Thermally activated delayed fluorescence sensitized phosphorescence: a strategy to break the trade-off between efficiency and efficiency roll-off.

Materials with thermally activated delayed fluorescence (TADF) realized 100% internal quantum efficiency (IQE) but suffered significant efficiency roll-off. Here, an exciton dynamics study reveals that materials with TADF may play opposite roles in affecting the efficiency roll-off: decreasing the triplet density due to the fast reverse intersystem crossing, on the one hand, and increasing the triplet density due to the weakened singlet radiation. We show theoretically and experimentally that TADF-sensitized phosphorescence can break this trade-off by exploiting the efficient Förster energy transfer and simultaneously achieve 100% IQE and low efficiency roll-off (with a critical current density of 460 mA cm(-2)).