W.G. Quirino

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.

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.