Manipulating Förster and Dexter Energy Transfer via Generation Regulation of Dendritic TADF Sensitizer for Solution‐Processable Narrowband Electroluminescence with EQE over 40%

A high-efficiency thermally activated delayed fluorescence (TADF) sensitized fluorophor, abbreviated as TSF, requires suppressing Dexter energy transfer (DET) while ensuring an effective Förster resonance energy transfer (FRET). The current general strategy for blocking DET is encapsulating the terminal emitter to increase the distance between the TADF sensitizer and emitter (RDA); however, there are few reports on increasing RDA by modifying the molecular structure of the TADF sensitizer. Here, a molecular design is proposed to modulate RDA from the perspective of TADF sensitizer, where three dendritic TADF sensitizers (nGPh-5CzBN, n = generation) are developed by increasing the generation of peripheral branches. On the one hand, TADF sensitizer with high-generation dendrons blocks DET and prevents excitons loss by enlarging RDA; on the other hand, the excessive cladding of dendritic TADF sensitizer, in turn, also hinders efficient FRET from itself to the emitter. As a result, a record-high maximum external quantum efficiency (EQEmax) of 40.6% is realized for 2GPh-5CzBN-based solution-processable TSF-OLEDs, with a full width at half maximum of 34 nm, but the EQEmax values are 23.9% and 16.0% for 1GPh-5CzBN-based and 3GPh-5CzBN-based counterparts, respectively. This work demonstrates the cutting-edge EQE of solution-processable narrowband devices, highlighting unique advantages of structural control for dendritic molecules.