J. Gmeiner

Tailoring of the electrical and optical properties of poly (p-phenylene vinylene)

The external quantum efficiency of monolayer light-emitting diodes (LEDs) prepared by poly(p-phenylene vinylene) (PPV) is usually in the range of 0.0005 and 0.002% for Al electrodes. These low efficiencies can be improved either by top electrodes having low work functions (e.g. Ca, Mg) or by the fabrication of multilayer structures. A third approach is to enhance the photoluminescence (PL) quantum efficiency of the emitting material. We reveal that large differences exist in the PL efficiency, strongly depending on the conversion conditions of the tetrahydrothiophene precursor polymer to the final polymer PPV. Following detailed investigations of this elimination reaction by different spectroscopic methods, like UV-Vis, PL, FT-IR, we were able to optimize the conversion conditions and thus obtain fully converted PPV films with high PL quantum efficiencies. In addition, a new elimination procedure, which was carried out in argon atmosphere and not as usually in vacuum, allowed us to reduce the required temperature for the elimination down to 160 °C (2 h). As a result, we have prepared flexible LEDs on poly(ethylene terephthalate) for the first time.

Vibrational analysis of different crystalline phases of the organic electroluminescent material aluminium tris(quinoline-8-olate)(Alq3)

The isomerism of the Alq3 molecule was investigated by applying infrared (IR) spectroscopy to two different crystalline phases of aluminium tris(quinoline-8-olate) (α-Alq3 and δ-Alq3). Significant differences between the two phases were found in the IR spectra, which can be explained in terms of the different symmetries of the facial and meridional isomer. Additionally, intermolecular interactions of the Alq3-molecules due to crystallinity were taken into account. The results suggest that α-Alq3 consists of the meridional isomer, while the recently discovered blue luminescent δ-phase is composed of the facial isomer of the Alq3 molecule.

Poly(p-phenylene vinylene) light-emitting devices prepared via the precursor route onto indium tin oxide and fluorine-doped tin dioxide substrates

For the preparation of organic light-emitting devices (LEDs) an optically transparent and electrically conducting thin film is needed as anode. Usually, a glass substrate coated with indium tin oxide (ITO) is used. We show that ITO is unsuitable in the case of poly(p-phenylene vinylene) (PPV) prepared by the precursor route. We have found that a reaction in which hydrogen chloride is eliminated during the thermal conversion to PPV and the ITO takes place. Scanning electron microscopy investigations of the ITO-PPV interface demonstrates that indium chloride compounds, e.g., InCl3 crystals with dimensions up to 40 μm, are produced. Photoluminescence measurements reveal that the fluorescence efficiency is quenched by a factor of 2–23 in the case of ITO compared with PPV converted onto usual glass. In a second step we have investigated LEDs prepared from PPV in the ITO/PPV/Al configuration in order to obtain information about the process responsible for the degradation of these devices. We shall show that the formation of the above-mentioned indium chloride compounds is one possible degradation mechanism and is responsible for the relative short lifetimes of these LEDs. To overcome this problem we propose to use fluorine-doped tin dioxide (FTO) instead of ITO. Finally, we show the results obtained for LEDs in the FTO/PPV/Al configuration and compare them with ITO/PPV/Al devices.

Polarized electroluminescence from rubbing-aligned poly(p-phenylenevinylene)

Abstract In this paper the rubbing-alignment of a poly( p -phenylenevinylene) (PPV) precusor polymer is described. Highly oriented films have been prepared leading to a dichroic ratio of 18 for photoluminescence corresponding to an order-parameter S PL =(PL ∥ −PL ⊥ )/(PL ∥ +PL ⊥ ) of 0.9. Two-layer LEDs with the configuration ITO/PPV/Poly(phenylquinoxaline) (PPQ)/Al that have been fabricated show dichroic ratios for electroluminescence (EL ∥ /EL ⊥ ) of 12 and a maximum brightness of 200 cd/m 2 .

