Synthetic Metals (v.150, #3)

Diodes based on blends of molecular switches and conjugated polymers by Peter Andersson; Nathaniel D. Robinson; Magnus Berggren (217-221).
Here we report polymer diodes based on a conjugated polymer host and a dispersed molecular switch. In this case, the molecular switch is a photochromic (PC) molecule that can be reversibly switched between low and high energy gap states, triggered by exposure to ultra-violet and visible light, respectively. While dispersed inside the conjugated polymer bulk and switched to its low energy gap state, the PC molecules act as traps for holes. Solid-state blends of this PC material and conjugated polymers have been demonstrated in diodes. The state of the PC molecule controls the current density versus voltage (JV) characteristics of the resulting diode. Both poly(2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) host materials have been studied. The two conjugated polymers resulted in differing JV switching characteristics. A more pronounced JV switch is observed with MEH-PPV than with P3HT. We postulate that the PC material, while switched to its low energy gap state, act as traps in both the conjugated polymers but at different trap depth energies.
Keywords: Polymer diodes; Molecular switch; Photochromic molecule; Charge trap depth; JV modulation;

Preparation of polydiacetylene single crystals based on physical vapor growth technique by Sadaharu Jo; Hitoshi Yoshikawa; Akane Fujii; Mitsuru Takenaga (223-226).
We have succeeded in obtaining 10-mm-size polydiacetylene (PDA) single crystals using the physical vapor growth technique followed by the irradiation of ultraviolet (UV) rays, for the first time. The variety of growth conditions, such as growth temperature, growth time, carrier gases, and flow rate of gases are examined. Especially, the growth temperature and growth time have clear effects on the quality of crystals. Morphologies and sizes with relation to the various growth conditions are also observed. They show only one type of plateletlike shape with a pair of large parallelopiped planes, regardless of variety of growth conditions. The crystal thickness was estimated as about 500 nm by the observation of atomic force microscopy. The solubility of diacetylene (DA) and PDA to some organic solvents, and the X-ray diffraction data indicate that the obtained crystals are single crystals.
Keywords: Physical vapor growth; Diacetylene; Polydiacetylene; Polymerization; Morphology;

Electrosynthesis of poly(biphenyl) (PBP) films was performed in an AOT [sodium bis(2-ethylhexyl) sulfosuccinate] reverse micelle by cyclic voltammetry and chronopotentiometry on a platinum electrode. An electroactive PBP film was formed either by cyclic voltammetry between −0.6 and 1.65 V/Ag/AgCl or under current control (j  = 0.2 mA/cm2). PBP films were characterized electrochemically and spectroscopically (MALDI-TOF mass spectrometry), NMR, Raman, FT-IR, X-ray photoelectron and UV–vis spectroscopy). The structure of the PBP films involved sequences of ramified trimer possessing quinoid forms. This type of sequences can give strongly condensed oligomers with PBP molar mass up to 2669.6 g/mol according to the electrosynthesis charge.
Keywords: AOT; Reverse micelle; Electropolymerization; Poly (Biphenyl) films;

We present a theoretical treatment of the triplet-pair exciton annihilation under microwave excitation in a sexithiophene crystal. This treatment gives a satisfactory fit of the observed effects and leads to the determination of the singlet annihilation rate constant λ and the effective decay rate β. The ZFS parameters D and E are also determined.
Keywords: Spin; Exciton; Triplet; Singlet; Fine structure constants; Fluorescence;

Structure and conductivity of unsymmetrical π-donor ethylenedithiodithiadiselenafulvalene iodomercurate (EDT-DTDSF)4Hg3I8 by E.I. Zhilyaeva; A.Yu. Kovalevskyi; S.A. Torunova; G.A. Mousdis; R.B. Lyubovskii; G.C. Papavassiliou; P. Coppens; R.N. Lyubovskaya (245-250).
New charge transfer salt of Se-containing unsymmetrical donor ethylenedithiodithiadiselenafulvalene (EDT-DTDSF) with iodomercurate anion, β-(EDT-DTDSF)4Hg3I8, has been synthesized. The structure of β-(EDT-DTDSF)4Hg3I8 has been determined from single-crystal X-ray analysis, which reveals alternating conducting donor layers of the β-type packing with insulating layers formed by infinite chains of [Hg2I6]2− anions and HgI2 molecules. Transport properties have been investigated.
Keywords: Organic conductors based on radical cation salts; Electrical conductivity; X-ray diffraction;

