Charge trapping and scattering by extrinsic gas dopants in tris(8-hydroxyquinoline) aluminum (Alq₃)

The effects of various ambient gases (N₂, O₂, H ₂O) to the electron mobility of tris(8-hydroxyquinoline) aluminum, Alq₃, were examined. For a clean Alq₃, the electron transport is non-dispersive and essentially trap-free as revealed by optical time-of-flight measurements. At 295K, the electron mobility (μℓ) spans the range 0.2-1×10^-6 cm²V^-1s^-1 when the field varies from 0.4-0.9 MV cm^-1. Exposing the sample to oxygen dosages in the range 1-100 Torr-s does not cause any noticeable changes in μℓ. The effect of oxygen on μℓ, can be observed only at much higher oxygen dosages (>10⁵ Torr.s). In contrast, exposing pristine Alq₃ to moisture of same dosages causes a gradual reduction in μℓ by a factor of 4. It is proposed that oxygen diffuses into Alq ₃ during exposure and subsequently acts as charge scattering centers for externally injected free electrons. On the other hand, water induces charge traps at high dosages. Moisture control appears to be the most critical factor for the long-term operation of Alq₃-based organic light-emitting diodes.