Direct silicon-nitrogen bonded host materials with enhanced σ-π Conjugation for blue phosphorescent organic light-emitting diodes

Silicon-containing ultrahigh-energy gap hosts (UGHs) have emerged as important candidates of high-performance host materials with high thermal stability and triplet energy for blue phosphorescent organic light-emitting diodes (PhOLEDs). However, the highest occupied molecular orbital (HOMO) of these UGHs are generally too deep to support balanced hole injection and transportation in devices. Here, we propose a new design strategy of UGHs by multiple introduction of strong electron-donating and high-triplet-energy units of carbazoles into the electron-accepting arylsilanes in the N-Si-N structure. The facilely synthesized carbazole-arylsilanes in one-step show high thermal stability, triplet energy and charge mobilities with high-lying HOMOs due to enhanced σ-π conjugation in the N-Si-N structure as revealed by combined experimental and theoretical investigations. Impressively, blue PhOLEDs hosted by these novel N-Si-N bonded UGHs exhibit an improved maximum current efficiency up to 39.5 cd A^-1, a power efficiency of 27.4 lm W^-1, and an external quantum efficiency of 24.2%, demonstrating significant advances in the design of UGHs by adjusting the d-orbital participation of π-conjugation to enhance the σ-π conjugation in donor (D)-acceptor (A) molecular architectures.