1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
PHOTOLUMINESCENCE SPECTROSCOPY OF PHOTOREACTIVE AROMATIC HYDROCARBONS. CASE: PENTAHELICENE
K.PALEWSKA
Institute of Physical and Theoretical Chemistry, Technical University of Wroclaw, Wyb. Wyspianskiego 27, PL-50 370, Wroclaw, Poland
Highly
resolved fluorescence and phosphorescence excitation and emission
spectra of pentahelicene, of its photocyclization product -
dihydropentahelicene (DHPH) and of benzo[g,h,i]perylene - a
product of subsequent dehydrogenation of DHBP in Shpol'skii
matrices were measured at 4.2 K.
The
photocyclization of pentahelicene to DHBP was found to be
reversible upon heating the sample in the dark beyond the melting
point of the matrix. The dehydrogenation of DHBP to BP was
inhibited in solid matrices. To the author’s knowledge, this is
the only confirmed case of a photochromic process at the liquid
helium temperature. Based on experimental results and on
quantum-chemical calculations, a reaction pathway for the
photocyclization of pentahelicene to DHBP via the lowest triplet
state of pentahelicene was put forward.
2,5-BIS(heptyloxy)-PV oligomers with vinyl OR but-1-en-1--yl end groups: preparation and microstructure-characterization of Two homologous series
A. Strachotaa, R. Peetzb, O. Narwarkc, E. Thorn-Csányic*
aInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, CZ-162 06 Praha 6, Czech Republic
bInstitute of Polymer Science, The University of Akron, Akron OH 44325-3909
cInstitute of Technical and Macromolecular Chemistry, University of Hamburg, Bundesstraße 45, D-20146 Hamburg
Poly- and oligo(p-phenylenevinylene) as well as their substituted derivatives belong to the conjugated systems with valuable opto-electronical properties [1]. Our way to synthesize easily processable p-phenylenevinylene (PV) oligomers (Fig.1) is based on metathesis polycondensation of ring-substituted p-divinylbenzenes [2,3]. The substituted PV oligomers proved to be valuable not only as model polymer compounds but may themselves serve as active materials [4].
Fig.1
(R =
alkyl, alkyloxy)
In this contribution we report on the preparation and microstructure characterization (NMR) of monodisperse 2,5-bis(heptyloxy)-substituted p-phenylenevinylene (PV)-oligomers. We present our results concerning the dependence of optical properties of these oligomers on the conjugation length. The preparation of a homologous series of thermally stabilized oligomers (stability investigated up to 250°C), with but-1-en-1-yl units instead of vinyl units as end groups, will also be presented.
Literature
[1] Friend, R. H., Gymer, R. W., Holmes, A. B., Burroughes, J. H., Marks, R. N., Taliani, C., Bradley, D. D. C., Dos Santos, D. A., Brédas, J. L., Löglund, M., Salaneck, W. R., Nature (1999) 397, 121
[2] Thorn-Csányi, E., Kraxner, P. (1997) Macromol. Chem. Phys. 198, 3828
[3] Thorn-Csányi, E., Kraxner, P., Strachota, A. (1998) Macromol. Rapid Commun. 19, 223
[4] EP 0 964 044 A1 R. Wehrmann, A. Elschner, E. Thorn-Csányi (Bayer AG)
INFRARED AND RAMAN STUDIES OF THE CHARGE ORDERING IN THE ORGANIC SEMICONDUCTOR κ-[Et4N](BEDT-TTF)4Co(CN)6.3H2O
R. ŚWIETLIKa,c, L. OUAHABb, J. GUILLEVICb, K. YAKUSHIc
aInstitute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland
bLCSIM-CNRS, UMR 6511, Université de Rennes I, 35042 Rennes cedex, France
cInsititute for Molecular Science, Myodaiji, Okazaki, 444-8585, Japan
In organic conductors the transfer integrals between neighbouring molecules are comparable to the on-site and inter-site Coulomb interaction energies, therefore, electron correlations play an important role in these materials. The Coulomb interactions in organic conductors can be responsible for changes of the average electron density on molecules, i.e. charge ordering (or charge disproportionation) phenomena.
The two-dimensional charge transfer salt formed by the bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) with hexacyanometalate trianions Co(CN)6 undergoes two phase transitions at T=240 K and 150 K. At T=240 K the unit cell is doubled, whereas at T=150 K the crystal structure remains nearly unchanged (so-called structureless phase transition). From the bond length analysis (X-ray data) it results that at room temperature all BEDT-TTF molecules possess the charge +0.5 but below T=240 K a charge redistribution takes place and molecules with average charges 0, +0.5 and +1 are observed.
Polarised IR reflectance spectra (600-10000 cm-1) and Raman spectra of the κ-[Et4N](BEDT-TTF)4Co(CN)6·3H2O crystals were measured as a function of temperature (300–40 K). Temperature dependence of the IR and Raman vibrational bands, especially those related to C=C stretching vibrations of BEDT-TTF, is studied to describe modifications of electron distribution inside the conducting BEDT-TTF layers. The analysis of charge-transfer bands in the IR spectra shows that both phase transitions have a considerable influence on the electronic structure of the salt. These electronic dispersions are analysed in terms of the Drude-Lorentz model to determine the optical transport parameters. Our spectroscopic studies provide an unambiguous evidence that the phase transition at T=150 K is due to the charge ordering. Possible reasons of the charge ordering in κ-type BEDT-TTF conducting layers are discussed.
This work was partially supported by KBN under grant 2 P03B 087 22
F.J.J.Janssen(a,c), J.M.Sturm(a,c), L.J.van IJzendoorn(a,c), A.W.Denier van der Gon(a,c), H.F.M. Schoo(b,c), M.J.A. de Voigt (a,c), H.H. Brongersma (a,c)
a Department of Applied Physics, Eindhoven University of Technology, Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, Netherlands, (f.j.j.janssen@tue.nl)
b TNO Institute of Industrial Technology, Eindhoven, The Netherlands
c Dutch Polymer Institute, Eindhoven University of Technology
The characteristics of Polymer Light Emitting Diodes (PLEDs) (ITO/PPV/Ca) depend strongly on the conditions during preparation and operation. PLEDs show a reduction of both the current and the light output to 40% after heat treatment for only 30 minutes on 65 °C. Electroluminsence spectroscopy experiments show that both the shape of the spectrum as well as the total light output are changed after heat treatment.
We studied the effects of heat treatment (during and after preparation) on PLEDs with OC1C10 PPV as active layer. Effects on I-V characteristics are studied by measuring single carrier devices (hole and electron only`s). The stability of the Ca/PPV interface is investigated with LEIS (Low Energy Ion Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) experiments.
Modeling of the I-V characteristics, obtained from measurements at ITO/PPV/Au hole only devices, shows that the field independent hole mobility is decreased after heat treatment. In contrast, Au/PPV/Au hole only devices do not show any changes. This indicates that the PPV itself does not degrade during heat treatment, but that e.g. oxygen diffuses from the ITO anode into the PPV.
Electron dominated devices also show reduced currents. Modeling of the I-V characteristics indicates that the field dependence (γ) of the electron mobility changes upon heat treatment. From LEIS and XPS spectra it is found that calcium diffuses from the PPV/Ca interface into the PPV. The reduced electron currents and the change of the field dependence of the electron mobility can be attributed to the changes of the Ca/PPV interface.
TIME AND FREQUENCY EVOLUTION OF INTRINSIC CONDUCTIVITY OF DIELECTRICS
J. MAŁECKI
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 PL Poznań, Poland
The creation-annihilation model of electric conductivity (CAMEC) was considered. The model assumes a competition between the rate b of current carrier creation, the rate a of carrier annihilation and the rate Eun/l of carriers capturing on electrodes; u and n denote the sum of carrier mobilities and their density, respectively. The model was applied for the study of the dependence of intrinsic electrical conductivity of dielectrics on time t, electric field intensity E and distance l between electrodes. The two characteristic times - time of flight tF = l/(Eu) and conductivity recovery time tR = (ab)-1/2 will be discussed. The numerical solutions of the interesting problem of transient currents observeď on first switching on and after switching off, as well as on polarity reversal, will be presented.
The theoretical analysis of transient states on the CAMEC model show that the latter can act as an alternative of the frequently applied injected current method in the experiments involving low densities of current flow. Such experiments can give an information on annihilation rate a, dissociation rate b and mobility u of current carriers naturally created in activation and/or dissociation processes, in contrast to time of flight method where the properties of injected carriers are studied.
SPACE CHARGE DISTRIBUTION IN LUMINESCENT CONJUGATED POLYMERS
D. GESCHKE a, F. FELLER a, A. P. MONKMAN b
a University of Leipzig, Faculty of Physics and Earth Sciences, Institute of Experimental Physics I, Polymer Research Group, Linnéstrasse 5, D-04103 Leipzig, Germany
e-mail : geschke@physik.uni-leipzig.de
b University of Durham, Department of Physics, Organic Electroactive Materials Research Group, South Road, Durham DH1 3LE, UK
e-mail : a.p.monkman@durham.ac.uk
There is to day a great interest in the use of conjugated polymers in light emitting devices. Prior to the emission of light, electrons and holes must be injected from the electrodes and transported through the polymer material under the influence of an external electric field. The space charge distribution influences the charge transport and, therefore, it is of fundamental interest to know whether the current is injection limited or transport limited .
We used at first time a well established thermal wave method (Laser Intensity Modulation Method; LIMM) 1 to resolve the depth profile of space charges and the distribution of the internal electric field in 3 to 4 : m films of the poly(p-phenylenvinylene ) derivative MEH-PPV and of the polymers poly(2,5-pyridinediyl) (PPY) and polyfluorene (PFO) 2. The results, obtained after deconvolution of the LIMM spectra (modulation frequency dependent pyroelectric currents) by use of a novel approximation technique 3 demonstrate that in PPY space charges of either sign can not only be created but also be stored permanently. For a positively biased Au/PPY/Au device e.g. the measurements indicate that the electric field in the bulk of the film is significantly reduced because of the existence of two space charge layers: a narrow, positively charged layer at the front electrode and a broader, negatively charged layer at about 500 nm. In MEH-PPV and in PFO space charge could also be observed in the presence of an externally applied bias but could not be stored within the material.
