Organic / Inorganic Surfaces, Interfaces, And Light Attenuation In Organic Light Emitting Devices

This thesis is devoted to the study of surface and interface problems in organic light-emitting devices (OLEDs) by means of photoelectron Spectroscopy (PES), especially synchrotron radiation PES. The main topics included in the thesis are:1) Organic/inorganic interfaces. The role of amphiphilic molecule sodium stearate (C₁₇H₃₅COONa) as a buffer at the metal/tris-(8-hydroxyquinoline) aluminum (Alq) interface has been explored. Shifts of the highest occupied molecular orbital and the secondary electron cutoff of the PES spectrum upon the adhesion of the Alq layer have been observed, revealing the existence of the interface dipole. Results of angle-resolved X-ray photoelectron spectroscopic measurements confirm that the C₁₇H₃₅COONa molecules tend to arrange themselves in an amphiphilic array at the interface. It is also found that there exists serious atomic diffusion at the Ag/Alq interface and that insertion of a very thin LiF layer between Ag and Alq can block the diffusion effectively. The phenomenon can be well explained by the tunneling mechanism.2) Photo-degradation of OLEDs under ultraviolet radiation. We find by electroluminescence (EL) and photoluminescence (PL) measurements that there exist two degradation ways in the process: decrease of EL excitons and increase of nonradiative recombination. PES results indicate that photo-oxidation or reaction with water and oxygen can be ruled out as the reasons for the photo-degradation phenomenon observed.3) Surface treatment of La_0.7Sr_0.3MnO₃ (LSMO), a spin injection material currently adopted in organic spin valves and OLEDs. It is found that exposure of the LSMO surface to activated oxygen can raise the sample work function-considerably. The carrier injection barrier formed upon the deposition of NPB, a hole transport material, onto the treated LSMO surface can be reduced to some extent.