Trap states in doped tris-8-(hydroxyquinoline) aluminum using thermally stimulated luminescence

The light emission from bi-layer organic devices (OLEDs) has been shown to be proportional to the current. Trap states have been speculated to contribute to the carrier transport in such devices. We will report on the bulk trap state properties of naphthyl-substituted benzidine derivative (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq(3)) using thermally stimulated luminescence (TSL). Using a general order TSL expression, the four peaks in NPB were modeled with trap states centered from 0.15 eV to 0.02 eV. The main Alq(3) peak is modeled as a distribution of trap states from 0.25 to 0.15 eV with two additional peaks observed at lower trap energies. For both materials, the trapping mechanism involves a combination of first and second order emission. Using TSL, the evolution of the trap states in Alq(3) has been studied as a function of coumarin 6 and NPB doping, with doping levels from 0.1% to 2.0%. For Ale doped with coumarin 6, we observe an almost 0.1 eV increase in the width of the trap states. Conversely, the Alq(3) samples doped with NPB do not show a change in the trap states. These trap depths are sufficient to support a trap charge limited model for the carrier transport in bilayer organic based light emitting diodes.