Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters

摘要

We investigate quenching processes which contribute to the roll-off in quantum efficiency of phosphorescent organic light-emitting diodes (OLED's) at high brightness: triplet-triplet annihilation, energy transfer to charged molecules (polarons), and dissociation of excitons into free charge carriers. The investigated OLED's comprise a host-guest system as emission layer within a state-of-the-art OLED structure—i.e., a five-layer device including doped transport and thin charge carrier and exciton blocking layers. In a red phosphorescent device, N,N′-di(naphthalen-2-yl)- N,N′-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium [Ir(piq)_3] as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,4′,4″-tris (N-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium [Ir(ppy)_3]. The triplet-triplet annihilation is characterized by the rate constant k_(TT) which is determined by time-resolved photoluminescence experiments. To investigate triplet-polaron quenching, unipolar devices were prepared. A certain exciton density, created by continuous-wave illumination, is analyzed as a function of current density flowing through the device. This delivers the corre- sponding rate constant k_P. Field-induced quenching is not observed under typical OLED operation conditions. The experimental data are implemented in an analytical model taking in account both triplet-triplet annihilation and triplet-polaron quenching. It shows that both processes strongly influence the OLED performance. Compared to the red Ir(piq)_3 OLED, the green Ir(ppy)_3 device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime τ and a thinner exciton formation zone w.