P‐198: Reduction of Efficiency Roll‐Off and Extension of Device Lifetime Through a Double Sensitizer Structure‐Controlling Triplet Diffusion

In this study, we propose a novel device architecture for organic light‐emitting diodes (OLEDs) by incorporating a double‐sensitizer system, which combines a disk‐shaped thermally activated delayed fluorescence (TADF) sensitizer and a phosphorescent sensitizer within the emitting layer. A double sensitized (TPDS) device was fabricated by introducing a phosphorescent sensitizer with energy levels strategically positioned between the TADF sensitizer and the terminal dopant. The TPDS device, utilizing the TADF sensitizer and a Pt‐based phosphorescent sensitizer, demonstrated enhanced performance by effectively controlling triplet exciton diffusion and reducing triplet exciton density. These improvements suppressed device degradation, leading to significantly lower roll‐off at high luminance levels compared to the conventional TADF sensitized fluorescence device, which employs only the TADF sensitizer and terminal dopant. Moreover, the operational lifetime of the TPDS device increased by 53.5%. Through this research, the potential of deep‐blue sensitization was explored without the need for a planar n‐type host in the TSF system. The double‐sensitizing approach provides new possibilities for achieving efficient deep‐blue OLEDs.