Polaronic Stabilities of Exciton Sensitizers Govern Operational Lifetime of Thermally Activated Delayed Fluorescence Assisted Fluorescence Organic Light‐Emitting Devices

Abstract Thermally activated delayed fluorescence (TADF) assisted fluorescence organic light‐emitting devices (TAF‐OLEDs), also known as hyperfluorescence OLEDs, sometimes have limited operational lifetimes, making them unsuitable for industrial applications. Although, this limitation is believed to originate from the irreversible degradation of constituent materials, the chemical processes underlying this degradation remain elusive. This study enables to better understand the chemical origins of the degradation processes by identifying key degradation intermediates in emission layers comprising 2,8‐di‐ tert ‐butyl‐5,11‐bis(4‐ tert ‐butylphenyl)‐6,12‐diphenyltetracene (TBRb) as the fluorescent emitter and 1,4‐azaborin‐based molecules exhibiting multi‐resonance (MR) TADF behaviors, 5,9‐diphenyl‐5,9‐diaza‐13b‐boranaphtho[3,2,1‐ de ]anthracene (DABNA‐1) and 9‐([1,1′‐biphenyl]‐3‐yl)‐ N , N ,5,11‐tetraphenyl‐5,9‐dihydro‐5,9‐diaza‐13b‐boranaphtho[3,2,1‐ de ]anthracen‐3‐amine (DABNA‐2), as exciton sensitizers. Despite the sensitizers facilitating energy transfer to TBRb, they also reduce the device's longevity. While the singlet and triplet excitons of TBRb, as well as excitons of the sensitizers, prove to be relatively stable, the radical ions of the sensitizers prove to be the key limit to device longevity. Comparisons with donor–acceptor‐type TADF exciton sensitizers, 2,4,5,6‐tetrakis(9 H ‐carbazol‐9‐yl)benzene‐1,3‐dicarbonitrile (4CzIPN), 10,10′‐(sulfonyldi‐4,1‐phenylene)bis[9,10‐dihydro‐9,9‐dimethylacridine] (DMAC‐DPS), and 10‐[4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl]‐9,10‐dihydro‐9,9‐dimethylacridine (DMAC‐TRZ), corroborate the findings, pointing to radical ions as primary degradation intermediates. This knowledge is essential for enhancing the durability of TAF‐OLEDs in future designs.