Coexistence of dissociation and annihilation of excitons on charge carriers in organic phosphorescent emitters

Abstract

We examined exciton quenching mechanisms in organic electrophosphorescent emitters. As an example we have performed detailed steady-state (dc) and alternating (ac) electric-field-modulated (photo)luminescence (EML) studies of the green phosphorescent iridium (III) complex, $\left[\mathrm{Ir}{\left(ppy\right)}_3\right]$, doped into a hole-transporting diamine derivative (TPD) blended with polycarbonate, varying electrode contacts from the hole-injecting indium-tin-oxide (ITO) and Au (dc EML) to the weakly electron-injecting Al on both sides (ac EML) of a sandwich type formed thin film samples. Analysis of ac EML results in terms of the three-dimensional (3D)-Onsager theory of geminate recombination shows that both the EML for the $\mathrm{Ir}{\left(ppy\right)}_3$ phosphorescence and TPD fluorescence components of the emission spectrum from the emitters is underlain by the dissociation of $\mathrm{Ir}{\left(ppy\right)}_3$ triplet and TPD singlet exciton precursors composed of an electron and a hole separated by a distance $r_0=2.1\pm 0.1\mathrm{nm}$. We find that for samples with the ITO/Au electrode combination, the dissociation is accompanied by exciton annihilation on injected charge. The annihilation proceeds by injected holes with the second order rate constant of ${\gamma }_{\mathrm{Sq}}=(2\pm 1)\times {10}^{-11}{\mathrm{cm}}^3∕\mathrm{s}$ characterizing the diffusivity of TPD singlets, and ${\gamma }_{\mathrm{Tq}}=(1\pm 0.5)\times {10}^{-14}{\mathrm{cm}}^3∕\mathrm{s}$ indicative of the relative diffusion motion of the $\mathrm{Ir}{\left(ppy\right)}_3$ triplet and a charge carrier. It is shown that the annihilation to dissociation efficiency has a minimum at an electric field of $F_{\min }\cong {10}^6\mathrm{V}∕\mathrm{cm}$ for the present system but the derived theoretical expression allows us to predict the evolution of $F_{\min }$ with various material and sample parameters, and by this to establish criteria to assist in the optimized design of quenching effects in organic electrophosphorescent devices.