J. Birgerson

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

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

Conjugated polymer surfaces and interfaces: a mini-review and some new results

Abstract Since the discovery of high electrical conductivity in doped polyacetylene in 1977, conjugated polymers have emerged as viable semiconducting electronic materials for numerous applications. In the context of polymer electronic devices, it is of critical importance to understand the nature of the electronic structure of the polymer surface and the interface with metals. It has been shown that, especially for conjugated polymers, photoelectron spectroscopy provides a maximum amount of both chemical and electronic structural information in one (type of) measurement. An overview of some details of the early stages of interface formation with metals on the surfaces of conjugated polymers and model molecular solids, especially in connection with polymer-based light-emitting diode (LED) devices, was presented at the symposium for Alan Heeger. Materials involved include poly (p-phenylenevinylene) (PPV), as well as a series of substituted PPVs, and a diphenylpolyene molecule, namely α,ω-diphenyltetradecaheptaene. Some general trends in the behavior of light-metal atoms on the clean surfaces of conjugated polymers was pointed out. Here, the core details of the oral presentation will be outlined. In addition, some new results presented are discussed, briefly, in order to indicate some recent new developments in this area. Finally, some details of one specific new issue are reviewed in slightly more detail. The choice of new results presented is influenced by recent publications by Alan Heeger and friends at Santa Barbara.

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.

Magnesium as electrode in polymer LEDs

Abstract Qualitative studies of the effect of the presence of oxygen during the vapor deposition of magnesium electrodes in the fabrication of single-layer polymer light-emitting diodes (poly-LEDs) in ultrahigh vacuum (UHV) have been carried out. Using a figure-of-merit based upon yield, intensity and lifetime, the best devices are obtained when fabricated in an essentially oxygen-free environment. The formation of oxides during the magnesium deposition contributes negatively to the charge injection characteristics and, therefore, leads to poor poly-LEDs. These results, that the poly-LED devices with magnesium as the negative electrode have the highest performance when fabricated in UHV, are in contrast to the case of calcium, where fabrication of electrodes is optimum in the presence of about 10 −6 mbar partial pressure of oxygen, as reported previously.

The interface surfaces of a CN-substituted poly(phenylenevinylene) light-emitting diode, a morphological study

Abstract The morphology of the polymer/electrode interfaces within polymer light-emitting diodes with a poly(cyanoterephthalylidene) polymer as the active layer, poly (2,5,2′,5′-hexyloxy-8,7′-dicyano-di- para -phenylenevinylene) (CN-PPV), was studied by scanning force microscopy (SFM). The structure and roughness of (i) the substrates (indium-tin oxide (ITO)), (ii) the CN-PPV films spin-coated from different solvents and concentrations, and (iii) the top electrodes of vapour-deposited aluminium were studied. Film thicknesses were determined by ellipsometry. CN-PPV films were also studied on smooth silicon substrates, to reduce the influence of substrate roughness on film morphology, and for comparison of SFM and ellipsometry results on ITO.

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 leas...

Electronic structure of some conjugated polymers for electron transport

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 π-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.

Doped polymeric cathodes for PPV/Al based LEDs

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-ethylenedioxythio ...