Large surface potential of Alq3 film and its decay

Abstract

The surface potential across Alq 3 [tris(8-quinolinolato)aluminum] evaporated films on a metal (Au, Al) electrode was measured by the Kelvin probe method and the results were discussed taking into account the orientational ordering of Alq 3 molecules, the presence of surface charges on Alq 3 films and the displacement of excess charge (electrons and holes) from the electrode into Alq 3 films. The very large surface potential established in as-deposited Alq 3 films was mainly due to the alignment of dipoles of Alq 3 molecules, whereas the small surface potential that remained in the films after photoirradiation was due to excess charge. In order to clarify the drastic decrease of surface potential by photoirradiation, the surface potential decay was examined.

Study of interfacial polarization phenomena in organic films by MDC, SHG and surface potential measurement

“…Interestingly, remained small surface potential is dependent on metal electrodes Au and Al, and Alq 3 films on Al electrode are more negatively charged than Alq 3 films on Au electrodes. Therefore, we may expect at the interface, excess charges are displaced from metal to Alq 3 films to some extent [17]. Taking into account these, we expect that before photoirradiation electron transfer happens at the nanointerface between metals and Alq 3 films, but this effect is very small compared with the effect of the alignment of dipoles.…”

Probing Metal/Organic Interfaces Using Doubly-Resonant Sum Frequency Generation Vibrational Spectroscopy

“…The interference effects of the SFG signal due to the multiple layers are well analyzed by Hirose et al Ishida et al, 1998) However, the SHG on the photo-irradiated Alq 3 films does not show such interference pattern. (Yoshizaki et al, 2005) Moreover, the decrease of in the intensity of the SHG signal was observed when the Alq 3 film was heated at a temperature lower than 100 °C. (Kajimoto et al, 2006) Although the part of the SFG signal might be influenced by the interference between the two interfaces, these SHG results suggest that the interference www.intechopen.com…”

“…On the other hand, the large surface potential is rapidly reduced by exposure to ambient light. (Yoshizaki et al, 2005;Ito et al, 2002) At first, this was considered with the photo-induced randomization of molecular orientation, (Sugi et al, 2004) however, the first-order electroabsorption measurements for the Alq 3 confirms that the non-centrosymmetric molecular orientation remains even after the light irradiation, indicating that the reduction of the large surface potential is not caused by the orientation randomization. (Isoshima et al, 2009) It is also suggested that the orientation polarization of Alq 3 film is maintained in OLED structure even under light illumination after the device fabrication.…”

“…However, such bulk contribution is not observed in the thickness dependence of the SHG on the photo-irradiated Alq 3 films. (Yoshizaki et al, 2005) Thus the bulk dipole-forbidden effects are excluded. Second, due to the existence of the two (air/Alq 3 and Alq 3 /CaF 2 ) interfaces, the interference of the SFG signal between the two interfaces will induces the enhancements of the SFG signal intensities.…”

“…It has been reported that a large potential is built in as-deposited Alq 3 thick film in dark and it decays rapidly by exposure to the ambient light observed by the Kelvin probe method. (Yoshizaki et al, 2005;Ito et al, 2002) According to the Kelvin probe experiments, (Ito et al, 2002) the surface potential increased rapidly, in the initial stage of Alq 3 deposition up to 1 nm on Al substrate, corresponding to the formation of the interfacial dipole layer. (Seki et al, 1997) After the initial shift, the surface potential increased linearly over a wide range of thickness from 5 to 550 nm.…”