For multicolor display applications, polymeric light emitters of the three primary colors of red, green and blue are required. Emitters of high luminescence efficiency and long lifetime stability for red and green have been found, but the search for a suitable blue emitter continues. 2,7-Disubstituted dibenzosilole monomers have been prepared by the selective translithiation of 4,4'-dibromo-2,2' diiodobiphenyl followed by silylation with dichlorodihexylsilane. Suzuki copolymerization of dibromo and bis(boronate) monomers afforded poly(9,9-dihexyl-2,7-dibenzosilole) which showed better color stability and efficiency than the corresponding polyfluorene in a single layer light emitting device. Preliminary studies demonstrated this to be a promising blue light emitting polymer.
Electroluminescence from conjugated polymers can be significantly improved by harnessing the energy of their nonemissive triplet states. Poly(2,7-dibenzosilole) has been prepared and its triplet energy has been measured as 2.14 eV, a figure that is slightly higher than that of polyfluorene (2.09 eV). Two new tris-cyclometalated iridium complexes with blue-to-green emission properties have been prepared and characterized.
Efficient electroluminescence is difficult to achieve using purely organic materials as only S0 <-> S* transitions (25 % quantum efficiency according to spin statistics) will be allowed when the spin selection rule is obeyed. In most strictly organic systems the triplet state energy will decay non-radiatively. In order to improve the electroluminescent quantum efficiency of light emitting polymers, triplet state emitting complexes have been introduced into the polymer either by blending or copolymerization. The strong spin-orbit coupling from the heavy metal atoms allows effective mixing of singlet and triplet states to provide strong radiative decay. Two approaches to the synthesis of solution-processible phosphorescent polymers have been developed and the degree of energy transfer from the polymer host to the triplet emitter has also been investigated. The phosphorescent core, [Ir(btp)2(acac)], where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate, can either be incorporated into the polymer chain through the extension of the ligands or tethered to the polymer backbone through an alkyl pendant. In addition, the synthesis of high band gap polymer hosts will also be discussed.
Control of charge interfacial charge transfer is central to the design of photovoltaic devices. We report herein the application of insulating metal oxide blocking layers to control the charge recombination kinetics at a solid-state dye sensitised nanocrystalline inorganic/organic semiconductor interface. We show that the conformal growth of a ~1 nm thick overlayer of Al2O3 on a preformed nanocrystalline TiO2 film results in a ~3-fold retardation in the rate of charge recombination at such an interface. This observation shows a good correlation with the current/voltage characteristics of dye sensitized nanocrystalline solar cells fabricated from such films, with the Al2O3 coating resulting in a 40% improvement in overall device efficiency
Further routes have been developed for the synthesis of a 1,4-bishalomethylbenzene derivatives for Gilch dehydrohalogenation polycondensation. Poly(2,3-dibutoxy- 1,4-phenylenevinylene) is a protoypical conjugated polymer which is thought to derive its high PL solid state fluorescence efficiency from the sterically twisted backbone and devices carrying this polymer have been further evaluated. Distyrylbenzene derivative carrying the structural feature of a 2,3-dibutoxy substitution pattern on the central ring have been prepared. One in particular has been copolymerized with a 9,9-dialkyl-fluorene-2,7- diboronate ester. The resulting conjugated polymer shows a good green emission maximum in an electroluminescent device.
The synthesis of poly(1,4-phenylene vinylene)s (PPVs) containing a 2,3-dialkoxy substitution pattern has been developed. Poly[2,3-bis(2-ethylhexyloxy)-1,4-phenylene vinylene] (BEH-PPV) 4 was prepared by Gilch polycondensation, and its optical properties were compared with the recently discussed poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) 1. The precursors for the Gilch method have traditionally been prepared by methods which have certain disadvantages. These can be overcome by the use of directed metallation reactions which are illustrated in the synthesis of some poly(2,5-disilyl-substituted 1,4-arylene vinylene) derivatives.
