Sudden death of organic light-emitting diodes

The degradation in light output of an Organic Light Emitting Diode (OLED) has been studied extensively and has been explained by different mechanisms, such as formation of chemical defects or electrical traps and by thermally induced inter-diffusion of dopants. However, there is an overlooked type of degradation, where the light output decreases rapidly with time. This catastrophic failure can often be attributed to a hard electrical short due to local defects. Here, we show that this "sudden death" can also occur in the absence of a hard electrical short. We investigate this phenomenon by current-voltage characteristics and small-signal impedance measurements on typical OLEDs with a LiF cathode interlayer. We show that in a short period of time the built-in voltage of the diode vanishes; the J-V characteristics become symmetric. The origin is a dramatic increase in the work-function of the LiF interlayer. The interlayer changes from an electron-injecting contact to a quasi-Ohmic hole-injecting contact. The pristine bipolar diode does not become electrically shorted, but suddenly transforms into a unipolar hole-only diode. Upon applying a high voltage the original diode is restored, manifesting the dynamic switching of the LiF contact. (C) 2015 Elsevier B.V. All rights reserved.