The dynamics of the poling process and of the accumulation of space charges have been investigated by means of measurements of the temporal pyroelectric response of the system to a switching of the external bias. Depending on the material poling can be achieved within the time scale of less than one second for shallow traps and more than one hour for deep traps.
1 Lang, S. B., Das-Gupta, D. K. J. Appl. Phys. 59, 2151 (1986)
2 Feller, F., Geschke, D., Monkman, A. P. Appl. Phys. Lett. 79, 779 (2001)
3 Feller, F., Geschke, D., Buttgereit, R., Roths, T., Honerkamp, J.
The Europ. Phys. J., Appl. Phys., submitted
LUMINESCENT LANTHANIDE COMPLEXES WITH ORGANIC AND POLYMER LIGANDS
D. VÝPRACHTICKÝ a, V. CIMROVÁ a, Y. OKAMOTO b, R. KOTVA a
a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 162 06 Prague 6, Czech Republic (e-mail: vyprach@imc.cas.cz ; cimrova@imc.cas.cz)
b Polymer Research Institute, Polytechnic University, Six MetroTech Center, Brooklyn, New York 11201, U.S.A. (e-mail: yokamoto@duke.poly.edu)
The ion binding properties of 4-methylpyridine (4MP) and poly(2- and 4-vinyl-pyridine)s (P2VP, P4VP) were investigated in methanol using Tb3+ as a fluorescence probe. The increase of terbium fluorescence intensity in [P2VP-Tb3+] and [P4VP-Tb3+] complexes in comparison with 4MP is due to both the replacement of the inner coordinated methanol molecules and ligand-to-metal energy transfer (polymer cooperative effect [1]). Poly(2- and 4- vinylpyridine N-oxide)s (P2VPNO, P4VPNO) were synthesized. The fluorescence intensities of P2VPNO were over three times that of P4VPNO in aqueous solution, which was accounted for by the difference in the microenvironment of the pyridine N-oxide fluorophore. The N-oxide polymers formed strong complexes with Tb3+ in aqueous solution, and the fluorescence intensities of terbium were greatly enhanced upon complex formation. P4VPNO was found to be a better complexing agent for Tb3+ than P2VPNO. Para-substituted aromatic acids were used as organic ligands in Tb3+ complexes and the influence of the substituent was studied. The polymers and their lanthanide complexes are tested as new materials for polymer light-emitting devices (LEDs). We synthesized formylated polymethylphenylsilane [2] and 1-(4-aminophenyl)-3-phenylpropane-1,3-dione will be attached as a b -diketone complexing unit. In this way the hole-transporting material with lanthanide luminescence are to be prepared.
We thank the Grant Agency of the Czech Republic for financial support by a grant 203/01/0512.
[1] D. Výprachtický, K. W. Sung, and Y. Okamoto J. Polym. Sci., Polym. Chem. 1999, 37, 1341-1345.
[2] D. Výprachtický and V. Cimrová Macromolecules 2002, 35, 3463-3473.
physical properties AND STructure of thin conducting Ion-beam-modified polymer FILMS
M. Guenthera, G. Gerlacha, G. Suchanecka, K. Sahreb,
K.-J. Eichhornb, B. Wolfc, A. Deinekad, L. Jastrabikd
aDresden University of Technology, Institute for Solid State Electronics, Mommsenstr. 13, 01062 Dresden, Germany
bInstitute of Polymer Research, Hohe Strasse 6, 01069 Dresden, Germany
cDresden University of Technology, Institute for Crystallography and Solid State Physics, Zellescher Weg 16, 01062 Dresden, Germany
dInstitute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
Thin films of aromatic polymers such as polyimide (PI) and polyethersulfone (PES) find extensive use in electronic application. The physical properties of these films may be tailored for special sensor applications. In order to increase the sensitivity to moisture uptake of PI and PES films applied in bimorphic humidity sensors 50, 130 and 180 keV boron ions with irradiation doses between 1013 and 1016 B+/cm2 were implanted. A complex investigation of the following features has been carried out: chemical changes in the surface regions by Attenuated Total Reflection (ATR)-FTIR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS); optical properties by spectroscopic ellipsometry; hardness and elastic modulus by depth-sensing low-load indentation technique; conductivity of modified polymer films. Volume and surface conductivity measurements were performed using electrode configurations consisting of evaporated NiCr bottom electrode, press measuring and guard electrodes as described in IEC 93.
It could be shown, that the partial destruction of chemical bonding under ion bombardment leads to the creation of amorphous and graphite-like structures, which increase the surface film conductivity by several orders of magnitude from an insulating to a semiconducting region and enhances the sensitivity of these polymer films to moisture uptake. The ion-beam irradiation increases the values of the refractive and absorption indices and destroys the refractive index anisotropy of polymer layers. The radiation-induced structural changes result in an enhancement of the hardness and the elastic modulus of the polymer layers up to 10 and 6 times, respectively. Hardness and refractive index depth profiles were determined. The effective modification depth was estimated from these depth profiles. The changes in structure and physical properties of polymer films are discussed in dependence on dose and energy of implanted ions and irradiation conditions optimal for sensoric properties of these films are determined.
SPECTRO-ELLIPSOMETRIC CHARACTERIZATION AND GASEOUS OCCLUSION OF FULLERENE C60 CRYSTALS
K. MATSUMOTO, H. MAEDA, K. POSTAVA, K. TAKAHASHI, M. AOYAMA, T. YAMAGUCHI
Research Institute of Electronics, Shizuoka University, Johoku 3-5-l, Hamamatsu 432-8011, Japan. E-mail: rokmats@ipc.shizuoka.ac jp
A single crystal of fullerene C60 has been grown from the vapor phase by sublimation in a silica ampoule. On one of its flat surfaces spectro-ellipsometric measurements have been carried out in vacuum and a spectrum of dielectric constant has been obtained as a function of photon energy. Ellipsometric measurements have also been carried out in gases such as He, Ar, H2, N2 and O2 with less than one atmospheric pressure. No differences were detected among the spectra obtained in these gases and in vacuum. Similar measurements have also been carried out in vacuum on a shiny flat surface that was prepared by polishing a lump of C60 polycrystal. The spectrum in this case was substantially different from that of the single crystal (Fig.1). Fitting calculation using Effective Medium Approximation (EMA) has revealed that this difference can be understood if there is a surface layer including a certain amount of voids. The ellipsometric spectra of this C60 polycrystal measured in gases with higher pressures distinctly differed from those obtained with lower pressures. These changes in the spectra were reversible with respect to gaseous pressure (Fig.2), and were interpreted by assuming that the gas molecules adsorbed on the microsurface of the voids involved in the surface layer would cohere to form a quasiliquid.
Fig.l Ellipsometric spectra of C60 single crystal and polycrystal.
Fig.2 Ar-pressure dependence of dielectric constant of C60 polycrystal (at 3.54 eV of imaginary part).
ULTRA WEAK SPECTRALLY RESOLVED THERMOLUMINSCENCE IN POLYMERS
E. MANDOWSKA, A.MANDOWSKI, J. ŚWIĄTEK
Institute of Physics, Pedagogical University, ul. Armii Krajowej 13/15, 42-200 Częstochowa, Poland
Thermoluminescence (TL) phenomenon was observed in many organic solids. The measurement is performed during heating of a sample after excitation at appropriately low temperature. Usually the level of the emitted light is very low therefore, the measurement technique requires using a very sensitive apparatus. Even stronger requirements are necessary while measuring spectrally resolved TL. However, the possibility to construct a three-dimensional representation of TL (see fig.1) adds materially to the ability to interpret the behaviour of the materials being studied. In this paper we report measurements of spectrally resolved TL in poly(n-vinylcarbazole) (PVK) blends and other polymers. Obtained spectra clearly indicate the influence of solvents on TL in PVK samples. The measurements were performed in a vacuum using a liquid nitrogen cooled CCD 1024E detector with SP150 spectrograph.
|
Fig. 1. Thermoluminescence of PVK after excitation at 363 nm. |
RECOMBIATION IN COMPLEX SYSTEMS – AN ANALYTICAL APPROACH
A.MANDOWSKI
Institute of Physics, Pedagogical University, ul. Armii Krajowej 13/15, 42-200 Częstochowa, Poland
Classical theories of trapping and recombination of charge carriers in dielectrics relate to the two analytically described cases. The first one relates to uniform distribution of traps. The second one relates to pairs of traps and recombination centres placed close to each other. No one of these cases is likely to prevail in complex organic solids. In such circumstances usually it is not possible to formulate a set of analytical equations describing charge carriers kinetics in such processes as phosphorescence (Ph), thermoluminescence (TL) and others. Until now, no one theory have been constructed that takes into account arbitrary spatial distribution of traps and recombination centres.
Some peculiarities of the kinetics that may take place in non-homogeneous systems (e.g. 1-D structures or clusters) were shown in a number of papers in last few years. Examples include apparently composite structure of monoenergetic peaks and additional 'displacement' peaks. The results were obtained numerically using Monte Carlo algorithms. These numerical results were hardly compared to the experiment due to the lack of an appropriate analytical theory. In this paper new equations are presented for TL in a system with an arbitrary spatial distribution of traps and recombination centres. It is shown that the distribution could be characterised by a single function G (n). This function depends solely on structural properties (e.g. spatial distribution, energy barriers, etc.) of metastable states in a solid. The G function have to be determined separately for traps and recombination centres. However, in most cases this process could be significantly improved by utilizing special symmetry properties. Examples of G functions for various cluster and one-dimensional structures are presented. A special emphasis is put on cases leading to the most common first-order kinetics.
