PPV Films Research Articles

Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices

Nanoscale two-dimensional (2D) organic materials have attracted significant interest on account of their unique properties, which result from their ultrathin and flat morphology. Supramolecular 2D ...

Organic field effect transistors (OFETs) of poly(p-phenylenevinylene) fabricated by chemical vapor deposition (CVD) with improved hole mobility

Abstract This study demonstrates organic field effect transistors (OFETs) of chemical vapor deposited poly(p-phenylenevinylene) (PPV). The optical and electrical properties of the films are reported. It is found that the grain sizes of the film can be engineered by surface treatment of the oxide of the Si/SiO2 substrate in the transistor with a self assembled monolayer (SAM) of octadecyltrichlorosilane (OTS). The enhancement of domain sizes of the grains with surface treatment leads to an increase in hole mobility to 1.4 × 10−4 cm2/V s, which is 1000 times higher than that reported for solution processed PPV field effect transistors. A comparison of the hole mobility between CVD grown PPV films and those reported for PPV and its derivatives, grown by spin coating or drop casting, shows that CVD provides an optimized alternative for growth of devices. It is shown that a low annealing temperature of 180 °C is enough to achieve polymerization. Film deposition by CVD has the advantage that up-scaling, with a fine control on the film thickness, is possible without compromising on the electrical/morphological properties. A vacuum based method of polymer deposition opens up the prospect of hybrid devices based on small molecules and polymers.

SYNTHESIS AND CHARACTERISTICS OF POLY(PARA-PHENYLENEVINYLENE)

Poly(para-phenylenevinylene) [PPV] films and nanotubes were prepared via chemical vapor deposition polymerization (CVDP) by vapor phase pyrolysis of α, α'-dichloro p-xylene on the quartz plate, silicon wafers and Al2O3 membrane substrate with the pore size f = 100 nm. We prepared graphitic carbon films and carbon nanotubes by carbonizing the poly(para-phenylenevinylene) [PPV] films and nano parterns under thermal treatment range from 500°C to 900°C. When the PPV films on quartz plate were treated at 900°C highly oriented graphitic carbon films were obtained. The characterization of the PPV nanotube and nanotube carbon has been investigated by means of IR, SEM, AFM and Raman scattering study.

Light-emitting flexible transparent paper based on bacterial cellulose modified with semiconducting polymer MEH:PPV

We demonstrated flexible light-emitting transparent bacterial cellulose (BC) paper modified with semiconducting polymer operated by pulse voltage. We prepared thin (∼ 35 μm) transparent BC films modified with luminescent conjugated polymer MEH:PPV with Ag electrodes. It was shown that the photoluminescence (PL) spectrum of BC/MEH:PPV films contains emission contributions from both BC and MEH:PPV components and that the PL intensity of BC and MEH:PPV maxima versus excitation power is sublinear. This indicates that exciton–exciton annihilation occurs in the light-emitting polymer at high excitation power. We found that the dependences of the spike-like electroluminescence (EL) intensity of BC/MEH:PPV films excited by the pulse voltage at different frequencies versus applied pulse bias and pulse frequency are superlinear. We assume that the turn-on and turn-off spikes originate from the recombination of charges accumulated in the bulk of the semiconducting polymer (bulk de-trapping) during the voltage pulse. The results demonstrate that such composites are prospective for application as flexible, transparent and environmentally friendly light-emitting cellulose-based papers for displays and lighting as well as biosensors.

Charge transport in thin films of MDMO PPV dispersed with lead sulfide nanoparticles

Enhanced hole mobility is observed in thin films of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO PPV) dispersed with Lead Sulfide nanoparticles. Changes in the polymer film morphology upon the dispersion of nanoparticles and its influence on the enhancement of carrier mobility is investigated using Photoluminescence (PL) and Raman spectroscopic studies. PL and Raman studies show that at low concentrations of nanoparticles there is an enhancement in polymer interchain interaction. At higher concentrations, the interhain interaction decreases. Raman studies show that the observed enhancement of polymer interchain interaction at low nanoparticle concentration is due to the tighter packing of polymer chains. Thus, at low nanoparticle concentration, the observed increase in mobility is assigned to both the enhancement in interchain interaction and tunneling of carriers through nanoparticles. At higher nanoparticle concentrations the observed enhancement of mobility can be totally attributed to the tunneling of carriers through nanoparticles.

Conjugation Length Distribution in Poly(p-phenylenevinylene) (PPV) Films.

