Slow recombination of the de-trapped carriers from doped OLEDs induced by spontaneous orientation polarization

Abstract

The spontaneous orientation polarization (SOP) induced by molecular permanent dipole moment has recently been discovered and plays an important role in the photoelectric performances of small molecular organic light-emitting diodes (OLEDs). However, the effect of SOP on the transient electroluminescence (TEL) dynamics of OLEDs is still vague. Herein, we find that the generation of the spike at the TEL falling edge in doped OLEDs depends remarkably on the SOP in the devices. That is, SOP is a disadvantage for spike formation and even causes the spike to disappear totally. In addition, a long-lived EL decay was observed in the OLEDs with strong SOP. Through a series of control experiments, it has been confirmed that the SOP-induced internal electric field in doped OLEDs would slow down the recombination process of the de-trapped carriers, leading to a suppression of the TEL spike and a long-lived EL decay. Accordingly, our findings not only supplement the prerequisites for the generation of the TEL spike, i.e., with weak SOP effects in an emissive layer, but also deepen the fundamental understanding of the SOP role in the TEL dynamics process of OLEDs.