M. Cremona

Growth and characterization of OLED with samarium complex as emitting and electron transporting layer

In this work, the growth and the characterization of new orange emitting triple-layer electroluminescent organic devices using vacuum deposited trivalent samarium complex [Sm(TTA)3(TPPO)2] as emission layer is described. The electroluminescence (EL) spectra of the devices show narrow bands arising from the 5G5/2→6HJ transitions (J=5/2, 7/2 and 9/2) of the Sm3+ ion with the hypersensitive 5G5/2→6HJ transitions as the prominent group. The hole transporting layer (HTL) was obtained using a thin film of 1-(3-methylphenyl)-1,2,3,4 tetrahydroquinoline-6-carboxyaldehyde-1,1′-diphenylhydrazone (MTCD), while the tris(8-hydroxyquinoline aluminum) (Alq3) is used as electron transport layer (ETL). Moreover, in order to use the Sm complex, two different kinds of OLEDs were prepared: the first one with a typical three layers architecture, MTCD/[Sm(TTA)3(TPPO)2]/Alq3, while the second one was a bi-layer device with an MTCD/[Sm(TTA)3(TPPO)2] design without the Alq3 ETL layer. In the last case, the EL emission was also observed, which indicates that the [Sm(TTA)3(TPPO)2] complex may be used as an electron transporting layer also.

Tunable blue organic light emitting diode based on aluminum calixarene supramolecular complex

In this letter, the results of supramolecular organic light emitting diodes using a calix[4] arene complex thin film as emitter and electron transporting layer are presented. The devices were grown onto glass substrates coated with indium-tin-oxide layer and aluminum thick (150nm) cathode. By applying a dc voltage between the device electrodes in forward bias condition, a blue light emission in the active area of the device was observed. It was found that the electroluminescent emission peak can be tuned between 470 and 510nm changing the applied voltage bias from 4.3 to 5.4V. The observed tunable emission can be associated with an energy transfer from the calixarene compound.

Low-voltage electroluminescence of europium in zinc oxide thin films

Europium(III)-containing zinc oxide thin films (1, 5, 8, and 10at.%) were prepared from Pechini’s solution and used as active layers in thin-film electroluminescent devices. By applying a low bias steady state voltage (10–50V) it was possible to observe electroluminescence of the device. By increasing the rate Eu3+∕Zn2+ the relative intensity of emission from the dopand Eu3+ ions increases, while the relative intensity of emission from the zinc oxide (ZnO) matrix decreases. This leads to ZnO:Eu3+ (10at.%) thin-film electroluminescent devices with low-voltage operation and high pure color.

Grazing incidence X-ray diffraction analysis of alkali fluoride thin films for optical devices

Abstract Alkali fluoride multilayered thin films are promising systems for different optical applications, including light generation and waveguide. Despite this, some of their physical properties are still not well known. The mechanical behaviour of these materials, when evaporated in the form of thin films, plays a fundamental role for the success of the related optoelectronic device. In this work, LiF, NaF and LiF/NaF polycrystalline thin films were deposited onto different substrates at temperatures varying from 50 to 200°C by physical vapour deposition (PVD). Using the X-ray diffraction technique with grazing incidence geometry (GIXRD), the residual stresses and the microstrains present in films deposited in different experimental conditions were measured. The lattice parameters and the mean domain were also estimated. The results show the dependence of the residual stresses in the films upon the cooling rate and the type of substrates and give, for the first time, an order of magnitude of these quantities for such systems.

In situ hard coatings strain measurement using a commercial strain-gage device

Abstract In this work we present a novel technique which can be used to measure in situ the induced substrate strain caused by the depositing film. The technique, based on the application of a commercial strain-gage, was used during the deposition of ultra-hard silicon carbide (SiC) films by RF magnetron sputtering, onto thin (95 μm) martensitic stainless steel substrates. The direct measurement of the electrical resistance of the strain-gage provides the strain values used to find the stress present in the film/substrate interface. Using a value of 480 GPa for the Young modulus of the SiC film, preliminary measurements furnish a value of the residual stress σ varying from 0.7 to 3 GPa, depending on the deposition conditions. These results are in good agreement with other ex situ measurements performed on the same material. The strain sensibility of this technique is about 10 μm/m.

Accurate and simultaneous measurement of thickness and refractive index of thermally evaporated thin organic films by surface plasmon resonance spectroscopy

We demonstrate that Surface Plasmon Resonance spectroscopy can be used for the accurate and simultaneous determination of the thickness and refractive index of transparent thin thermally deposited organic films. The experimental approach is based on a two-metal deposition or a two-thickness method. These methods have been applied to an encapsulated sample containing a thin film of commercial tris(8-hydroxyquinoline) (Alq3). The accuracy of the measurement depends on the control of the film deposition process and suggests the use of SPR spectroscopy as inexpensive and valuable metrology tool for small molecule organic thin films.

