Effect of Ag nanoparticles on the electron energy structure and electrical properties of poly(p-phenylene vinylene) (PPV)

The effects of inorganic nanoparticles on a conjugated polymer were investigated by measuring the electronic properties of poly(p-phenylene vinylene) (PPV) and PPV/Ag nanocomposites. Through hybridization, an enhancement in current density was achieved with PPV/Ag nanocomposites due to an increase in the electron affinity with Ag nanoparticle content. Furthermore, roughening of the surface morphology was observed with incorporation of Ag nanoparticles. This roughness induces an enhanced applied field at the thinner region of the film and an increase in the surface area with a resulting increase of electron injection, leading to current enhancement.     

Syntheses and optical properties of poly(o-phenylene vinylene)s

Poly(p-phenylene vinylene) (PPV) derivatives have been extensively studied as photoluminescence and electroluminescence materials. Recently, meta-phenylene has been incorporated into PPVs for conjugation-length control and luminescence enhancement. Here, we report three poly(o-phenylene vinylenes) (oPVs). Among them, Polymer II exhibits high molecular weight, good processibility and high photoluminescence quantum efficiency in both solution and solid state, making it an attractive candidate for light-emitting diode applications.     

Synthesis and electroluminescent properties of heterocycle-containing poly(p-phenylene vinylene) derivatives

Two novel conjugated polymers were synthesized by the Wittig reaction. The multilayer light emitting diodes were fabricated and showed green to blue electro-luminescence.     

Influence of the counter ion on the properties of poly(o-toluidine) thin films and their Schottky diodes

The influence of various counter ions on the electrochemical, morphological and optical properties of poly(o-toluidine) (POT) thin films and on the electrical properties of POT-based diodes was investigated. The POT thin films were deposited on indium tin oxide (ITO)-coated glass substrates by electrochemical polymerization under cyclic voltammetric conditions from o-toluidine monomer in aqueous solutions of HCl, H₂SO₄ and HNO₃. The deposited films were characterized by cyclic voltammetry, scanning electron microscopy and UV–vis spectrophotometry. It was observed that the current densities and optical absorption spectra are influenced by the counter ions of the electrolyte solutions.On the other hand, ITO/POT/Al devices were fabricated by thermal evaporation of aluminum circular electrodes on the as-deposited POT films. The current–voltage (C–V) characteristics of these devices are non linear. The diode parameters were calculated from current–voltage (I–V) characteristics using the modified Shockley equation. The C–V and capacitance–frequency (C–F) characteristics were also measured. These measurements revealed that the junction electrical parameters depend strongly on the type of supporting electrolytes used in the synthesis of the polymer.     

Poly (p-pyridine) - and poly (p-pyridyl vinylene) -based polymers: their photophysics and application to SCALE devices

Photophysics and light-emitting device applications of poly (p-pyridine) - and poly(p-pyridyl vinylene)-based polymers are presented. Extensive time-resolved (ps to ms) photoluminescence, stimulated emission and photoinduced absorption studies of solutions, powders and films demonstrate that the primary photoexcitation of these polymers is an intrachain singlet exciton. The presence of (n,π^*) states leads to enhanced intersystem crossing to triplet excitons for the powder form, while aggregate formation plays a key role in the films. Polarons are important at longer times. These polymers were used to fabricate ‘conventional’ polymer light-emitting diodes. In addition, these polymers were used to demonstrate a novel light-emitting structure, the symmetrically configured a.c. light-emitting (SCALE) device. These new devices have potential advantages in their use with high workfunction electrodes, such as gold, and also in their a.c. operation.     

