Exciton-exciton annihilation in poly (p-phenylenevinylene) films

We have measured the saturation of fluorescence intensity with increasing excitation intensity and the time dependence of fluorescence intensity decay at high excitation intensities in poly (p-phenylenevinylene) films. The experimental results are consistent with the hypothesis that exciton-exciton annihilation reduces the exciton lifetime at high exciton concentrations and that the exciton diffusion coefficient is quite high. We conclude that the lowest energy electronic transition creates a Frenkel exciton rather than a free-carrier pair in a band-to-band transition.     

Temperature effect on the electronic spectra of poly(p-phenylenevinylene)

The experimental temperature-dependent absorption, phololuminescence and photoluminescence excitation spectra of poly (p-phenylenevinylene) (PPV) are presented and a molecular theory is used to analyze the results. In order to model the complicated system such as PPV, the structural inhomogeneity has to be included. The structural inhomogeneity is represented by the different Huang-Rhys factors associated with different chain lengths. The theoretical spectra agree qualitatively with the experimental results.     

Electrical bistability and memory applications of poly(p-phenylenevinylene) films

Devices based on oriented and unoriented poly(p-phenylenevinylene) films showed presence of electrical bistability. Depending on the voltage sweep direction, current through the device at any voltage had two values. Switching between a low- and a high-conducting state occurred when a suitable voltage pulse was applied. Impedance spectroscopy identified a change in bulk resistance of the device during the transition between the two states. Role of orientation of the polymer film on switching and electrical bistability has been studied. The bistability in these devices had an associated non-volatile memory. We have demonstrated read-only and random access memory applications in such devices using current–voltage and impedance characteristics.     

Synthesis and characterization of a novel side-chain liquid crystalline poly(p-phenylenevinylene)

We describe the synthesis of soluble liquid crystalline poly(p-phenylenevinylene)s with cyanobiphenyl as the mesogenic group linked to the vinylene group via a flexible alkyl chain. The polymers were well characterized by spectroscopic methods (NMR, IR, UV—Vis, photoluminescence (PL)) and showed PL emission of green light with a maximum around 540 nm. Polarized UV—Vis spectroscopy of polymer films prepared on oriented substrates indicated a high orientation of the backbone and mesogenic units.     

Application of spectroscopic ellipsometry to probe the environmental and photo-oxidative degradation of poly(p-phenylenevinylene) (PPV)

We report on the spectroscopic ellipsometry (SE) characterization and modeling of the light and environmental induced degradation of Poly(p-phenylenevinylene) (PPV) prepared by xanthate precursor route. We find a decrease in the refractive index as well as absorption coefficients after exposure to light in air ambient. Further, a decrease in film density and increase in film thickness is also observed on light exposure in air. On the other hand, optical constants are not affected by light exposure in vacuum.     

Electroluminescence in conjugated polymers: excited states in cyano-derivatives of poly(p-phenylenevinylene)

Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics both as electroluminescent diodes and as photoconductive diodes. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and, thus, bring about electron-hole capture to generate excitons at this interface. We present results on the photophysical properties of the cyanosubstituted poly(phenylenevinylene)s which provide electron transport layers if these heterostructure devices; we find evidence for strong interchain interactions (excimer formation) for the neutral excited states in these polymers, with strong red shifts of the luminescence, though preserving high photoluminescence efficiencies.     

Detection of delamination in IC packages using the phase of microwaves

To detect the delamination in IC packages, a microwave nondestructive testing method that utilizes an open-ended coaxial line sensor has been developed. The coaxial line sensor acts both as a source and receiver of microwave signal that is transmitted into and reflected from the package. The phase of the effective reflection coefficient measured at the aperture of the coaxial line sensor is varied with the thickness of the delamination layer. The phase difference in the case of package with and without delamination was calculated for evaluation of the delamination effectively. The high sensitivity of the present technique makes it very useful to detect delaminations in IC packages. Also, the method has the potential for inspection of other dielectric materials.     

Synthesis,spectroscopy and electrochemistry study on a novel di-silyl substituted poly(p-phenylenevinylene)

A novel di-silyl substituted poly(p-phenylenevinylene) (PPV) derivative is synthesized through the Gilch route. The polymer is solution processable and shows high thermal stability in air. UV–Vis spectra for the film and solution samples illustrate some different features, in which the absorption peak corresponding to the π–π* transition of the delocalized electrons along the polymer back bone is red shifted and broadened in the film state compared to the solution one. The fluorescent emission spectra are quite similar for the polymer in solution and in the solid states. Cyclic voltammetric investigation reveals a partially reversible n-doping process and an irreversible p-doping process. The cathodic current is much greater than that of the anodic one, which means the polymer has the tendency to be charged through n-doping rather than p-doping process. The improved affinity of the polymer to electrons may be owing to the low interface barrier between the polymer films and the electrode surface. The HOMO and LUMO energy levels have also been estimated from the data of oxidation and reduction onset potentials. All of the results indicate that the synthesized polymer is a promising green emissive material for the light-emitting device application.     