NaF films: Growth properties and electron beam induced defects

Abstract Polycrystalline NaF films were grown by e-beam assisted physical evaporation on amorphous silica substrates kept, during film growth, at constant temperatures ranging from 40°C to 400°C. The structural characterization of the films was performed by X-ray diffraction and by scanning electron microscopy. Irradiating the films with a 15 keV electron beam induced the formation of F and F-aggregate colour centres stable at room temperature. Absorption and photoemission measurements were performed and indicated a dependence of coloration on the deposition conditions.

Spectroscopic evidence of photodegradation by ultraviolet exposure of tris(8-hydroxyquinoline) aluminum (Alq3) thin films

The extrinsic degradation of organic light emitting diodes (OLEDs) remains a critical issue especially concerning degradation due to exposure to light. Very few studies exist and little is known about the related photodegradation products and mechanisms, responsible for quenching of luminescence. In order to gain insight into the degradation mechanisms caused by light exposure of Alq3 thin films, used successfully as electron transport layer and emissive material in the fabrication of OLEDs, we have monitored UV photodegradation through synchrotron radiation-based photoabsorption and photoemission techniques at the carbon, nitrogen, and oxygen 1s edges as well as at the aluminum 2s and 2p edges. The influence of light exposure was simulated using three different wavelengths, namely 254, 365 and 307 nm, the first two nearly corresponding to absorption maxima in the UV spectrum of Alq3. After exposure all spectra show decrease of the photoabsorption and photoemission signals. However, while irradiation at 307 nm causes lesser changes in the total electron yield NEXAFS spectra, strong spectral modifications are observed for 254 and 365 nm exposures. Core level photoemission measurements from non-irradiated and irradiated Alq3 thin films at 307 nm were also performed. While the Al peaks maintained almost intact, changes in peak intensities as well as shifts for carbon, nitrogen and oxygen are much more dramatic.

Photoluminescence, photoabsorption and photoemission studies of hydrazone thin film used as hole transporting material in OLEDs

A fotoluminescencia de filmes finos de 1-(3-metilfenil)-1,2,3,4-tetrahidroquinolina-6carboxialdeido-1,1’-difenilhidrazona foi monitorada em funcao da irradiacao com luz UV. A intensidade da emissao decresce exponencialmente com o tempo de exposicao, sugerindo degradacao das amostras. Com o objetivo de investigar os mecanismos de degradacao e determinar a estrutura eletronica desse material orgânico usado com sucesso como camada transportadora de buracos na fabricacao de diodos orgânicos emissores de luz (OLEDs), foram empregadas as tecnicas de fotoabsorcao e de fotoemissao nas bordas 1s do carbono e do nitrogenio bem como na banda de valencia. A influencia da luz solar foi simulada usando radiacao sincrotron nao-monocromatica. Apos exposicao, todos os espectros apresentam um decrescimo nos sinais de fotoabsorcao e de fotoemissao, que e menos acentuado na borda do carbono, apresentando, entretanto, um decrescimo drastico na borda do nitrogenio e na regiao de valencia. O estudo sugere que a perda de nitrogenio e a principal causa para a quebra do sistema π, levando, dessa forma, a falha do dispositivo fabricado com esse composto. Photoluminescence (PL) emission of 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6carboxyaldehyde-1,1 ’ -diphenylhydrazone (MTCD) thin films was monitored as a function of UV irradiation, and it was found to decrease exponentially with the exposure time. In order to gain insight into the degradation mechanisms and evaluate the electronic structure of this organic material used with good results as hole transporting layer (HTL) in the fabrication of organic light emitting diodes (OLEDs), synchrotron radiation-based photoabsorption and photoemission techniques at the carbon and nitrogen 1s edges as well as at the valence band were employed. The influence of sunlight was simulated using non-monochromatized synchrotron radiation. After exposure all the spectra show a decrease of the photoabsorption and photoemission signals, however, while it is less accentuated at the carbon edge, at the nitrogen edge and at the valence region it decreases drastically. The loss of nitrogen is suggested to be the main step in the disruption of the π system, leading to the failure of the devices fabricated with this compound as hole transporting layer.

Realisation and characterisation of LiF/NaF thin film planar waveguides

Guided light propagation in two-layer LiF/NaF structures grown on glass substrates is reported for the first time. LiF/NaF based waveguides have been grown at different temperatures. The films texture analysis revealed both the influence of the underlying NaF buffer layer on the structure of the guiding LiF film and the importance of the film microstructure on the light propagation properties.