Sterically shielded blue thermally activated delayed fluorescence emitters with improved efficiency and stability

Blue emitters with thermally activated delayed fluorescence (TADF) have the potential to achieve 100% internal quantum efficiency, though the strategy to design stable blue TADF emitters remains unexplored. Here, high efficiency and improved stability are achieved simultaneously for blue or sky blue TADF emitters by wise designing of molecular structures using tert-butyl units. The tert-butyl substituents act like a shield surrounding the luminance core, not only promoting the photoluminescence efficiency, but also improving the stability of the compounds. Consequently, the device with a sterically shielded emitter achieves a maximum external quantum efficiency as high as 21.2% and a record long T50 (time to 50% of initial luminance) of 770 h at an initial luminance of 500 cd m(-2), corresponding to a half lifetime of 12 873 h at 100 cd m(-2).