Phosphorescence response to excitonic interactions in Ir organic complex-based electrophosphorescent emitters

The phosphorescence (PH) response to increasing excitation intensity $\left(I\right)$ has been studied from an efficient electrophosphorescent iridium (III) complex, fac tris(2-phenylpyridine) iridium $\left[\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3\right]$ , dispersed in a diamine derivative (TPD)-doped polycarbonate (PC) hole-transporting matrix and in the form of neat vacuum-evaporated films. It is demonstrated that the observed decrease in relative PH efficiencies at increasing $I$ is principally due to triplet-triplet $\left(T\text{-}T\right)$ interactions that include mutual annihilation of the TPD host, $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ guest, and host-guest triplets. The effective annihilation rate constants $\left[{\gamma }_{TT}^{\mathrm{eff}}\right]$ fall in the range $(1–3)\times {10}^{-12}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ depending slightly on the matrix composition. The lower and upper limits of ${\gamma }_{TT}^{\mathrm{eff}}$ correspond to TPD-free $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ -doped PC samples and high-content TPD or neat $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ solid films, respectively. A deviation from the second-order kinetics of $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ triplets observed with neat films is attributed to a saturation of nonradiative excited sites (e.g., molecular aggregates) populated by energy transfer from the triplets. From extrapolation of $I_{\mathrm{crit}}$ at which $T\text{-}T$ interactions become the triplet lifetime controlling process to electrical excitation in $\mathrm{Ir}{\left(\mathrm{ppy}\right)}_3$ -based light-emitting-diodes, the onset current of the roll off in electrophosphorescence (EPH) quantum efficiency (QE) is calculated. Its values exceed at least one order of magnitude the experimental data, supporting previous suggestions of the large current density EPH QE roll off to be substantially underlain by the field-assisted dissociation of emissive states and their precursors.