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

Abstract 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 (Φ_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 Φ_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.