Evoking Synergetic Effect of Dual Thermally Activated Delayed Fluorescent Hosts for High-Efficiency Sensitized Fluorescent Organic Light-Emitting Diodes

Thermally activated delayed fluorescent (TADF) sensitized fluorescent organic light-emitting diodes (FOLEDs) provide the possibility of achieving 100% exciton utilizing efficiency for remarkably enhanced device efficiency. However, the TADF sensitization process to harvest triplet excitons for electroluminescence is still unclear; thus, TADF-sensitized FOLEDs remain to be further explored and modified. Herein, a dual-TADF cascade sensitization strategy was proposed to promote the sensitization process through creating an additional efficient sensitizing channel for high-performance TADF-sensitized FOLEDs. It was found that the synergetic effects of two introduced TADF hosts would be established only when the Förster resonance energy transfer (FRET) processes from the 1st TADF host to the 2nd TADF host and then to the fluorescent dopant occurred efficiently. Compared to the traditional single-TADF sensitization process, the constructed dual-TADF cascade sensitization strategy, i.e., 1st TADF → 2nd TADF → fluorescent molecule energy-transfer process, supplies a more valid energy funneling route with the assistance of the 2nd TADF host. Thus, an efficient dual-TADF-sensitized FOLED device with a significant 20% efficiency enhancement was achieved based on two commercial TADF host molecules. The rational selection of TADF hosts and utilization of dual-TADF sensitization strategy would open a new paradigm for the design of high-performance FOLEDs.