We studied the absorption line-shape of poly(p-phenylenevinylene) (PPV) films deposited via spin coating and Langmuir-Blodgett techniques with the intent of identifying the conjugation length distribution in these two types of films, a key morphological aspect of conjugated polymer films. We treated the excitons in the polymer as independent oligomer excitons and modeled the absorption spectra of the individual oligomers using simple expressions for the oligomer size dependence of the gap energy, the line-broadening factor, the transition dipole moment and the Huang-Rhys parameter. We validated these expressions by independent measurements on phenyl-based oligomers and Density Functional Theory calculations. Our results show clear evidence that, for both types of PPV films, the conjugation length distribution depends exponentially on the segment size. Our results also set a lower limit, of about ten repeat units, for the maximum exciton length of three different phenyl-based oligomers.

Dynamics of charge migration in poly(para-phenylene vinylene) films and nanocomposites with single walled carbon nanotubes

We present in this paper a comprehensive study of the migration dynamics of the charges underlying transient photoluminescence (PL) processes in poly(para–phenylene vinylene) (PPV) samples from room temperature to 13 K. In order to interpret experimental data, we have modelled the long–time PL decays (from 100 to 1000 ps) using a time function proportional to in which the parameter α is evaluated in a Monte Carlo simulation on polymeric chains. The one dimensional chains (2000 sites long) are formed by random sequences of long and short conjugated segments whose bimodal distributions have been elaborated in previous works in order to reproduce the PL band shapes and peak positions. Intra-chain and inter-chain dynamics are taken into account in the migration of the photogenerated charges from short to long conjugated segments. The statistical analysis is performed by averaging over a total of 106 trials for each initial conditions. The values of α have been determined for pristine PPV films and PPV composite films with single-walled carbon nanotubes. This theoretical analysis is in good agreement with experimental data and provides a coherent description for the migration of the photogenerated charges in such inhomogeneous polymeric systems.

Morphology and Luminescence of Poly ( p -Phenylene Vinylene) Films Prepared by Chemical Vapor Deposition

The films of poly( p -phenylene vinylene) (PPV) were prepared by chemical vapor deposition (CVD) of α , α ′-dichloro- p -xylene on the surface of crystalline silicon (001) wafer. Atomic force microscopy (AFM) and reflective IR were used for measuring the topographic images and the orientations of PPV chains, respectively. Photo-luminescent (PL) property of the PPV films indicates that the details of PL spectrum are dependent on the thickness of the films deposited on the wafer surface. The UV-vis spectrum was also measured for the PPV films deposited on the amorphous quartz substrate.

Polarized emission from stretched PPV films viewed at the molecular level.

We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PPV), non-stretched and uniaxially stretched. The experimental study was carried out using linear polarized light as the excitation beam, oriented either parallel or perpendicular to the stretch axis (S). The results showed that when the excitation light source has polarization perpendicularly oriented to the stretch direction, the emitted PL presents maximum intensity in the orientation S, and a minimum in the direction orthogonal to S. In order to understand this interesting phenomenon, we employ theoretical simulations at the atomistic level. We use classical molecular dynamics to simulate amorphous PPV films, non-stretched and stretched, from which we find a tendency of overall alignment of PV units to S, and of local clustering in herring-bone and π-stacking partial symmetries. Our study of optical activity of these kinds of clusters, performed through a quantum semi-empirical method, allows us to explain this polarization conversion behavior, and indicates the possibility of using underivatized PPV as the active layer for polarized electroluminescent devices.

Numerical modeling of current-voltage characteristics to extract transport properties of organic semiconductors

The current vs. voltage (I-V) characteristics of single crystal rubrene Organic Field-Effect Transistors (OFETs) and polycrystalline poly(p-phenylenevinylene) (PPV) films are modeled using the polaron transport theory presented in a previous work [A. F. Basile et al., J. Appl. Phys. 115, 244505 (2014)]. The model is first applied to rubrene OFETs, where transport is two-dimensional and is confined near the interface between the insulator and the organic semiconductor. By considering the effect of image charges in the insulator and by assuming a constant intrinsic mobility, we reproduce both the positive and the negative temperature dependences of the channel mobilities measured on OFETs having a gate dielectric and an air-gap insulator, respectively. In addition, we adapt this model to the three-dimensional transport in PPV films, characterized by effective mobilities which depend on temperature, charge density, and electric field. We show that the I-V characteristics of these materials can be matched by the numerical solution of the Poisson and drift-diffusion equations assuming a constant intrinsic mobility. The polaron binding energy can account for the thermally activated behavior of the I-V characteristics and for the increase of the effective mobility at high applied voltages. Therefore, this model enables to extract the intrinsic transport parameters of organic semiconductors, independent of the device structure, and of the measurement conditions.