Temperature and Field Dependence of the Mobility in Liquid-Crystalline Conjugated Polymer Films
S. J. Martin, A. Kambili, * and A. Walker
Department of Physics, University of Bath,
Claverton Down, Bath BA2 7AY, UK
We investigate the transport properties of organic light-emitting diodes in which the emissive layer is composed of conjugated polymers in the liquid-crystalline phase. Liquid-crystalline polymer films are in principle characterized by a high degree of order, both spatial and energetic, which constitutes them highly desirable candidates for applications. We have performed calculations of the current as a function of time via a Monte Carlo model, and from the transit times we have extracted the mobility of the charge carriers as a function of both the electric field and the temperature. We demonstrate that the transport properties of films made of highly aligned polymer chains are very different from their disordered counterparts, and caution has to be taken when describing interchain hopping. The case of from perfect alignment is also investigated and their influence on the charge carrier mobilities is being established. The differences on the temperature and field dependence of the mobility from the highly aligned case could be a measure of identifying in real systems the degree of purity within the polymer films.
* pysak@bath.ac.uk, agapi.kambili@physik.uni-regensburg.de
LUMINESCENCE PROPERTIES OF CARBAZOLE DERIVATIVE OF MODIFIED EPOXY RESINS
E. MANDOWSKAa, W. MAZELAb, P. CZUBb,
A.MANDOWSKIa,
J. PIELICHOWSKIb, J.
ŚWIĄTEKa
aInstitute of Physics, Pedagogical University, ul. Armii Krajowej 13/15, 42-200 Częstochowa, Poland
bDepartment of Chemistry and Technology of Polymers, Cracow University of Technology, ul. Warszawska 24, 31-155 Kraków, Poland
Epoxy resins are interesting materials due to their
excellent mechanical properties, high thermal stability and high
chemical resistivity. The materials would be even more attractive
for photonic applications if they would join these features with
good optical properties. For this purpose a new class of
materials was synthesized containing carbazole groups. This
modification does not influence superior mechanical properties of
epoxy resins, but significantly improves their luminous capacity.
Absorption and photoluminescence (PL) measurements were performed
to study the dependence of the optical properties on various
chemical and technological conditions including the content of
carbazole groups, various additives, sample thickness and other
factors. Absorption curves were similar for most studied
materials, having edge at about 355 nm. Photoluminescence in
modified epoxy resins is very strong. It is much higher than PL
in poly(n-vinylcarbazole) (PVK) polymer. The emission is shifted
down to shorter wavelengths (see fig. 1). Detailed numerical
analysis of the spectra exhibits several emission bands. It was
found that all the materials show also long-lived (in the range
of seconds) phosphorescence.
Redox composited polymer coatings on carbon paper electrodes and their suitability for use within solid-state supercapacitors.
Andrew M. White 1 and Robert C. T. Slade 2
Department of Chemistry, University of Surrey, Guildford GU2 7XH, United Kingdom.
[1a.white@surrey.ac.uk and 2r.slade@surrey.ac.uk]
Redox composited polymers have been chemically fabricated and evaluated for potential use as electrodes within a solid-state supercapacitor. A hydrated perfluorosulfonic acid, Nafion®, membrane acts as supercapacitor solid electrolyte, giving an environmentally ‘green’ system.
Successful combinations with conductive polymer gave black, conductive direct addition and thin-film products; oxidation involved inorganic oxidants.
Single electrodes were deposited onto a carbon paper support. When multiple coats were applied, smooth surfaces were obtained, which are essential for good contact with the Nafionâ 115 membrane. Cyclic voltammetry investigations were carried out in aqueous acid, yielding capacitances as high as 776 F g-1 (with respect to active polymer material) and 0.79 F cm-2 (geometric area of the electrode).
An evaluation of membrane electrode assemblies (MEAs, which are essentially capacitor devices without a casing) was carried out. MEAs were made by hot pressing of the electrodes onto the solid electrolyte membrane in a ‘sandwich’ configuration. Investigations employed a.c. impedance techniques and dc charge-discharge cycling.
ELECTROLUMINESCENCE AND CHARGE PHOTOGENERATION IN Poly(9,9-dihexadecylfluorene-2,7-diyl) and its Blends
V. Cimrová, D. Výprachtický, R. KOTVA
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, CZ-162 06 Praha 6, Czech Republic
(e-mail: cimrova@imc.cas.cz, vyprach@imc.cas.cz )
Photoluminescence (PL), electroluminescence (EL) and charge photogeneration of soluble poly(9,9-dihexadecylfluorene-2,7-diyl) (SPF), poly[(2,5-dihexadecyl-1,4-phenylene)(1,4-phenylene)] (SPPP), modified poly[methyl(phenyl)silanediyl] (PMPSi) and their blends were studied. SPF and SPPP exhibited intensive blue luminescence with high PL efficiency. PMPSi was modified with p -conjugated (pyrene, biphenyl) [1] or electron-transporting (1,3,4-oxadiazole) moieties. Efficient blue LEDs with EL maxima at 420 - 480 nm were prepared from polymer blends composed of SPF and SPPP or SPF and polysilane. Blending led to significant improvement in the EL efficiency and stability compared with the devices fabricated from neat SPF or SPPP. An increase in the EL efficiency of up to two orders of magnitude was observed, which indicated successful modification of the charge transport and recombination. Modification of transport properties was studied by measurements of the dark surface potential decay. A model of trap-controlled hopping was used to describe the experimental curves. The model parameters such as the effective hole mobility (the mobility of charge carriers controlled by energetically shallow traps in thermal equilibrium with the transport states) and the capture and release rates of charge carriers in a deep trap were determined by curve fitting. The highest values of the effective mobility were determined in SPF (1-2 orders of magnitudes higher than that in SPPP and polysilanes). Depending on the contents of the components in the blend, the effective mobility was modified. The charge photogeneration in thin polymer films was investigated by the method of photoinduced surface potential decay. Experimental data were analyzed by the Onsager theory of geminate recombination with the Gaussian distribution function of separation distances of electron-hole pairs. The results were in a good agreement with the results of PL and EL studies.
We would like to acknowledge support of the Grant Agency of the Czech Republic (Grant No. 203/01/0512) and the Academy of Sciences of the Czech republic (AVOZ 4050913).
[1] D. Výprachtický and V. Cimrová, Macromolecules 35 (2002) 3463.
POLYPYRROLE ELECTRODEPOSITION ON INORGANIC SEMICONDUCTORS CuInSe2 AND CuInS2 FOR PHOTOVOLTAIC APPLICATIONS
S. BEREZNEVa, I. KONOVALOVb, J. KOISc, E. MELLIKOVc, A. ÖPIKa
aDepartment of Basic and Applied Chemistry, Tallinn Technical University, Ehitajate tee 5, Tallinn 19086, Estonia, e-mail: bereznev@edu.ttu.ee
bInstitut für Solartechnologien, Im Technologiepark 7, D-15236 Frankfurt (Oder), Germany, e-mail: ikono@ist-ffo.de
cDepartment of Materials Technology, Tallinn Technical University, Ehitajate tee 5, Tallinn 19086, Estonia, E-mail: enn@edu.ttu.ee
Combination of transparent conductive polymers with inorganic semiconductors, e.g. CuInSe2 (CISe) and CuInS2 (CIS), is attractive for use in thin-film photovoltaic cell structures. The polymer layer of p-type is considered as an alternative for the traditional buffer layer and window layer in the conventional cell structure. Among various techniques of preparation of these structures, step-by-step electrodeposition of the polymer deserves special attention because it has been shown to be an inexpensive, low-temperature and non-polluting method.
Thin polypyrrole (PPy) layers with an average thickness of about 0.5 m m were deposited using potentiostatic and galvanostatic techniques onto glass/ITO/CISe substrates and on CIS structures fabricated using CISCuT technique on Cu tape substrates. The deposition proceeded from an aqueous solution containing sulfonates as a dopant. In order to prepare stable PPy films of high quality with a good adherence to the surface of inorganic semiconductors CIS and CISe, the optimal concentration of reagents, current density and electrodepositing potential were selected experimentally. It has been found that β-naphthalene sodium sulfonate is highly suitable as a dopant for the PPy deposition onto the CIS and CISe layers. Electrochemical polymerization of pyrrole to PPy on CIS and CISe surfaces is faster under white light irradiation, and the polymerisation starts at lower potential, than in the dark. It has been found that annealing of the PPy films at 100 ° C markedly improves its adherence to CIS and CISe surfaces. The surface morphology of the layer stack has been studied using SEM and AFM methods. Significant photovoltage and photocurrent of the fabricated CISe/PPy and CIS/PPy structures have been observed under tungsten-halogen white light illumination.
SIMULTANEOUS AND INDIVIDUAL EFFECTS OF MECHANICAL AND THERMAL STRESSES ON ELECTRIC DEGRADATION CHARACTERISTICS OF POlyoleFins AND THE COMPOSITES PREPARED ON THEIR BASIS AND CROSSLINKED XLPE MATERIALS
S. V. MAMEDOV1, V. A. ALEKPEROV1, N. CAN1, S. Y. GUNES1, F. ARAS2, G. YILMAZ3
1Yildiz Technical University, Department of Physics, 34210, Esenler, Istanbul, Turkey
2 Kocaeli University, Technical Faculty, Electrical Department, 41100, Kocaeli
3 Ar-Ge, Turk Pirelli, Mudanya, Bursa
Polymers, polymer composites and crosslinked polymers are used extensively in all types of power networks, devices and substation equipment, showing attractive features such as excellent dielectric properties and good thermomechanical behaviour. However, undergoing (being exposed to) the individual effects of external factors (electrical strength (E), mechanical stress (s ), temperature (T), etc.) all the materials mentioned above degrade and become unusable. There are some studies about the individual effects of these factors in the literature but the simultaneous effects are a bit complex and the influence and mechanism of them are still being discussed. In this abstract we give experimental results and influence mechanism of simultaneous and individual effects of E, s and T on polyethylene (PE), polypropylene (PP) and the composites prepared on their basis. The following conclusions are obtained from the experimental results:
Growth of perfluorinated phthalocyanines on thin polymer films for use as n-conducting channels in organic field effect transistors
W. Michaelisa, A. Hirthb, D. WÖhrleb, D. Schlettweina
aPhysical Chemistry 1, University of Oldenburg, 26111 Oldenburg, Germany
bDept. of Chemistry, University of Bremen, 28359 Bremen, Germany
Thin films of different polymers (polystyrene, polymethylmethacrylate, polyvinylcarbazol and polyvinylchloride) were deposited by spin-coating on ITO-electrodes. On these polymers, thin films of perfluorinated phthalocyanines were prepared by physical vapour deposition. The growth of these films was monitored in-situ by optical absorbance and electrical conductance measurements.
