X-ray photoemission and photoabsorption of organic electroluminescent materials

Abstract

Thin films of tris-(8, hydroxyquinoline) aluminum (Alq3) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) were measured using synchrotron radiation-based core and valence level photoemission and core level photoabsorption to elucidate the element-specific electronic structure of organic electroluminescent materials. The energy level alignment of an Alq3/TPD interface is given for both occupied and unoccupied states. A comparison of freshly evaporated films of Alq3 and TPD with films that have been exposed to intense radiation or oxidative conditions sheds light on possible damage mechanisms of the molecular solid.

Electronic Structure in Thin Film Organic Semiconductors

“…5 Due to the long acquisition time required for the weak signal from this second order process, any organic material can suffer significant synchrotron beam damage, and Alq 3 is no exception. 4 We circumvent this by continuously translating our samples during the measurement, at a rate of approximately 40 microns per second, and explain the difference between our results and previous x-ray emission studies in terms of the latter exhibiting beam damage. 5 Valence band XPS (VB-XPS) of thin Alq 3 films was found to be in agreement with earlier studies by Curioni et al…”

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

“…This behavior can be explained by the formation of new carbon containing species, as observed for Alq 3 . 4 Although MTCD has unequivalent nitrogen and carbon atoms it was not possible to distinguish them due to the low resolution at the present experimental conditions. The damage produced by the irradiation affects directly the valence band (see Figure 6), where there is a dramatic change in the occupied electronic structure leading to the disappearance of almost all original transitions.…”

“…5, 2008 are commonly used in the OLED fabrication is still the principal weakness of these devices. Many efforts have been made in order to understand the factors that influenced the different degradation mechanisms of the OLEDs and their organic materials, principally for TPD [N,N´-diphenyl-N,N´-bis (3- [2][3][4][5][6] Another important aspect of the OLED operational stability, especially for the implementation in portable devices, is their stability against radiation originated from the environment, such as intense sunlight. In order to better understand the degradation processes which occur when these organic materials are submitted to intense radiation a series of investigations using spectroscopic techniques were carried out.…”

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

“…To better understand the degradation processes, which occur when organic materials used in OLEDs are submitted to intense radiation a series of investigations using spectroscopic techniques were carried out. [2][3][4][5][6][7][8][9] Among the materials used in OLED fabrication, tris-(8-hydroxyquinoline) aluminum (Alq 3 ) is one of the most attractive electroluminescent material, used successfully as electron transport and emissive material in organic devices, and therefore has stimulated many studies focusing in a better comprehension of the degradation processes due to light exposure. Thangaraju et al 10 studied the influence of light in the photoluminescence from Alq 3 thin films.…”

“…Similar results were obtained by exposing Alq 3 films to intense broadband synchrotron radiation. 4,8 The loss of nitrogen followed by damage of the molecular structure may cause disruption of the conjugated p system and consequently interruption of the charge conduction through the molecule, degradating the performance of the device. …”