Synthetic Metals (v.157, #8-9)

Top emission organic light emitting diode with a Cr/Al/Cr anode by Sung Mook Chung; Chi-Sun Hwang; Jeong-Ik Lee; Sang Hee Ko Park; Yong Suk Yang; Lee-Mi Do; Hye Yong Chu (327-331).
A top emission organic light emitting diode (TEOLED) comprised of a Cr anode on PES film/NPB/Alq3/cathodes has been fabricated. The triple layer structure of Cr/Al/Cr allowed for fabrication of a crack-free anode and provided better thermal stability and higher work function than a conventional ITO anode. For the Cr/Al/Cr anodes, a series of Cr layers with various surface morphology has been tested. A Cr layer with a smooth surface morphology was found to be optimal. The TEOLED fabricated on PES film having good anode surface morphology showed similar device characteristics to that on a Si wafer. TEOLEDs on PES film and Si wafer exhibited a maximum luminous efficiency of 2.87 and 3.0 cd/A, respectively, at 1000 cd/m2 with a NPB/Alq3/LiF/Al/Ag structure on a Cr/Al/Cr anode.
Keywords: A top emission organic light emitting diode (TEOLED); Triple layer structure; Anode; Thermal stability; High work function;

Synthesis of p-type conjugated dendrimers bearing phenothiazine moiety at the periphery and their light-emitting device characterization by Dong Hoon Choi; Kyu Il Han; In-Hee Chang; Suk-Ho Choi; Xiao-Hang Zhang; Kwang-Hyun Ahn; Yong Kyun Lee; Jin Jang (332-335).
New dendrimers bearing a phenothiazine heterocyclic building block have been successfully prepared and their absorption and emission properties have been investigated in terms of photoluminescence (PL) and electroluminescence (EL). Particularly, the EL has been studied precisely in the heterostructural devices that are fabricated with two dendrimers. The spectrum and the intensity of EL and PL are varied with the number of generation. Significant difference of the IVL characteristics and the external quantum efficiency in the dendrimer devices reveals that the EL performance of the higher-generation dendrimers is higher than that of the lower-generation ones. These results suggest that the site isolation between the chromophores suppresses the non-radiative decay process and reduces the interchain molecular excimer emission from the aggregated dendrimers.
Keywords: Phenothiazine; Dendrimer; Absorption; Photoluminescence; Electroluminescence;

We succeeded in the chemical preparation of nano-level thick polyaniline (PANI) emeraldine salt films on plastic substrate by an in situ vapor-phase deposition (VDP) polymerization method under ambient conditions, using a self-assembly method which is unprecedented. Homogeneous conductive PANI thin films were uniformly fabricated at nano-level thickness (20–100 nm), but their morphologies could grow as polycrystalline grains of a highly ordered structure, depending on the deposition conditions. The grain size was also controlled between 30 and 100 μm depending on the deposition time/temperature. The surface resistance of PANI films was enhanced up to 104  Ω/square with crystallization and light transmittance was increased up to 90% in the case of a film less than 30 nm thick. A typical spectrum for the oxidized PANI, the emeraldine salts form, showing π–π* transition and a polaron lattice were observed by UV–visible/IR and infrared /Raman spectroscopy.
Keywords: Polyaniline; Vapor-phase polymerization; Nano-films; Polycrystalline;

Multilayer blue polymer light-emitting devices with spin-coated interlayers by Lian Duan; Byung Doo Chin; Nam Choul Yang; Mu-Hyun Kim; Hye Dong Kim; Seong Taek Lee; Ho Kyoon Chung (343-346).
Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m2 at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m2.
Keywords: Blue-emitting polymer; Interlayer; Exciplex; Charge balance; Stability;

