Insight into Design of Sterically Hindered Multi-Resonance Induced Charge Transfer Molecule for Highly Efficient Deep-Blue OLEDs

Abstract

The search for narrowband emission, highly efficient pure-blue organic light-emitting diodes (OLEDs) has been left as an exigent issue for the development of ultrahigh-definition displays for decades. To set the stage for this intractable problem, herein, we report the highly efficient pure-blue OLEDs with the multi-resonance (MR) induced thermally activated delayed fluorescence (TADF) material, o-Tol-ν-DABNA-Me, achieving relatively deep blue light with CIE xy coordinates of (0.11, 0.12), narrow emission (full-width at half-maximum of 18 nm), and high molecular anisotropy of 0.95, leading to the maximum maximum external EL quantum efficiency (EQE) ~ 33 %. Employing hyperfluorescent architecture, used in boron-based charge-transfer (CT) type TADF (DBA-SAF), as an assistant dopant in the ternary emission layer, the pure-blue OLED performance can be further enhanced up to EQE of 35.2 % (56.1 % with a half-cylinder lens) with mitigated efficiency roll-off behaviors. Through comprehensive spectroscopic and theoretical results in this study, above all, the steric hindrance introduced in the proposed molecular design promotes high degree of horizontal TDM orientation with little dependence on blue host molecules and tends to reduce the problematic long-wavelength excimer peak, suppression of which is critical for deeper blue emission.