Resonant Raman scattering dispersion in poly(dithieno[3,4-b:3′,4-d]-thiophene): 2Ag spectroscopy by E. Ehrenfreund; A. Cravino; H. Neugebauer; N.S. Sariciftci; S. Luzzati; M. Catellani (251-253).
Small band gap semiconducting π-conjugated polymers may, in principle, be more efficient as the active media in solar cells, due to their better adaptability to the solar spectrum. In luminescent π-conjugated polymers, interband photon absorption results in the radiative recombination of the photogenerated excitons, which competes with charge separation. The photoluminescence (PL) quantum efficiency is largely determined by the relative energies of the optically allowed lowest odd parity (LOP, e.g. 1Bu) excited state and the optically forbidden lowest even parity (LEP, e.g. 2Ag) excited state. If E(LEP) <  E(LOP), the quantum efficiency is small and charge separation following photoexcitation is more likely than in efficient PL polymers. The determination of the relative position of LEP and LOP states is therefore essential for the possible utilization of small band gap π-conjugated polymers in practical devices. It was previously shown that resonant Raman scattering dispersion might serve as a spectroscopic tool for the determination of the LEP states, since the dispersion is solely determined by the dependence of the LEP state on the conjugation length, rather than that of the optical gap, or LOP state. In this work, we use the Raman spectra measured in poly(dithieno-thiophene) (PDTT1) in order to estimate the LEP energy level. We find that the LEP energy level is in the vicinity of the optical gap.
Keywords: Even parity states; Low gap conjugated polymers; Resonant Raman scattering;

Electrochemical growth of poly(3-dodecylthiophene) into porous silicon layers by N. Errien; G. Froyer; G. Louarn; P. Retho (255-258).
Electrochemical growth of poly(3-dodecylthiophene) into mono or double layer of porous silicon used as working electrode is described. The polymer content may be optimized by adjustment of the current density even in a double layer with two different porosities leading to structures, such as active optical guides.
Keywords: Porous silicon; Polyalkylthiophene; Nanocomposite; Electrochemistry;

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the evolution of the valence electronic states as a function of conjugation length for thiophene, bithiophene, terthiophene, and sexithiophene films deposited in-vacuum on gold substrates at 130 K. The peaks due to localized thiophene ring electronic states are found to be at ca. 8 eV below the vacuum level, regardless of the number (n) of thiophene rings in the chain. Peaks due to orbitals delocalized along the oligomer backbone progress to lower binding energies as the chain length increases from n  = 2 to 6. Electron loss spectroscopy (ELS) has also been performed using an electron gun typically employed for Auger electron spectroscopy, and it is found that the lowest energy loss feature shifts to lower energy as conjugation length increases. The results obtained are in general agreement with theoretical and higher resolution experimental studies, and the thresholds of the lowest energy ELS peaks provide estimates of the bandgaps of the films. These results show that the combination of UPS and ELS provides a convenient means of evaluating the conjugation length of short-chain oligothiophenes.
Keywords: Electron energy loss spectroscopy; Electronic structure; Oligothiophenes; Photoemission; Polythiophene and derivatives; Semiconducting surfaces;

To investigate the effects of introducing the iron compound on the carbonization behavior polyacrylonitile (PAN)-based electrospun nanofibers were carbonized with or without iron(III) acetylacetonate (AAI) over the temperature range of 900–1500 °C in nitrogen atmosphere. The morphological characteristics of the carbon nanofibers were investigated using X-ray diffractometer (XRD), Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. The electrical conductivity of the carbon nanofiber web was measured by four-point probe method. The iron catalyst had a profound effect on the crystal structure of the carbonized nanofiber. In the presence of AAI the nanofibers carbonized at 1300 °C developed graphite structure, which could be obtained at the temperature higher than 2000 °C in the absence of the catalyst. The in-plane size of the graphite crystals (L a) was measured to be about 6.5 nm by Raman spectroscopy and the (0 0 2) spacing by XRD was 0.341 nm.
Keywords: Electrospinning; Polyacrylonitrile; Catalytic graphitization; Carbon nanofiber; Graphite structure; Iron(III) acetylacetonate;

