Investigation of the luminescent properties of metal quinolates (Mqx) for use in OLED devices

Since Tang and VanSlyke developed the first organic light emitting diode (OLED) in the late 80’s using tris-(8-hydroxyquinoline) aluminium (Alq3) as both the emissive and electron transporting layer, a lot of research has been done on Alq3 and other metal quinolates (Mqx). The optical, morphological and electrical properties of these Mqx have been studied extensively. Alq3 has, however, a disadvantage as it tends to degrade when stored under atmospheric conditions. These degraded products are non-luminescent and lead to poor device performance. A good understanding of what happens during the degradation process and ways of eliminating this process are needed. In this study different Mqx compounds were synthesized and their degradation behavior was studied to see what effect it has on their luminescent properties. One way to tune the emissive colour of Alq3 is to introduce electron-withdrawing or electron-donating groups (EWG and EDG) onto the hydroxyquinoline ligands. These groups 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 an EDG (-CH3) substituted at position 5 and 7 ((5,7-dimethyl-8-hydroxyquinoline) aluminium) (5,7Me-Alq3) and EWG (-Cl) at position 5 ((5-chloro-8-hydroxyquinoline) aluminium) (5Cl-Alq3). A broad absorption band at ~ 380 nm was observed for un-substituted Alq3. The bands of the substituted samples were red shifted. The un-substituted Alq3 showed a high intensity emission peak at 500 nm. The 5Cl-Alq3 and 5,7Me-Alq3 samples showed a red shift of 33 and 56 nm respectively. Optical absorption and cyclic voltammetry measurements were done 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. The photon degradation of the samples was also investigated and the 5,7Me-Alq3 sample showed the least degradation to the UV irrad
National Research Foundation (NRF)
University of the Free State