Photoluminescent and electroluminescent properties of DCM-1 dispersed poly(p-phenylene vinylene) derivatives

The single layer devices utilizing poly[2-(carbazol-9-yl)-5-(2-ethylhcxyloxy)-1,4-phenylene vinylene] (CzEh-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-l,3,4-oxadiazolyl]-phenyl}-5-(2-ethythexyloxy)-l,4-phenylene vinylene] (OxdEh-PPV) doped with varying weight percent of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM-1) were fabricated and their photo luminescence (PL) and electroluminescence (EL) properties were discussed in this investigation. The PL spectra of DCM-1 doped polymers show that the emission is mostly from DCM-1 and Förster energy transfer may occur between DCM-1 and the two polymers. On the other hand, field-dependence of the emission spectra of EL devices was observed in detail. For the CzEh-PPV/DCM-1, the emission at the wavelength of 534 nm remains unchanged as the level of DCM-1 increases, whereas the peak at 572 nm is intensified with increasing both the additive level and applied electric field. For the OxdEh-PPV/DCM-1, the main peak is red-shift as the level of DCM-1 increases and blue-shift as the applied electric field does.     

Spectroscopy of conducting and insulating ladder-type poly(para-phenylene) device structures

We have applied the electroabsorption (EA) and charge-induced absorption (CIA) spectroscopic techniques to methyl substituted ladder-type poly(para-phenylene) (m-LPPP) sandwich device structures. The device structures have been either of conducting (ITO/m-LPPP/Al) or insulating (ITO/SiO/m-LPPP/Al) type. For both devices we have observed a CIA band with its 0–0 transition at 1.95 eV. From a comparison to doping induced and photoinduced absorption spectra we assign this band to polarons. In addition, in the insulating device we observed a derivative-like band with zero crossing at 1.15 eV. In the conducting device two derivative bands are observed with zero crossing at approximately 0.8 and 1.2 eV, respectively. Their intensity increases linearly with the modulus of the applied electric field and the features are independent of the modulation frequency. We tentatively assign these derivative-like bands to a Stark-effect related to charge transfer transitions at the ITO and Al electrodes. At high currents in the conducting device we also observed a relatively broad, structureless CIA band peaking at 1.3 eV, that we assign to biplarons.     

Synthesis and optical properties of poly(fluorene)-based alternating copolymers

The novel poly(9,9-bis(2-ethylhexyl)fluorene-alt-2′,5′-thiophene), and the similar poly(9,9-bis(2-ethylhexyl)fluorene-alt-2′,5′-thiophene-1,1-dioxide) and poly(9,9-bis(2-ethylhexyl)fluorene-alt-1′,4′-phenylene) luminescent copolymers were synthesised by the Suzuki coupling reaction. Within this series, we show that an adequate choice of the unit combined with the fluorene monomer, effectively controls the energetic position of the frontier levels of the copolymer. This effect is evidenced by the different optical energy band gaps and different emission colours. Furthermore, the results of cyclic voltammetry (CV) studies show that the S,S-dioxide substitution on the thiophene ring increases both the ionisation potential (IP) and the electron affinity (EA) in relation to the similar thiophene-based copolymer.     

Synthesis and electrical properties of poly(p-azoarylene) films

Polymers and copolymers containing p-azoarylene and p-azoxyarylene groups were synthesized by oxidative coupling of various aromatic diamines. Films were cast directly from the reaction mixtures or from the polymer solutions. The films were n-doped by sodium naphthalide or p-doped by iodine. They showed electrical conductivities of 10⁻⁴ ohm⁻¹ cm⁻¹ to 10⁻⁵ ohm⁻¹ cm⁻¹.     

Efficient blue light-emitting diodes from a soluble poly (para-phenylene) internal field emission measurement of the energy gap in semiconducting polymers

Polymer light-emitting diodes with poly (2-decyloxy-1,4-phenylene), DO-PPP, as the luminescent polymer emit blue light with external quantum efficieny of ∼3%. Using internal field emission (Fowler-Nordheim tunneling) in single carrier devices, the top of the π band and the bottom of the π^* band have been determined for DO-PPP, and for poly (2-methoxy-5-(2'-ethyl-hexyloxy)-1,4phenylene vinylene), MEH-PPV. For DO-PPP, the single particle π-π^* energy gap is 3.4–3.6 eV; for MEH-PPV, the π-π^* energy gap is 2.2–2.3 eV. Comparison with optical absorption (the optical gap) implies that the exciton binding energy is small; for both polymers, E_exc ≤ 0.2 eV. Such small residual values are probably not significant in the presence of disorder broadening which is also 0.1–0.2 eV and in the presence of lattice relaxation with self-localization energies of approximately 0.2 eV.     

