Analysis of unusual temperature dependence of Ir complexes

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We established an absolute PL quantum efficiency measurement system using an integrating sphere. We carefully measured absolute PL quantum efficiency and internal system crossing (ISC) efficiency of Ir complexes which are useful for organic light emitting diodes. In particular, we clarified non-radiative decay process from the triplet excited state to the ground state.We observed that green emitting Ir(ppy)_3 has PL quantum efficiency of η_<PL>=89±2%, and blue emitting FIrpic has η_<PL>=89±2%, indicating suppression of non-radiative decay processes in their doped films. On the other hand, red emitting Btp_2Ir(acac) showed rather low η_<PL>=32±3%. In order to clarify the non-radiative deactivation process, we measured the ISC efficiency by photoacoustic spectroscopy (PAS). As a result, Btp_2Ir(acac) was revealed to have nearly 100% of ISC efficiency, and we concluded that non-radiative deactivation efficiency from the triplet level is 64%.