Triplet-triplet annihilation process in organic light-emitting diodes doped with fluorescent dyes

Abstract

At room temperature, the magnetic field dependent electroluminescence (EL) was investigated in red fluorescent dye doped organic light-emitting diodes (OLEDs) of ITO/N,N′-bis(naphthalen-1-y)-N,N′-bis(phenyl) benzidine (NPB)/ tris(8-hydroxyquinolato) aluminum (Alq3):4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM)/ Alq3/LiF/Al. It is observed that the applied magnetic field induces a sharp increase of EL in low field regime (<italic>B</italic>≤50 mT) and then a remarkable decrease at higher fields (50 mT&lt;<italic>B</italic>≤500 mT). Furthermore, the high-fields decrease in EL will be stronger with higher doping concentration and bias voltage. While in another type of OLEDs doped with green fluorescent dye of 5,12-dihydro-5,12-dimethylquino [2,3-b]acridine-7,14-dione (DMQA), the low-fields rapid increase is also presented while the high-fields effect exhibits a slow increase and gradual saturation. These results demonstrate that the triplet-triplet annihilation (TTA) process hardly occurred in DMQA doped devices because its Förster energy transfer process is not conducive to the generation of triplet excitons; while in DCM doped devices whose EL is dominated by charge carrier trapping process, the probability of TTA process is enhanced due to the prolonged lifetime of triplet excitons. Therefore, based on two different emission mechanisms, the applied magnetic field plays different roles in controlling the evolvement of triplet pairs (T…T) in OLEDs.