The preparation and properties of sodium and organomodified-montmorillonite/polypyrrole composites: A comparative study by Miroslava Mravčáková; Mária Omastová; Katarína Olejníková; Béla Pukánszky; Mohamed M. Chehimi (347-357).
Two montmorillonites, an inorganic sodium montmorillonite (NaMMT) and an organo-modified montmorillonite (OMMT), were used for the preparation of montmorillonite/polypyrrole (MMT/PPy) composites. MMT particles were modified by the in situ polymerization of pyrrole in water, in aqueous solution of dodecylbenzenesulfonic acid (DBSA) used as anionic surfactant, and in water/methanol. Ferric chloride was used as oxidant in each case. Wide angle X-ray scattering (WAXS) measurements proved the intercalation of PPy into the galleries of NaMMT regardless the reaction media. In contrast, for OMMT/PPy composites, the increase of interlayer spacing depends on the preparation conditions, the highest increase in interlayer spacing was achieved in water/DBSA solution. The WAXS patterns of OMMT/PPy composites synthesized in methanol/water showed no change in interlayer spacing and the electrical conductivity of these composites was low, similar to that of NaMMT/PPy composites prepared under the same conditions. Conductivity about 1.1 S cm−1 was reached for OMMT/PPy composites containing 13.3 wt% PPy prepared in the presence of DBSA. The NaMMT/PPy composite containing 15.6 wt% PPy and prepared under the same conditions showed a conductivity of 0.26 S cm−1. X-ray photoelectron spectroscopy (XPS) proved that the surface of NaMMT/PPy composites is rich in MMT, whereas more PPy was found on the surface of OMMT/PPy composites. The conductivity of composites correlated with the N/Si atomic ratio determined from XPS results, which was taken as a semi-quantitative measure of the PPy surface fraction.
Keywords: Polypyrrole; Montmorillonite; Composites; Electrical conductivity; XPS; WAXS;

Conducting textiles from single-walled carbon nanotubes by Marc in het Panhuis; Jian Wu; Syed A. Ashraf; Gordon G. Wallace (358-362).
In this paper we present a simple and straightforward route to prepare conducting textiles by incorporating carbon nanotubes through a dyeing approach. This process consists of immersing textiles in an aqueous sulfonated polyaniline-carbon nanotube dispersion acting as a dye. Incorporating carbon nanotubes into textiles increased the conductivity by four orders of magnitude and doubled the capacitance compared to textiles dyed with sulfonated polyaniline. Textile durability and their applicability as strain gauges were also demonstrated.
Keywords: Carbon nanotubes; Conducting polymers; Conducting textiles; Electronic textiles; Polyaniline;

Charge transfer in PDI-derived systems studied with light-induced electron spin resonance by Henryk Manikowski; Danuta Wróbel; Kornelia Lewandowska; Andrzej Graja (363-367).
Dissymmetrical perylene-3,4,9,10-tetracarboxydiimide building block (PDI) was used to obtain the dyads with fullerene (PDI-C60) and tetrathiafulvalene (PDI-TTF). Photoinduced electron transfer in these dyads was investigated with light-induced electron spin resonance (LESR) spectroscopy. The LESR signals of the samples in CHCl3 solutions were recorded in the dark and under light illumination, at room temperature and at about 80 K. It was stated that the LESR signal increases stepwise with the incident light and decreases slowly when the light is switched off. The photogeneration of PDI• +, TTF• and C60 ion-radicals was observed and analyzed. The principal parameters of the LESR lines of the ion-radicals are determined.
Keywords: PDI dyads; Fullerene; Tetrathiafulvalene; Light-induced electron spin resonance;

Dielectric properties of thiophene based conducting polymers by Mustafa Okutan; Yusuf Yerli; S. Eren San; Faruk Yılmaz; Okan Günaydın; Murat Durak (368-373).
Dielectric properties of some conducting polymers were investigated and their electrical properties are compared with each other. Temperature dependencies of their electrical properties are investigated throughout the work. Electrical behaviors of the synthesized systems constitute a good infrastructure for both understanding the intrinsic mechanisms and optimizing new synthesizes. It is found that, it can be possible to attain desired electrical properties by small synthesis modulations in thiophene based polymer materials.
Keywords: Thiophene; Conducting polymer; Dielectric spectroscopy;

Polypyrrole coated carbon nanotubes: Synthesis, characterization, and enhanced electrical properties by Nanda Gopal Sahoo; Yong Chae Jung; Hyang Hwa So; Jae Whan Cho (374-379).
We describe a simple approach to the synthesis of MWNT/polypyrrole nanotubes by the in situ chemical polymerization of pyrrole on the carbon nanotubes using ferric chloride as an oxidant. The effects of pyrrole concentration on the coating and properties of the resulting complex nanotubes were studied by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, and thermal gravimetric analysis. The coated PPy layers could be controlled easily by adjusting a feed ratio of pyrrole to MWNTs. FT-IR results suggested an existence of interaction between the –COOH groups of chemically modified MWNTs and =NH groups of the PPy. SEM and TEM studies indicated that each individual MWNT could be coated with PPy. The resultant nanotubes enhanced electrical conductivity compared to PPy and MWNT which was strongly influenced by the feed ratio of pyrrole to MWNTs.
Keywords: Carbon nanotube; FT-IR; Polypyrrole; Raman spectroscopy; TEM; X-ray;

