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Inorganic-organic hybrid polymers (ORMOCER®s) have been synthesized by sol-gel processing. The materials can be functionalized such that their physical and chemical properties can be reproducibly tailored towards the desired application, i.e., electronics, optics (passives/actives) or passivation technology. Besides, the materials which show negative resist behavior, can be patterned by UV exposure with good resolution. The materials are well-suited for thin and thick film technology using conventional coating technology, applied in multi-layer technology. We here particularly focus on materials for passivation against environmental influences, which additionally exhibit very good dielectric properties.
In order to obtain efficient blue, green and red light-emitters as active layers in polymer light-emitting diode (PLEDs), we report herein the design, the synthesis, and the characterization of new polyesters and copolymers derived from fluorene. We will focus on a new approach to obtain a stable, high luminance blue emitter via polyesters derived from fluorene. The versatile synthetic approach allows the easy design of a whole class of new and tunable electroactive and photoactive aromatic polyesters. Poly(1,6-hexane- 9,9′,9′,9′′,9′′-hexahexyl-7,2′;7′,2′′-terfluorene-2,7′′-dicarboxylate) (PTFHHC6) and Poly(1,6-hexane-2,7-Bis(1,1′-biphenylen-4-yl-4′-dicarboxylate)-9,9-dioctylfluorene) (PE-BP-DOF-BP) exhibit strong emission in the blue range, both in solution and in the solid state. Moreover, these polyesters present some advantages over other polyfluorene derivatives. For example, their solid-state fluorescence spectrum does not show the formation of any excimer and does not vary upon thermal treatment. We will also present copolymers derived from fluorene which emit green light and a new approach to obtain red polymer light-emitting-diode will also be shown. Optical properties such as light absorption, light emission and quantum yield of fluorescence in solution as well as electrical properties such as cyclic voltammogram and conductivity in-situ are reported. Finally, some of the polymers presented have been tested in PLEDs devices and the preliminary results seem promising for the development of multicoloured displays made from the same class of polyfluorene derivatives.
A method of solvent-enhanced dye diffusion in polymer films for organic light-emitting diode (OLED) application is introduced. After an initial dye transfer from a dye source substrate into the top of the electractive polymer film, the device substrate is then exposed to solvent vapor. Due to solvent absorption by the polymer film, the glass transition temperature of the polymer is significantly decreased, which leads to greatly enhanced diffusion of the dye in the polymer film. Secondary ion mass spectroscopy shows that the temperature for dye diffusion can be decreased by 150 °C. OLEDs with 0.4% external quantum efficiency were demonstrated. The materials used are the polymer poly(9-vinylcarbazole) (PVK) combined with electron transport molecules (PBD), and the dyes coumarin 47, coumarin 6 and Nile red.
Taking advantage of the multifunctional characteristics of carbazole along with rational molecular design, several two-dimensional carbazole chromophores were acquired. Doping the chromophores into poly(ether imide), a series of NLO-active guest-host polymers were obtained. The compatibility between chromophores and poly(ether imide) was investigated by SEM and extraction experiment. After poling, these NLO poly(ether imide)s with large second-order nonlinearities were achieved. Correlation between structure and nonlinear optical characteristics, and the chromophore size effect on the thermal and temporal stability have been probed. Large rotational cone volumes make the two-dimensional chromophores possessing excellent orientational stability at 60.
Research and development of displays and image sensors based on semiconducting polymers require design of new polymer materials and evaluation of film properties. Optimizing device performance using a one-factor-at-a-time (OFAT) requires screening the effect of several process parameters, running numerous samples, and may consume more scarce new material than desired. This paper investigates the effects of Indium-tin oxide (ITO), alkoxy-poly(pphenylene vinylene) (OC1C10-PPV) and poly(3,4-ethylene dioxythiophene) (PEDOT) layer preparation on polymer LED brightness and power efficiency by performing and analyzing a two-cubed full-factorial design experiment with 3 replicated center points. Full-factorial design evaluates all main factors and all interactions. Design of experiments (DOE) showed that correct selection of ITO anneal temperature can significantly improve brightness. Atomic Force Microscopy (AFM) measurements affirm that the increased brightness correlates with a reduction in ITO average surface roughness.
The polymerization of a fused 2,5-dibromothieno-TTF derivative by Yamamoto procedures gave a highly electroactive polymer; doping the material with TCNQ increased the conductivity of the polymer by approximately two orders of magnitude.
