P. Bröms

Polymer-based light-emitting devices: investigations on the role of the indium—tin oxide (ITO) electrode

Abstract We have studied the cleaning procedure dependence of the chemical composition and work function for different indium—tin oxide (ITO) samples using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Also, the surface morphology of ITO was investigated by atomic force microscopy (AFM). Despite the pronounced differences in surface morphology and the In Sn ratio, the variation of the work function between different ITO samples was very small after each cleaning procedure. The work functions of ITO samples cleaned with organic solvents and hydrogen peroxide were 4.4–4.5 and 4.7–4.8 eV, respectively. Ne-ion sputtering preferentially removed oxygen, which resulted in a partial reduction of the surface and a lowering of the work function to 4.0–4.1 eV. Used as hole-injecting electrodes in organic and polymer light-emitting devices (LEDs), different ITOs resulted in pronounced differences in the LED performances despite their almost identical work functions after the hydrogen peroxide treatment. This reveals that the work function of the ITO is not the only factor to determine the hole-injection characteristics in polymer-based LEDs.

Calcium electrodes in polymer LEDs

Abstract The qualitative results of a study of the fabrication of Ca/CN-PPV/ITO polymer light-emitting diodes (LEDs) in ultrahigh vacuum (UHV) are reported, where CN-PPV is a cyano-substituted poly(p-phenylenevinylene). If the calcium electrode is deposited on a clean polymer surface under UHV conditions, the device yield is very low, and, for the devices that do function, the electroluminescence lifetimes are extremely short. If the calcium electrode is deposited in the presence of a high partial pressure of oxygen, the diodes are much better in terms of yield, lifetime and luminescence.

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.

Electronic structure of pristine and sodium‐doped cyano‐substituted poly(2,5‐dihexyloxy‐p‐phenylenevinylene): A combined experimental and theoretical study

The electronic structure of cyano‐substituted poly(2,5‐dihexyloxy‐p‐phenylene‐vinylene), or CN‐PPV, has been studied in both pristine and doped states. Ultraviolet photoelectron spectroscopy (UPS) and x‐ray photoelectron spectroscopy (XPS), as well as optical absorption spectroscopy have been carried out under ultrahigh vacuum (UHV) conditions, and the results have been interpreted with the help of quantum‐chemical calculations. For the pristine polymer, the addition of cyano groups to the vinylene units does not affect the width of the π‐bands closest to the Fermi level; however, the positions of the flat parts of the upper π‐bands are shifted by approximately 0.4 eV towards higher binding energies relative to the Fermi energy, as compared with the corresponding bands of other alkoxy‐substituted poly(p‐phenylenevinylene)s. On the other hand, there are only marginal differences in the optical absorption spectra; the interband absorption onset is comparable to the values for alkoxy‐substituted poly(p‐pheny...

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.

Solid state amplified spontaneous emission in some spiro-type molecules: A new concept for the design of solid state lasing molecules

Abstract Here are presented initial results on spectral narrowing in submicron thick neat films, of 2,2′,7,7′-tetraphenyl-9,9′-spirobifluorene (spiro-quarterphenyl) and 2,2′,7,7′-tetrakis(biphenyl-4-yl)-9,9-spirobiflourene (spiro-sexiphenyl). The luminescence effects reported here are common to several other molecules which belong the class of molecules known as the spiro-type molecules. The Amplified Spontaneous Emission (ASE) is evidenced by a dramatic collapse of the line width, when pumped with sufficient energy, down to 2.2 and 2.9 nm for spiro-quarterphenyl and spiro-sexiphenyl, respectively. These molecules have the ability to form stable, amorphous films with glass transition temperatures at around 200 °C. Thus, these molucules are suitable candidates as the active material in molecular solid state injection lasers.

XPS and UPS study of charge transport material/electrode interface of light emitting diodes.

The electronic structure of the interfacial region between tris(8-hydroxy-quinoline) aluminum (Alq) and alkali metal (Li and K) was studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). Condensed molecular solid films of Alq were doped by alkali metal deposited on the surface. The alkali metal atoms were found to distribute uniformly through out the depth to which the sample could be studied. A new feature of the density of states was observed in the band gap, 1.7eV higher than the HOMO of the pristine Alq. The binding energy of N(1s) core level of the quinoline ligand of Alq was shifted by doping to lower binding energies, of 1.6 eV and 1.8 eV from the original peak for Li and K doping, respectively. The intensity of the new shifted peak was saturated when Alq was fully doped. The full doping level, determined by chemical analysis (XPS), was Alq:metal=1:3.

The electronic structure of neutral and alkali metal-doped poly[3-(4-octylphenyl)thiophene] studied by photoelectron spectroscopy

The electronic structure of poly [3-(4-octylphenyl)thiophene] (POPT) has been studied by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), as well as by quantum chemical calculations. Both temperature-dependent effects on the electronic structure of the neutral system, as well as the generation of new electronic states induced by doping with alkaline metals, have been observed. The experimental results are in good agreement with the results of the quantum chemical calculations.

Optical absorption studies of sodium doped poly(cyanoterephthalylidene)

The effects of doping poly(cyanoterephthalylidene) with sodium in ultrahighvacuum been studied by optical absorption spectroscopy. Upon doping, new optical transitions are observed within the bandgap; the characteristics of these transitions are consistent with the formation of bipolarons. The optical absorption results are confirmed by direct measurements of the doping-induced gap states using ultraviolet photoelectron spectroscopy.