Optimization of yellow phosphorescent organic light-emitting devices based on triplet exciton diffusion length

Study on the diffusion of triplet excitons from light emitting layer (EML) in organic light-emitting devices doped with yellow phosphorescent material to the adjacent hole transporting layer (HTL) is carried out, in which a non-doped red phosphorescent ultra-thin layer as an exciton-sensing layer is set in various position of HTL. A diffusion length of triplet exciton between 3 nm and 5 nm is inferred from the observed and disappeared red light emission in electroluminescent spectra. For further device optimization, either 5 nm 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) or 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as a exciton blocking layer is introduced to block the exciton diffusion. The maximum current efficiency and power efficiency of the optimized device are reached to 24.6 cd/A and 16.3 lm/W, respectively. The high performance is ascribed to confined diffusion of triplet excitons from light-emitting zone and higher radiation efficiency of the triplet exciton.