Interfacial engineering of organic light-emitting diodes with NaCl-incorporated anodic buffers

Abstract

Power efficiency and device stability of organic light-emitting diodes (OLEDs) are crucial factors for practical applications such as displays and lightings. Many efforts have been made to improve the external quantum efficiency of OLEDs in various aspects such as synthesis of phosphorescent materials and design of device architecture. In this thesis, the driving voltages and device lifetimes are improved through the interfacial engineering at anodic side by introducing a new interlayer of NaCl:organic composite. The mechanistic origins of the improved efficiency are investigated with ultraviolet photoemission spectroscopy and atomic force microscope. Because metal halides have been widely applied at the interface of cathode and organic layer, the application of NaCl-incorporated organic composite at anode/organic interface implies simpler fabrication of highly efficient OLEDs by introducing the same metal halide for both electron and hole injection efficiency. In chapter I, the physics of OLED device is briefly introduced to give background information for understanding and analyzing the experimental results for the electrical characteristics of OLED devices. Power efficiency and device degradation mechanisms are mentioned based on the previous reports. Then, the electrode/organic interfacial engineering approaches for enhancing device performance are reviewed to support the importance of current study. In chapter II, the effects of ultraviolet-ozone (UVO) and O2-plasma surface treatments of indium tin oxide (ITO) anode surface are compared for OLED performance. While the OLED device with UVO-treated ITO anode shows a lower drive voltage than that of O2-plasma treated ITO device during device operation, the lifetime is shorter. Through photoelectron emission spectroscopy measurements, decrease in tin (Sn) concentration was observed after the UVO treatment, which is attributed to the increased work function of ITO and to the decrease in surface conductivity, resulting in a lowered drive voltage. The shorter lifetime of the hole injection-efficient device is attributed to larger charge imbalance and more carbon contaminants. In chapter III, we show that the composite buffer layers of N,N-bis(naphthalene-1-yl)-N,N-bis(phenyl)benzidine (NPB) and NaCl at the anode/organic interface are very effective for improving both hole injection efficiency and device stability. Furthermore, two maxima of current injection are observed with compositional variation of the buffer, implying that there exist multiple charge injection mechanisms such as tunneling effect via insulating property of NaCl and interfacial energetics. In chapter IV, we suggest the mechanistic origins of the improved OLED performance upon the use of a NaCl-containing organic interlayer between the ITO anode and NPB through the studies with ultraviolet photoelectron spectroscopy and atomic force microscopy. While a pure NaCl interlayer has a high hole-injection barrier (1.40 eV), the NPB:NaCl composite layer exhibits a substantially lower barrier (0.84 eV), which is comparable to the value at bare ITO/NPB interface. Furthermore, the wettability of the composite onto ITO is enhanced due to significant adhesive interactions of NaCl with both ITO and NPB (work-of-adhesions of 252 mJ/m2 and 168 mJ/m2, respectively), leading to effective electrical contacts. The two key factors of the plausible hole-injection barrier and the better wettability of the NPB:NaCl composite contribute to the improved hole injection efficiency and life time.