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| 2. |
- Birgerson, J., et al.
(författare)
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Doped polymeric cathodes for PPV/Al based LEDs
- 2002
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Ingår i: Synthetic metals. - 0379-6779 .- 1879-3290. ; 132:1, s. 57-61
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Tidskriftsartikel (refereegranskat)abstract
- The effect of Li-doping in poly(para-phenylenevinylene) (PPV) based light emitting devices has been studied. In a standard structure with an indium tin oxide (ITO) anode, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT-PSS)-layer and an active PPV-layer, the effects of a thin (around 1 Å) Li-layer and a thin layer, (50 Å), of a large bandgap polymer, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP) between the PPV and the aluminum cathode have been studied in terms of IV-characteristics and efficiency. The Li-atoms dope the interfacial layer of the PPV as seen by photoelectron spectroscopy. A thin layer of Li improves the charge balance by decreasing the energy barrier for injection of electrons for the Al/Li/PPV/PEDOT-PSS/ITO device. The efficient electron injection originates from a Fermi level alignment between the doped polymer and the aluminum cathode, which reduces the energy barrier. A thin layer of the large bandgap polymer P14NHP, between the PPV and Al contact, increases the light output and efficiency by blocking the holes. In addition, it may also reduce the light quenching by moving the region of recombination away from the Al-contact. The addition of a Li-layer on top of P14NHP leads to an increase of the quantum efficiency, because of better electron injection. © 2002 Elsevier Science B.V. All rights reserved.
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| 3. |
- Keil, M., et al.
(författare)
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High levels of alkali-metal storage in thin films of hexa-peri-hexabenzocoronene
- 2002
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:24, s. 10854-10860
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Tidskriftsartikel (refereegranskat)abstract
- The results of the doping of molecular solid films of the medium-sized aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC), using lithium or sodium atoms were presented. The evolution of the valence band electronic structure was monitored by x-ray photoelectron spectroscopy and studied by ultraviolet photoelectron spectroscopy. Results indicated that n-doping depended on the type of alkali-metal atom employed and on the molecular order of the film. The smaller Li ion was found to have a stronger influence on the electronic structure than Na ions. These results revealed the potential use of the graphene materials in lithium-ion batteries with a high charge-storage capacity.
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| 4. |
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| 5. |
- Samori, P., et al.
(författare)
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Growth of ordered hexakis-dodecyl-hexabenzocoronene layers from solution : A SFM and ARUPS study
- 2001
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:45, s. 11114-11119
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Tidskriftsartikel (refereegranskat)abstract
- The layer growth of a polycyclic aromatic hydrocarbon moiety, hexakis-dodecyl-hexabenzocoronene (HBC-C12), from solution onto a conductive flat solid substrate has been studied. Scanning Force Microscopy (SFM), together with the analysis of the intensities of p-structures in spectra of Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) measurements, revealed that the HBC-C12 molecules can self-assemble as dry layers with the conjugated disklike molecules lying flat on the (0001) plane of highly oriented pyrolitic graphite (HOPG). By varying the rate of the molecular physisorption it was possible to orient these molecular architectures along preferential directions according to the symmetry of the substrate. Additionally, the film morphology is affected by the concentration of the solution. This indicates that the growth of these organic layers on HOPG is a kinetically governed process which, if carried out sufficiently slowly, leads to the growth of hetero-epitaxial crystallites.
