| 1. |
- Crispin, Annica, et al.
(författare)
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Influence of dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors
- 2002
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:18, s. 8159-8167
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors was studied by means of photoelectron spectroscopy. With lithium atoms as dopants, two charges were stored on the same spiro branch in the form of bipolarons, for spiro-quarterphenyl and spiro-sexiphenyl. For doping with the sodium atoms, the size of the counter ions made it less energetically desirable to store two charges onto a single branch, and the charged species were polarons independent of the level of doping which was confirmed by optical absorption data.
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| 2. |
- Crispin, Annica, 1972-, et al.
(författare)
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Transition between energy level alignment regimes at a low band gap polymer-electrode interfaces
- 2006
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 89:21
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Tidskriftsartikel (refereegranskat)abstract
- The energy level alignment at interfaces between a low band gap conjugated polymer and various electrodes is investigated using ultraviolet photoemission spectroscopy. When the electrode work function is lower (higher) than the negative (positive) polaronic level of the polymer, the Fermi level is pinned to the negative (positive) polaronic level. These Fermi level pinning regimes suggest a spontaneous electron transfer from or towards the electrode resulting in an interfacial dipole of different orientation. On the contrary, when the substrate work function is intermediate, there is no charge transfer and the energy level alignment across the interface follows the Schottky-Mott limit. © 2006 American Institute of Physics.
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| 3. |
- Crispin, Xavier, et al.
(författare)
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Characterization of the interface dipole at organic/metal interfaces
- 2002
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:27, s. 8131-8141
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Tidskriftsartikel (refereegranskat)abstract
- In organics-based (opto)electronic devices, the interface dipoles formed at the organic/metal interfaces play a key role in determining the barrier for charge (hole or electron) injection between the metal electrodes and the active organic layers. The origin of this dipole is rationalized here from the results of a joint experimental and theoretical study based on the interaction between acrylonitrile, a p-conjugated molecule, and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces is investigated experimentally by photoelectron spectroscopies, while quantum mechanical methods based on density functional theory are used to study the systems theoretically. It appears that the interface dipole formed at an organic/metal interface can be divided into two contributions: (i) the first corresponds to the "chemical" dipole induced by a partial charge transfer between the organic layers and the metal upon chemisorption of the organic molecules on the metal surface, and (ii) the second relates to the change in metal surface dipole because of the modification of the metal electron density tail that is induced by the presence of the adsorbed organic molecules. Our analysis shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chemical potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the organic layer.
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| 4. |
- Crispin, Xavier, 1972-, et al.
(författare)
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Electronic delocalization in discotic liquid crystals : A joint experimental and theoretical study
- 2004
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:38, s. 11889-11899
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Tidskriftsartikel (refereegranskat)abstract
- Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2′,3′-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper π-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the π-electrons along the discotic stacks.
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| 5. |
- Crispin, Xavier, 1972-, et al.
(författare)
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The origin of the high conductivity of poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) (PEDOT-PSS) plastic electrodes
- 2006
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 18:18, s. 4354-4360
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Tidskriftsartikel (refereegranskat)abstract
- The development of printed and flexible (opto)electronics requires specific materials for the device's electrodes. Those materials must satisfy a combination of properties. They must be electrically conducting, transparent, printable, and flexible. The conducting polymer poly(3,4-ethylenedioxythiophene) - poly-(styrenesulfonate) (PEDOT-PSS) is known as a promising candidate. Its conductivity can be increased by 3 orders of magnitude by the secondary dopant diethylene glycol (DEG). This "secondary doping" phenomenon is clarified in a combined photoelectron spectroscopy and scanning probe microscopy investigation. PEDOT-PSS appears to form a three-dimensional conducting network explaining the improvement of its electrical property upon addition of DEG. Polymer light emitting diodes are successfully fabricated using the transparent plastic PEDOT-PSS electrodes instead of the traditionally used indium tin oxide. © 2006 American Chemical Society.
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| 6. |
- Crispin, Xavier, et al.