Carbon nanotube/polyaniline core-shell nanowires prepared by in situ inverse microemulsion by Yijun Yu; Bo Che; Zhihua Si; Liang Li; Wei Chen; Gi Xue (271-277).
By the in situ inverse microemulsion, we prepared multi-walled carbon nanotubes/polyaniline composites (MWNTs/PANI). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that the nanotubes were coated with a PANI layer. Fourier transform infrared (FT-IR) spectra suggested that the π-bonded surface of the carbon nanotubes (CNTs) interact strongly with the conjugated structure of the PANI shell layer. The thermal stability and electrical conductivity of the MWNTs/PANI composites were examined by thermogravimetric analysis (TGA) and conventional four-probe method. In comparison with the pure PANI, the decomposition temperature of the MWNTs/PANI (1 wt% MWNTs) composites increased from 360 to 400 °C and the electrical conductivity of MWNTs/PANI (1 wt% MWNTs) composites was increased by one order of magnitude.
Keywords: MWNTs; Polyaniline; Inverse microemulsion; Composite;

Doped and de-doped nanotubes and nanowires of polypyrrole, polyaniline, and poly(3,4-ethylenedioxythiophene) were synthesized by the electrochemical polymerization method, using Al2O3 nanoporous templates. The electrical and optical properties of the nanotubes and nanowires were controlled through various synthetic conditions, such as doping level, dopant, and template-dissolving solvents. The diameters and wall thicknesses of the nanotubes were 100–200 nm and 10–30 nm, respectively. To determine the electrical and optical properties of the nano-systems, IV characteristic curves with gate bias, dc conductivity, and UV–vis spectra were measured. We observed that the nano-systems were transformed from a conducting state to a semiconducting (or insulating) state through the process of de-doping using the template-dissolving solvents. Possible applications of these nanotubes and nanowires in the construction of nanotip emitters in field emission displays and polymer-based transistors are presented.
Keywords: Conducting polymer; Nanotube; Nanowire; Transistor; Field emission;

Self-doped polyaniline (PANI) possesses superior electrochemical properties and processability in relation to the PANI due to an acid group substituted on the polymer backbone. However, the polar acid group causes the self-doped PANI to undergo dissolution in aqueous, in particular, acidic solutions. To prevent the solubility, poly(anthranilic acid), PANA, is confined in the cavities of the Nafion membrane by a novel electrochemical as well as a conventional chemical polymerization of anthranilic acid. The PANA is characterized by electrochemical, optical, spectroscopic and scanning electron microscopic studies.
Keywords: Anthranilic acid; Polymerization; Poly(anthranilic acid); Cyclic voltammetry; UV–visible spectroscopy;

Role of diffusion on SCLC transport in double injection devices by F. Neumann; Y.A. Genenko; R. Schmechel; H. von Seggern (291-296).
A theoretical study of SCLC transport in double injection insulators is presented. It will be demonstrated, that the inclusion of charge carrier diffusion, neglected in many previous studies of transport in organic light emitting diodes (OLEDs), is essential to obtain physical meaningful spatial charge carrier densities and field distributions. Only the knowledge of such correct spatial distributions enables one to compute the correct position of the charge carrier recombination zone. In previous calculations without diffusion the recombination process often takes place in the vicinity of both electrodes, even for equal mobilities of holes and electrons. In the present calculation including diffusion it is demonstrated that only one recombination zone exists. For equal mobilities of electrons and holes the recombination zone is found as expected in the centre of the device whereas for different mobility values it may be strongly shifted to one of the electrodes. The resulting IV characteristics indicate that, in double injection devices, the well-known Mott–Gurney law holds only at sufficiently high voltages and only if recombination is taken into account. For small voltages, an ohmic-like behavior is observed in any case, however, if no recombination is assumed a transition to an I  ∼  V 3 law is obtained for higher voltages. Due to the inclusion of diffusion, all IV characteristics exhibit temperature dependence.
Keywords: Organic light emitting diodes; Simulation; Diffusion; Space charge limited current;