Light-emitting electrochemical cells and light-emitting diodes based on ionic conductive poly(phenylene vinylene): a new chemical sensor system

Red-orange light-emitting electrochemical cells (LECs) based on poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene] also named poly[2,5-bis (triethoxy-methoxy)-1,4-phenylene vinylene], (BTEM-PPV) are fabricated and characterized. BTEM-PPV combines good electronic conductivity with ionic conductivity in the oxidized and reduced state due to its conjugated backbone and oligo(ethylene oxide) attached as side chains. When applying this polymer in LECs one obtains devices of moderate brightness and with fast response times. This high performance is achieved without blending an additional ionic conductive polymer into the film. The response time of such devices driven with square waveform voltage pulses was determined to be 480 μs. The turn-on voltage for electroluminescence occurs at 2 V and at 3 V a brightness of about 35 cd/m² was obtained. Due to the covalent linkage of glyme like side chains to the PPV backbone, BTEM-PPV, complexed with metal ions, shows an ionochromic effect in the absorption spectra and also in the electroluminescence spectra, which can be a new approach to chemical sensors.     

Energy transfer from poly(2-carboxyphenylene-1,4-diyl) to poly(p-phenylene vinylene) in bilayer films and its effects on luminescent properties

Luminescent properties of poly(p-phenylenevinylene) (PPV) in bilayer films were found significantly enhanced by energy transfer from poly(2-carboxyphenylene-1,4-diyl) (PCPD) to PPV prepared by alternatively spin-coating the poly(xylylene tetrahydrothiophenium chloride) (PXT, dubbed as PPV-precursor) aqueous solution and PCPD pyridine solution followed by heat treatments. The energy transfer process, as verified by the photoluminescent excitation (PLE) spectroscopy and the analysis of time-resolved photoluminescence (PL) decays, was attributed to the chemical interlocking between two polymers in the interfacial region. Accordingly, the efficiency of energy transfer from PCPD to PPV was calculated. Consequently, the (glass)PPV/PCPD configuration exhibited higher energy transfer efficiency than the (glass)PCPD/PPV, resulting in higher PL and electroluminescent (EL) quantum efficiencies. Besides, the improved EL emission of indium-tin-oxide (ITO)/PPV/PCPD/Al device was attributed to the energy barrier in the PPV/PCPD interfacial region, which trapped the majority carriers, holes, to form the excitons in situ, that decayed radiatively.     

Alteration of the photo and electroluminescent properties of poly(p-phenylene vinylene) upon addition of indium chloride

We investigate the modification of the optical properties of poly(p-phenylene vinylene), PPV, upon addition of indium chloride, InCl₃, to its precursor solution. We find that the most significant decrease of the photoluminescence efficiency occurs after addition of ≈0.5% molar of InCl₃ to the PPV precursor. The PL efficiency either remains constant or increases, depending on the conversion conditions, for further increase of the salt content. Interestingly, we find instead that the electroluminescence efficiency increases upon addition of InCl₃. Absorption and photo-deflection spectroscopy yield no evidence of charge-transfer doping of PPV+InCl₃ blends converted on spectrosil. We discuss these results in the context of previous reports on the indium–tin oxide influence on the transport and luminescent properties of PPV.     