The optical absorbance spectra are analyzed based on the coupling of transition dipoles to obtain information of the intermolecular arrangement of chromophores in the films. In all of these samples the molecules are oriented with their molecular plane perpendicular to the substrate surface. This gives the desired overlap of the pi-systems for electrical conductance parallel to the substrate. Before such behaviour was only found on the polar surface of glass. Normally the perfluorinated phthalocyanines are positioned flat on the substrate surface like most phthalocyanines. Differences in the interactions were detected when deposition temperatures below or above the glass transition temperature of a given polymer were compared.
The measurements of electrical conductance give insight into the growth mode of the substituted phthalocyanine films. The morphology of the polymer films and the deposited semiconductors were controlled by atomic force microscopy and scanning electron microscopy. The influence of the chosen substrate on the film structure is determined. The optical and electrical properties of the films could thereby be influenced and the applicability of such films as active layers in organic thin film transistors is discussed.
UV-VIS CHARACTERIZATION OF SUBSTITUTED POLYANILINES
I. MAVa, M. ŽIGONa, J. VOHLÍDALb
aLaboratory for Polymer Chemistry and Technology, National Institute of Chemistry, Hajdrihova 19, POB 660, SI-1001 Ljubljana, Slovenia; ida.mav@ki.si
bDepartment of Physical and Macromolecular Chemistry, Laboratory of Specialty Polymers, Faculty of Science, Charles University, Albertov 2030, CZ-128 40 Prague 2, Czech Republic
Polyaniline (PANI) and its derivatives exist in various forms which differ from one another in oxidation state and extent of doping (protonation). The oxidation state and the extent of doping are control factors for the electroactive properties of PANIs. Many properties of substituted polyanilines differ from those of the parent polyaniline (PANI), for instance, their solubility is considerably improved. Water-soluble PANI derivatives, where sulfonate or carboxyl groups bound to the polymer backbone act as internal dopants (self-doping), are especially interesting. In the present work, we present the UV-vis study on the copolymers (PANIAOMA) of 2-methoxyaniline (OMA) with four different acidic substituted anilines (ANIA): 2- and 3-aminobenzensulfonic acid, 2- and 3-aminobenzoic acid, synthesized in the presence of oxidant in hydrochloric acid (HCl) aqueous medium at various reaction conditions.
Absorption signals in UV-vis spectra of copolymers PANIAOMA are characteristic of various transitions of p -electrons; excitations of p -electrons in benzenoid units, quinoid units or semi-quinoid units of polymer chain. The ratio between intensities of these signals it a measure of the ratio between individual units, depending on the oxidation state and doping level of polymer as well as the basicity of solvent, chosen for UV-vis measurements. The degree of conjugation (delocalization) of p -electrons can be deduced from the wavelength of UV-vis signals as well.
The reaction temperature and the amount of oxidant, in addition to the composition of reaction mixture, have the largest effects on the structure and composition of copolymers. The copolymers synthesized at a low temperature and those synthesized at a low amount of oxidant, have similar composition like samples with high OMA feed fraction, where copolymers with a low portion of ANIA units are formed. According to the UV-vis spectra reaction time and HCl concentration in reaction mixture do not affect structure of polymers very much.
THERMAL AND NEAR-IR PHOTOCHEMICAL GENERATION OF POLARONS IN m-NITROANILINE SINGLE CRYSTALS. DIELECTRIC AND ELECTRICAL MEASUREMENTS AND BAND STRUCTURE CALCULATIONS
M.M. SZOSTAKa, E. STARYGAb, M. DŁUŻNIEWSKIb, G.BĄKb,H.CHOJNACKIa
aInstitute of Physical and Theoretical Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
bInstitute of Physics, Technical University of Łódź, ul. Wólczańska 219, 93-005 Łódź, Poland
Previous investigations on the solid m-nitroaniline (m-NA) revealed unpaired spin generation under heating and near IR (NIR) irradiation by EPR [`1], NIR[1], and 1H-NMR [2] spectroscopic methods. Recent FT-NIR studies has suggested that radical-ions generated thermally and photochemically (NIR) have different nature and that they behave like polarons prapagating in different directions in the m-NA crystal [3]. In this work, the influence of temperature and frequency changes as well as of NIR irradiation on the crystal electrical conductivity was examined. The current-voltage characteristics obtained may indicate the domination of the hopping conduction mechanism.
The dielectric measurements were carried out in the frequency range 0.1 Hz¸ 1 MHz and in the temperature range 190K¸ 370K. Some evidence of change of the temperature dependence of the dielectric constant at about 210K was detected.
Irradiation by 980 nm laser diode caused a small signal increase and the heating to about 320K, accompanied by the irradiation, further enhanced the current. The experimental data will be discussed along with the semiempirical quantum mechanical calculation results on the m-NA band structure revealing its strong anisotropy.
1.. M.M. Szostak, B.Jakubowski and M. Komorowska. Mol. Cryst.
Liq. Cryst. 229,
7-12 (1993).
2. M.M. Szostak, G. Wójcik, J. Gallier, M. Bertault, P. Freundlich, H.A. Kołodziej, Chem. Phys. 229, 275-284 (1998).
3. M.M. Szostak and M.A. Czarnecki, Polish J. Chem. 76 ,419 (2002).
ELECTROMAGNETIC SHIELDING OF CARBON FIBRES COATED WITH NON-CONDUCTING POLYANILINE IN POLYMER MATRIX
M. PALIGOVÁa, J. VILČÁKOVAa, P. SAHAa, V. KRESALEKa,
J. STEJSKALb and O. QUADRATb
aTomas Bata University in Zlín, Faculty of Technology, 762 72 Zlin, Czech Republic
bInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,
162 06 Prague 6, Czech Republic
The electric and the dielectric properties of composites with short carbon fibres (CF) have been studied. Two types of fibres bare and coated with an insulating layer of polyaniline base were embedded in an epoxy resin matrix. Due to the fibrous character of the filler the percolation area of the composite with uncoated fibres was very low (@ 1-2 vol. %). In contrast, the percolation threshold of coated-fibre composite was as high as 20 – 25 vol. %. As a result DC conductivity below the percolation limit corresponding to a well-insulating material (108 - 1010 S cm-1) was found only bellow 1 vol. % CF for the composite with uncoated fibres, while for the coated fibres such low conductivity appeared for the system containing as much as 20 vol. % CF.
The comparison of dielectric characteristics below the percolation threshold revealed that both the permitivity and AC conductivity of the material with coated fibres in radiofrequency range are much higher than those for the uncoated-fibre system. Consequently, absorption of electromagnetic waves in the composite with coated fibres proved to be more effective. This means that while due to the low DC conductivity this material does not short-circuit the surface of electronic devices in the protective layer, it can transmit high-frequency radiation.
BLUE-LIGHT-EMITTING POLYMERS BEARING BIPHENYLENETHYNYLENE MOIETIES IN THE MAIN CHAIN
SUNG-HOON JOO, AND JUNG-IL JIN
Center for Electro- and Photo-Responsive Molecules and Division of Chemistry and Molecular Engineering, Korea University, 5-1, Anam Dong, Seoul 136-701, Korea, e-mail: jijin@korea.ac.kr
Electroluminescence (EL) properties of a new poly(aryleneethynylene) and a copolymer were studied. They are Poly[4,4’-(3,3’-dimethyl-1,1’-biphenylene)ethynylene-(2,5’-dioctyl-1,4-phenylene)ethynylene] (PPEBE) and its copolymer(PPEBE-co-mP) containing 20 mole % of the 1,3-phenylene unit. Both polymers are blue light emitters and the former was found to perform better than the latter when light-emitting diode (LED) device had the configuration of ITO/PEDOT/polymer/Li:Al. The device constructed with the former polymer exhibited the external quantum efficiency of 0.05 and the maximum brightness higher than 400 cd/m2 with its EL spectrum showing maxima at l = 445 and 472 nm. The performance of the device constructed with the copolymer was about one fifth of the device fabricated with the homopolymer
Structures of polymers.
TRANSFORMATIONS OF CHROMOPHORIC METHACRYLATES UNDER INFLUENCE OF LIGHT AND ELECTRIC FIELD
S. KUCHARSKI, R. JANIK
Institute of Organic and Polymer Technology, Technical University of Wrocław, 50-370 Wrocław, Poland
A series of methacrylate polymers and copolymers containing chromophore fragments in side chains were obtained. The chromophores were azobenzene derivatives containing sulfonyl group and heterocyclic ring and typical examples are shown in the figure. All the materials were orange to red solid substances and their mean molecular weight was between 20 000 and 30 000. The polymers were processed by forming thin films by Langmuir-Blodgett technique or by spin coating. They were investigated as those sensible to illumination with light from different sources: white light, polarized white light and laser beams. The maximum absorption band of these substances either in solution or in films was at ca. 430 – 460 nm. The absorption of light caused changes in their material structure owing to trans – cis isomerisation of the azobenzene group. Using of linearly polarized light caused domains in the films enriched with cis form which were detected by
polarized UV-vis spectroscopy. Ellipsometric characterization of the films under illumination with blue (absorbed) light showed reversible changes of refractive index which followed illumination. This indicated a possibility of grating formation in the materials. The degenerate 4 wave mixing with two beams of λ = 512 nm and a probe beam of λ=632 nm resulted in refraction index modulation with 4-7 % yield. The polymers deposited on ITO glass plate by L-B technique and placed in KCl solution in a system of three electrodes showed reversible effect of photocurrent generation when illuminated with light absorbed by trans form. This process was much faster than that observed in isomerisation tests.
