Study on triplet exciton diffusion length of mCP in phosphorescent organic light-emitting devices using electroluminescent spectra

Electroluminescent (EL) spectra was employed to probe the triplet exciton diffusion length (LT) of a commonly used host material of N,N′-dicarbazolyl-3,5-benzene (mCP) in phosphorescent organic light-emitting devices (OLEDs). By varying the film thickness of bis [2-(4-tertbutylphenyl) benzothiazolato-N,C2], iridium (acetylacetonate) [(t-bt)2Ir(acac)] phosphor doped layer within 30 nm thick mCP layer, a series of devices were fabricated to investigate the EL characteristics. The results showed that with the increasing doped layer thickness (d), both (t-bt)2Ir(acac) emission peaks at 562 nm and mCP emission centered at 403 nm were observed. Moreover, the relationship between mCP EL intensity and d was detected. The LT was induced by an abrupt decrease in variation of mCP EL intensity when d is increased from 10 to 15 nm, and the reason to cause this phenomenon was investigated. The LT of mCP approximately to 15 nm was perfectly consistent to the result of 16±1 nm, which was calculated by the traditional steady-state diffusion model.