Deep-Level Characterization of Tris(8-hydroxyquinoline) Aluminum with and without Quinacridone Doping

Abstract

We have investigated band gap states in tris(8-hydroxyquinoline) aluminum (Alq3) with and without quinacridone (Qd) doping on fabricated indium–tin-oxide (ITO)/Alq3(:Qd)/LiF/Al devices by a deep-level optical spectroscopy (DLOS) technique. A nondoped Alq3 sample shows a discrete trap level located at ∼1.39 eV below the lowest unoccupied molecular orbital band in addition to near-band-edge transitions at 2.2–3.6 eV. This pronounced 1.39 eV level is attributable to the intrinsic nature of Alq3 and can be activated as an efficient generation-recombination (GR) center that may affect photophysical properties. On the other hand, a Qd-doped Alq3 sample exhibits a new deep level at ∼2.40 eV with increasing double-carrier injection rate, corresponding to the highest occupied molecular orbital band of Qd. Simultaneously, this GR center is subject to charge up positively owing to the presence of holes injected into the Qd doping level of the Qd-doped Alq3 sample.