A synthetic route to highly luminescent organic semiconductors with curable alkoxysilyl groups is described. Monodisperse oligo(phenylenevinylene)s are rigidly connected to di- and triethoxysilanes via Heck reactions or via cross-metathesis. Hydrolysis of the silicic esters yields silanols condensing to linear and cyclic oligo-OPV-siloxanes or to three-dimensional networks, thus allowing the transformation of small molecules to fluorescent materials with well-defined chromophores. Transparent films are obtained by casting of soluble cyclosiloxanes and from OPV-silanetriols, the latter can be cured to insoluble networks.
We report on a scanning probe spectroscopy study of the electronic properties of model organic/metal interfaces. The experiments allow us to determine parameters that are critical in charge carrier injection and transport, as are the energy gap between positive and negative polaronic states and the height of the barrier for charge carrier injection at metal/organic interfaces. In combination with optical absorption measurements, we gauge the exciton binding energy, a parameter determining energy transport and electroluminescence efficiency. The study was performed on thin films of tris(8-hydroxyquinolato)aluminum (Alq3) deposited on clean and LiF-covered Au(111), and on N,N'-di(naphthalen-1-yl)-N,N'-diphenylbenzidine (NPB) on Ni(111) and substrates.
In this study bi-layers of metal/polyaniline (PAni) films were deposited and characterized in order to investigate the interaction of thin film metals with doped and de-doped PAni. The bilayers were fabricated by depositing PAni films on flat substrates via solution chemistry and then depositing metallic films by physical vapor deposition. The oxidation state of the PAni was varied from the doped emeraldine salt to a de-doped emeraldine base to a de-doped and fully reduced leucoemeraldine base. Aluminum and iron thin films were then magnetron sputter deposited onto the PAni films to form bi-layer structures. Characterization of the fabricated bi-layers by scanning electron microscopy, current-voltage measurements, and Auger electron spectroscopy was done to investigate the morphology, electrical properties and chemical composition of the samples. Results from the study indicate that the type of metal and the doping level of the PAni influence the interactions and properties of metal/PAni interfaces and films.
We studied the electronic transport properties of conjugated polymer/fullerene based solar cells by means of temperature and illumination intensity dependent current-voltage characteristics, admittance spectroscopy and light-induced electron spin resonance. The short-circuit current density increases with temperature at all light illumination intensities applied, i.e., from 100 mW/cm2 to 0.1 mW/cm2 (white light), whereas a temperature independent behavior was expected. An increase of the open-circuit voltage from 850 mV to 940 mV was observed, when cooling down the device from room temperature to 100 K. The fill factor depends strongly on temperature with a positive temperature coefficient in the whole temperature range. In contrast, the light intensity dependence of the fill factor shows a maximum of 52% at intermediate illumination intensities (3 mW/cm2) and decreases subsequently, when increasing the intensity up to 100 mW/cm2. Further studies by admittance spectroscopy revealed two frequency dependent contributions to the device capacitance. One, as we believe, originates from trapping states located at the interface between composite and metal electrode with an activation energy of EA=180 meV, and the other one is from very shallow bulk states with EA=10 meV. The origin of the latter is possibly the thermally activated conductivity. The photo-generation of charge carriers and their fate in these blends have been studied by light-induced electron spin resonance. We can clearly distinguish between photo-generated electrons and holes in the composites due to different spectroscopic splitting factors (g-factors). Additional information on the environmental axial symmetry of the holes located on the polymer chains as well as on a lower, rhombic, symmetry of the electrons located on the methanofullerene molecules has been obtained. The origin of the signals and parameters of the g-tensor have been confirmed from studies on a hole doped polymer.
We developed an alternative approach to produce self-assembled (SA) thin films of poly(p-phenylene vinylene) (PPV) by controlled substitution of the chloride counterion of the precursor poly(xylylidene tetrahydrothiophenium chloride) (PTHT) by a long chain dodecylbenzenesulfonate (DBS) anion. The main advantage of this novel procedure is that thermal conversion through the elimination of DBS may be performed at considerably lower temperatures (80-100 °C) in few minutes. In addition it provides PPV films with better optical properties and low incorporation of structural defects, like carbonyl groups. An important feature of these SA-PPV films is the well resolved vibronic structures in the photoluminescence and absorption spectra. We observe a dramatic improvement in quantum efficiency of PPV when the conversion temperature is decreased from 230 to 80 °C. This effect may be explained by the decrease of extrinsic defect incorporations (carbonyl groups) detected by infrared (IR) measurements. This improvement in the optical properties at low temperatures may help us to understand basic phenomena, such as the nature of excitons in PPVs.
