Triplet State Phosphorescence in Tris(8-hydroxyquinoline) Aluminum Light Emitting Diode Materials

Abstract

Time-dependent density functional theory (TD-DFT) with the quadratic response technique is applied to the tris(8-hydroxyquinoline) aluminum complex (Alq 3 ) to calculate spin–orbit coupling (SOC) effects and the main mechanism of the Alq 3 phosphorescence. This compound exhibits weak phosphorescence which provides additional emission in organic light emitting diodes (OLEDs) besides the main fluorescence band. The phosphorescence affords to overcome the efficiency limit imposed by the formation of triplet excitons in the emissive layer of OLEDs. The zero-field splitting (ZFS) parameters are also calculated taking into account the spin–spin coupling in the first-order perturbation theory and the SOC in the second order. On the basis of the results obtained, we propose that an efficient spin-polarized injection and transport in long channels of Alq 3, which have been recently achieved with new hybrid organic–inorganic interfaces, can proceed with the triplet electronic state admixture to the charge carriers. Our results also indicate that weak spin–orbit coupling may be responsible for the room-temperature magneto-resistance at the ferromagnetic–Alq 3 interfaces and the spin-dependent charge transport.