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| 6. |
- Birgerson, Jonas, 1969-
(författare)
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A study of materials and devices for polymer electronics
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, much research has been conducted in the field of conjugated polymers. A very promising direction for applications of this research is toward polymerbased light emitting devices. Conjugated, semi-conducting polymers, otherwise known as "conducting polymers" in their oxidized or reduced states, have been shown to be very suitable for use as active material in light emitting devices. Thus, much of the research activity has been concentrated on the design and fabrication of light emitting devices. In such devices, there are many important parameters that determine device performance. In addition to the properties of materials, the interfaces between the different materials are very important in determining performance. In this work, we have studied organic materials, polymers and molecules, used in devices, as well as the devices themselves. The main experimental method to study the materials has been photoelectron spectroscopy. We have combined the experimental method with the results of quantum chemical calculations in order to obtain a better understanding of the chemical and electronic structure of the materials. Devices have also been made in attempts to extend the results from the basic studies of these materials and to get a better understanding of how the materials interact in a device.Much recent research has been focused on to increasing the quantum efficiency of the polymer based light emitting devices. In the first polymer based light emitting devices only one active emitting layer was used. In this thesis, one work is aimed at showing to increase the quantum efficiency of the device by using a polymer blend, with two different polymers in the emitting layer. Another way to increase the efficiency is to use several layers in the device. Normally there are two layers in common devices; one active emitting layer and one extra layer that decrease the energy barrier for the positive charges between the anode and the emitting polymer. The most common polymer used as a hole injecting layer is poly(3,4-ethylenedioxythiophene) blended with poly(4-styrenesulfonate), PEDOTPSS. Here we have studied the electronic structure of the monomer of PEDOT-PSS, namely ethylenedioxythiophene. The molecules have been studied in gas phase, in solid state, and interacting with a gold substrate. The understanding of interactions between polymers and metals is important for patterning, making printed circuits, in more advanced devices. Another way to increase the quantum efficiency is to incorporate an extra layer for injecting of electrons. This is normally the case in devices based on molecules. Here we have studied the electronic and chemical structure of some polymer very similar to some molecules that are used elsewhere in molecular based devices.
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| 7. |
- Birgerson, J., et al.
(författare)
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A study of the electronic structure of ethylenedioxythiophene in gas phase using NEXAFS and quantum chemical calculations
- 2004
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 392:1-3, s. 100-104
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Tidskriftsartikel (refereegranskat)abstract
- Near-edge X-ray absorption fine structure spectroscopy spectra of ethylenedioxythiophene has been recorded in gas phase at the carbon K-edge, sulphur L-edge and oxygen, K-edge. The experimental data has been interpreted with the help of a modified density functional code deMon. The good agreement between the calculated spectra and the measured one allows us to assign all observed resonances. The existence of pi* resonances in both the measured and the calculated OK-edge adsorption spectrum demonstrate that the delocalized pi-system of the thiophene part of the molecule is extended up to the oxygen atoms of the molecule.
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| 8. |
- Birgerson, J., et al.
(författare)
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Electronic structure of some conjugated polymers for electron transport
- 2001
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Ingår i: Synthetic metals. - 0379-6779 .- 1879-3290. ; 122:1, s. 67-72
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Tidskriftsartikel (refereegranskat)abstract
- The chemical and electronic structure of three different, strictly alternating copolymers, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP), poly(2,5-diheptyl-1,4-phenylene-alt-2,6-naphthylene) (P26NHP) and poly(2,5-diheptyl-1,4-phenylene-alt-9,10-anthrylene) (P910AHP), have been studied by photoelectron spectroscopy and optical absorption spectroscopy. The experimental results have been analyzed using the results of quantum chemical calculations. In the geometrical structure of all three of the polymers there are large torsion angles between the phenylene unit and the naphthylene or anthrylene units. These large torsion angles lead to localization of the p-electron wave functions, and minimal conjugation along the polymer backbone. For all three polymers, the highest occupied molecular orbital is completely localized to the naphthylene or anthrylene unit. The frontier molecular orbital wave functions are very reminiscent of the highest occupied orbitals of the isolated naphthalene or anthracene molecules. The optical absorption spectra of all three polymers verify the existence of large optical band gaps, consistent with the large torsion angels. The first several optical transitions in the polymers are also very reminiscent of the transitions in single naphthalene and anthracene molecules.
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| 9. |
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| 10. |
- Friedlein, Rainer, et al.
(författare)
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Role of electronic localization and charge-vibrational coupling in resonant photoelectron spectra of polymers : Application to poly(para-phenylenevinylene)
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:12
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Tidskriftsartikel (refereegranskat)abstract
- A combination of x-ray absorption and resonant photoemission (RPE) spectroscopy has been used to study the electronic structure of the one-dimensional conjugated polymer poly (para-phenylenevinylene) in nonordered (as prepared) thin films. The dispersion of RPE features for the decay to localized and delocalized bands are qualitatively different. A theory for band dispersion of RPE in polymers is given, showing the important roles of electronic state localization and vibrational (phonon) excitations for the character of the dispersion.
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| 11. |
- Jönsson, Stina, et al.