We report the synthesis and characterization of a novel series of aryl-substituted PPVs, which have shown excellent processability, good thermal stability, high photoluminescence quantum efficiency and low content of structural defects. The substituents of the polymers were designed with different degree of hindrance effect on the main chain. 1H NMR measurement indicates that the defect structure in the polymer main chain can be effectively depressed by introducing bulk and hindrance substituents.
The stabilizing effect of dendritic boxes (DBoxes) as a means to protect against photo- degradation of encapsulated dye molecules was experimentally investigated. We focused on photoinduced oxidation and studied the ability of DBoxes to protect dyes inside them from reactive singlet oxygen. Rubrene was captured in a DBox, and then the box was closed at the surface by other stable molecules. A singlet oxygen generator was added to a Rubrene solution to exaggerate photo-oxidation in the complicated total-photodegradation process, and then the solution was exposed to a laser beam to generate singlet oxygen. Bleaching of the encapsulated Rubrene in a DBox was 50 times slower than that of Rubrene alone. We also compared the dependence of surface molecules of a DBox with other kinds of molecules that have different efficiencies of quenching singlet oxygen. Further improved stability was observed by attaching singlet-oxygen-quencher molecules on the DBox surface.
Visible and ultraviolet photoconductivity (PC) measurements have been performed on thick (7[.proportional]m) and thin (170nm) films of poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV) sandwiched between indium tin oxide (ITO) and semitransparent gold (Au) electrodes. PC spectra were obtained for samples under different bias polarities and for illumination through the ITO electrode and through the Au electrode. The results obtained indicate that the absorbed photons form excitons that dissociate extrinsically at or near the electrodes to form hole polarons or intrinsically at higher photon energies in the bulk polymer.
The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar.
Poly(2,5-pyridinediyl) was prepared from 2-bromo-5-iodopyridine, by a method which combines organomagnesium and organonickel chemistry, and leads to the regioselective formation of poly(2,5-pyridinediyl) (rPPY). The product of the reaction was compared to conventional poly(2,5-pyridinediyl) (PPY), thus enabling us to estimate that rPPY consists of 84±6% head-to-tail linkages. Photophysical properties of rPPY were also measured, and found to be generally similar to those of PPY, although there are some significant differences.
Effect of photo-irradiation on the color change of diarylethene polymer films was investigated. Diarylethene polymer films were prepared from photopolymerization of a radical curable mixture of diarylethene compound and a fluoroalkylacrylate containing a photo initiator. 1-[6'-(Methacryloxyethyloxycarbonyl)-2'-methylbenzo[b]thiophene-3'-yl]-2- (2"-methylbenzo[b]thiophene-3"-yl)hexafluorocyclopentene (MMBTF6) was synthesized from 2,3-bis(2-methylbenzo[b]thiophene-3-yl)hexafluorocyclopentene (BTF6) in three steps. 2,2,3,3-Tetrafluoro-1,4-butyldiacrylate (TFBDA) was synthesized from the corresponding diol and acryloyl chloride in the presence of a base. The photocurable mixture was coated on a substrate and subjected to antinic irradiation, to afforded homogeneous transparent film. A mask image was recovered on the film by a light of 365 nm and read by a visible light (λ > 700 nm) without destruction of the image. To erase the recorded image, a white light or a visible light was irradiated. Photochromic quantum yield and photo-induced refractive index change of the diarylethene bound polymer film were determined as 0.12 and 0.0006 respectively.
Ultrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo-excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to hν = 6.2 eV indicate an apparent increase in the “photoconductivity” at hν > 3-4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO2+SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of π-π* absorption, behavior that is inconsistent with a large exciton binding energy.
Poly(p-phenylene vinylene) (PPV) films are usually photodegraded in air. We report on the enhancement of photoluminescence (PL) intensity of PPV films induced by Ar ion laser in the presence of air. The PL spectra were measured as function of laser power, time exposure and sample thickness. The initial PL intensity increases up to 300% without significant changes in peak positions by optimizing such parameters. This effect is accompanied by a blue-shift in the absorption spectrum resulting in shortened effective conjugation length and by a formation of defects such as carbonyl groups. Mechanisms for the observed photoluminescence intensity enhancements are suggested.
Silver phenylacetylide prepared by one-step process was characterized as a linear coordinated polymeric compound by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy. Its prominent third-order optical nonlinearity was detected by heterodyned ultrafast optical Kerr effect (OHD-OKE) measurement. The second-order hyperpolarizability was enhanced by 2 orders of magnitude by means of incorporation of proper electron-pushing groups. The structure status was further investigated by quantum chemical calculation and molecular modeling.