Exploring the Key Factors of TADF Materials as Sensitizers: Toward High-performance Triplet Fusion Upconversion

Employing metal-free thermally activated delayed fluorescence (TADF) materials as novel photosensitizers (PSs) for triplet-triplet annihilation upconversion (TTA-UC) systems has received great attention. Nevertheless, how to choose suitable TADF materials is still a gap that deserves further in-depth research. Herein, by systematically investigating the three TADF-based TTA-UC systems, it is found that TADF materials with multiple resonance (MR) effect are more suitable PSs for high-performance TTA-UC systems than those with intramolecular charge transfer (ICT) properties. Therefore, unlike a negligible maximum UC quantum efficiency (phi(')(UC), with a theoretical maximum of 100%) of 0.05% for ICT-1 with a highly twisted molecular structure, MR-TADF materials MR-1 and MR-2 offer superior UC performance with phi(')(UC) values above 12.7%. All these results suggest that employing MR-TADF materials as the PS is the key to achieving efficient TTA-UC performance.