The change of refraction index of the materials deposited onto ITO glass could also be observed when the film was placed in an electric field during so called corona poling. This phenomenon might be ascribed to orientation of side-chains dipoles.
KINETICS OF REFRACTIVE INDEX MODULATION IN POLYMERIC FOTOCHROMIC FILMS
EWELINA ORTYL, STANISŁAW KUCHARSKI*
*Institute of Organic and Polymer Technology, Technical University of Wrocław, 50 – 370 Wrocław, Poland. E-mail: kucharski@itots.ch.pwr.wroc.pl
Methacrylate homopolymers and copolymers containing azobenzene groups with heterocyclic sulfonamide: 4-amino-N-[2,6-dimethylpyrimidyn-4-yl] benzenesulfonamide and 4-amino-N-[5-methylisoxazol-3-yl]benzenesulfonamide substituents in side chains were investigated. The polymers were deposited as thin films onto glass plates by spin coating technique. The materials show a strong absorption band in the visible region with maximum at ca.440 – 460 nm and the isomerisation of the polymers carried out during ellipsometric measurements showed the reversibility of the process in repeated illumination cycles with white light (ca. 2mW cm-2 power). The trans – cis isomerisation of the azobenzene derivatives results in change of dielectric properties of the materials as cis-isomer formed has different dipole moment and molecular polarizability. From measured values of ellipsometric parameters and (eqn.1) it was possible to calculate the film thickness and complex refractive index: n = nr + ik.
(1)
Depending on the polymer, the change of real part of refractive index (nr) during illumination with white light, was between 0.013 to 0.03. Biexponential functions were used to describe the dynamics of growth and decay of refractive index changes. The biexponential functions were:
for photoinducing and
for the relaxation process in the absence of illumination, where nr is the refractive index change observed at time t; ka, kb, kc and kd represent the rate constants with the amplitudes of A, B, C and D respectively. The rate constants of the fast growth process and stability of the photoinduced refractive index changes (E) depend on the kind of chromofore side group as well as on the kind comonomer used: buthyl 2-methylacrylate and 2-ethylhexyl acrylate.
QUANTUM MANIFESTATION OF CLASSICAL STOCHASTICITY IN THE MIXED STATE
V.P.Berezovoj, Yu.L.Bolotin, V.A.Cherkaskiy
NSC KIPT ITP, Academicheskaya Str. 1, 61108 Kharkiv, The Ukraine
A Hamilton system with a few local minima in the potential energy surface represents a model in frame of which one can describe the dynamics of transitions between different equilibrium states, including such important transitions as chemical reactions, nuclear fission and others. Systems of such type allow existence of several critical energy values even for a fixed set of potential parameters. That leads to a possibility of existence of so-called mixed state [1] for such potentials: different dynamic regimes (regular or stochastic) are realised in the same energy interval in different local minima. It gives new possibilities for studies of the quantum manifestations of classical stochasticity (QMCS), using the following objects: statistical properties of energy spectrum (nearest-neighbour spacing distribution), structure of the wave functions (nodal curves, probability density) and wave packet dynamics.
We consider such possibilities in application to quadrupole surface oscillations of nuclei, described by the lowest terms of the deformation potential decomposition in deformation parameters. In that potential the mixed state is observed in that part of the parameter space, where the equilibrium shape of the nucleus can be either spherically symmetric or deformed, i.e. for the potentials with a few local minima.
In the case of the potential energy surface of complicated topology numerical calculation based on matrix diagonalization becomes not effective, but the so-called spectral method [2] can become an inspiring alternative. Since the spectral method is fundamentally based on numerical solutions of a time-dependent differential equation, its implementation is always straightforward. Neither special ad hoc selection of basis function is required, nor is it necessary for the potential to have a special analytic form. The spectral method is in principle applicable to problems involving any number of dimensions.
REFERENCES
[1] Yu. L. Bolotin, V. Yu. Gonchar, E. V. Inopin, V. V. Levenko, V. N. Tarasov and N. A. Chekanov, Fiz. Elem. Chastits and At. Yadra 20 (1989) 878
[2] M. D. Feit, J. A. Fleck, Jr., and A. Steiger
Journal of Computational Physics 47 (1982) 412
QUANTUM CHEMICAL STUDY OF OXIDATION PROCESSES IN METAL–PHTHALOCYANINES
P. TOMANa, S. NEŠPŮREKa, K. YAKUSHIb
aInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, CZ-162 06 Praha 6, Czech Republic
bInstitute for Molecular Science, Myodaiji–cho, Okazaki, Aichi 444–8585, Japan
Phthalocyanine (Pc) molecules are stacked one-dimensionally in a metal-over-metal mode. Owing to this structure, they have metal and ligand channel in the same molecular column. Martinsen et al. ascribed the distinct properties of CoPcI to the difference in the doped channel: CoPcI had a metal-oxidized channel, whereas the NiPcI had a ligand-oxidized channel [1]. If this is the case, the infrared (IR) spectra of metal-oxidized and ligand-oxidized compounds should be different. Yakushi et al. prepared CoPc(AsF6)0.5 and found that the IR spectrum was essentially the same as that of the ligand-oxidized salt NiPc(AsF6)0.5 [2]. Because the IR spectra of both compounds were substantially different from those of ligand-unoxidized compounds, CoPc and NiPc, they concluded that the ligand was also oxidized in CoPc(AsF6)0.5.
The experimental IR spectra of NiPc, CoPc, NiPc+, and CoPc+ were compared with those obtained by quantum chemical calculations. The experimental changes in the IR spectra during the oxidation are in good agreement with the theoretical results. Both the NiPc+ and CoPc+ IR patterns indicate the ligand oxidation in NiPc(AsF6)0.5 and CoPc(AsF6)0.5, respectively. It was found that the main differences in atomic charge distribution during the oxidation are located on the ligands. The charge on the metal in the oxidised state is only slightly more positive than that in the neutral molecule. The main differences between the atomic spin densities of the cations and neutral molecules are also located on the ligand. The calculations strongly support the ligand oxidation both in NiPc and CoPc.
The financial support by grants A1050901 of the Grant Agency of the Academy of Sciences of the Czech Republic and ME 17/2001 of the Ministry of Education, Youth and Sports is gratefully appreciated.
[1] J. Martinsen, J.L. Stanton, R.L. Greene, J. Tanaka, B.M. Hoffman, and J.A. Ibers, J. Am. Chem. Soc. 107, 6915 (1985).
[2] K. Yakushi, H. Yamakado, T. Ida, and A. Ugawa, Solid State Commun. 78, 919 (1991).
Poly(9,9-Dialkylfluorene) degradation induced by UV irradiation - changes in polyfluorene-type semiconducting materials
PATRICIA SCANDIUCCI DE FREITAS, ULLRICH SCHERF
Institut für Chemie, Polymerchemie, Universität Potsdam,
Karl-Liebknecht-Str. 24/25, Haus 25, D-14476 Golm, Germany.
FAX: ++49 331 977 5059, e-mail: scherf@rz.uni-potsdam.de
Poly(2,7-(9,9-bis(3,7,11-trimethyldodecyl)fluorene (PF) is one promising conjugated polymer that can be used as polymer light emitting diode (PLED). Polyfluorene films in the solid state display an absorption maximum at ca. 380 nm, the photoluminescence spectrum peaks at ca. 420 nm. There is an additional more or less intense emission band between 500 and 550 nm which was described as exciton emission or originating from charge trapping keto defects sites [1]. Such defects can be a result from a photodegradation process of the PF, as shown in Figure 1. The degradation was induced by UV radiation (1h, 750 W/m2, final dose: 2700 kJ/m2). The experiments were carried out under atmospheric oxygen or in an inert atmosphere (argon). This degradation occurs only in the presence of oxygen. The degradation cycle results in fluorenone moieties that causes an intense bathocromic shift and a broadening of the PF photoluminescence spectrum turning the emission colour from blue to yellow.
Fig. 1. Absorption and photoluminescence
spectra of irradiated PF.
[1] E. J. List, R. Güntner, P. Scandiucci de Freitas, U. Scherf, Adv. Mater. 2002, 14, 374.
ATMOSPHERIC AGING OF POLY[METHYL(PHENYL)SILANEdiyl] (PMPSi) MONITORED BY FTIR SPECTROSCOPY
O. MESZÁROŠ, P. SCHMIDT, J. POSPÍŠIL, S. NEŠPŮREK, I. KELNAR
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague, Czech Republic
Electric properties of polysilanes are based on σ-conjugation of the Si-Si backbone and are affected by a phototriggered degradation of the backbone accounting for breaking of the conjugation. The basic chemical character of degradation caused by environmental stress is not exactly understood. We performed a detailed study of irreversible chemical changes in thin PMPSi films prepared by spin coating and exposed in a Weather-Ometer Atlas Ci 3000+ (xenon lamp, dry conditions, borosilicate filter, 40 oC bpt) and in a proprietary illumination apparatus (mercury discharge, water filter, air or argon atmosphere). Chemical changes were monitored by FTIR.
The measured data reveal minor time-dependent changes at 3750-3100 cm-1 (free and bonded HO groups), at 1730 cm-1 (carbonyl species) and in the range of 800-700 cm-1 (structural changes near the benzene rings). Typical major changes are in the 1200-1000 cm-1 region. The calculated intensity increase with the time caused by the formation of siloxane species and loss of absorbance at 1098 cm-1 is presented. The measured data and the pattern of curves are influenced by attacking environment (hυ, air oxygen, argon atmosphere) and the presence of additives interfering with light absorption of PMPSi. The measured and calculated data are used for interpretation of the assumed transformation mechanism.
Acknowledgement. Financial support by grant A1050901of Grant Agency of the Academy of Sciences of the Czech Republic and technical cooperation of Ms M. Brunclíková are gratefully appreciated.
