Impedance spectroscopy of organic hetero-layer OLEDs as a probe for charge carrier injection and device degradation

摘要

Impedance spectroscopy (IS) is a powerful method for characterizing the electrical properties of materials and their interfaces. In this study we use IS to investigate the charge carrier injection properties of different anodes and anode treatments in bottom-emitting organic light-emitting diodes (OLEDs). These are ITO-based (indium tin oxide) hetero-layer devices with TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4-diamine) as hole transporting layer (HTL) and Alq_3 (tris-(8-hydroxyquinoline) aluminum) as emission and electron transporting layer (EML and ETL, respectively). A detailed analysis of the capacitance as function of frequency and DC bias yields information about trapped and interfacial charges as well as the dynamics of injected charges. Furthermore, we use IS to study degradation processes in OLEDs.