| Indium tin oxide (ITO) thin film, as a kind of n type degenerate transparent conductive oxide (TCO), has been attracted increasing attention in the field of materials and electronics due to its unique characteristics, e.g. low resistivity, high transparency in the visible light region, wide energy band gap and relative high work function. Nowadays, ITO has been successfully applied in photoelectron devices, such as solar cells, liquid crystal display (LCD), photovoltaic and organic light emitting diodes (OLED) etc. OLED is a kind of solid light emitting device by carrier injection which could directly converse electrical energy into luminous energy. Compared with LCD, light emitting diodes (LED) and plasma display panel (PDP), OLED has been reputed as the most ideal and potential flat panel display of the next generation, because it takes advantages of high luminous efficiency, low energy consumption, solid-state, easy to fabricate, ultrathin and capable realization of flexible display. Meanwhile, OLED could also be fabricated as new type of solid energy-saving lamp for its characteristics of illuminate uniform, light soft, spectrum wide, low drive voltage, anti-conflict, no glaring and high color rendering, so it also shows strong competitiveness in lighting market.ITO is one of the most widely used anode material for OLED fabrication, which facilitate the injection of holes into the nearby organic semiconductors.The properties of the interface between ITO and organic material affect directly the whole performance of OLED. Generally, OLED fabricated directly with laboratorial deposited ITO always performs very poor because of its low work function. The performance of OLED can be greatly improved by modifying the surface of ITO, because it can increase the surface work function of ITO and low the energy barrier for hole injection. Therefore, various studies have been conducted to modify the ITO surface and subsequently improve the overall operational efficiency of OLED. It is found that ITO can get a relatively high work function increment by0.4eV by one of the most effective method oxygen plasma. However, the problem is how to further increase ITO surface work function because the energy barrier for hole injection at the ITO/organic material interface is still very high. Meanwhile, a number of interpretations have been reported to explain the change of surface work function, and the mechanism of the work function is unclear and debatable.Moreover, the enhanced work function caused by oxygen plasma treatment has been proved decay rapidly with a time-dependent curve and return to its original value within hours due to the aging effect, and the explanation and physical meaning of it is also unclear.In order to solve these problems, an approach called plasma immersion ion implantation (PⅢ) was proposed in this doctoral dissertation to modify the surface of ITO, PⅢ is a new surface modification technology emerged with the development of semiconductor technics which possess advantages of good capability to treat three dimensional work pieces and retaining the treatment effect. In this work, the author designed and built a PⅢ system with wide applicability. In order to improve the surface work function of ITO and explain the mechanism, the surface properties of ITO treated by PⅢ were deeply and systematically studied from the aspects of experiment and theory.1. Commercially used ITO thin film has been surface treated by PⅢ technology, the influence of working gas, amplitude of bias voltage, treatment time and duration of pulse on the surface properties of ITO have been studied respectively, and the optimized experimental parameters have been obtained. A comparison has also been made among PⅢ modification, inductive coupling plasma (ICP) treatment and ultrasonic solvent pretreatment. The surface properties of ITO before and after the treatment have been measured by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscope (AFM), Ultraviolet-visible spectrophotometer (UV-vis), four-point probe detector and Kelvin probe (KP). The results show that oxygen PⅢ is more effective that oxygen ICP in modifying ITO surface, because it can effectively remove the surface contamination and increase surface oxygen content, thus increase surface work function substantially. But oxygen PⅢ will cause no pronounced influence to the crystal orientation, surface resistivity, transmittance and surface morphology.2. ITO thin film had been surface treated by oxygen PⅢ by using the obtained optimal experimental parameters. A base pressure of5×10-4Pa was pumped. The pressure in the chamber was held at4Pa with oxygen flow rate of50sccm during plasma discharge. The13.56MHz radio frequency power with66W was coupled into the chamber by the horn-shaped antenna with no measurable reflection. In the PⅢ process, a negative pulse bias with amplitude of1kV, frequency of I kHz and pulse duration of10μs was applied on the substrates, the measured rise time of the pulse was less than40ns. The XPS and KP measurement results showed that ITO surface work function can be enhanced by0.4-0.5eV by oxygen ICP treatment on the basis of ultrasonic pretreatment, which is in good agreement of the reported value.However, compared with the ultrasonic pretreated sample, an increment of surface work function by1.1～1.2eV could be achieved by using oxygen PⅢ treatment. Therefore, it can be concluded that the surface work function of ITO could be further enhanced by oxygen PⅢ method on the basis of oxygen ICP, and the final work function of ITO can possibly match with the HOMO level of hole transport material in OLED.3. An interpretation of ITO surface work function increase by surface oxidation treatment has been proposed, which is based on the experimental observation that ITO surface work function has been substantially enhanced by oxygen PⅢ modification. It was concluded that the concentration of oxygen vacancies in ITO system was decreased by adsorbing oxygen from the ambient during surface oxidation treatment and the concentration of carrier in ITO was also decreased, thus work function increased. This interpretation can also give a reasonable explanation to the substantial enhancement of work function of ITO by oxygen PⅢ. Meanwhile, an extra experiment has been carried out to testifying the reliability of this interpretation, the results show an good agreement with the proposed interpretation.4. The aging effect of ITO surface work function after oxygen PⅢ and ICP has been studied by KP direct measurements. The results show a rapidly decrease of work function with time after oxygen PⅢ and ICP treatment and exposed in the air. Actually, the surface work function of oxygen ICP treated ITO sample return to its initial value five days later, but the decrease trend of the work function of oxygen PⅢ treated sample is much slower than that and will not get back to the initial value, and an increment of0.3eV will always be performed. A tailed discussion on the mechanism of this phenomenon has been also made in this doctoral dissertation.
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