Assistant dopant system in red phosphorescent OLEDs and its mechanism reveal

We have constructed an assistant dopant system in red phosphorescent OLEDs. In the system, we chose the 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) as the host material, a thermally activated delayed fluorescence (TADF) 4CzIPN as the assistant dopant and a red phosphor Hex-Ir(phq)2(acac) as the guest. The assistant dopant system can remarkably reduce the turn-on voltage and increase device brightness. Meanwhile, devices with assist dopant system show negligible efficiency roll-off even in high luminance and current density. The current efficiency of optimized device with 15 wt% 4CzIPN only drops to 12.7 cd/A at 74,532 cd/m2 from maximum 13.7 cd/A. By further investigation on the exciton generation mechanism and the energy transfer process in the system, the main cause of the improvement has been unveiled.