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

• The optical band gaps of Alq3, 5,7-Me-Alq3 and 5-Cl-Alq3 were determined.
• Cyclic voltammetry, Tauc’s relation and the absorption onset were used and compared.
• The values of the optical band gap of 5,7-Me-Alq3 and 5-Cl-Alq3 were lower than that of Alq3.
• A red shift was observed in the absorption spectra of 5,7-Me-Alq3 and 5-Cl-Alq3.

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.