Mechanical properties of PPV film in thermal elimination process

This paper deals with the relationships between the structure of poly(p-phenylene vinylene)(PPV) film and treatment temperature and their mechanical properties. The chemical structures and aggregated structures of PPV precursor(PPVpre) and PPV treated at three node temperatures(125, 155 and 230 °C) were investigated by FTIR spectrometry and X-ray diffraction. The results show that the content of trans-conformation and the size of the crystalline domains increased with the increase of treatment temperature. The mechanical performances of the corresponding polymers were studied by nanoindentation. The elastic modulus and hardness of PPV films decreased with the increases of the content of trans-conformation and the size of the crystalline domains. These results suggest that the conformation and the size of the crystalline domains are able to induce the elastic modulus and hardness variation of PPV films.

Nanomechanical properties of poly(para-phenylene vinylene) determined using quasi-static and dynamic nanoindentation

Mechanical properties of poly(p-phenylene vinylene) (PPV) were determined by quasi-static and dynamic nanoindentation. PPV films were prepared by thermolytic conversion of poly[p-phenylene (tetrahydrothiophenium)ethylene chloride] precursor films, at different temperatures. The mechanical properties of the films showed a dependence on the extent of conversion and chemical composition of the films. The storage modulus, Er′, and plasticity decreased with an increase in conversion, whereas the loss modulus, Er″, showed the opposite trend. Both the precursor and the fully-converted PPV films were found to have significantly lower Er″ than Er′, consistent with the glassy nature of the polymers at room temperature. The fully-converted PPV polymer was found to have room-temperature storage and loss moduli of 4.4 and 0.2 GPa, respectively. A comparison of the contact mechanical properties of the isotropic PPV films synthesized in this work with previously reported values for stretch-oriented anisotropic films determined by conventional tensile testing methods is also discussed.

Photoluminescence properties of poly (p-phenylene vinylene) films deposited by chemical vapor deposition

Photoluminescence spectra of PPV at varying thicknesses and temperatures have been studied. A study of the quenching of the polymer film using a modified version of fluorescence spectroscopy reveals interface effects dominating at thicknesses below about 600Å, while bulk effects dominate at higher thicknesses. The application of the Stern–Volmer equation to solid film is discussed. Stern–Volmer plots were nonlinear with downward deviations at higher thickness of the film which was explained due to self-quenching in films and larger conformational change and increased restriction from change in electron density due to electron transition during excitation in bulk polymer films over 60nm thick. PPV deposited into porous (~4nm in diameter) nanostructured substrate shows a larger 0–0 than 0–1 transition peak intensity and decreased disorder in the films due to structure imposed by substrate matrix. Temperature dependent effects are measured for a film at 500Å, right on the border between the two areas. PPV films deposited on porous methyl silsesquioxane (MSQ) were also examined in order to compare the flat film to a substrate that allows for the domination of interface effects. The enthalpies of the first two peaks are very similar, but the third peak demonstrates a lower enthalpy and a larger wavelength shift with temperature. Films deposited inside pores show a smaller amount of disorder than flat films. Calculation of the Huang–Rhys factor at varying temperatures for the flat film and film in porous MSQ shows large temperature dependence for the flat film but a smaller amount of disorder in the nanostructured film.

Spectroscopic ellipsometry as a complementary tool for studying luminescent polymers: Poly(p-phenylenevinylene) as a particular case

A novel approach for studying the thermal conversion of poly-p-xylylidene tetrahydrothiophenium chloride (PTHT) into poly(p-phenylenevinylene) (PPV) films has been proposed on the basis of calculated optical parameters. Spin-cast PTHT films were thermally converted into partially conjugated PPV films over conversion temperatures ranging from 22 °C to 200 °C. A strong correlation between refractive index (n) and damping constant extinction coefficient (k) was verified by spectroscopic ellipsometry (SE) for the films studied in the visible range of energy. The conversion temperature has a strong influence on the values of n and k, which were theoretically modeled using a Lorentz harmonic oscillator analysis. The films were also studied by spectroscopic techniques (photoluminescence, FTIR, and UV–VIS), and it was verified that the values of n decrease with increasing the amount of CC groups present along the polymer chains, as a strong evidence that both structure and optical properties of thermally converted PPV films can be directly compared. The optical properties of the PPV films were also compared with those of two electroluminescent derivatives in which side chains are introduced into the phenyl ring of PPV, poly(2-methoxy-5-hexyloxy)-p-phenylenevinylene) (MH-PPV) and poly(3-methoxy-5-dodecyloxy-p-phenylene–vinylene) (MD-PPV).