(författare)
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Understanding the initial stages of polymer grafting on metals : A photoelectron spectroscopy study of acrylonitrile adsorption on transition metal surfaces
- 2001
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - 0368-2048 .- 1873-2526. ; 121:1-3, s. 57-74
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Tidskriftsartikel (refereegranskat)abstract
- X-ray and UV photoelectron spectroscopies show that acrylonitrile is chemisorbed on iron, nickel and copper polycrystalline surfaces via the carbon and nitrogen atoms. Depending on the conditions used, different adsorption geometries are found. The molecules can either be adsorbed flat on the surface and chemically bound by a (2pp)-(3d/4s) overlap via both the C=C double bond and the C=N nitrile group or they can be adsorbed perpendicular to the surface via a covalent interaction between the nitrogen lone pair and the 3d-4s levels of the metals. Analysis of the XPS data obtained on molecular mono-layers chemisorbed on metal surfaces emphasizes the importance of initial-state effects (charge transfer upon chemisorption, contribution of the metal surface dipole) and final-state effects (metal screening and polarization effect within the mono-layer). The correlation between the XPS and UPS data illustrates the importance of the metal surface dipole in understanding the workfunction changes upon molecular adsorption on metal surfaces. © 2001 Elsevier Science B.V. All rights reserved.
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| 7. |
- Fahlman, Mats, et al.
(författare)
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Electronic structure of hybrid interfaces for polymer-based electronics
- 2007
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:18
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Forskningsöversikt (refereegranskat)abstract
- The fundamentals of the energy level alignment at anode and cathode electrodes in organic electronics are described. We focus on two different models that treat weakly interacting organic/metal (and organic/organic) interfaces: the induced density of interfacial states model and the so-called integer charge transfer model. The two models are compared and evaluated, mainly using photoelectron spectroscopy data of the energy level alignment of conjugated polymers and molecules at various organic/metal and organic/organic interfaces. We show that two different alignment regimes are generally observed: (i) vacuum level alignment, which corresponds to the lack of vacuum level offsets (Schottky-Mott limit) and hence the lack of charge transfer across the interface, and (ii) Fermi level pinning where the resulting work function of an organic/metal and organic/organic bilayer is independent of the substrate work function and an interface dipole is formed due to charge transfer across the interface. We argue that the experimental results are best described by the integer charge transfer model which predicts the vacuum level alignment when the substrate work function is above the positive charge transfer level and below the negative charge transfer level of the conjugated material. The model further predicts Fermi level pinning to the positive (negative) charge transfer level when the substrate work function is below (above) the positive (negative) charge transfer level. The nature of the integer charge transfer levels depend on the materials system: for conjugated large molecules and polymers, the integer charge transfer states are polarons or bipolarons, for small molecules' highest occupied and lowest unoccupied molecular orbitals and for crystalline systems, the relevant levels are the valence and conduction band edges. Finally, limits and further improvements to the integer charge transfer model are discussed as well as the impact on device design. © IOP Publishing Ltd.
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| 8. |
- Friedlein, Rainer, et al.
(författare)
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Electronic structure of conjugated polymers and interfaces in polymer-based electronics
- 2003
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Ingår i: Synthetic Metals (Proceedings of the International Conference on Science and Technology of Synthetic Metals). - 0379-6779. ; 135:1-3, s. 275-277
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Konferensbidrag (refereegranskat)abstract
- The electronic structure of conjugated polymers and interfaces in polymer-based electronics were analyzed. Fine structure were observed in the region of the first resonance with pi-final state symmetry, between 284.1 eV and 285.8 eV. The electronic transitions from the non-dispersed C(1s) level to specific parts of the unoccupied band structure were generated. It was found that for a dispersing valence band, in the presence of a core-hole, a given photon energy corresponded to an excitation into a state with a distinct wave vectors.
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| 9. |
- Crispin, Annica, et al.
(författare)
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Aluminum-barium interfaces on some processable poly(p-phenylene vinylene) polymers studied by photoelectron spectroscopy
- 2001
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 115:11, s. 5252-5257
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Tidskriftsartikel (refereegranskat)abstract
- The control of the deposition of metals on the surface of conjugated polymers determined the electronic properties of the electrical contacts. X ray photoelectron spectroscopy (XPS) and ultraviolet photon spectroscopy (UPS) was used to study the influence of an intermediate layer of barium atoms on the chemical effects that occured during the deposition of aluminium atoms of substituted poly(p-phenylvinylene) polymers. The thin films were prepared on gold substrates for spectroscopy by spin coating techniques. The diffusion of the aluminium atoms occured when the side grops were bulky and therby resulted in porous polymer films.