The random copolymers of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PV) and 2,5-thienylene-vinylene (ThV) were synthesized by Gilch method [H.G. Gilch, W.L. Wheelwright, J. Polym. Sci. Part A, Polym. Chem. 4 (1996) 1337; H. Becker, H. Spreitzer, K. Ibrom, W. Kreuder, Macromolecules 32 (1999) 4925; J.A. Mikroyannidis, Chem. Mater. 15 (2003) 1865], the alternate copolymer of MEH-PV and ThV was synthesized by Hornor–Emmons reaction, and poly(3-hexyl-thienylene-vineylene) was synthesized by Stille coupling reaction. The absorption characteristics of the copolymers were modulated by changing the ratio of ThV to MEH-PV units in the copolymers. With the increase of the ThV ratio, the absorption spectra of the copolymers were broadened and red-shifted in comparison with that of MEH-PPV. Hole mobility of the random copolymer with 18% ThV is an order higher than that of MEH-PPV. The polymer photovoltaic cells based on the blend of the copolymer and C60 were fabricated and characterized by IV and input photon to converted current efficiency (IPCE) measurements. Energy conversion efficiency under a white light (70 mW/cm2) of the photovoltaic cell based on the copolymer containing 18% ThV increased by 47% in comparison with that of the device based on MEH-PPV. The performance improvement of the polymer photovoltaic cell based on the random copolymers should benefit from the broader absorption and higher hole mobility of the copolymers.
Keywords: Random copolymers; Alternate copolymer; MEH-PPV; Poly(3-hexyl-thienylene-vineylene); Polymer photovoltaic cells; Absorption spectra;

Surface photovoltaic properties of an expanded aza H2Pc containing four 1,10-phenanthroline subunits by Xiaoke Hou; Xiguang Du; Chunyu Ma; Yan Li; Qinglin Zhang; Xu Wang; Yuchun Chang; Wenhai Jiang; Chuanhui Cheng; Shijun Shan; Guotong Du (305-308).
The photovoltaic properties of an aza H2Pc containing four 1,10-phenanthroline subunits were investigated by means of surface photovoltage spectroscopy and electric-field-induced surface photovoltage spectroscopy. The surface photovoltaic response was enhanced under positive electric field and weakened under negative electric field. Thus, the expanded aza H2Pc was judged to be p-type semiconductor. From the similar response behavior, the Q-band and high-energy side of Soret-band were assigned to π–π* transition. Interestingly, the low-energy side of Soret-band exhibits blue shift under external electric field. According to the influence of electric field on the ground state of n-orbital with high polarizability, it was assigned to n–π* transition. We think the peripheral substituent units play a key role in the electronic transition and photovoltaic processes.
Keywords: Semiconductors; Phthalocyanine; Surface photovoltage spectroscopy; Electronic transition;

For the purpose of balancing charge carriers’ transport, we designed and synthesized two new compounds that are composed of both electron- and hole-transporting structural moieties: electron-transporting moieties are diaryl-1,3,4-oxadizole (Oxa) groups and hole transport moieties carbazole (Cz), or triphenylamine (TPA) moieties. The compounds formed amorphous glassy films when vacuum deposited and their glass transition temperature (T g) was close to or higher than 150 °C. Their electronic structures (the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and E g values) and hole and electron mobilities in the compounds were studied. A couple of compounds bearing oxadiazole moieties revealed the electron mobility greater than 1.0 × 10−4  cm2/(V s) at the electric field of 7.5 × 105  V/cm. The single layer light-emitting electroluminescence (EL) devices show that the external quantum efficiencies of the devices fabricated with those compounds having balanced carrier mobilities are much higher than those of the compounds composed of predominantly hole- or electron-transporting moieties.
Keywords: Electroluminescence; Carrier mobility; LEDs; Hole-transporting; Electron-transporting;

CETBz-TTF (4,5-bis(cyanoethylthio)benzo-tetrathiafulvalene) and CESBz-TTF (4,5-bis(cyanoethylseleno)benzo-tetrathiafulvalene) 5 have been synthesized from cyano precursor via a cross-coupling reaction. Radical salts with paramagnetic Cu(II) complex anions, such as (CETBz-TTF)2CuCl4 6 and (CESBz-TTF)2CuCl4 7 have been prepared. X-ray crystal structure and physical properties of these new compounds are presented.
Keywords: Single-crystal growth; Magnetic measurements; X-ray diffraction; Organic conductors based on radical cation;