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, $N,N^{\prime }$-di(naphthalen-2-yl)- $N,N^{\prime }$-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium $\left[\mathrm{Ir}\right(\mathrm{piq}{)}_3]$ as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,$4^{\prime }$,$4^{″}$-tris ($N$-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium $\left[\mathrm{Ir}\right(\mathrm{ppy}{)}_3]$. 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 corresponding 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 $\mathrm{Ir}(\mathrm{piq}{)}_3$ OLED, the green $\mathrm{Ir}(\mathrm{ppy}{)}_3$ device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime $\tau$ and a thinner exciton formation zone $w$.

• Received 11 July 2006

DOI:https://doi.org/10.1103/PhysRevB.75.125328