| 1. |
- de Jong, Michel P, 1970-, et al.
(author)
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Orbital-specific dynamic charge transfer from Fe(II)-tetraphenylporphyrin molecules to molybdenum disulfide substrates
- 2005
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:3, s. 35448-
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Journal article (peer-reviewed)abstract
- Orbital-specific femtosecond charge transfer dynamics between Fe(II)-tetraphenylporphyrin molecules and semimetallic molybdenum disulfide substrates is investigated using core-level resonant photoemission spectroscopy. The electronic coupling to the substrate and the efficiency of charge transport across the interface is found to be different for the individual molecular electronic subsystems. In particular, electrons excited at the phenyl substituents are transferred within 3–6 fs, while hopping from the porphyrin ring is slower than 30 fs.
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| 2. |
- Osikowicz, Wojciech, 1974-, et al.
(author)
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Energetics at Au top and bottom contacts on conjugated polymers
- 2006
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:19
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Journal article (peer-reviewed)abstract
- Photoelectron spectroscopy was employed to examine the energetics, and therefore charge injection barriers, at top and bottom contact configurations of gold and conjugated polymers, i.e., polymer spin coated on gold and vapor-deposited gold on polymer interfaces. Very similar results are obtained for both ex situ (contaminated) and in situ (clean) prepared interfaces: a 0.7-0.8 eV decrease in the vacuum energy levels is consistently observed as compared to bare polycrystalline gold. These observations are explained by changes of the metal work function upon contacting either polymers or contaminants, associated with the reduction of the electron density tail that extends outside the metal surface. © 2006 American Institute of Physics.
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| 3. |
- Tengstedt, Carl, 1974-, et al.
(author)
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Fermi-level pinning at conjugated polymer interfaces
- 2006
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:5, s. 53502-
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Journal article (peer-reviewed)abstract
- Photoelectron spectroscopy has been used to map out energy level alignment of conjugated polymers at various organic-organic and hybrid interfaces. Specifically, we have investigated the hole-injection interface between the substrate and light-emitting polymer. Two different alignment regimes have been observed: (i) Vacuum-level alignment, which corresponds to the lack of vacuum-level offsets (Schottky–Mott limit) and (ii) Fermi-level pinning, where the substrate Fermi level and the positive polaronic level of the polymer align. The observation is rationalized in terms of spontaneous charge transfer whenever the substrate Fermi level exceeds the positive polaron/bipolaron formation energy per particle. The charge transfer leads to the formation of an interfacial dipole, as large as 2.1 eV.
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| 4. |
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| 5. |
- Tengstedt, Carl, 1974-
(author)
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Materials study of organic electronics
- 2005
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Doctoral thesis (other academic/artistic)abstract
- The electronic structure of materials for use in organic electronics is studied in this thesis. The first part includes applied research in the form of studies of polymers for use in polymer light emitting devices. The second part is more directed toward organic based spintronics and contains research regarding a room temperature organic ferrimagnetic material. Common for the studies, apart from that all regard organic material, are the use of electron spectroscopy techniques. The studies give new spectroscopic evidence of how the energy level alignment occurs between electrical conductors and spin coated semiconducting polymers, i.e. alignment at the anode side of polymer light emitting devices. The studies prove theoretical predictions regarding spontaneous charge injection forming positive polaronic species in the semiconducting polymer, pinned to the Fermi level of the substrate. The first part also includes studies of novel conducting polymers, based on PEDOT and polyaniline, with work functions spanning from 4.2 eV to 6.4 eV. In the case of organic magnets, our design and construction of the purpose built vacuum deposition system allowed for the first time oxygen free films of the extremely reactive organic ferrimagnet, vanadium tetracyanoethylene (V(TCNE)x), to be studied by means of several different electron spectroscopies, proving or disproving several previous results and assumptions.
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| 6. |
- Tengstedt, Carl, 1974-
(author)
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Physics of materials in organic electronics
- 2004
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Licentiate thesis (other academic/artistic)abstract
- The research included in this thesis is regarding materials physics in organic electronics. The thesis consists of two projects: one applied and one basic science in nature. The applied project concerns modification of charge injection properties in organic electronics. The choice of the materials used as anodes and cathodes in polymer light emitting devices, PLEDs, plays a crucial role in device performance. The electrodes, often multi-component/layer systems, need to be chosen in such a way that that they fit the properties of the active material. Here, the electronic, chemical and physical properties of materials for so-called hole transporting layers (HTL) were studied and used to explain and improve the performance of polymer-based light emitting diodes (PLED). The study focused on the properties most important for devices, e.g. film morphology, work function and chemical composition, and the results were compared to PLED 1-V characteristics and luminescence efficiency. The second project aimed to provide basic understanding of issues concerning charge confinement (and charge injection) in small molecules. The n-doping of an component of an organic-based magnetic semiconductor was studied as a first step towards understanding the electronic and magnetic properties of the actual magnetic material.
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| 7. |
- Tengstedt, Carl, 1974-, et al.
(author)
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Study and comparison of conducting polymer hole injection layers in light emitting devices
- 2005
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In: Organic electronics. - : Elsevier BV. - 1566-1199 .- 1878-5530. ; 6:1, s. 21-33
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Journal article (peer-reviewed)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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| 8. |
- Tengstedt, Carl, 1974-, et al.
(author)
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Ultraviolet light-ozone treatment of poly(3,4-ethylenedioxy-thiophene)-based materials resulting in increased work functions
- 2006
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 515:4, s. 2085-2090
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Journal article (peer-reviewed)abstract
- We describe a simple method to increase the work function of poly(3,4-ethylenedioxy-thiophene)-poly(perfluoroethylene sulfonic acid), PEDOT-PFESA, and poly(3,4-ethylenedioxy-thiophene)-poly(styrene sulfonic acid), PEDOT-PSS, by short exposure to ultraviolet light and ozone. The creation of carbonyl groups in the surface region forms a dipole layer shifting the vacuum level with a followed increase in work function. It has been shown that the work function of PEDOT-PFESA can be increased by as much as ∼ 0.4 eV to the absolute value of 6.3 eV and by at least ∼ 0.2 eV for PEDOT-PSS to the absolute value of 5.4 eV. The increase in work function has also proven to be time dependent with the largest increasing rate occurring for short exposure times. Upon ozone treatment, both PEDOT and PSS are oxidized whereas PFESA seems to be unaffected.
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