Abstract
Two kinds of organic light-emitting diodes with structure of ITO/NPB/Alq<sub>3</sub>/LiF/Al (Alq<sub>3</sub> bilayer device) and ITO/ NPB/Alq<sub>3</sub>:DCM/Alq<sub>3</sub>/LiF/Al (Alq<sub>3</sub>:DCM device) were fabricated. The mechanisms of delayed fluorescence (DF) in the two devices were studied by transient electroluminescence. These results show that the DF in the Alq<sub>3</sub>-based bilayer device decays fast after removing the pulsed bias. We suggest that the DF in bilayer device is caused by the decay of the singlet excitons which were formed by the delayed charge injection after removing the pulsed bias. In contrast, the DF in the Alq<sub>3</sub>:DCM device is very strong. The dependences of DF in the Alq<sub>3</sub>:DCM device on reverse bias after removing the pulsed bias and on the pulsed width were also analyzed. We found that the detrapping and then recombination of the trapped charge can only generate a small fraction of DF intensity in Alq<sub>3</sub>:DCM device. Large fraction of the DF intensity results from triplet-triplet annihilation process.
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