induced absorption techniques to identify the excited states present within an electroluminescent device and to estimate their concentrations.

We have constructed electroluminescent diodes using several layers of conjugated polymers with differing energy gaps; these provide a range of different color light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene), PPV, and derivatives have been used, with indium tin oxide as hole-injecting electrode and calcium as electron-injecting electrode. For this selection of materials, we show that the sequence of the polymer layers allows control of the color of device emission. Emission from more than one layer can be produced simultaneously. The position and breadth of the light-emitting region of the device provides information about the mechanisms of charge transport and of exciton motion. Various models for multilayer emission are discussed in the paper.

It is now established that conjugated polymers can be used to provide charge transport and to act as the emissive layer in thin-film light-emitting diodes (LEDs). The operation of these devices provides important information about the semiconductor physics of these materials. We discuss here the progress made in the design, fabrication and measurement of these devices, and in the understanding of the basic properties that determine device performance.

We report time-resolved photoluminescence measurements on partially conjugated poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4phenylenevinylene] (MEH-PPV). By controlling the average conjugation length via the incorporation of saturated units we can reduce the amount of excimer emission. This results in an increase in the photoluminescence quantum yield of the polymer.

Poly(1,4-phenylenevinylene) (PPV) and derivatives have been studied extensively for use in polymer based LEDs. It has been shown that the introduction of low levels of saturated units into the polymer backbone can give LEDs with improved efficiency. We have developed a new route to partially conjugated PPV derivatives enabling the final polymer to be formed with ethylene links bearing no leaving groups. This was achieved by the treatment of the chloro precursor to poly{2-(2'-ethyl)hexyloxy-5-methoxy-1,4-phenylenevinylene} (MEHPPV) with different amounts of tri-n-butyltin hydride to form copolymers which contain selected proportions of unsubstituted and chlorine substituted ethylene linkages. The subsequent thermal conversion gave rise to conjugated copolymers containing ethylene and vinylene linkages.

Solution state absorption and fluorescence of samples of fully and partially conjugated MEH-PPV at 77K and 300K have been measured to probe the distribution of conjugated segments within the polymer. The absorption of the partially conjugated polymer is at shorter wavelength and has a larger full width at half maximum at all temperatures than that of the fully conjugated polymer. The spectra are analysed by considering the polymer chain to be made up of a number of non interacting conjugated segments. Using this approach it is possible to explain all the features seen in the spectra.

We report a study on a novel family of conjugated dendrimers suitable for organic electroluminescence applications. By increasing the branching of the dendrimer, the separation between adjacent cores in dendrimer films is increased, which in turn results in a slowing of charge transport. We investigate this using the time of flight technique and observe non-Gaussian transport in thin films of dendrimers.

Dendrimers can be used as the emissive layer in organic light-emitting diodes. We have synthesised conjugated dendrimers containing meta-linked stilbene dendrons and luminescent porphyrin or triarylamine cores. The HOMO energy levels of the amine-cored dendrimers were studied by cyclic voltammetry and modelling of device characteristics. Both techniques showed that charge was injected directly into the core and importantly that the energy levels did not change with dendrimer generation.

We present an experimental characterisation of cyano-substituted p-phenylene vinylene model compounds and polymers. The photophysics of these samples in toluene solution are investigated using steady state and time-resolved photoluminescence spectroscopy The influence of cyano and methoxy substitution on the energetics and the natural radiative lifetime are studied.

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3-n-hexylthiophene-2,5-diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.

We have used neutron reflectivity (NR) measurements to probe the physical structure of phosphorescent dendrimer films. The dendrimers consisted of fac-tris(2-phenylpyridyl)iridium(III) cores, biphenyl-based dendrons (first or second generation), and perdeuterated 2-ethylhexyloxy surface groups. We found that the shape and hydrodynamic radius of the dendrimer were both important factors in determining the packing density of the dendrimers. “Cone” shaped dendrimers were found to pack more effectively than “spherical” dendrimers even when the latter had a smaller radius. The morphology of the films determined by NR was consistent with the measured photoluminescence and charge transporting properties of the materials.

