The competitive behavior of exciton reactions in Rubrene doped system

Abstract

In this work, organic light-emitting diodes (OLEDs) have been fabricated with 1 percent of tetraphenyldiben- zoperiflanthene (DBP) doped into 5,6,11,12-tetraphenylnaphthacene (Rubrene) as active layer, which had excellent properties such as strong electroluminescence, high stability and low threshold voltage. The magneto-eletroluminescence (MEL) curves of this devices have been measured under various injection currents at room temperature. The MEL curves exhibit a complicated line shape: the MEL increases sharply with a tiny amplitude first with increasing magnetic field from 0 mT to 27 mT and then decreases rapidly with a large amplitude until at about 200 mT followed by a slow increase in the high field, and this type of shape shows substantially changes during different currents. The non-monotonic increase and decrease of the MEL suggested that there are abundant exciton reacitons inside the devices, such as singlet-triplet annihilation (STA), triplet-triplet annihilation (TTA, or triplet fusion) and singlet fission (STT). These three reactions could be tuned by changing the injection current. The STA and TTA play the mainly role when the injection current is large enough in the devices, and the exciton reaction gradually changes from TTA to STT when the injection current decreases. Furthermore, The MEL curves have also been measured by altering the thickness and location of the doped layer in the devices. Result shows that the STT increases as the thickness of doped layer decreases while the STA and TTA becomes more dominant when the location of doping layer is more closer to the cathode. In conclusion, this study gives insight into the microscopic evolutions of the interactions between singlet and triplet excitons and provides a feasible pathway to control the competition among STA, TTA and STT in OLEDs.