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

Abstract

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 (T_d) up to 506 °C and outstanding morphological stability with a glass transition temperature (T_g) exceeding 145 °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 (E_T) 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 (EQE_max) 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 EQE_max of 29.8% and low efficiency roll-off.