The thickness dependence of the absorption spectrum of spin-coated films of poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] has been studied using reflectivity and variable angle spectroscopic ellipsometry measurements. It is found that, for films with thicknesses in the range of 18–178 nm, a single set of optical constants is sufficient to simulate accurately all the experimental data used, including the absorption spectra, independently of the film thickness or the processing conditions. Thus, the observed changes in the absorption spectrum with thickness can be fully accounted for by reflectivity and interference effects alone without the need to invoke morphology differences between films.

The strong electroluminescence (EL)-detected magnetic resonance of PPV-based light emitting diodes is compared to the conductivity ([sigma])- and photoluminescence (PL)-detected resonances. It provides direct evidence that polaron-to-singlet exciton conversion is responsible for the EL. In contrast to the narrow PL-enhancing resonance assigned to polaron recombination, strong EL- and [sigma]-quenching resonances are attributed to the spin-dependent polaron-to-bipolaron decay. The half-field EL- and [sigma]-detected resonances of two distinct triplet excitons is believed to result from triplet-triplet fusion to singlets.

We report transient absorption measurements using the pump and probe technique on poly(2,5 dimethoxy-p-phenylene vinylene), DMPPV, and poly(p-phenylene vinylene), PPV. We observe a transient absorption in both materials, although a transient bleaching would be expected, and we discuss possible explanations for this behaviour.

Studies of the effect of different electrode combinations on the device characteristics of simple three layer light-emitting diodes (LEDs) prepared with poly(ρ-phenylenevinylene) (PPV) as the emissive layer sandwiched between two metal contacts have shown that it is generally more difficult to inject electrons than holes. In order to improve the efficiency of such devices it is, therefore, necessary to develop methods to enhance the injection of electrons and we illustrate here one example where we have successfully achieved this by the introduction of a further, electron transport, layer. The result is an eight fold increase in efficiency over our best three layer PPV devices. The efficiency is also dependent on the details of the polymer electronic structure and using a family of copolymers we have been able to produce enhancements in efficiency to values of up to 30 times that of the corresponding PPV devices. Variations in the polymer electronic structure also affect the colour of emission and the same family of copolymers allow control of emission colour from blue/green to orange/red. Supramolecular control of the copolymer electronic structure can be achieved by lithographic patterning and we show that it is possible to produce regions within a single polymer film that possess different π-π* energy gaps.

Precursor route poly(arylene vinylene) polymers are of considerable interest as electro-optic materials. They show electroluminescent (EL) light emission, charge carrier injection dependent sub-gap optical absorption, and a large electric field dependence of the refractive index close to the π — π* absorption edge. In this paper we focus on the EL response and the factors that limit device performance and we report the results of a specific experiment aimed at enhancing the EL efficiency through the use of copolymer “ quantum-well” structures.

We present work which has been directed towards the understanding of the factors which affect the degree of order and orientation in the family of conjugated polymers based on poly(phenylene vinylene). We have found that by increasing the bulk of the leaving group and/or the rigidity of precursor polymers to poly(phenylene vinylene) the intra-chain order increases whereas the inter-chain order decreases. This has allowed us to prepare poly(phenylene vinylene)s which have more one-dimensional character.

Excitation energy transfer from host to guest is studied in spin-cast blends of 4,4′-bis(N-carbazolyl)biphenyl (CBP) and a phosphorescent fac-tris(2-phenylpyridyl)iridium(III)-cored dendrimer using time resolved fluorescence. The kinetics of energy transfer are consistent with homogeneous dispersion of the dendrimers in the CBP host. Diffusion-controlled singlet-singlet exciton annihilation is observed in the CBP host at moderate excitation densities, similar to those encountered in high brightness light-emitting devices and organic lasers. The results are important for organic lighting and the understanding of exciton diffusion in guest-host systems for electrophosphorescence.

