Determination of the optical band gap of Alq₃ and its derivatives for the use in two-layer OLEDs

Tris-(8-hydroxyquinolinato)aluminium (Alq₃) was used as both the emission and electron transporting layer in organic light emitting diodes. One way to tune the emissive color of Alq₃ is to introduce substituents onto the hydroxyquinoline ligands. These substituents will have an effect on the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. In this study, Alq₃ powders were synthesized with -CH₃ substituted at position 5 and 7 (tris-(5,7-dimethyl-8-hydroxyquinolinato)aluminium) and -Cl at position 5 (tris-(5-chloro-8-hydroxyquinolinato) aluminium). Optical absorption and cyclic voltammetry measurements were performed on the samples. The optical band gap was determined from these measurements. The band gap did not vary with more than 0.2 eV from the theoretical value of Alq₃.