Charge transport and trapping in Cs-doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes

Al/Cs/MDMO-PPV/ITO (where MDMO-PPV stands for poly[2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4phenylene vinylene] and ITO is indium tin oxide) light-emitting diode (LED) structures, made by physical vapor deposition of Cs on the emissive polymer layer, have been characterized by electroluminescence, current-voltage, and admittance spectroscopy. Deposition of Cs is found to improve the balance between electron and hole currents, enhancing the external electroluminescence efficiency from 0.01 cd A-1 for the bare Al cathode to a maximum of 1.3 cd A-1 for a Cs coverage of only 1.5×1014 atoms/cm2. By combining I-V and admittance spectra with model calculations, in which Cs diffusion profiles are explicitly taken into account, this effect could be attributed to a potential drop at the cathode interface due to a Cs-induced electron donor level 0.61 eV below the lowest unoccupied molecular orbital. In addition, the admittance spectra in the hole-dominated regime are shown to result from space-charge-limited conduction combined with charge relaxation in trap levels. This description allows us to directly determine the carrier mobility, even in the presence of traps. In contrast to recent literature, we demonstrate that there is no need to include dispersive transport in the description of the carrier mobility to explain the excess capacitance that is typically observed in admittance spectra of p-conjugated materials

Modification of PEDOT-PSS by low-energy electrons

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

Inkjet printing of well-adapted PEDOT-PSS dispersions

Poly (3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) is a popular conductive polymer that finds widespread use in the fields of polymeric electronics and display applications. Its major advantages lie in the suitability for flexible electrical devices and the relatively simple deposition process capabilities with techniques such as inkjet printing. However, the deposition of PEDOT-PSS dispersions with inkjet printheads can cause a number of problems due to possible interaction of the fluids with the printhead, and due to poor inkjet functionality and layer formation on the substrate. In the present work we have adapted PEDOT-PSS dispersions for improved performance in a Xaar-type piezoelectric inkjet printhead, in particular for high quality drop formation and for reduced corrosion of the inkjet printhead. At the same time the best PEDOT-PSS inks regarding layer formation and conductivity were identified. As an application example, a passive LCD device incorporating an inkjet printed PEDOT-PSS electrode pattern that was produced with an adapted PEDOT-PSS ink is presented

Traitement morphinique au long cours et hyperalgésie post-opératoire (étude de la consommation morphinique après vertébroplastie et évaluation de l'effet du protoxyde d'azote)

BORDEAUX2-BU Santé (330632101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

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

The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

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

High levels of alkali-metal storage in thin films of hexa-peri- hexabenzocoronene

The affects of alkali-metal atoms on the electronic structure of disordered and highly ordered thin films of the medium-size aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC) have been investigated by valence and core level photoelectron spectroscopies-ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS)-and accompanying quantum-chemical calculations. Deposition of Li or Na atoms in situ leads to new spectral features in the UPS spectra, which are related to formerly unoccupied molecular states. The binding energies and intensities of these features depend on the nature of the counterion. The smaller Li ion exhibits a stronger influence on the electronic structure than its sodium counterpart. In the intercalation of sodium into ordered films, a high degree of molecular order is preserved, and, at high deposition levels, a surface dipole is formed that is associated with the layered structure of the compound. Remarkably, high levels of alkali-metal storage of at least one alkali-metal atom for each four carbon atoms have been observed, indicating clearly the potential use of these graphene materials in lithium-ion batteries with a high charge- storage capacity. (C) 2002 American Institute of Physics

Role of electronic localization and charge-vibrational coupling in resonant photoelectron spectra of polymers: Application to poly(para-phenylenevinylene)

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