Structure control of poly(p-phenylene vinylene) in layer-by-layer films by deposition on a charged poly(o-methoxyaniline) cushion

In this paper, we demonstrate that the intrinsic electric field created by a poly(o-methoxyaniline) (POMA) cushion layer hinders the changes in molecular conformation of poly(p-phenylenevinylene) (PPV) in layer-by-layer with dodecylbenzene sulfonic acid (DBS). This was modeled with density functional theory (DFT) calculations where an energy barrier hampered molecular movements of PPV segments when they were subjected to an electric field comparable to that caused by a charged POMA layer. With restricted changes in molecular conformation, the PPV film exhibited Franck-Condon transitions and the photoexcitation spectra resembled the absorption spectra, in contrast to PPV/DBS films deposited directly on glass, with no POMA cushion. Other effects from the POMA cushion were the reduced number of structural defects, confirmed with Raman spectroscopy, and an enhanced PPV emission at high temperatures (300 K) in comparison with the films on bare glass. The positive effects from the POMA cushion may be exploited for enhanced opto-electronic devices, especially as the intrinsic electric field may assist in separating photoexcited electron-hole pairs in photovoltaic devices. © 2013 American Institute of Physics.

Langmuir and Langmuir-Blodgett films of poly[2-methoxy-5-(n-hexyloxy)-p-phenylenevinylene]

Langmuir films have been fabricated from poly[(2-methoxy-5-n-hexyloxy)-p-phenylenevinylene] (OC1OC6-PPV). The stability and the area per monomer for condensed films indicate the formation of true monolayers with a very small extent of aggregation, which is unusual for polymer films. This is attributed to the linearity of the alkyl side chain. The Y-type Langmuir-Blodgett (LB) films produced from Langmuir films of OC1OC6-PPV have distinctive features compared to those of cast films, probably due to the organization in the LB films whereas the molecules are randomly oriented in cast films. Infrared absorption spectra recorded for both transmission and reflection modes indicate that OC1OC6-PPV molecules are anchored to the substrate by the lateral groups. This is confirmed by the Raman spectrum, in which a distortion of the vinylene group was observed, and by surface enhanced fluorescence (SEF) on an LB monolayer deposited onto Ag nanoparticles. The more homogeneous nature of the LB films in comparison with the case of cast films was demonstrated by optical microscopy and fluorescence measurements where the emission spectra were essentially the same for different regions of an LB film but showed dispersion in cast films. The LB films also displayed reversible photoconductivity.

Langmuir and Langmuir-Blodgett (LB) films of poly [(2-methoxy,5-n-octadecyl)-p-phenylenevinylene] (OC1OC18-PPV)

A PPV derivative, poly(2-methoxy,5-(n-octadecyl)-p-phenylenevinylene) (OC1OC18-PPV), has been synthesized via the Gilch route and used to fabricate Langmuir and Langmuir-Blodgett (LB) films. True monomolecular films were formed at the air/water interface, which were successfully transferred onto different types of substrate. Using UV-visible absorption, FTIR, fluorescence and Raman scattering spectroscopies we observed that the polymer molecules were randomly distributed in the LB film, with no detectable anisotropy. This is in contrast to the anisotropic LB films of a previously reported PPV derivative, poly(2-methoxy-5-n-hexyloxy)-p-phenylenevinylene (OC1OC6-PPV), which is surprising because the longer chain of OC1OC18-PPV investigated here was expected to lead to more ordered films. As a consequence of the lack of order, LB films of OC1OC18-PPV exhibit lower photoconductivity and require higher operating voltage in a polymer light-emitting diode (PLED) in comparison with LB films of OC1OC6-PPV. This result confirms the importance of molecular organization in the LB film to obtain efficient PLEDs. (c) 2005 Elsevier Ltd. All rights reserved.