Influence of stereoregularity on the photoinitiated electrical conductivity of poly (3-alkylthiophenes)

Several kinds of synthesis methods leading to regie and stereo chemical control of polymer architecture and also to polymers derived from oligomers possessing internal symmetry or containing suitably spaced alkyl chains have been performed. The different structures have been studied in relation to their electrical conductivity behaviour. Doping of polymers has been photoinitiated with optically dissociative materials (triarylsulfonium salts) which were incorporated with the basic poly(3-alkylthiophene) polymer into the spinning solution. Conditions of light exposure (wavelength range, exposure dose) and concentration of photodopant used have been optimized in order to enhance the electrical conductivity and its thermal stability. High-resolution conducting patterns have been realized by microlithography techniques. Their possible use in microelectronics is investigated.     

Iron doping effects on the high-resolution 13C NMR spectra of solid poly(para-phenylene)

A comparative study of the electrical conductivity, electron spin density and ¹³C high-resolution solid-state NMR linewidth of FeCl₃-doped poly(para-phenylene) (PPP) was made as a function of Fe³⁺ concentration. It is concluded that the NMR linewidth changes are related to the interaction between bipolarons in the polymeric chain.     

Structure and electrical properties of chemically modified poly(3-alkylthiophenes)

The aim of this work was to determine the effects of substituting the thiophene aromatic hydrogen atom in the 4-position in poly(3-alkylthiophene) (P3AT) by a nitro group (-NO₂), or by a chlorine atom (-Cl) in a highly dispersed state of polymer. The feature of such substituted polymers is lower conductivity in comparison with the initial polymer. Modified P3ATs remain semiconducting in character. The nitration and chlorination of P3ATs take place mainly in the 4-position of the thiophene ring. The rates of both processes increase with an increase in the chain length of the alkyl side group.     

Onset of thermal degradation in poly(p-phenylene vinylene) films deposited by chemical vapor deposition

We report on the thermal degradation characteristics of heat treated poly(p-phenylene vinylene) (PPV) films deposited using chemical vapor deposition. It was found that no decrease in the thickness of the films (110 nm) occurred after isothermal heat treatment in vacuum at 450 °C for up to 1 h, while films treated at 500 °C for 1 h decreased to 70% of their original thickness. In situ mass spectrometry during heat treatment of the film at 500 °C showed the release of significant amounts of material including toluene and xylene ion fragments. However, Fourier transform infrared spectroscopy shows no significant change in bond structure, indicating that the decrease in film thickness after heat treatment is due to release of material and densification, not crosslinking or bond breaking.     

Synthesis and characterization of low bandgap poly(dithienosilole vinylene) derivatives

Two low bandgap conjugated polymers containing vinylene unit on their main chains, poly(dithienosilole vinylene) derivatives P1 and P2, were synthesized by Pd-catalyzed Stille-coupling method. The two polymers show good solubility in common organic solvents, broad absorption bands from 500 nm to 700 nm and lower energy bandgap of 1.72 eV for P1 and 1.77 eV for P2. The electronic energy levels of the polymers were measured by electrochemical cyclic voltammetry. The HOMO and LUMO energy levels of P1 and P2 are at ?5.03 eV, ?5.18 eV and ?3.39 eV, ?3.43 eV, respectively. The polymers P1 and P2 containing dithienosilole units show lower HOMO and LUMO energy levels, in comparison with other poly(thienylene vinylene) derivatives.     

The origin of the delayed emission in films of a ladder-type poly(para-phenylene)

We report on the observation of delayed fluorescence (DF) and phosphorescence (P) from films of the ladder-type poly(para-phenylene) (MeLPPP). Upon pulsed optical excitation into the S₁←S₀ transition of the MeLPPP, delayed emission occurs on a time scale of μs to ms. The DF varies linearly with pump light intensity, while the phosphorescence shows a sublinear intensity dependence. We will argue that singlet–singlet (SS) annihilation is responsible for the sublinear phosphorescence intensity (P^int) dependence, since the intensity dependence of the prompt fluorescence shows the same behaviour above excitation intensities of 1 μJ/pulse. Both, DF and P decay according to a power law. The data analysis reveals that the DF is caused by recombination of geminate electron hole pairs rather than triplet–triplet (TT) annihilation. This conclusion is supported by investigations of the response of the DF to an applied electric field.