(författare)
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The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films
- 2003
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Ingår i: Synthetic metals. - 0379-6779 .- 1879-3290. ; 139:1, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Films of poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS), prepared by coating the aqueous PEDOT–PSS dispersion and by coating a mixture of the PEDOT–PSS dispersion and different solvents, have been studied using four-point probe conductivity measurements, atomic force microscopy and photoelectron spectroscopy. The electrical conductivity of thin films of the second type (further on called PEDOT–PSS–solvents) was increased by a factor of about 600 as compared to films of the first type (further on called PEDOT–PSS–pristine). Morphological and physical changes occur in the polymer film due to the presence of the solvent mixture, the most striking being that the ratio of PEDOT-to-PSS in the surface region of the films is increased by a factor of ∼2–3. This increase of PEDOT at the surface indicates that the thickness of the insulating PSS ‘shell’ that surrounds the conducting PEDOT–PSS grains is reduced. The (partial) reduction of the excess PSS layer that surrounds the conducting PEDOT–PSS grains is proposed to lead to an increased and improved connectivity between such grains in the film and hence a higher conductivity. When PEDOT–PSS–solvents receives a post-coating heat treatment, the increased PEDOT-to-PSS ratio at the surface is maintained or even slightly improved, as is the increase in electrical conductivity, even though spectroscopy show that the solvent molecules are removed. This suggests that screening or doping by the solvents throughout the film are not likely to be the key mechanisms for the improved conductivity and supports our proposed mechanism of improved conductivity through improved connectivity between the conducting grains.
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| 12. |
- Marciniak Braun, Slawomir, et al.
(författare)
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Light Induced Damage in Poly(3,4-ethylenedioxythiophene) and its Derivatives Studied by Photoelectron Spectroscopy
- 2004
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 141:1-2, s. 67-73
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Tidskriftsartikel (refereegranskat)abstract
- Poly(3,4-ethylenedioxythiophene), usually known as PEDOT, and derivatives have attracted significant interest because of their high electrical conductivity. This electric property, however, deteriorates upon exposure to solar radiation. X-ray photoelectron spectroscopy (XPS) has been used to study the UV-light-induced chemical changes in doped PEDOT, as well as in both neutral and doped forms of its alkylated derivative—PEDOT-C14H29. Analysis of the XPS data indicates an oxidation of the sulfur in the thiophene ring. Apparently, photo-oxidation leads to the formation of sulfon groups, SO2, resulting in a disruption of π-conjugation in PEDOT, which there by diminishes the conductivity of the organic layer. This hypothesis is supported by the results of a study of model molecules for pristine and the oxidized PEDOT unit: 3,4 ethylenedioxythiophene (EDOT) and 3,4 ethylenedioxythiophene and S-dioxide (EDOT-SO2), respectively.
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| 13. |
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| 14. |
- Van, Der Gon A.W.D., et al.
(författare)
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Modification of PEDOT-PSS by low-energy electrons
- 2002
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Ingår i: Organic electronics. - 1566-1199 .- 1878-5530. ; 3:3-4, s. 111-118
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Tidskriftsartikel (refereegranskat)abstract
- The stability of conjugated organic materials under electron transport is of great importance for the lifetime of devices such as polymer light-emitting diodes (PLEDs). Here, the modification of thin films of poly(3,4- ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate) (known as PEDOT-PSS, often used in the fabrication of PLEDs) by low-energy electrons has been studied using X-ray photo-electron spectroscopy. Thin films of PSSH and molecular solid films of EDOT molecules also have been studied. We find that electrons with kinetic energies as low as 3 eV result in significant modification of the chemical structure of the materials. For thin films of PSSH, the electron bombardment leads to a strong loss of oxygen and a smaller loss of sulfur. In addition, a large amount of the sulfur atoms that remain in the films exhibits a different binding energy because of scissions of the bonds to oxygen atoms. For condensed molecular solid films of EDOT molecules, we find that the carbon atoms bonded to oxygen react and form additional bonds, as evidenced by a new component in the C(1s) peak at a higher binding energy. In the PEDOT-PSS blend, we find both effects. The importance of these observations for light-emitting diodes incorporating PEDOT-PSS films is discussed. This work demonstrates that the combination of in situ low-energy electron bombardment in combination with photo-electron spectroscopy is a powerful method to simulate and study certain processes, associated with low-energy electrons, occurring in organic based devices, which cannot be studied directly otherwise. © 2002 Elsevier Science B.V. All rights reserved.
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| 15. |
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