INFLUENCE OF MICROCRYSTALLINITY ON DIELECTRIC RELAXATION OF THE ALTERNATING TERPOLYMERS OF ETHENE, PROPENE AND CARBON MONOXIDE
V.P.Privalkoa,*, P. Pissisb, G. Polizosb, V.V. Korskanova, E.G. Privalkoa, V.Yu.Kramarenkoc, V.B. Dolgosheya, e, W. Huhnd, F. Hollmannd and B.Riegerd
aInstitute of Macromolecular Chemistry, National Academy of Sciences of Ukraine, Kharkiv Chaussee 48, 02160 Kyiv, UKRAINEbNational Technical University of Athens, Zografou Campus, GR 15780 Athens, GREECE
cPolytechnic University, Frunze 21, 61002 Kharkiv, UKRAINE
dDepartment of Inorganic Chemistry II, Materials & Catalysis, University of Ulm, D-89069 Ulm (Donau), GERMANY
eInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, CZ-162 06 Praha 6, Czech Republic
The alternating terpolymers of ethylene, propylene and carbon monoxide were characterized by WAXS, DSC, TSDC and dielectric relaxation spectroscopy (DRS). At room temperature, the terpolymers turned out X-ray amorphous ; however, the existence of weak endothermal effects over fairly broad temperature intervals above the glass transition suggested that they should be classified as ”microcrystalline” (in a sense that the ultimate level of crystallinity lies beyond the resolution limits of ordinary WAXS instruments).
The DRS data in the temperature intervals of α- and sub-glass relaxations were quantitatively fitted to the Havriliak-Negami equation. The characteristic parameters for the α-relaxation were consistent with classification of terpolymers as extremely fragile glass formers ; however, their high fragility was attributed to steric constraints to the motion of chain dipoles by the residual network of microcrystals. Strong asymmetry of the dielectric sub-glass relaxation was regarded as the additional experimental evidence for the interference of the spatial network of microcrystallites with the motion (in this case, non-cooperative) of chain segments.
*To whom all correspondence should be addressed ; E-mail : privalko@iptelecom.net.ua
HYBRID ORGANIC–INORGANIC FILMS AND COATINGS CONTAINING SUBMICROMETRE POLYANILINE PARTICLES
M. ŠPÍRKOVÁ, J.STEJSKAL
Institute of Macromolecular Chemistry, Academy of Science of the Czech Republic, Heyrovského nám. 2, CZ-162 06 Praha 6, Czech Republic
The preparation of polyaniline (PANI) films in situ during the polymerization of aniline or PANI films prepared by dispersion polymerization of aniline in the presence of colloidal silica led to conducting polymer films of rough granular structure and the thickness limited to a submicrometer scale. This study presents an alternative approach to the synthesis of hybrid organic–inorganic coatings and free-standing films with improved mechanical and surface properties:
Submicrometre PANI particles stabilized with colloidal silica were prepared by dispersion polymerization [1,2]. These particles were incorporated into the hybrid organic–inorganic matrix produced from trialkoxysilanes containing organic functional (glycidyl) groups. Sol–gel process involving the hydrolysis and polycondensation reactions of alkoxysilanes led to the build-up of inorganic structures of the coatings while the organic structure was formed by classic polyaddition reactions.
Mechanical and electrical properties of products can be tuned by varying reaction conditions. Stress at break, strain at break, Young modulus, toughness, shear moduli, loss factor and electrical conductivity were determined. The surface morphology was assessed with atomic force microscopy. The films have regular and smooth surface that could be in principle applicable as various types of substrate protection, e.g., in the mar or corrosion protection of materials.
1. J. Stejskal, P. Kratochvíl, S.P. Armes, S.F. Lascelles, A. Riede, M. Helmstedt, J. Prokeš, I. Křivka, Macromolecules 29, 6814–6819 (1996).
2. A. Riede, M. Helmstedt, V. Riede, J. Zemek, J. Stejskal, Langmuir 16, 6240–6244 (2000).
Acknowledgement. The authors wish to thank the Grant Agency of the Czech Republic (203/01/0735) for financial support.
ELECTRICAL CONDUCTIVITY OF THE POLYCRYSTALLINE FILMS OF p-SEXIPHENYL
S.W. TKACZYK, J.ŚWIĄTEK
Institute of Physics, Pedagogical University, Al. Armii Krajowej 13/15, 42-200 Częstochowa, Poland, e-mail:s.tkaczyk@wsp.czest.pl
Recently one could observe an increasing interest of microelectronic industry in organic materials possessed non-linear optical and electronic properties. Thin films of these one became very promising. The dielectric molecular complex materials and organic semiconductors are especially attractive.
The internal structure of organic thin films (like concentration of defects and changes of external electrostatic potential) has a strong influence on its electrical properties. By changing of the concentration of defects is possible to obtain material, where conduction charge carriers move by hopping or tunneling between neighboring localized states. The polycrystalline materials are interesting objects to study hopping conduction in a wide range of temperature and electric field.
The thin films of p-sexiphenyl were obtained by controlled vacuum sublimation process on BK-7 glass substrate. The thickness of the investigated material varied from 0,2 to 2 m. The DC conductivity measurements were performed by measuring the current flowing through the bulk of p-sexiphenyl between gold and aluminum electrodes (Au-Al). The measurements were carried out for different electrode polarities in the voltage and temperature range 0-100 V and 15-325 K, respectively. The activation energy was varying from kT for low temperature up to 0,4 eV for temperature above 250 K.
In carried out experiments the influence of external electric field on electrostatic potential barrier of localized charge carrier was defined (Poole – Frenkel effect).
Analyzing the obtained results we may conclude that:
PHOTOVOLTAIC PROPERTIES OF TETRACENE AND PENTACENE LAYERS
R. SIGNERSKI, G. JAROSZ*, J. GODLEWSKI
Gdansk University of Technology
ul. G.Narutowicza 11/12, 80-952 Gdansk, Poland
*e-mail: zofia@mif.pg.gda.pl
Attractive photoelectrical properties of organic materials such as interaction with visible radiation and transport capability of charged excitations promise cheap manufacture of optoelectric devices. Therefore, recently we have observed a huge interest of both polymers and low weight organic compounds, among which tetracene and pentacene due to their photoelectric properties are examined intensively.
One of the purposes of our study is to compare photovoltaic properties of pentacene and tetracene layers evaporated in the same conditions (pressure, temperature of substrates and evaporation speed) and provided with the same couple of electrodes: Au and Al. The former electrode effectively injects holes into both investigated materials, while the latter one blocks this process. The thickness of organic layers was 0.1m m to 0.4m m. The samples were illuminated by monochromatic light from the range of singlet-exciton absorption or by white light.
Our experimental researches concern spectra of photovoltaic short-circuit currents and photocurrents in the presence of external voltage for illumination through either the Au electrode or through the Al electrode as well as voltage and light intensity dependences of currents.
Big difference between the efficiencies of hole injection by Al and Au has been confirmed experimentally by rectification of dark currents. Moreover, for both materials in the range of singlet-exciton absorption the symbatic relations between spectra of short-circuit current obtained for illumination through Al and absorption spectra denote photogeneration of charge carriers in the vicinity of Al electrodes. In the case of pentacene we have also observed photogeneration of charge carriers in the bulk of samples in the short-wave range of absorption.
Acknowledgements
Work was supported by KBN under Program No. 4 T11B 057 22.
THERMALLY STIMULATED CARRIER TRANSPORT DUE TO STEP SAMPLE HEATING
W. TOMASZEWICZ
Faculty of Technical Physics and Applied Mathematics, Gdansk Institute of Technology, Narutowicza 11/12, 80-952 Gdansk, Poland, wtomasze@sunrise.pg.gda.pl
The
communication concerns non-isothermal dispersive carrier
transport in an insulating solid with traps. The approximate
solutions of transport equations derived in [1] are extended to
the case of step sample heating. The specific features of
thermally stimulated currents (TSCs) can be attributed to
non-linear dependence of the demarcation level on temperature. In
particular, initial TSC rise has thermally activated character
where activation energy equals to the demarcation energy at the
end of previous heating cycle. The accuracy of the formulae
describing TSCs is verified by Monte Carlo calculations for
exponential trap distribution. Exemplary results are presented in
Fig. 1 (lines and points – analytical and numerical results,
respectively).
FIG. 1. TSCs for step sample heating and exponential trap distribution.
[1] W. Tomaszewicz, J. Phys.: Condens. Matter 4 (1992) 3985
PHOTOELECTRIC PROPERTIES OF TETRACENE-PENTACENE HETEROJUNCTION
R. SIGNERSKI, G. JAROSZ, J. GODLEWSKI
Gdansk University of Technology, ul.G.Narutowicza 11/12, 80-952 Gdansk, Poland
The subject of our investigation is heterojunction formed from vacuum-evaporated layers of pentacene (Pn) and tetracene (Tc). The properties of both materials have been well researched although the heterojunction has not been investigated, yet.
The thickness of a single organic layer was 0.1m m or 0.2m m. The samples were provided either with electrodes of Au from the Pn side and CuI from the Tc side or with two Au electrodes. The heterojunctions were analyzed with the usage of spectra of photocurrents, as well as voltage and light intensity dependences of currents.
The enclosed Figure presents exemplary spectra of short-circuit currents of the Au/Pn/Tc/CuI system illuminated through Au or CuI electrode and absorption spectrum of the Pn/Tc bilayer. The observed relation between absorption and short-circuit currents suggests that for the light from the range of Tc absorption the area of charge carrier photogeneration is situated near the Tn/Tc interface whereas for the light from the range of Pn absorption photogeneration of charge carriers occurs also in the bulk of Pn layer.
Fig. Short-circuit current spectra of Au/Pn/Tc/CuI cell illuminated through Au (1) or CuI (2) and absorption spectrum of Pn/Tc bilayer.
Acknowledgements
Work was supported by KBN under Program No. 4 T11B 057 22.
