|| In this dissertation, the electrical characteristics of the interface between the metal cathode and organic layer in OLEDs are detailed investigated. Currently, surveying on the literature, it is limited in understanding the interfacial characteristics and the injection process of electron at interface, therefore the carrier injection phenomena investigated here is still based on the traditionally inorganic semiconductor physics.|
As a thin LiF layer inserted between the Al and Alq3 layers, the performance of OLED shown a great deal of improvement, such as lowering the driving voltage and increasing the luminescence efficiency. At first, we study how a very thin LiF layer affecting the performance of OLED device, and the feasible mechanisms attributed to this improvements. Then, the further discussion should be focused on the injection model built for the charge at metal/organic interfaces. Finally, the relationship that the injection model related to the variety of LiF layer thickness could be investigated.
From the experimental data, the Al/LiF/Alq3 devices with the LiF thickness of 0.5nm have shown the best performance, and the device performance decay as the thickness of LiF layer increased over 0.5nm. In this study, it assumes that the LiF layer just forms an “integrated” thin insulating film and lowing the charge injection as the layer thickness over 0.5nm, and it also assume that in model derived process is independent on the metal work function.
Since the molecular structure of organic materials is quite difference from the valence band structure in inorganic semiconductor materials, it could be assumed no band bending like that the p-n junction at the interface of inorganic semiconductor under thermal equilibrium. After theoretical approach, we get the reasonable results by comparing with the literatures reported recently.
The conclusion of this study reveals that the charge injection is independent on the metal work function, but is determined by the interface structure characteristic of interface structure at metal/organic interfaces. Furthermore, in the Al/LiF/Alq3 structure, the chemical reaction is saturated at the interface as the LiF layer forms a “integrated” thin insulating film. Therefore, the device performance decay is the effect due to the insulating LiF layer when the thickness of LiF layer is over 0.5nm.