Analysis of Intrinsic Degradation Mechanism in Organic Light-Emitting Diodes by Impedance Spectroscopy

The organic light-emitting diodes (OLEDs) in the sandwiched structures ITO/poly3,4-ethylenedioxythiopene (PEDOT)/poly9,9-dioctylfluorene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole (PFO-DBT15)/Ba/Al are fabricated. We use impedance measurements to investigate the degradation of aged OLEDs. A detailed analysis of the impedance spectra as functions of frequency and dc bias yields information about the injection of the interfacial electrode and the transport properties of emissive layer changes. Morphology differences between degraded and undegraded devices can also be observed by a scanning electron microscope. We perform comparative studies of the impedance spectroscopy (IS) of undegraded and degraded devices by both experiment and simulation approaches to explain the degradation mechanism for OLEDs. The IS of the undegraded device can be well understood by simply adopting the conventional RC equivalent circuits. For the degraded device, however, the successful model of the impedance spectroscopy results needs to take into account the more complicated situations, and we adopt a constant phase element to meet the experimental and simulated data and discuss the reasons caused by the problem.