Formation of protein monolayer film on solid/liquid interfaces and its function observed by slab optical waveguide spectroscopy
N. Matsudaa, J. Santosa, Z. Qia, A. Takatsub, K. Katob
aNanoarchitectonics Research Center, AIST, Central 5, 1-1-1, Higashi, Tsukuba, 305-8565, Japan
bNational Methodological Laboratory, AIST, Central 4, 1-1-1, Umezono, Tsukuba, 305-8563, Japan
Many researches on surface immobilized protein systems have been carried out for the development of nano-devices and sensors. The properties of immobilized proteins on interfaces are different from those in solution due to the protein-surface interaction. For proper understanding of their properties and to characterize their structure on interfaces, it is essential to obtain real time (in situ) information about the monolayer or submonolayer of adsorbed protein on the surface. However, it is difficult to observe absorption spectra of thin layer of proteins adsorbed on electrodes because of the lack of sensitivity using conventional techniques
Recently we have developed a novel technique using SOWG in which the absorption spectra of adsorbed materials were observed to conveniently investigate the properties of surface-bound species. Additionally we utilized ITO electrode on a SOWG (ITO-SOWG) for in situ observation of absorption spectra from adsorbed materials on electrode surface. In this study, the monitoring of the protein monolayer film formed on SOWG surface was performed by SOWG spectroscopy. In addition, the function of the protein film immobilized on SOWG surfaces was also investigated.
The adsorption process of proteins (cytochrome c and hemoglobin, for example) from dilute solutions onto the quartz surface usually reached steady state within a few minutes by SOWG spectroscopy. This time scale was much shorter compared to those obtained using quartz crystal microbalance (QCM) method and may be attributed to the unique interaction of the evanescent wave with the surface. In the case of SOWG, soon after the slight attachment of proteins to surface the evanescent wave recognized them as being adsorbed, so it took only a few minutes to reach the absorbance plateau. However, the conformational change of proteins on surfaces after slight attachment might be induced by the vibration of the QCM surface and therefore may lead to much longer time before reaching the equilibrium.
It was confirmed that the surface immobilized protein films on SOWG surface kept its activity for electron transfer reaction. Additionally, the function of protein inside the nano-tube with diameter of sub-micrometer and high-axial-ratio is investigated to understand the relation between the circumstance and its function.
EXOTIC EXCITONIC EFFECTS IN SUPRAMOLECULAR ARCHITECTURES: THE CASE OF POLAR-POLARIZABLE CHROMOPHORES
FRANCESCA TERENZIANI, ANNA PAINELLI
Dipartimento di Chimica GIAF, Universitr di Parma,
viale delle Scienze 17/A, 43100 Parma, Italy; INSTM, UdR Parma
-conjugated electrons largely respond to external perturbations including molecular vibrations and environmental fields. Intermolecular interactions play an important role in dense samples of organic molecules, so that molecular properties acquire a large and non-trivial dependence on the supramolecular architecture. Here we discuss the case of donor-acceptor (DA) conjugated chromophores, whose polarity immediately indicates electrostatic interactions as the most important supramolecular effect. Specifically, each chromophore is described within a two-state picture that fully accounts for its large (hyper-)polarizability. Surrounding chromophores originate at each molecular location an effective electric field, that affects the properties of the chromophore, leading in turn to a variation of the effective electric field, in a feedback mechanism that largely amplifies the non-linearity of the system.
Exotic effects of intermolecular interactions in the ground-state of interacting polar-polarizable chromophores are easily predicted, including the observation of collective charge-transfer phase transitions. Here we will mainly concentrate on the effects of aggregation on spectral properties. Standard excitonic models as applied to aggregates of symmetric (cyanine-like) dyes do not account for the molecular polarity nor for its (hyper-)polarizability and are inadequate to model aggregates of polar-polarizable chromophores. New and intriguing excitonic effects, including exciton strings, will be demonstrated with no counterpart in standard excitonic models.
The coupling of the electronic system to molecular vibrations introduces additional effects that are thoroughly investigated in the case of two interacting chromophores. Exact non-adiabatic diagonalization allows us to discuss not only the strong and weak coupling case, but also intermediate cases. Molecular vibrations are demonstrated to play an important role in driving the electronic properties of the system, even when they do not show up as evident features in electronic spectra.
ELECTRON-PHONON COUPLING IN PENTACENE CRYSTAL STRUCTURES
M. MASINOa, A. GIRLANDOa, R.G. DELLA VALLEb , E. VENUTIb , L.FARINAb and A. BRILLANTEb
aDip. di Chimica G.I.A.F., Università di Parma, Viale delle Scienze 17/A, I-43100 Parma, Italy
bDip. di Chimica Fisica e Inorganica, Università di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
Pentacene is among the most promising organic semiconductors for microelectronics. Recent improvements in the crystal growth/purification has yielded crystals exhibiting even more interesting properties: The appearance of huge charge carriers mobilities on the order of 104 to 105 cm2/ V-1s-1 at very low temperature, the observation of the quantum Hall effect and charge-injected superconductivity have revived interest in basic questions regarding the issue of transport properties in molecular organic crystals and the role of electron-phonon interactions.
However, it is not yet clear how the conditions under which the molecules assemble to form the solid determine these outstanding physical properties.
We have addressed the above problem by calculating the pentacene crystal structures by Lattice Dynamics calculations [1]. We have shown that two distinct bulk crystalline phases of pentacene exists, with very subtle structural differences but clearly different phonon spectra. We then compute the temperature dependence of both crystal structures, together with the corresponding changes in the intermolecular hopping integrals between pairs of pentacene molecules.
It appears that these integrals (and hence the bare band structure) change very little by lowering the temperature. Therefore the observed huge increase in the charge carriers mobilities cannot be ascribed to a simple bare bandwidth increase: other factors, such as the coupling to phonons, are probably able to strongly affect the effective bandwidth. Here, the relevance of these factors in establishing the charge transport and the superconducting properties in pentacene polymorphs is checked by calculating the temperature and structure dependence of the phonon spectra and of the coupling strength of lattice phonons with electrons and holes (Peierls interactions).
[1] E. Venuti, R. G. Della Valle, A. Brillante, M. Masino and A. Girlando; JACS 124, 2128 (2002).
PIEZOELECTRIC BEHAVIOUR IN PVC/BaTiO3 COMPOSITES
M. OLSZOWY
Institute of Physics, University of Zielona Gora, Podgorna 50, PL-65 246 Zielona Gora, Poland; e-mail: M.Olszowy@if.uz.zgora.pl
Flexible piezoelectric composite was prepared in the thin film form (200-400 µm) by hot-pressed method (2.8 MPa pressure at ~398 K). Barium titanate (BaTiO3) ceramic powder was dispersed in a suspension poly(vinyl chloride) (PVC) matrix, providing a composite with 0-3 connectivity. The circular gold electrodes were evaporated on both sides of the films for dielectric constant, piezoelectric parameters and thermally stimulated current (TSC) measurements. These films were polarized in a strong DC electric field at elevated temperatures. The changes of dielectric constant and piezoelectric parameters on composites of PVC/BaTiO3 with different contents of BaTiO3 up to 40 % vol. were investigated. The obtained parameters were compared to the model of two-phase polymer-ceramic system. TSC thermograms of the composites were studied in the temperature range between room temperature and 378 K. The piezoelectric properties and the origin of their piezoelectric behaviour for PVC/BaTiO3 composites with different contents of BaTiO3 are reported.
Effect of a dipolar self-assembly monolayer formation on Indium-Tin Oxide on the performance of single-layer polymer-based light-emitting diodes
J. Morgado*a, A. Charas a, N. Barbagallo a, L. Alcácer a, M. Matos b, F. Cacialli c
aDepartamento de Engenharia Química, Insituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa; *jmorgado@gcsi.ist.utl.pt
bDepartamento de Engenharia Química, Instituto Superior de Engenharia de Lisboa, Rua Conselheiro Emídio Navarro – 1, P-949-001 Lisboa
c Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK, and
Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
We report on the effect of indium-tin oxide, ITO, surface modification on the electroluminescence efficiency and on the light-onset voltage, Lon, of single-layer light-emitting diodes, LEDs, with aluminum cathodes. The modification involves the grafting of highly polar p-disubstitutedbenzene molecules creating a dipolar monolayer on the surface of ITO. This modification brings about an increase of the ITO workfunction and, therefore, a reduction of the barrier for hole injection into the HOMO of the luminescent polymers. The polar molecules possess –COCl or –SO2Cl grafting groups and CF3 or NO2 as highly electronegative groups.
Poly[(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene], MEH-PPV, and polyfluorenes are used as luminescent polymers. We observe that the surface modification of ITO with these self-assembled monolayers, brings Lon down to about 2.2 eV, close to the energy gap voltage of MEH-PPV (≈2.13 eV), in spite of the low electron injection ability of the aluminum cathode. Correspondingly, the current flowing through the devices is significantly increased. Similar studies have been carried out in polyfluorene-based LEDs and the results will be reported.
STUDIES OF ELECTRICAL AND MECHANICAL PROPERTIES OF POLY (VINYL CHLORIDE) MIXED WITH CONDUCTIVE CARBON BLACK
A.I.G. SAADa , O.I.H. DIMITRYb AND H.A. AZIZb
a Microwave Physics Dept.,National Research Centre, Dokki, Cairo, Egypt.
bPetrochemical Dept.,Egyptian Petroleum Research Institute,Nasr City, Cairo,Egypt.
Electrically conducting polymers have excited considerable interest because many applications can be envisioned for electroactive materials. In this study, a preparation consists of poly(vinyl chloride) ( PVC) and a polar plasticizer such as dioctylphthalate (DOP) was mixed with variable proportions of conductive FEF carbon black (CB) in order to obtain a semiconductive PVC with good mechanical properties. For comparison, samples of PVC-CB system were also prepared by dispersing different concentrations of CB into PVC matrix. Electrical conductivity (6) measurements at temperatures from 20 to 60°C show that as C loading increases, samples of PVC-DOP-CB system become more conducting and the conductivity increases as the temperature is increased due to the increase of the mobility of ionic bodies that takes place as a result of the excitation by heat. For PVC-CB system, 6 shows much higher increase with the increase in the concentration of CB. This may be attributed to the Percolation Phenomenon which is based on the fact that when CB is present in abundance in the PVC matrix, the PVC will be saturated with CB which forms tunnels of electrons between adjacent C clusters and through the PVC matrix. When the electric current is applied, it is easier for the current to pass through these tunnels. Accordingly, there will be a high increase in electrical conductivity. Mechanical measurements also show that a high degree of loading brings about deterioration of the mechanical properties (tensile strength and ultimate elongation). The activation energy (E) for electrical conductivity was calculated by plotting the logarithm of the conductivity (σ) against the reciprocal of the absolute temperature where a straight line was obtained for samples of PVC-CB system , showing that they tend to behave as semiconductors. From the slope of the straight line, E was calculated using the equation:
σ=σ0e-E/RT
The period of excitation depends on the activation energy needed to make the substance conducting. If the activation energy is low the PVC composition becomes semi.conducting at room temperature or in presence of indirect light. Accordingly, it can be better employed for electronic devices working at normal temperatures.
