Pierre Destruel

Ultrathin films of homeotropically aligned columnar liquid crystals on indium tin oxide electrodes

We report the achievement of very thin films (thickness of about 50 nm) of thermotropic columnar liquid crystal in homeotropic (columns normal to the interface) orientation on indium tin oxide (ITO) electrodes. The face-on alignment of the discotic compound has been obtained by thermal annealing without any intermediate coating between the mesophase and the ITO substrate. Such a columnar mesophase alignment is thus shown on a substrate of technological interest in open supported thin film reaching the thickness range suitable for organic photovoltaic devices.

Red organic light emitting device made from triphenylene hexaester and perylene tetraester

Saturated red light emission from organic light emitting diodes is less common than emission in the green or the blue. Most organic red light emitting devices are based on rare earth complexes, mainly europium, which are known to exhibit stability problems. The present article describes new diodes made of indium tin oxide-coated glass/triphenylene hexaether/perylene tetraester/aluminum. The band diagram was determined by ultraviolet photoemission spectroscopy, cyclic voltammetry, scanning tunneling microscopy, and absorbance measurements. The interfaces between electrodes and organic layers were investigated by x-ray photoelectron spectroscopy. The current–voltage and luminance–voltage characteristics are very reproducible from device to device, with an emission peak at 620 nm and a full width at half maximum of 80 nm, a current rectification ratio of about 30, I∼V2 at low voltages and I∼Lum∼V6 at higher voltages.

Electrophosphorescent homo- and heteroleptic copper(I) complexes prepared from various bis-phosphine ligands

Homo- and heteroleptic copper(I) complexes obtained from various chelating bis-phosphine ligands and Cu(CH3CN)4BF4 have been used for the preparation of light emitting devices.

An all-columnar bilayer light-emitting diode

Abstract The first organic light-emitting diode with both an electron-rich columnar liquid crystal as hole transport material and an electron-deficient fluorescent columnar liquid crystal as electron transport material is presented. Red fluorescence is observed above 10 V from the configuration indium–tin oxide (ITO)/hexabutoxy-triphenylene/tetraethyl perylenetetracarboxylate/Al. The excellent charge transport properties of columnar phases are thus exploited for light-emitting diodes. Both organic materials exhibit their columnar liquid crystal phase well above room temperature and the underlying ordered solid columnar phase is used in the diode. The I ( V ) and L ( V ) characteristics are very reproducible from device to device, with an emission peak at 620 nm and an FWHM of 80 nm, a current rectification ratio of about 30, I ∼ V 2 at low voltages and I ∼ L ∼ V 7 at higher voltages. Initial lifetime measurements are encouraging.

XPS and sputtering study of the Alq3/electrode interfaces in organic light emitting diodes

Abstract The interface formed between tris(8-hydroxyquinoline) aluminum (Alq3) and electrodes (Al and ITO) of light emitting diodes was examined by X-ray photoelectron spectroscopy (XPS). Upon deposition of aluminum layer, Alq3 reacts partially with the metal, forming metallic carbide and/or Al–O–C complex in the interfacial region. On the Alq3/ITO side, no noticeable change in the spectra was observed. Analysis of the organic material/electrode interface was also performed on the devices after several working cycles up to their complete destruction. Compared to non-degraded samples, the interface between Alq3 and Al of degraded samples was modified by the diffusion of indium from the ITO base electrode to the upper Alq3/Al interface and aluminum from the upper electrode to the Alq3 layer. In the ITO/Alq3 interface, partial decomposition of the oxide layer occurred, leaving indium to diffuse throughout the emitting layer. The structural changes of the contact region is proposed to be one of the possible causes of the diode failure.

A new plastic scintillator with large stokes shift

Abstract We have developed a new plastic scintillator with the novel characteristic of highly localized light emission; scintillation and wavelength shifting take place within a few tens of micrometers of the primary ionization. The new scintillator consists of a scintillating polymer base [polyvinyl toluene (PVT) or polystyrene (PS)] doped with a single wavelength shifter, 1-phenyl-3-mesityl-2-pyrazoline (PMP), which has an exceptionally large Stokes shift and therefore a comparatively small self-absorption of its emitted light. In other characteristics (e.g. scintillation efficiency and decay time) the performance of the new scintillator is similar to a good quality commercial plastic scintillator such as NE110.

Characterization of ITO/CuPc/AI and ITO/ZnPc/Al structures using optical and capacitance spectroscopy

Metal-substituted phthalocyanine (MPc) thin films as zinc- or copper-phthalocyanine are often used as charge injection layers for organic electroluminescent or photovoltaic devices. It is then important to characterize their electronic defect density and band structure near their gap. In this work the monolayer structures were prepared by vacuum sublimation of the organic thin film sandwiched between indium tin oxide (ITO) and aluminum electrodes. Electrically active defects were investigated with space-charge capacitance spectroscopy, as a function of temperature and frequency, in the range 80–330 K and 40 Hz to 10 MHz, respectively. Organic materials are best described on the basis of individual molecular orbital (HOMO and LUMO) energies instead of valence and conduction band. Such energies were derived from cyclic voltammetry and optical absorption spectroscopy measurements. Experimental results were correlated to the electrical J–V characteristics of these MPc based devices to gain more insight on the charge injection processes and their limitations.

PRESSURE DEPENDENCE OF ELECTRICAL AND OPTICAL CHARACTERISTICS OF ALQ3 BASED ORGANIC ELECTROLUMINESCENT DIODES

Organic electroluminescent diodes based on thin organic layers are one of the most promising next-generation systems for the backlighting of liquid crystal screens. In the literature, conduction has been interpreted in terms of tunneling or as a space charge limited process. To clarify this point we carried out an experimental study of the electrical and optical characteristics, as a function of the applied voltage, for different hydrostatic pressures. Results show that tunneling is the dominant mechanism which controls the organic light emitting diodes efficiency.

Organic scintillators with large Stokes shifts dissolved in polystyrene

Abstract Since several years, PMP and 3-HF have been known as scintillators with large Stokes shifts. We have synthesized four new aromatic pyrazoline compounds

Degradation in organic light-emitting diodes

Abstract Light-emitting diodes using tris(8-hydroxyquinoline) aluminum complex (Alq 3 ) as an active layer and N,N′ -diphenyl- N,N′ -bis(3-methyl-phenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and 2-(4-biphenyl)-5-(4- tert -butylphenyl)-1,3,4-oxadiazole (PBD) as hole and electron transport layers, respectively, have been studied by electrical and optical measurements. The degradation effects of these diodes have been examined by measuring the current density-applied field characteristics as a function of time. It has been demonstrated that the three-layer diodes have the best stability and that the degradation occurred through the formation of dark points did not modify the injection mechanism. A discussion on the degradation mechanism is given, and the results are compared to those reported in the literature.