Assistant dopant system in solution processed phosphorescent OLEDs and its mechanism reveal

We have developed an assistant dopant system in phosphorescent OLEDs. The system is consisted of a conventional host material 4,4′-Bis(carbazol-9-yl)biphenyl (CBP), a thermally activated delayed fluorescence (TADF) 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) and a red phosphor Bis[2-(4-n-hexylphenyl)quinoline](acetylacetonate)iridium(III) (Hex-Ir(phq)2(acac)), it has achieved maximum current efficiency and external quantum efficiency of 21.52 cd/A and 9.22%, respectively. The intrinsic mechanism and energy transfer process in the system have been discussed in detail. Moreover, a satisfactory critical current density J0 of 106.1 mA/cm2 has been obtained when the assistant dopant concentration ascends to a suitable level, which indicated low efficiency roll-off of the device. By investigating the exciton generation mechanism and the transient electroluminescence (EL), the main cause of the improvement has been revealed.