Crafting host materials for narrowband blue OLEDs with low efficiency roll-off by the medium-ring strategy

Abstract

The pursuit of high-performance narrowband blue organic light-emitting diodes (OLEDs) is of paramount importance in both academic research and industrial applications. While obvious strides have been made in the design of narrowband blue emitters, the development of appropriate host materials has evidently trailed behind. Herein, we introduce a medium-ring strategy for crafting host molecules based on the heptagonal tribenzo[b,d,f]azepine (TBA) unit. The twisted three-dimensional (3D) architecture of the TBA framework not only endows the host molecules with fast hole-transporting pathways but also effectively reduces exciton quenching. Equipped with two TBA units, DTBA, synthesized with ease, demonstrates a huge horizontal orientation factor (Θ_//) of 93.5% and a broad emission spectrum for accelerating the energy transfer process in the emitting layer, which contributes to enhancing device performance. We have fabricated high-performance narrowband blue OLEDs using DTBA as the host, FIrpic as the phosphor sensitizer, and the widely used boron-nitrogen-containing multiple resonance emitter BCz-BN. These devices exhibit a maximum external quantum efficiency (EQE_max) as high as 31.0% with an impressively low-efficiency roll-off. Even at a high luminance level of 10,000 cd m⁻², the EQE value remains noteworthy at 20.3%, marking a significant advancement in BCz-BN-based devices.