Interfacial electronic structures of Ca/MEH–PPV/Au studied by one and two photon photoemission spectroscopy

Interfacial electronic structures and photoemission spectral characteristics of Ca/MEH–PPV/Au were studied by one and two photon photoemission spectroscopy. For a pristine MEH–PPV film, the work function was measured to be 4.0 eV. This value is 1.1 eV lower than that of a bare Au substrate. At a Ca coverage of 3 Å, the work function was sharply decreased to 2.6 eV. Upon further deposition, it was decreased to 2.3 eV, which is 0.6 eV lower than that of metallic Ca. CaO formation was revealed at the interface of Ca and MEH–PPV.

Optical Properties of Mdmo-Ppv and MDMO-PPV/ [6,6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester Thin Films

Thin films of a conjugated polymer Poly [2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) were prepared from chloroform, 1,2dichlorobenzene and toluene solutions by spin coating technique on quartz substrates. Absorption and photoluminescence (PL) spectra of the polymer thin films prepared from different solvents were measured. The UV -vis absorption and PL spectra of MDMO -PPV films was affected by solvents used for spin coating. Further, with Atomic Force Microscope (AFM) it has been demonstrated that the surface morphology of MDMO -PPV: [6,6]-Phenyl C61-butyric Acid 3ethylthiophene Ester thinfilms depends strongly on preparation condition (solvents).

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.

A new carbon precursor: synthesis and carbonization of triethylammonium-based poly(p-phenylenevinylene) (PPV) progenitors

Unsubstituted poly(p-phenylene vinylene) (PPV), when heated under an inert atmosphere, has very high char yields at temperatures up to 1800 °C and is a potential source of carbonaceous material. However, using unsubstituted PPVs in the synthesis of carbon materials is hindered by their limited processability into fibers or free-standing films before carbonization. To circumvent processability problems, the synthesis of soluble ammonium-based PPV precursors was accomplished, and the first ammonium-based PPV films were carbonized. 1-(Chloromethyl)-4-[(triethylammonium)methyl]benzene chloride was polymerized via an analogous approach to the Gilch reaction under basic conditions. Significant elimination of ammonium groups occurred during polymerization, but the 5–20% remaining ammonium functionalities allowed for solubility in polar aprotic solvents such as dimethylsulfoxide (DMSO). A post-polymerization exchange of the ammonium functionality to a larger counter anion, p-toluenesulfonate, provided more flexible, film-forming PPV-based polymers. Additionally, the low fraction of ammonium groups on the backbone facilitated a low weight loss during carbonization, up to 51 wt% remaining at carbonizations up to 1800 °C, and preservation of the final film structure. These films were carbonized at temperatures of 1000, 1400, and 1800 °C and were analyzed with scanning electron microscopy, Raman, Fourier-transformation infrared spectroscopy, X-ray diffraction, and conductivity measurements to examine the development of the carbon structure.

Abnormal Anti-Stokes Raman Emission and Infrared Dichroism Studies on Poly(p-phenylenevinylene)/Single-Walled Carbon Nanotube Composites

Using surface-enhanced Raman scattering (SERS) and Fourier transform infrared spectroscopy (FTIR) in the grazing angle incident reflection geometry, new data concerning the abnormal anti-Stokes Raman emission (AASRE) and the molecular orientation of poly(p-phenylenevinylene) (PPV) deposited on metallic supports of Ag and Au are reported. The particular dependencies of the anti-Stokes Raman intensity on the Raman shift, the sample thickness, and the incident pump intensity are highlighted in this paper. In the spectral range of 800–1000 cm–1, the FTIR spectra of PPV films deposited on Ag and Au supports show a dichroism similar to that reported for the free-standing PPV film. Significant differences are reported in the spectral range of 1400–1700 cm–1 for the s-and p-polarized FTIR spectra of PPV films deposited on Au and Ag supports. The annealing treatment at 110 °C of the PPV precursor solution in the presence of single-walled carbon nanotubes (SWNTs) induces a noncovalent functionalization of carbon na...

Preparation of Fluorescent Conjugated Polymer Films with Different Emission Colors: Investigation into the Influences of Chemical Structure and Morphology

AbstractA series of PPV films with different fluorescent emission colors are prepared using a facile approach. Modified from the conventional Wessling route, the labile sulfonium groups in the polymer precursor (pre‐PPV) are partly substituted to various degrees by relatively stable methoxy groups, followed by a regular thermal elimination step to obtain PPV. Different chemical structures among these films are confirmed by IR and photophysical studies. Two film‐preparative processes are compared and the films are studied by SEM and confocal fluorescent microscopy. From the same polymer precursor, different preparative processes result in PPV films with different morphologies but the same photophysical properties.