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| 10. |
- Crispin, Annica, 1972-
(författare)
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Chemical and electronic structure of interfaces in organic-based devices
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with some of the main physical and chemical phenomena involving organic semiconductors in (opto )electronic devices: (i) the fabrication and characteristics of organic light emitting diodes (OLEDs), (ii) the chemistry occurring at metal-polymer and polymer-metal interfaces, (iii) the nature of electronic levels induced upon doping in molecular organic semiconductors.In the first paper, fluorine tin oxide (FTO) electrodes have been studied as an alternative to indium tin oxide (ITO) for hole-injection in polymer-based light emitting devices. There appear to be several advantages in using FTO instead of ITO electrodes: (i) FTO is less expensive, (ii) there is a lower sensitivity to surface cleaning methods, and, most importantly, (iii) more light is obtained for a given voltage. This study points out that the work-function of the hole injecting electrode is not the only factor in determining hole injecting characteristics at the corresponding electrode-interface.In Paper II-IV, we show that chemical reactions occur at the metal-polymer and polymer-metal interfaces, which is expected to affect the device performance.In Paper II, chemical reactions at the interface between a precursor polymer for poly(p-phenylenevinylene) and ITO are identified using X-ray photoelectron spectroscopy (XPS). The HCl eliminated in the conversion process reacts with the surface of the ITO substrate leading to the formation of indium chloride, which then diffuses into the polymer.XPS is used to study chemical reactions at a polymer ITO interface under 100 A polymer (Paper III). This has been possible by partially replacing indium ions by gallium ions. The immediate formation of GaCl3 at the ITO/precursor- PPV interface can be seen.In OLEDs, the cathode is usually deposited on the conjugated polyme layer. In Paper IV we show that the detrimental effects arising from the physical vapor deposition of metal atoms on conjugated polymer surfaces can be controlled by using a ultra-thin protection layer of barium.For some organic semiconductor applications, as for inorganic devices, the charge mobility within the active material must be enhanced in order to achieve better devices efficiency. This can be done typically by doping the semiconductors. Paper V deals with doping of molecular organic semiconductors, composed of oligophenyls involved in propeller-shaped spiro-type molecules. The nature of electronic levels induced upon chemical doping is investigated by a joint theoretical and experimental study using UV-vis absorption spectroscopy and photoelectron spectroscopy. With lithium atoms as the dopant, bipolarons are formed, while sodium atoms give rise to polarons independent of the level of doping.
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| 11. |
- 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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| 12. |
- Tengstedt, Carl, 1974-, et al.
(författare)
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Study and comparison of conducting polymer hole injection layers in light emitting devices
- 2005
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Ingår i: Organic electronics. - : Elsevier BV. - 1566-1199 .- 1878-5530. ; 6:1, s. 21-33
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Tidskriftsartikel (refereegranskat)abstract
- A set of polyaniline- and poly(3,4-ethylene dioxythiophene)-based materials were studied as hole injection layers in polymer light emitting devices. The choice of polymeric counterion/dopant poly(styrenesulfonic acid), and poly(acrylamido-2-methyl-1-propanesulfonic acid), and poly(acrylamide) blended with polyaniline/poly(acrylamido-2-methyl-1-propanesulfonic acid) was found to influence both work function and film morphology, which in turn affects device performance. The work functions of the polymer films spanned the range of over 1 eV and the surface region of the films were found to be low in conducting polymer content compared to the bulk. This was particularly the case of the polyaniline/poly(acrylamido-2-methyl-1-propanesulfonic acid) blended with poly(acrylamide) which showed device efficiency equal to that of the poly(3,4-ethylene dioxythiophene)–poly(styrenesulfonic acid) reference. The turn on voltage, however, was significantly larger, likely due to the insulating poly(acrylamide)-rich surface region of the polyaniline/poly(acrylamido-2-methyl-1-propanesulfonic acid)/poly(acrylamide) film. The polymer blend of polyaniline/poly(styrenesulfonic acid) yielded the highest work function (5.5 ± 0.1 eV).
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