Substituent effects on poly(p-phenylenevinylene) backbone,interrupted with silicon atoms: molecular orbital theory

The electronic structures and the structural deformations of silicon containing poly(p-phenylenevinylene) (PPV) polymers have been studied theoretically. The results of the calculations indicate that the regular π conjugated system is effectively interrupted by the organosilicon units yielding a blue emission. It has also been observed that substitution of methoxy on vinylene group caused significant structural deformation of the PPV lumophore, and subsequently, decreased the ionization potential (IP) and increased the electron affinity of the SiBu PPV. Experimentally, it is known that light emitting diodes made of silicon containing copolymers operate at low voltages, due to the reduction of the LUMO level in the luminescence polymers. The flat deformation of the PPV moiety induced a red shift of the absorption maxima. The substitution of phenyl ring on silicon atoms constrained free rotation around the Si C bond increasing the torsional angles of the lumophore and caused slight blue shift of absorption peak. Substitution of the cyano group on the vinylene moiety increased the ionization potential as well as the electron affinity. On the other hand, the electron donating methoxy group decreased the ionization potential and electron affinity. Those calculated results agree well with the experimental observations. The density functional calculation method, which account for electron correlation, yielded much more accurate values for the HOMO–LUMO gap than the Hartree–Fock method.     

Luminescent monomer of substituted tetrastyrylpyrene and poly(p-phenylenevinylene) derivative with pyrene segments: Synthesis and photophysics

1,3,6,8-Tetrabromopyrene reacted with 1-tert-butyl-4-vinylbenzene to afford the luminescent monomer M. In addition, the reaction of 1,6-dibromopyrene with 1,4-didodecyloxy-2,5-divinylbenzene yielded a poly(p-phenylenevinylene) (PPV) derivative P. M dissolved partially in chloroform and completely in THF. P showed an excellent solubility being readily soluble in common organic solvents. M emitted intense blue–green light in THF solution with photoluminescence (PL) maximum at 507 nm and green–yellow light in thin film with PL maximum at 570 nm. Polymer P displayed in THF a PL maximum at 464 nm and a shoulder at 513 nm. Thin film of this polymer exhibited PL maximum at 572 nm. Both M and P emitted from excimers in solid state. The PL emission quantum yields in THF were 0.35 for M and 0.32 for P. The emission maximum of M thin film was red shifted by 9 nm after annealing at 150 °C for 24 h as a result of conformational changes of the molecules with increase in temperature.     

Fullerene/poly(methylphenylsilane) (PMPS) organic photovoltaic cells

Fullerene (C₆₀)/poly(methylphenylsilane) (PMPS) photovoltaic cells were fabricated with various concentrations of C₆₀ and their photovoltaic properties were investigated. The characteristic PL peak of PMPS was significantly quenched in C₆₀/PMPS and the photocurrent of a C₆₀/PMPS device was much higher than that of a PMPS single layer device. This indicates that the addition of C₆₀ to PMPS enhanced the separation efficiency of electron-hole pairs. It was found that the performance of a PMPS device was dramatically improved by blending C₆₀.     

Synthesis and characterization of poly (aniline-co-acrylonitrile) using organic benzoyl peroxide by inverted emulsion method

An inverted emulsion process was adopted to synthesize conducting copolymers of aniline and acrylonitrile using benzoyl peroxide (BPO) as a novel oxidizing agent. The influence of polyacrylonitrile (PAN) ratio on the properties of conducting copolymers is reported. The chemical structures of the copolymers were characterized spectroscopically by UV-Vis, FT-IR, FT-Raman and EPR. The solubility of polyaniline (PAni) and PAni–PAN copolymers in dimethyl sulphoxide (DMSO) was confirmed by electronic absorption spectra. The conductivity of these copolymers lie in the range (1.26–4.20)×10⁻² S/cm. TGA thermogram of copolymers showed multi-step thermal degradation behaviour. X-ray scattering studies reflected that the copolymers are semicrystalline and showed two crystalline peaks at 17 and 25 (2θ).     

Electropolymerization and characterization of poly(N-methylaniline) and poly(N-butylaniline) in mixtures of aqueous and organic solvents

Electropolymerization of N-methylaniline (NMA) and N-butylaniline (NBA) was studied on glassy carbon and optically transparent tin oxide electrodes in mixtures of aqueous and organic solvents. Five different organic solvents, dimethyl formamide, dimethyl sulfoxide, tetrahydrofuran (THF), propylene carbonate (PC) and acetonitrile (ACN) were mixed with aqueous 1.0 M HClO₄ solutions. Our results show that NMA and NBA can be electropolymerized in aqueous–organic solvent mixtures with maximum 30 and 50% (v/v) organic solvents, respectively. The choice of organic solvent strongly influences the film formation. With addition of 10–20% THF, ACN and PC to the aqueous polymerization solution, the film formation was highly improved. The poly(N-methylaniline) and poly(N-butylaniline) films were characterized with cyclic voltammetry, in situ UV–vis spectroscopy, size exclusion chromatography and with electrical conductivity measurements.     