CURRENT-VOLTAGE CHARACTERISTICS OF THIN POLYMER FILMS
A.A. BUNAKOVa, A.N. LACHINOVb, R.B. SALIKHOVa
aBashkir State Pedagogical University, Oktyabrskoy revolyutsii 3a, Ufa, 450000 Russia, gen_phys@bspu.ru
bInstitute of Molecular and Crystal Physics URC RAS, Prospekt Oktyabrya 151, Ufa, 450075 Russia, lachinov@anrb.ru
In thin electroactive polymer films the transitions from a dielectric to metal state are observed. These transitions may be induced by electric field, small uniaxial pressure, temperature etc. The features of charge carriers transport were investigated near to a threshold of the transition, which was induced by a small uniaxial pressure. In the present paper the current-voltage (I-V) characteristics of thin polymer films at different pressure (P) were measured and analyzed.
The poly(diphenylenephtalide)s was used as a polymer samples. The sample thickness were in the range from 0.8 μm to 1.6 μm.
The current-voltage characteristics were convenient to approximate by power function like I ~ Un. Under P = 0 the I-V characteristic is possible to divide into two areas: linear with n = 1 at small voltages and square-law with n = 2 at major voltages. The transition from first to second area happens at some voltage U1. The increase of pressure is given in the following change of the I-V characteristic view. Two new regions appear on I-V characteristics additionally. There are practically vertical sites at reaching value U2>U1; behind which the curves are close to square-law. The increase of pressure leads to reduction of the value U1 and to the U2 value increasing.
For interpretation a current-voltage characteristics the Space Charge Limited Conduction (SCLC) model was used. The model permits us to estimate following parameters vs pressure: the charge carrier mobility, the concentration of thermally generated equilibrium charges, the concentration of trapping levels and others.
The calculations display that the increase of pressure leads to decrease of equilibrium carrier concentration, growth of charge carrier mobility, increase of deep traps concentration and decrease of traps filling.
The analysis of obtained results allows us to make the following preliminary conclusions. Pressure increasing promotes to form the narrow trap band close to the Fermi level by means of strengthening of injection level. It can give a sharp magnification of the charge carrier mobility and to transition to a metal state even.
COHERENT ELECTRONIC TRANSPORT THROUGH THE MOLECULAR SYSTEMS
W. I. BABIACZYK AND B. R. BUŁKA
Molecular Crystals Department, IFM PAN, Smoluchowskiego 17/19 Street,
60-374 Poznań, Poland
iwo@ifmpan.poznan.pl
Recent experiments show that carefully fabricated contacts connecting molecules to the electrodes can make it possible to occur the coherent electronic transport. The numerous effects have been observed including quantum interference and the Fano effect. In this work, we present results for coherent electonic transport through molecules consisting of the few aromatic rings as well as the single walled carbon nanotubes (SWCNT).
The methods used for calculations are based on the nonequilibrium Green’s functions formalism what allows to include the multiscatterings and interference processes. Our models work for the different number of aromatic rings and length of the SWCNT and couplings to both electrodes. The calculated conductances show the discrete spectrum of peaks, which width and height depend on the coupling with the electrodes. In case of the single molecule systems we focus on the current-voltage dependencies, obtaining the negative differencial resistance. We also obtain Fano resonances in the conductance spectrum which is due to the destructive quantum interference. For SWCNT, we obtain the big value of the conductance for strong couplings, which is close to the ideal transmission (of G ≈ 4e2/h) as in the experiment. Our calculations, however, indicate the negligible role of the interference effects between the both conductance channels for the sake of the orthogonality of the wave functions. The structure of the conductance rather results from superposition of the conductance for both channels. We also consider the ferromagnetic electrodes and analize the tunneling magnetoresistance in the coherent regime of conductance. One can observe strong oscillations of the magnetoresistance for the applied gate voltage.
Small molecule organic solar cell incorporating fullerene/phthalocyanine mixed layer and wide-gap transport layers
J. Drechsel, B. Maennig, M. Pfeiffer and K. Leo
Institut of Applied Photophysics, TU Dresden, D-01062 Dresden, Germany
Organic solar cells are a promising way towards large area and low price photovoltaic systems. Although the doping process is crucial for inorganic solar cells, organic solar cells are normally still prepared without controlled doping. Our doping concept by coevaporation of two organic compounds allows us to increase the conductivity of the transport layers by more than four orders of magnitude. Common cell structures containing a mixed layer of zinc-phthalocyanine (ZnPc) and buckminsterfullerene C60 as photoactive part between a zinc-phthalocyanine layer on one side and a dimethyl-perylene-tetracarboxylic-diimide (MPP) layer on the other side. Here, we replace these transport layers by doped wide-gap materials like m-MTDATA to improve the internal quantum efficiency. The devices show a power conversion efficiency of about 1% and an open circuit voltage of 0.44V at 100 mW/cm2. The main advantage of these devices is the much higher internal quantum efficiency which opens the way for further improvements. We present a detailed study mainly focusing on the p-side of the organic solar cell.
PHOTOCHROMIC LIQUID CRYSTALLINE STRUCTURES CONTAINING AZOBENZENE GROUPS
M. Studenovskýa, Z. Sedlákováa, G. Wanga, S. Nešpůreka, K. Janusb, F. Kajzarc
a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 162 06 Prague, Czech Republic
b Institute of Physical and Theoretical Chemistry, Technical University of Wroclaw, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland
c Commissariat á l’Energie Atomique, DRT–LIST, DECS/SEMM/LCO, CEA/Saclay, 91191 Gif sur Yvette, France
Novel liquid crystalline photochromic materials of the type
|
(I) |
where R1 is NO2, H, Cº N, O-C8H17, phenyl, phenyl-NO2, and R2 is OH, O(CH2)n-N(CH2CH2OH)2, O(CH2)n-CH(CH2OH)2, O(CH2)n-N(CH2CH2O-CO-NH-phenyl)2 were prepared. Some of them are photoconductive. These materials were used for preparation of light sensitive polymers in which the photoactive moieties (I) were chemically attached to polyurethane chain. Some of these polymers are liquid crystalline, photoconductive and photochromic. The photochromism is based on trans-cis isomerization of azobenzene group.
The photochromic activity of the material (I) [R1 =
O-C8H17, R2 = O(CH2)5-N(CH2CH2OH)2]
in poly(methyl methacrylate) was examined. As prepared
spin coated film shows absorption typical of trans-azobenzene,
with a maximum at
360 nm, and a shoulder at 430 nm. After irradiation into the 360
nm nm band, its intensity decreases, accompanied by a build-up of
bands 310 nm and 450 nm, characteristic of cis-azobenzene. The
thermal cis-trans reaction shows an exponential decay with room
temperature life-time being about t =
2 ´ 104 s. Its activation
energy and frequency factor amount to Ea = 93 kJ/mol
and n = 2 ´
1011 s-1, respectively.
The financial support from the Grant Agency of the Academy of Sciences of the Czech Republic (grant A1050901), the Ministry of Education, Youth and Sports of the Czech Republic (grant OC D14.30), and the Polish State Committee for Scientific Research (grant No 3T091714419) is gratefully appreciated.
Optical and electrical studies of novel azobenzene derivatives in ordered thin films
I.MUZIKANTEa, L.GERCAa, A.Tokmakova, E.FONAVSa, E.MARKAVAb, D.Gustinab
aInstitute of Physical Energetics, Academy of Sciences of Latvia, Aizkraukles Str. 21, LV-1006 Riga, Latvia
bLatvian Institute of Organic Synthesis, Aizkraukles Str. 21, LV-1006 Riga, Latvia
In the last years we are searching for new azobenzene derivatives, which are able to form organized photoactive thin layers. We have focused our attention to Langmuir –Blodgett (LB) multilayers and self-assembled monolayers (SAMs). In the design of a new molecular structure we have developed our investigations in the field of modified azobenzene derivatives showing high cis/trans photoreactivity due to looser packing of azo unites in the films as a result of steric requirements of the bulky hydrophobic groups. In this report we have presented the synthesis and some results of the investigation of compound 1 in LB multilayers and compound 2 in SAMs.
The Y-type LB layers were deposited onto quartz or ITO glass substrates. The number of mono-layers was of 19. The SAM of compound 2 was deposited onto vacuum evaporated Au layer. Both LB and SAM samples were dried at least in air for a day.
Thin films were irradiated by Xenon lamp using filters centered at 360nm for UV light and at 450nm for visible light. The surface potential was measured in air by Kelvin probe technique applying vibrating gold electrode.
The macroscopic effects of switching, namely, optical absorption and surface potential are determined by the density of the molecules and their orientational order in the film. The trans-isomer of azobenzene 1 in chloroform solution exhibits the main absorption band at 362nm and a shoulder at 440nm. Cis-isomer shows two spectrally well-separated bands at 362nm and 440 nm. The absorbances of these bands are correlated with the density of trans and cis-isomers.
The LB multilayer and SAMs exhibit reversible trans/cis photoisomerization on alternating UV and visible irradiation. The time of the response of the decrease of the absorbance in absorption bands at 362nm to irradiation with light centered at 360nm was of 15s. The reversible cis-to-trans isomerization process continued more than 4min. The fast decrease of 7s of the surface potential of 20mV on UV irradiation was obtained.