MetaVInsulatorPolymer - LEDs Based on PPV

Abstract We investigated the current-voltage (I-V) and electroluminescence (EL) characteristic of metal/polymer and metal/insulator/polymer (MIP) LEDs based on poly (1,4-phenylene vinylene) (PPV). The I-V- and EL characteristics of the MIP structures display a pronounced dependence of the insulator thickness and we measure an increase of the quantum efficiency of more than a factor of 40 at an AlOX layer thickness of 3–5nm. The device characteristic is qualitatively understood within inorganic metal/insulator/semiconductor (MIS) theory and can be explained by a voltage dependent barrier for minority carrier injection in connection with a hole blocking barrier at the PPV/insulator interface. With MIP structures we reveal external quantum efficiencies up to 0.01%, comparable to values achieved on monolayer Ca LEDs. The MIP structures, however have the advantage, that a more stable device performance is obtained.

The thermal conversion of the tetrahydrothiophene-precursor polymer to poly(p-phenylene vinylene)

This paper reports the thermal conversion of the tetrahydrothiophene (THT)-precursor to poly(p-phenylene vinylene) (PPV). Detailed investigations of the conversion process show that the leaving groups THT and HCl do not eliminate simultaneously. Moderate temperatures (≤125 °C) are sufficient to eliminate the THT while a higher temperature of ≈150 °C is necessary for the leaving group HCl. Furthermore, the THT groups split off at two characteristic temperatures. Our investigations have shown that a consistent picture of the reaction mechanism can only be obtained if the configuration of the polymer chain is considered. For the total reaction of the THT-precursor to PPV a reaction mechanism is suggested that consists of at least four steps. Copyright

Subgap absorption in tris (8-hydroxyquinoline) aluminium

Photothermal deflection spectroscopy was performed on films of tris (8-hydroxyquinoline) aluminium. The presence of an exponential part of the absorption spectrum was found followed by a broad defect absorption. The former is ascribed to HOMO and LUMO tail states and their relevance to the photoluminescence is discussed. The intensity of the defect absorption depends on the used source material. The optically detected defect states are correlated with trap states seen in transport measurements.

Doping and trap states in PPV light-emitting devices

The influence of different substrates used for the fabrication of poly-(p-phenylene-vinylene) light-emitting devices on the device characteristics is investigated. In devices prepared on indium-tin oxide substrates doping with InCl 3 leads to states with a depth of about 0.15 eV and a concentration of 10 16 - 10 17 cm -3 . This doping is responsible for the observed Schottky diode behaviour in PPV devices on ITO but also leads to considerable photoluminescence quenching. The use of fluorine-doped tin dioxide as anode material causes much lower doping and avoids photoluminescence quenching. An improvement of device properties can be achieved by controlled doping or using partially conjugated PPV.

Observation of charge density wave transport phenomena in the organic conductor (FA)2PF6

Abstract The organic conductor (FA) 2 PF 6 with its high dc conductivity up to 1000 S/cm is a model system for quasi-one-dimensional organic conductors. Below and slightly above the 180 K metal to semiconductor phase transition nonlinear conductivity has been observed above a sample dependent threshold field. Studies of the differential resistance clearly indicate that with the onset of nonlinearity the voltage noise in the crystal increases. In the whole temperature range investigated the noise power follows a f −α dependence with α ≈ 1–1.1. Above the threshold field voltage steps are created in the sample. With increasing field the pulse frequency and the pulse height increases. Additionally, at higher electrical fields low frequency oscillations have been observed.

Impedance spectroscopy of polymeric light emitting devices based on different poly(p-phenylene-vinylene) derivatives

Abstract Light emitting devices fabricated from different poly(p-phenylene-vinylene) (PPV) derivatives prepared by the precursor route (the unsubstituted homopolymer, a dimethoxy-substituted copolymer and an acetate-substituted copolymer) and devices based on a soluble dialkoxy-substituted homopolymer are compared with respect to their temperature-dependent impedance characteristics. Although similar activation energies of the conductivity in the range 0.4–0.5 eV are found for all polymers, the magnitude of the dielectric loss is significantly different in the polymers indicating a substantial difference in the amount of doping and/or impurities.