Dynamic analysis of hole injection mechanism in multi layer organic devices with the structures of Al/NPB/ITO and Al/NPB/CoPc/ITO

다국어 초록 (Multilingual Abstract)

Organic electronic devices (OEDs) have been of interest for over a decade and many techniques have been developed to improve device performance. The most important progress is the adoption of organic multilayer structures. An appropriate hole injection layer (HIL) is necessary to achieve highly efficient hole injection from an anode to a hole transport layer (HTL) in OEDs. The various working mechanisms of HIL have been studied in the thermal equilibrium state, e.g. the energy level alignments with photoelectron spectroscopy (PES) measurements. Comprehensive models have been suggested. However, a working device should be understood with an applied bias through the device, that is, dynamic analysis is highly required
We analyzed the hole injection in hole only devices with the structures of Al/N,N’- bis(1-naphthyle)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine (NPB)/ITO and Al/NPB/cobalt phthalocyanine (CoPc)/ITO. Using the combined dynamic analysis of current density-voltage and impedance spectroscopy, we investigated the charge transport mechanism based on the injection limited current model. We found that the NPB single layer shows Richardson-Schottky type thermionic emission in the entire applied bias range. On the other hand, the device with the CoPc hole injection layer (HIL) shows thermionic emission until the applied bias reaches 3.7 V. Increasing the bias further, Fowler-Nordheim tunneling dominates the charge transport. We discussed the hole transport mechanism by evaluating the energy level alignment in the dynamic charge flow condition.