The critical role of the small singlet-triplet energy gap in heavy-atom containing TADF emitters for high performance electroluminescence

Thermally activated delayed fluorescent (TADF) molecules are promising emitters for organic light-emitting diodes (OLEDs). However, they often experience significant efficiency roll-off at high brightness levels due to a relatively slow reverse intersystem crossing rate (k_RISC). Both a small singlet-triplet energy gap (ΔE_ST) and the heavy-atom effect have been shown to effectively accelerate the k_RISC of TADF emitters. Despite this, the interplay and relative importance of these two factors have rarely been explored. In this study, we designed two TADF emitters by connecting a selenium-containing electron donor with a triazine acceptor. Although both emitters share a similar molecular framework, the heavy-atom effect in the control emitter, which has a larger ΔE_ST, negatively impacts its optical properties, leading to a lower photoluminescence quantum yield (PLQY) and weaker TADF characteristics. In contrast, the emitter with a smaller ΔE_ST exhibits significantly better optical properties, including higher PLQY, much faster k_RISC, and consequently, improved device performance. This work highlights that a small ΔE_ST is the prerequisite for the heavy-atom effect to enhance k_RISC and optimize TADF molecule design.