The measurement of the velocities of oil and water phases in the bore-hole of a producing well, together with volume fraction measurement, gives the flow rate of each fluid phase. Even though radioactive tracers methods provides a good velocity measurement, safety concerns and regulations makes their use unattractive. We propose a method of measuring fluid velocity using local optical fiber probes and tracer techniques. A fluorescence dye is injected into the bore-hole where it is mixed with the fluid of interest. An optical fiber probe is positioned down-stream from the point of injection detects the passage of the tracer with the flow. The time between injection and detection enables one to calculate the flow velocity for a given distance between injector and detector. The same local probes cold also be used for hold-up estimation. We describe laboratory experiments using a flow loop, fluorescence dye as tracer and an optical fiber probe. The dependency of the injection to detection distance to the flow mixing is observed. Best results for average velocity estimation can be achieved for long distances. Measurements at short distances are possible but some kind of flow model may be needed to interpret the results.
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.
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.
The high luminescence efficiencies and significant blue shift of the 2,3-disubstituted poly(1,4-phenylene vinylene) polymer 4 have prompted further investigation, and in this paper the synthesis and characteristics of the homopolymer 9 and copolymers 10 and 12 are described. Semiempirical calculations and single x-ray crystallography offer further insight into the explanation of the properties of this class of polymers. A promising organic semiconductor 15 has been prepared and used as the active layer in a thin film transistor. This material exhibited excellent device characteristics, including a field effect mobility of 0.02- 0.05 cm2 V-1s-1 and a high On/Off ratio.
Changes as small as 1 ppm in the real and imaginary components of the complex permittivity resulting from flash photoexcitation of dilute solutions of (pi) -bond conjugated oligomers and polymers can be measured with nanosecond time resolution using the time-resolved microwave conductivity technique. The results provide information on the extent of delocalization of singlet and triplet state photoexcitations. Data are presented for oligomeric and polymeric derivatives of phenylene vinylene, paraphenylene and thiophene. For the polymers excess polarizability volumes, (Delta) Vp, of the singlet exciton of 2000, 180 and 570 angstroms3 are found, respectively. (Delta) Vp for the triplet exciton of the polythiophene derivative is found to be only 25 angstroms3.
We report a study of the photophysics of solutions and films of the conjugated polymer poly(3-hexylthiophene) (P3HT). We have performed measurements of absorption, photoluminescence (PL), PL quantum yield, and time-resolved PL. For P3HT in a good solvent the PL decays with a time constant of 510 +/- 20 ps, and the PL quantum yield is 0.42 +/- 0.04, we assign the PL to the intra-chain singlet exciton, and deduce a natural radiative lifetime for this species of 1.2 +/- 0.1 ns. We explore the use of solvent mixture as a way of controlling the degree of intermolecular interaction. We find that there is a red-shift of absorption and PL in poor solvent mixtures, and the spectra and PL quantum yield become similar to the film. The PL decay becomes faster and non-exponential in poor solvent mixtures and the thin film.
The synthesis and luminescent properties of the homopolymers (4,5) and copolymers (9,10) carrying ion-transporting side chains are reported. When fabricated as alight emitting electrochemical cell the copolymer 10 exhibited a significant reduction in turn-on voltage and improved luminous efficiency compared with a conventionally fabricated polymer light emitting device. Similar results were observed with the pyridine copolymer 13. The model meta-linked trifluoromethyl substituted distyrylbenzene derivative 16 has been synthesized and its crystal structure has been determined with a view to evaluating the related polymers as charge transporting materials.
Simple light emitting diodes can be constructed using fluorescent organic materials. Conjugated polymers can be used both for charge transport and for light emission. It is considered necessary for maximum device efficiency to balance the rates of electron and hole injection. We report the synthesis of a poly(cyanoterephthalylidenene) that was designed to exhibit an increased electron affinity. Electrochemical measurements showed a significant shift in the oxidation and reduction potentials due to the cyano functionality. The use of this polymer in a range of electroluminescent devices is described. Internal quantum efficiencies of up to 4% can be achieved in a bilayer device using stable electrode materials. The route used to synthesize this polymer is amenable to considerable variation in the subunits employed. This allows tuning of both the band-gap and the electron affinity of the resulting polymer.
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.
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.
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