Spirobifluorene-based hole-transporting materials for RGB OLEDs with high efficiency and low efficiency roll-off

In this work, we designed and synthesized three spirobifluorene (SBF)-based hole-transporting materials (HTMs) by incorporating the di-4-tolylamino group at different positions of the SBF skeleton. These materials demonstrate excellent thermal stability with thermal decomposition temperatures (Td) up to 506 degrees C and outstanding morphological stability with a glass transition temperature (Tg) exceeding 145 degrees C. The meta-linkage mode between the conjugated skeleton and functional groups in the molecular structure results in electronic decoupling, giving these 3,6-substituted SBFs higher triplet energies (ET) compared to 2,7-substituted SBFs. This makes the 3,6-substituted SBFs suitable as universal HTMs for red, green, and blue (RGB) organic light emitting diodes (OLEDs). Among the three HTMs, 3,3 ',6,6 '-tetra(N,N-ditolylamino)-9,9 '-spirobifluorene (3,3 ',6,6 '-TDTA-SBF) exhibits the best device performance, achieving maximum external quantum efficiencies (EQEmax) of 26.1%, 26.4%, and 25.4% for RGB phosphorescent OLEDs, with extremely low efficiency roll-off in both green and blue devices. Utilizing 3,3 ',6,6 '-TDTA-SBF as the HTM, we have also fabricated narrowband blue OLEDs based on the widely used multiple resonance emitter BCz-BN, which exhibits a EQEmax of 29.8% and low efficiency roll-off. A new kind of 3,3 ',6,6 '-tetrasubstituted spirobifluorene-based hole-transporting material (HTM) is designed and synthesized, which enables excellent device performance for RGB OLEDs with high efficiency and low efficiency roll-off.