Thermally activated delayed fluorescence host materials with bulky indolocarbazole derivatives acceptors for high-performance solution-processed OLEDs

The design and development of large steric-hindrance host materials can effectively solve the quenching of triplet excitons, so as to improve the utilization of excitons and the performance of organic light-emitting diodes (OLED) devices. In this work, four large volume thermally activated delayed fluorescence (TADF) host materials were synthesized using triazine derivatives as electron acceptors and indolocarbazole derivatives as electron donors. Systematic studies demonstrated that introducing tert-butyl carbazole into indolocarbazole derivatives not only increased the steric hindrance of the molecule but also improved its thermodynamic stability. The decomposition temperatures (T_d) of TRZ-tCzICz and MeTRZ-tCzICz reached 449 °C and 481 °C, and there was no significant glassing transition temperature (T_g) in the range of 20–200 °C. The strategy of introducing methyl into triazine effectively separated HOMO and LUMO of molecule, resulting in smaller singlet-triplet splitting (ΔEST) and larger reverse intersystem crossing rates (k_RISC). Among the four molecules, the k_RISC of TRZ-tCzICz and MeTRZ-tCzICz reached 2.07 × 10⁶ s⁻¹ and 2.16 × 10⁶ s⁻¹, respectively. All four synthesized molecules had relatively high triplet energy levels and can serve as host materials for green TADF compounds. With 4CzIPN as the dopant, the solution processable OLED devices based the four compounds as the host materials achieved maximum external quantum efficiency (EQE_max) of 11.13 %.