Efficiency and stability of different tris(8-hydroxyquinoline) aluminium (Alq3) derivatives in OLED applications

In this work, we have investigated the material stability and organic light emitting diode (OLED) performance for tris-(8-hydroxyquinoline) aluminium (Alq3) and for three of the Alq3 derivatives. For each material, electron spin resonance (ESR) measurements were performed and the degradation of photoluminescence (PL) with atmosphere exposure was studied. Performance of OLEDs based on these materials was also studied. The device structure was ITO/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) (50 nm)/Alq3 or Alq3 derivatives (60 nm)/LiF (0.3 nm)/Al (60 nm). It was found that the rate of PL decay of the films correlates strongly with the existence of ESR signal. However, stability of OLEDs fabricated from these materials showed no direct relationship with PL decay in the films. This indicates that the dominant degradation mechanism in OLEDs is related to the passage of electric current through the device. Detailed discussion of the performances of four light emitting materials in terms of efficiency and stability is given.