Triplet decay-induced negative temperature dependence of the transient photoluminescence decay of thermally activated delayed fluorescence emitter

The photophysical properties of three TX-based D–A isomers (TXO-PhCz1, TXO-PhCz3, and TXO-PhCz4) with a PhCz donor at different substitution positions of phenyl group on a TXO unit were investigated. The substitution position of the PhCz unit significantly impacts the photophysical properties of these isomers. TXO-PhCz1 exhibits a very weak emission in both undoped and doped films, while TXO-PhCz3 and TXO-PhCz4 exhibit a strong emission with the requisite properties for TADF emitters, including a small ΔEST and transient PL decay curves with a prompt and delayed fluorescent component. TXO-PhCz4 exhibits a much stronger orbital coupling than TXO-PhCz3 and then the phosphorescent emission causes the inverse temperature dependence of the transient PL decay, which is contrary to that of TXO-PhCz3 and other TADF emitters. TXO-PhCz4 exhibits a small ΔEST of 23 meV and a short decay time of 14 μs at room temperature, which are much smaller and shorter than those of TXO-PhCz3. Multilayer OLEDs based on TXO-PhCz4 exhibit a very low-efficiency roll-off with a maximum current efficiency of 49.2 cd A−1, a maximum power efficiency of 47.7 lm W−1, and a maximum EQE of 16.3%.