Polaron formation dynamics in conducting polymers by Xiaojing Liu; Kun Gao; Yuan Li; Jiyong Fu; Jianhua Wei; Shijie Xie (380-385).
In the framework of the extended Su–Schrieffer–Heeger model, the formation mechanism of a polaron is studied by calculating the interaction force between a hot electron and the lattice in a conducting polymer. It is found that the formation of a polaron depends upon the energy of the hot electron. Regardless of whether an electron comes from an injection, doping or a dissociated state of a polaron, it will be trapped by the lattice to form a polaron only when its energy is lower than a critical value. The force field between the lattice and the hot electron with a lower energy has a localized characteristic, which causes polaron formation in a conducting polymer. For a hot electron with much higher energy, the force field is extended over the whole polymer chain, which cannot trap the electron and therefore a polaron does not form in this case.
Keywords: Organic polymer; Polaron;

Polypyrrole-polyethylene glycol conducting polymer composite films: Preparation and characterization by Lim Mei Yee; H.N.M. Ekramul Mahmud; Anuar Kassim; Wan Mahmood Mat Yunus (386-389).
The preparation and characterization of polypyrrole-polyethylene glycol (PPy-PEG) composite films are reported in this present paper. The polypyrrole-polyethylene glycol composites were synthesized by electrochemical method, using p-toluene sulfonate as a dopant in aqueous medium. The composite films were synthesized with various concentration of PEG and were characterized by optical microscopy, x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, photoacoustic spectroscopy and electrical conductivity measurement. Both the electrical conductivity and thermal diffusivity exhibited the highest values with the process condition of 0.2 M pyrrole, 0.1 M p-toluene sulfonate and 1×10−3  M PEG at 1.2 V (versus SCE). The optical microscopy of PPy-PEG shows the globular surface morphology. The XRD results demonstrated that PPy-PEG composite films are amorphous. The FTIR result reveals the successful incorporation of PEG into the PPy structure forming PPy-PEG composite films.
Keywords: Polypyrrole; Polyethylene glycol; Electrochemical method; p-Toluene sulfonate;

A polyurethane inserted multi-wall carbon nanotube (MWCNT) composite conductive film was prepared by in situ dispersed polymerization reaction using hydroxyl-terminated poly(butadiene-acrylonitrile) liquid rubber as a linear diol, toluene diisocynate as a curative, ethylene glycol or glycerine or triethanolamine as a chain-extending agent and MWCNT as a conducive filler. The effect of various curing temperatures and chain-extending agents on vapor-induced electrical responsiveness of the conductive films was investigated. The structural characterization of the cured film was conducted by Fourier transformation infrared spectrophotometer (FTIR), differential scanning calorimeter (DSC), polarization microscope (POM) and wide angle X-ray diffraction (WAXD). The experimental results showed that the conductive composite film obtained in the present work exhibited a microphase separation resulting from the soft-hard segment domains, and possessed some crystalline behavior from the hard segment. The response intensity was enhanced with the curing temperature increased, while the reversibility could be improved at a relatively low curing temperature. The responsivity of the film produced by a linear difunctional group chain-extending agent was lower than that prepared by trifunctional group curatives, and the reversibility was vice versa. The experimental phenomena were explained from the viewpoint of the microphase separation, crystalline behavior, the structural characteristics of the soft-hard segment, and the electronic properties of multi-wall carbon nanotubes as well as a weak electrostatic or noncovalent interaction between polymer or analyte molecules and MWCNTs.
Keywords: Conductive composites; Carbon nanotubes; Vapor sensitive response; Electric properties; Characterization;

Carbazolyl- and diphenylamino substituted fluorenes as hole transport materials by A. Kruzinauskiene; A. Matoliukstyte; A. Michaleviciute; J.V. Grazulevicius; J. Musnickas; V. Gaidelis; V. Jankauskas (401-406).
Four fluorene-containing aromatic amines with carbazole and diphenylamine moieties were synthesized by Ullmann coupling. Comparative study on their thermal, optical and photoelectrical properties is presented. One synthesized compound is found to form glass with the glass transition temperature 58 °C as characterized by differential scanning calorimetry. The ionization potentials established by electron photoemission technique are in the range 5.9–6.1 eV. Hole-drift mobilities of 50% solid solution of 2,7-dicarbazolyl-9,9-dihexylfluorene in bisphenol Z polycarbonate established by the xerographic time-of-flight technique exceed 10−4  cm2  V−1  s−1 at electric field of 106  V cm−1.
Keywords: Aromatic amine; Ionization potential; Hole-drift mobility;