Green- and blue-emitting tris(8-hydroxyquinoline) aluminum(III) (Alq3) crystalline polymorphs: Preparation and application to organic light-emitting diodes

Tris(8-hydroxyquinoline) aluminum(III) (Alq₃) is known to have two isomeric states, namely meridional and facial isomers. Typical Alq₃ crystals composed of meridional and facial isomers are α- and δ-form crystals, respectively. First, we investigate the temperature change in the crystalline forms of α-Alq₃ and δ-Alq₃ in X-ray diffraction experiments in a vacuum. α-Alq₃ remains in α-form up to 300 °C, immediately before sublimation. In contrast, δ-Alq₃ is found to transform into γ-form at ∼180 °C, and remain in γ-form immediately before sublimation. Both γ-Alq₃ and δ-Alq₃ are composed of facial isomers and emit blue luminescence, which is different from the typical green emissions of α-Alq₃. Second, we fabricate organic light-emitting diodes (OLEDs) from different crystals as source powders; i.e., from (1) α-Alq₃, (2) δ-Alq₃, and (3) a mixture of α-, γ-, and δ-Alq₃. All the OLEDs exhibit green electroluminescence with almost the same maximum wavelength, suggesting that some facial isomers become meridional while Alq₃ is in the gas phase. In contrast, electroluminescence efficiency depends on the Alq₃ crystalline polymorph; the OLED fabricated from the mixture of α-, γ-, and δ-Alq₃ has up to 1.4 times the efficiency of the OLED fabricated from α-Alq₃ for the same device structure.