Cascading Energy Transfer for Highly Efficient Deep−Red OLED Emission with Cyclometalated [3+2+1] Iridium Complexes

Abstract The promising cyclometalated iridium (III) complexes have been proved to possess great potential in vacuum−deposited organic light−emitting diodes (OLEDs) applications for full−color displays and white solid−state lighting sources. Herein, based on the unique bidentate ligand of dibenzo[a,c]phenazine (dbpz) group with strong conjugated effect of aromatic rings for red emission, four novel [3+2+1] coordinated iridium (III) emissive materials have been rationally designed and synthesized. The monodentate ligands of −CN and −OCN have been effectively employed to tune the deep−red emission of 628−675 nm with high photoluminescence quantum yields up to 98%. Moreover, all devices displayed deep−red color coordinates ranging from (0.675, 0.325) to (0.716, 0.284), which is close to the standard−red color coordinates of (0.708, 0.292), as recommended by International Telecommunication Union Radiocommunication (ITU−R) BT.2020. The device based on n BuIr(dbpz)CN with an exciplex cohost has exhibited maximum external quantum efficiencies of 20.7% and good stability. With n BuIr(dbpz)CN as an effective sensitizer, the n BuIr(dbpz)OCN based phosphorescent OLED devices have successfully demonstrated cascading energy transfer processes, contributing to pure red emission with maximum luminance as high as 6471 cd m −2 . Therefore, this work has been successfully demonstrated rational molecular design strategy of [3+2+1] iridium complexes to obtain highly efficient deep−red electrophosphorescent emission.