Blue TADF Emitters Based on <i>B</i>-Heterotriangulene Acceptors for Highly Efficient OLEDs with Reduced Efficiency Roll-Off
Young Hoon Lee, Woochan Lee, Taehwan Lee, Donggyun Lee, Jaehoon Jung, Seunghyup Yoo, Min Hyung Lee
Abstract
The design of robust boron acceptors plays a key role in the development of boron-based thermally activated delayed fluorescence (TADF) emitters for the realization of efficient and stable blue organic light-emitting diodes (OLEDs). Herein, we report a set of donor (D)–acceptor (A)-type blue TADF compounds (1–3) comprising triply bridged triarylboryl acceptors, the so-called B-heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (1), dimethylmethylene (2), or oxo (3). The X-ray crystal structures of 2 and 3 reveal a highly twisted D–A connectivity and a completely planar geometry for the B-heterotriangulene rings. All compounds exhibit blue emissions with the unitary photoluminescence quantum yields and small singlet–triplet energy splitting (<0.1 eV) in their doped host films. The compounds exhibit a fast reverse intersystem crossing rate (kRISC ≈ 106 s–1) with short-lived delayed fluorescence (τd ≈ 2 μs), which is found to be promoted by the strong spin–orbit coupling between the local triplet excited state (3LE, T2) and singlet (S1) states. Using compounds 1–3 as the emitters, highly efficient blue TADF-OLEDs are realized. The devices based on the emitters with B-heterotriangulenes exhibit better performances than the device incorporating a singly bridged reference emitter over the whole luminance range. Notably, the device based on the fully dimethylmethylene-bridged emitter (2) achieves the highest maximum external quantum efficiency (EQE) of 28.2% and the lowest efficiency roll-off, maintaining a high EQE value of 21.2% at 1000 cd/m2.