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

► Pure δ-Alq3 is obtained by vacuum-encapsulation and thermal annealing. ► Thermal changes of crystalline forms for α-Alq3 and δ-Alq3 are investigated. ► Effects of Alq3 polymorphs as source powders on OLED performance are examined. ► An OLED from αγδ-Alq3 has 1.4 times higher EL efficiency than that from α-Alq3.