Tilted Spiro-Type Thermally Activated Delayed Fluorescence Host for ≈100% Exciton Harvesting in Red Phosphorescent Electronics with Ultralow Doping Ratio

Despite promising efficiency, the high fabrication cost due to the required high concentrations of noble metal based phosphors is still problematic for phosphorescent organic light-emitting diodes (PhOLEDs). This situation becomes even serious considering some practical applications need high luminance, which in turn requires a higher concentration of emitters. A paradigm shift to circumvent these issues can be development of materials that are able to operate efficiently in very low concentrations (<1 wt%). Here, two thermally activated delayed fluorescence hosts (OSTFPCN and OSTFPB) with tilted spiro geometry, small singlet-triplet splitting (Δ E _st), and effective resonance energy transfer are developed. Within expectation, record performances with a power efficiency of 63.6 lm W⁻¹ and an external quantum efficiency (EQE) of 31.2% for the red phosphor Ir(MDQ)₂(acac) (2.0 wt%) are achieved with OSTFPCN as host. Additionally, a high power efficiency around 58 lm W⁻¹ is also gained even at an ultralow dopant concentration of 0.5 wt% for a OSTFPB based device. Mechanism studies demonstrate that efficiency roll-off can be effectively suppressed in such low concentrations. These findings pave a new way to exploit low cost and high efficiency PhOLEDs.