Preparation of light-emitting devices with poly(p-phenylenevinylene): effects of thermal elimination conditions and polymer layer thickness on device performance

The optimum thermal elimination conditions for poly(p-phenylene vinylene) light-emitting diodes were sought. The precursor films should be heated to 230°C and kept at this temperature for 5 min under a N₂ flow of 50 ml/min. By the heat treatment the degree of conversion to PPV was about 70%. A method determining the degree of conversion to PPV was proposed with IR measurement. About 8% of the THT resides in the fully converted PPV. A high external quantum efficiency of 0.0078% was achieved for the ITO/partially converted PPV/Al devices. The optimum thickness for the partially converted PPV layer as a electroluminescent was about 150 nm.     

Field effect transistor using poly(o-metoxyaniline) films

A thin-film transistor (TFT) with good carrier mobility and environmental stability has been fabricated using poly(o-metoxyaniline) (POMA) as the active layer. The carrier mobility has been determined to be approximately 2×10⁻⁴ cm² V⁻¹ s⁻¹, for the best transistors with low conductivity (<10⁻⁷ Ω⁻¹ cm⁻¹). High dynamic ranges have been found, with source–drain currents varying four orders of magnitude with different gate voltages.     

Tunable photoluminescence of poly(phenylene vinylene) nanofibers by doping of semiconductor quantum dots and polymer

In this paper, we report the effects of polyvinyl alcohol (PVA) and CdS quantum dots (QDs) on the photoluminescence (PL) properties of poly(phenylene vinylene) (PPV) nanofibers. The composite nanofibers were fabricated by coupling self-assembly and electrospinning method. A significant blue shift in both PL spectra and fluorescence images of PPV fibers was observed after doping of CdS QDs and PVA in succession. It was found that the changes in PL spectra of PPV originated from the chemical interactions between PPV and PVA, and the changes in the crystallization of PPV itself. A possible PL mechanism was proposed accordingly.     

Doping of poly(acetylene) to the metallic state using thionyl chloride

SOCl₂ has been found to be an effective oxidant for poly(acetylene), increasing the electrical conductivity by several orders of magnitude. The oxidized (CH)_x sites are thought to be compensated with chloride ion liberated by reduction of SOCl₂. The use of poly(acetylene) as a current collector in lithium/oxyhalide cells is reported elsewhere [14].     

Light-emitting diodes based on alternating copolymers containing fluorene and oligo(p-phenylenevinylene)

We synthesized poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-cyanostyryl)benzene-1,4-diyl)] [P(FOPV-CN)] and poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-diphenylaminostyryl)benzene-1,4-diyl)] [P(FOPV-Am)]. These polymers have two axes of longitudinal π-conjugation: one in the direction of the polymer backbone and the other in the direction of the oligo(p-phenylenevinylene) chain. From the spectroscopic study, the emissive color of poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(benzene-1,4-diyl)] changed from blue to green by substituting cyanostyryl or diphenylaminostyryl group at the 2,5-positions of the benzene ring. The electronic structures were significantly influenced by the substituents at the ends of oligo(p-phenylenevinylene) moiety. The diphenylamino substituent which has more extended π-conjugation resulted in red-shifted electronic absorption and emission spectra compared to the cyano substituent groups. In the light-emitting diodes (LEDs) based on these two polymers, the turn-on voltage (ca. 9 V) of the device based on P(FOPV-Am) was lower than that of P(FOPV-CN) (ca. 12 V) due to the electron-donating effect of the amino groups. The maximum brightness of the P(FOPV-Am)-based LED was 3100 cd m⁻² at 22 V which is much higher than that of the P(FOPV-CN)-based LED (27 cd m⁻² at 26 V).     

Nanostructured layers of a new cross-linkable poly(3-hexylthiophene) in organic photovoltaic cells

We report on the synthesis of a new cross-linkable poly(3-hexylthiophene) copolymer, P3HT-Ox10, consisting on regioregular blocks of 3-hexylthiophene and 10% molar of an oxetane-functionalized thiophene comonomer. This copolymer was used in the fabrication of organic photovoltaics (OPVs) upon combination with 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C₆₁ (PCBM), either as blends or in bilayer structures. Bilayer devices with a nanostructured donor/acceptor interface were fabricated using a patterning process based on the phase separation in P3HT-Ox10:polystyrene blends. Columnar-grain films of cross-linked P3HT-Ox10 with diameters of ca. 140 nm were thereby obtained. These are the lowest dimensions achieved so far by this approach. Despite the modest power conversion efficiencies achieved, we believe that this easily implemented approach is very effective to prepare OPVs with well-defined morphologies.     

The application of soluble and regioregular poly(3-hexylthiophene) for organic thin-film transistors

We have investigated the effect of solvent medium, thermal annealing and dedoping reducing agent such as hydrazine on the performance of poly(3-hexylthiophene) (P3HT) device. The performance of P3HT device varied significantly from different solvents, among them, p-xylene proved to be the best solvent medium, and also hydrazine as dedoping agent made progress in the mobilities by one or two orders of magnitude. Especially, in our experiments, solution preparing, deposition and device measurements were all performed in the air for the application of P3HT without further purification to the ink-jet printing, spin coating or solution casting process for large area and flexible organic thin-film transistors (OTFTs).