| 1. |
- Cacialli, F., et al.
(författare)
-
Synthesis and characterisation of poly(distyrylbenzene-block-hexa(ethylene oxide)) and its fluorinated analogue - Two new block copolymers and their application in electroluminescent devices
- 2002
-
Ingår i: Polymer. - 0032-3861 .- 1873-2291. ; 43:12, s. 3555-3561
-
Tidskriftsartikel (refereegranskat)abstract
- Two new soluble block copolymers are reported in which chromophores and hexa(ethylene oxide) units alternate along the polymer backbone. In polymer 1 the chromophore was the distyrylbenzene unit. The polymer was synthesised via the Wittig reaction and the ionization potential of 5.4 ± 0.2 eV was measured by cyclic voltammetry and photoelectron spectroscopy. Polymer 1 showed a high solid-state photoluminescence efficiency (34%) and was used to make efficient (0.5 cd/A) light emitting diodes (LEDs). Polymer 1 was also used in light emitting cells, these showed luminescence in reverse bias and a reduced turn-on voltage compared to the LEDs. Polymer 2, in which the chromophore was dodecafluoro-distryrylbenzene, was prepared via the Horner-Wittig reaction and showed an ionization potential of 6.25 ± 0.15 eV and a solid-state photoluminescence efficiency of 17%. It was used as electron-conducting layer in a LED but failed to give significant electroluminescence. The optical energy gap for both polymers was 3.0 eV. © 2002 Published by Elsevier Science Ltd.
|
|
| 2. |
- Crispin, Xavier, et al.
(författare)
-
Characterization of the interface dipole at organic/metal interfaces
- 2002
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:27, s. 8131-8141
-
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.
|
|
| 3. |
- Crispin, Xavier, 1972-, et al.
(författare)
-
Electronic delocalization in discotic liquid crystals : A joint experimental and theoretical study
- 2004
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:38, s. 11889-11899
-
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.
|
|
| 4. |
- Crispin, Xavier, et al.
(författare)
-
Understanding the initial stages of polymer grafting on metals : A photoelectron spectroscopy study of acrylonitrile adsorption on transition metal surfaces
- 2001
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - 0368-2048 .- 1873-2526. ; 121:1-3, s. 57-74
-
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.
|
|
| 5. |
- Kim, J.S., et al.
(författare)
-
Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: : a comparison
- 2000
-
Ingår i: Synthetic metals. - 0379-6779 .- 1879-3290. ; 111-112, s. 311-314
-
Tidskriftsartikel (refereegranskat)abstract
- We report a comparison of the work functions of thin films of indium tin oxide (ITO), carried out by means of ultraviolet photoelectron spectroscopy (UPS) and by measurements of the contact potential difference with respect to a gold reference electrode (Kelvin probe (KP) method). We investigated commercially available ITOs both "as-received", and after certain surface treatments, such as oxygen plasma. First, we find measurable discrepancies between KP values measured with three different instruments, and between the KP and the UPS values. Secondly, and unexpectedly, we find that the KP, although more sensitive than UPS, does not detect certain differences between ITOs with different surface treatments. We discuss the results in view of the different environments in which the measurements are carried out (UHV for the UPS and air/Ar for the Kelvin method), of the effects which may be induced by the high-energy photon irradiation in the UPS measurement, and of the stability of the gold probe work function in gas ambient. We conclude that UPS is better-suited for absolute work function determination, although KP remains a convenient and inexpensive tool for fast screening of contact potential differences. (C) 2000 Elsevier Science S.A. All rights reserved.
|
|
| 6. |
- Minkov, Ivaylo, et al.
(författare)
-
Core-excitations of naphthalene : Vibrational structure versus chemical shifts
- 2004
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 121:12, s. 5733-5739
-
Tidskriftsartikel (refereegranskat)abstract
- The initial state chemical shifts and vibrational fine structure of core excitations of naphthalene were analyzed using high-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra. The carbon atoms at peripheral sites were found to experience a small chemical shift and exhibit similar charge-vibrational coupling. The C-H stretching modes provide significant contributions to overall shape of spectra in the XPS spectra. The results show that vibrational fine structure dominates by particular C-C stretching modes, and in XPS of C2 and C3 sites also by high-energy C-H stretching modes.
|
|
| 7. |
- Osikowicz, Wojciech, et al.
(författare)
-
A joint theoretical and experimental study on the electronic properties of phenyl-capped 3,4-ethylenedioxythiophene oligomers
- 2003
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 119:19, s. 10415-10420
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of a series of phenyl-capped EDOT oligomers was studied using ultraviolet photoelectron spectroscopy, in combination with quantum-chemical methods. The bulk IP of the neutral PEDOT polymer was estimated to be 4.2 eV. The frontier band structue was predicted from the evolution of the spectral features in the studied series of oligomers.
|
|
| 8. |
- Osikowicz, Wojciech, et al.
(författare)
-
Electronic structure of a novel alkylidene fluorene polymer in the pristine state
- 2004
-
Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 385:3-4, s. 184-188
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of a novel conjugated polymer, polyalkylidene fluorene has been studied using a combined experimental-theoretical approach. The densities of states in the valence band region of the new derivative, poly(9-(1'-decylundecylidene)fluorene), were measured by ultraviolet photoelectron spectroscopy and compared with electronic band-structure calculations performed in the valence effective Hamiltonian framework. The results are compared with those of similar studies on the reference polymer poly(9,9-dioctylfluorene). We report the experimentally determined ionization potential for this new material and discuss the role of substitution in altering the electronic properties of the polymer backbone. © 2003 Elsevier B.V. All rights reserved.
|
|
| 9. |
- Birgerson, J., et al.
(författare)
-
A study of the electronic structure of ethylenedioxythiophene in gas phase using NEXAFS and quantum chemical calculations
- 2004
-
Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 392:1-3, s. 100-104
-
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.
|
|
| 10. |
- Birgerson, J., et al.
(författare)
-
Doped polymeric cathodes for PPV/Al based LEDs
- 2002
-
Ingår i: Synthetic metals. - 0379-6779 .- 1879-3290. ; 132:1, s. 57-61
-
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.
|
|
