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 (L_T) 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 L_T 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 L_T 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.