We report the trace vapor detection of the molecule 1,4-dinitrobenzene, a model analyte for the explosive substance 2,4,6-trinitrotoluene, via fluoresence quenching of a first generation conjugated dendrimer containing a 2,2′-bis[9,9-di-n-hexylfluorene] core. We show that much greater sensitivity can be obtained by using the material as a surface emitting distributed feedback laser. We find that the slope efficiency of the laser is a convenient and sensitive indicator of the presence of the analyte. The slope efficiency decreases by a factor 50 in the presence of 1,4-dinitrobenzene.

New high triplet energy dendrons based on 1,2-diphenylethylene with and without 2-ethylhexyloxy surface groups have been developed for deep blue phosphorescent iridium(III) dendrimers. The fac-tris[1-methyl-5-(4-fluoro)phenyl-3-n-propyl-1H-[1,2,4]triazolyl]iridium(III)-cored dendrimers, bearing first generation 1,2-diphenylethylene dendrons on the ligand triazolyl and phenyl rings, were prepared in excellent yields, employing Sonogashira cross-couplings and palladium catalysed hydrogenation as the key synthetic steps. Both dendrimers showed good thermal stability although the flexible nature of the dendrons led to the materials having low glass transition temperatures. Dendrimer 15 (without the surface groups) emitted good blue phosphorescence with a solution photoluminescence quantum yield (PLQY) of 46%, Commission Internationale de l'Eclairage (CIE) co-ordinates of (0.15, 0.14) and photoluminescence peaks at 441 and 468 nm. The solution PLQY was 50% higher than the parent iridium(III) complex showing that the high triplet energy of the diphenylethylene dendrons does not quench the luminescence of the iridium(III) complex core. Dendrimer 34, which has the surface groups, had a film PLQY of 49% and CIE co-ordinates of (0.16, 0.19) with PL peaks at 441 and 469 nm.

We have prepared a poly(styrene) [poly(Irppy2acac)] that has a bis(2-phenylpyridyl)iridium(III) acetylacetonate complex attached to every ‘monomer’ via a methylene unit. The polymer is solution processable and can be spin-coated to give good quality neat thin films. We show that despite the close proximity of the phosphorescent chromophores along the polymer backbone there is only a modest reduction in the solution photoluminescence quantum yield (PLQY) relative to a monomer model compound. The PLQY of the 4-ethylbenzyl substituted bis(2-phenylpyridyl)iridium(III) acetylacetonate complex was 52% while the poly(Irppy2acac) had a PLQY of 34%. Time-resolved photoluminescence measurements showed that the photoluminescence decay of the poly(Irppy2acac) in solution was bi-exponential indicating the presence of more than one emissive species, which was attributed to intra-polymer chromophore interactions. In contrast to the solution measurements the solid-state PLQY was very low (

The use of phenylenevinylene based dendrimers for organic LEDs was investigated. We have found when a family of distyrylbenzene centred dendrimers were used as the light-emitting layers in LEDs that the efficiency was dependent on the dendrimer generation number. A maximum efficiency of 0.09% was observed in an ITO/ second generation dendrimer/Ca device.

Conjugated polymers based on poly(l,4-phenylenevinylene) (PPV) have provided one of the foundations for polymer based LED's. One of the important routes to these materials is to polymerise a substituted 1,4-bis(halomethylene)benzene to form a soluble halo precursor polymer which is then thermally treated to give the final conjugated polymer. We have been investigating the effect of varying the vinylic substituent on the preparation of sterically congested polymers. We have found that electronic effects play a major role in controlling the polymer molecular weight by comparing the syntheses of PPV's substituted on the vinylene moiety.

The use of conjugated dendrimers as a novel molecular architecture for organic LEDs is investigated. Control of colour is demonstrated using materials based on a porphyrin, anthracene and distyrylbenzene core with stilbene dendrons. The resulting dendrimers are solution processible and give photoluminescence and electroluminescence in the red, green and blue regions of the spectrum.