Blends of poly(3-hydroxybutyrate) and poly(p-dioxanone): miscibility, thermal stability and biocompatibility

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Efficiency of extrinsic and intrinsic charge-carrier photogeneration processes obtained from the steady-state photocurrent action spectra of poly(p-phenylene vinylene) derivatives

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Light emitting diodes containing Langmuir-Blodgett films of copolymer of a poly(p-phenylene-vinylene) derivative and poly(octaneoxide)

The properties of Langmuir and Langmuir-Blodgett (LB) films from a block copolymer with polyethylene oxide and phenylene-vinylene moieties are reported. The LB films were successfully transferred onto several types of substrates, with sufficient quality to allow for evaporation of a metallic electrode on top of the LB films to produce polymer light emitting diodes (PLEDs). The photoluminescence and electroluminescence spectra of the LB film and device were similar, featuring an emission at ca. 475 nm, from which we could infer that the emission mechanisms are essentially the same as in poly(p-phenylene) derivatives. Analogously to other PLEDs the current versus voltage characteristics of the LB-based device could be explained with the Arkhipov model according to which charge transport occurs among localized sites. The implications for nanotechnology of the level of control that may be achieved with LB devices will also be discussed.

Poole-Frenkel effect in amorphous poly(p-phenylene sulfide)

The conductivity of poly(p-phenylene sulfide) (PPS) amorphous samples sandwiched between metallic electrodes has been studied as a function of applied voltage, temperature, and electrode material. The voltage (U) dependence of the currents for electric fields within the range 10(3)-10(6) V/cm exhibits exp beta U-1/2 behavior with beta = beta(Schottky) below the glass transition temperature (T-g congruent to 90 degrees C), and beta = beta(Poole-Frenkel) above T-g. Coordinated temperature measurements of de currents with different metallic contacts and thermally stimulated currents (TSC) indicate, however, that the conductivity at T < T-g is consistent with the so-called ''anomalous'' Poole-Frenkel effect rather than the Schottky effect. Consequently, the p-type conductivity in amorphous PPS is proposed to be a bulk-limited process due to ionization of two different types of acceptor centers in the presence of neutral hole traps. (C) 1996 John Wiley & Sons, Inc.

Electrical transport properties of doped poly (p-phenylene)

An experimental study of the temperature dependent dc electrical conductivity of doped poly (p-phenylene) in the range of 50-300 K has been presented. The results have been analyzed in the framework of some hopping models. We have observed that hopping models are not consistent with the temperature dependence of the conductivity data over the entire temperature range of measurement. We find that the logarithmic conductivity is proportional to T-beta, wherethe exponent beta is independent of temperature. It is shown that the most probable transport process in this material for the entire range of temperature is due to multiphonon-assisted hopping of the charge carriers that interact weakly with phonons. The parameters obtained from the fits of the experimental data to this model appear reasonable.

Spectroscopic and thermal characterization in poly(p-phenylene vinylene)/sol-gel silica sample

The absorption and luminescence spectra for the poly(p-phenylene vinylene)/sol-gel silica with different thermal treatments were measured. A considerable increase in the luminescence was observed for the polymer introduced into SiO2 matrix with thermal treatment at 120 degreesC. The thermal diffusivity of these samples was measured using the thermal lens technique, and the obtained value 3.3 x 10(-5) cm(2)/s (sample treated at 37 degreesC) is practically independent of the thermal treatment (37-150 degreesC). (C) 2003 Elsevier B.V. All rights reserved.

The application of thermally stimulated processes to the study of poly p-phenylene sulfide

The electrical properties of poly p-phenylene sulfide (PPS) samples sandwiched between metallic electrodes are studied as a function of the applied voltage, temperature, time, electrode materials, and sample thickness. Superlinear current-voltage characteristics are observed, which are explained in terms of Schottky effect and space-charge limited currents (SCLC). The conductivity data for variable-range hopping have also been studied, but the calculated values of density of states are approximately one order of magnitude higher than those obtained by SCLC measurements. From thermally stimulated polarization currents we observed a current peak around 80°C that was related with the glass transition temperature of PPS. © 1993.

