Using light-emitting dyes as a co-host to markedly improve efficiency roll-off in phosphorescent yellow organic light emitting diodes

We discovered in this study the feasibility of using regular light-emitting dyes as an effective co-host, rather than a sensitizer, to markedly improve the efficiency of phosphorescent organic light emitting diodes. At 10 000 cd m(- 2), for example, the efficacy of a yellow emitter containing device was increased from 11.7 lm W-1 to 15.4 lm W-1, an increment of 32%, as a sky-blue phosphorescent dye, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), was blended into a host of 4,4',4"-tri(N-carbazolyl)triphenylamine (TCTA). The efficacy at 1000 cd m(-2) was 26.7 lm W-1, the highest among all reported yellow OLEDs with a solution-processed emissive layer. The marked efficiency improvement may be attributed to the co-host having an electron trapping character, enabling excitons to generate on itself instead of on the guest, creating an additional efficiency-effective energy transfer route, and having a very efficient co-host to guest energy transfer. The most effective co-host may vary with the variation of the host employed, depending on the energy level pairing of the co-host and host.