Transient electronic processes of phosphorescent Ir(ppy) 3 under endothermic energy transfer from fluorescent TPD host material

Abstract

Temperature-sensitive photoluminescence (PL) of Ir(ppy) 3 is observed in a spin-coated film of TPD containing 5 wt% Ir(ppy) 3. Numerical calculation is made for the emission intensity using rate equations for three zero-field splitting substates of the triplet T 1 state. Good agreement is obtained for the temperature dependence between the calculated and measured PLs. It is concluded that (1) the increase of PL intensity observed at 10–100 K is due to the spin-phonon transitions among the three substates and the exothermic energy transfer from Ir(ppy) 3 to TPD, (2) the endothermic energy transfer from TPD to Ir(ppy) 3 is responsible for the increase above 100 K, and (3) the decrease of PL intensity observed above 200 K is caused by the energy diffusion from TPD to neighboring unexcited TPS.