Effect of a Polymeric Protective Coating on Optical and Electrical Properties of Poly(p-phenylene vinylene) Derivatives

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

Power-conversion efficiencies of organic heterojunction solar cells can be increased by using semiconducting donor-acceptor materials with complementary absorption spectra extending to the near-infrared region. Here, we used continuous wave fluorescence and absorption, as well as nanosecond transient absorption spectroscopy to study the initial charge transfer step for blends of a donor poly(p-phenylenevinylene) derivative and low-band gap cyanine dyes serving as electron acceptors. Electron transfer is the dominant relaxation process after photoexcitation of the donor. Hole transfer after cyanine photoexcitation occurs with an efficiency close to unity up to dye concentrations of similar to 30 wt%. Cyanines present an efficient self-quenching mechanism of their fluorescence, and for higher dye loadings in the blend, or pure cyanine films, this process effectively reduces the hole transfer. Comparison between dye emission in an inert polystyrene matrix and the donor matrix allowed us to separate the influence of self-quenching and charge transfer mechanisms. Favorable photovoltaic bilayer performance, including high open-circuit voltages of similar to 1 V confirmed the results from optical experiments. The characteristics of solar cells using different dyes also highlighted the need for balanced adjustment of the energy levels and their offsets at the heterojunction when using low-bandgap materials, and accentuated important effects of interface interactions and solid-state packing on charge generation and transport.

Nanostructured organic layers via polymer demixing for interface-enhanced photovoltaic cells

Significant progress is being made in the photovoltaic energy conversion using organic semiconducting materials. One of the focuses of attention is the morphology of the donor-acceptor heterojunction at the nanometer scale, to ensure efficient charge generation and loss-free charge transport at the same time. Here, we present a method for the controlled, sequential design of a bilayer polymer cell architecture that consists of a large interface area with connecting paths to the respective electrodes for both materials. We used the surface-directed demixing of a donor conjugated/guest polymer blend during spin coating to produce a nanostructured interface, which was, after removal of the guest with a selective solvent, covered with an acceptor layer. With use of a donor poly(p-phenylenevinylene) derivative and the acceptor C-60 fullerene, this resulted in much-improved device performance, with external power efficiencies more than 3 times higher than those reported for that particular material combination so far.

Electrical properties of polymer/metal interface in polymer light-emitting devices: electron injection barrier suppression

The electrical characterization of a high efficient multilayer polymer light emitting diode using poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene] as the emissive layer and an anionic fluorinated surfactant as the electron transport layer was performed. For the sake of comparison, a conventional single layer device was fabricated. The density current vs. voltage measurements revealed that the conventional device has a higher threshold voltage and lower current compared to the surfactant modified device. The effective barrier height for electron injection was suppressed. The influence of the interfaces and bulk contributions to the dc and high frequencies conductivities of the devices was also discussed. (c) 2006 Springer Science + Business Media, Inc.

Effect of ion concentration of ionomer in electron injection layer of polymer light-emitting devices

Polymer light-emitting devices (PLEDs) with poly(2-methoxy-5-hexyloxy)-p-phenylenevinylene (OC1OC6-PPV) as the emissive layer were studied with an electron injection layer of ionomers consisting of copolymers of styrene and methylmethacrylate (PS/PMMA) with 3, 6 and 8 mol% degree of sulfonation. The ionomers were able to form very thin films over the emissive layer, with less than 30 nm. Additionally, the presence of ion pairs of ionomer suppresses the tendency toward dewetting of the thin film of ionomer (similar to 10 nm) which can cause malfunction of the device. The effect of the ionomers was investigated as a function of the ion content. The devices performance, characterized by their current density and luminance intensity versus voltage, showed a remarkable increase with the ionomer layer up to 6 mol% of ionic groups, decreasing after that for the 8 mol% ionomer device. The study of the impedance spectroscopy in the frequency range from 0.1 to 10(6) Hz showed that the injection phenomena dominate over the transport in the electroluminescent polymer bulk. (c) 2006 Elsevier B.V. All rights reserved.