Pascale Jolinat

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.

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.

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.

Photoemission study of the ITO/triphenylene/perylene/Al interfaces

Abstract The interface formation in a ITO-coated glass/triphenylene hexaether/perylene tetraester/aluminum heterostructure was studied by ultra-violet photoemission spectroscopy (UPS). The interfaces were built step by step by successive evaporation of thin (few nm) material layers. Each step was followed by UPS characterization which allowed determination of the evolutions of the valence bands (VBs) and that of the vacuum level. An electronic energy diagram has been deduced giving the metal/organic barriers and the band offset between the two organic semiconductors. Major differences were observed between the two metal/organic interfaces which can have consequences for light emitting diodes based on these materials.