Physical and chemical aging of aluminum(III) 8-hydroxyquinoline: Failure and materials design in organic light-emitting diodes

The chemical and physical aging of aluminum(III) 8-hydroxyquinoline (Alq3) films was studied by a variety of techniques. The products of these mechanisms produce electron trap states below the conduction band, or lowest unoccupied molecular orbital level (LUMO), of Alq3. The distribution of these trap states was shown to affect the charge transport and luminescence characteristics of organic light-emitting diodes (OLEDs) based on this material. ^ The chief route of chemical decomposition is the substitution of water with one of the ligands of Alq3. This reaction was characterized by gas chromatography, and quantitative kinetics were obtained to measure the impact of chemical aging in real device architectures. Electrochemical reduction of either Alq3 or the free ligand, 8-hydroxyquinoline (Hq), facilitates dissociation of the complex, especially when oxygen is present. The reduced form of Hq is unstable and forms quinones, hydroquinones, and charge transfer complexes, the latter of which will cause an additional loss in the performance of OLEDs due to luminescence quenching. This instability is particularly important because Hq traps electrons during device operation. ^ Physical aging and crystallization were observed by X-ray diffraction, calorimetry, and fluorescence spectroscopy. Films which were amorphous upon deposition, crystallized rapidly upon annealing at temperatures below their glass transition. Blends of Alq3 with aluminum(III) 5-methyl-8-hydroxyquinoline were proposed for thermally stable amorphous emitting layers in OLEDs. Films coevaporated at a 1:1 ratio did not show evidence of crystallization even after long annealing periods at temperatures as high as 160°C. ^ Conduction in Alq3 was considered based on trap-charge limited conduction of electrons in the bulk. The evolution of a narrow Gaussian distribution of localized trap states below the LUMO, lying against a natural exponential background, was used to explain the changes in the current-voltage characteristics and electroluminescence efficiency observed over time in OLEDs by many researchers. Chemical aging kinetics were used to predict the rate at which OLED performance was compromised.