by Meng-Huan Ho, Chang-Yen Wu, and Shang-Yu Su

Organic light-emitting-diode (OLED) technology and its application for high-information-content displays continue to be a focus of many research programs around the world. Creating an active-matrix OLED (AMOLED) display requires the use of thin-film transistors (TFTs) such as amorphous-oxide TFTs. However, there are compatibility issues with a-si TFTs and OLED materials, as our student team from the National Chiao Tung University in Taiwan discovered. With guidance from our professor, we sought to find a solution to this compatibility problem by developing an inverted-type OLED structure called an IOLED, as well as by implementing the solution on a flexible substrate to demonstrate the full range of innovation possible using our ideas.

Our efforts, as described in the paper "Flexible inverted bottom-emitting organic light-emitting devices with a semi-transparent metal-assisted electron-injection layer," were recognized by the receipt of the 2010 Outstanding Student Paper award from the Journal of the Society for Information Display, bestowed each year to a published student paper on the basis of originality, significance of results, organization, and clarity.

Background

During the time that our team members, Chang-Yen Wu, Shang-Yu Su, and Meng-Huan Ho (Fig. 1), were pursuing their graduate studies at the National Chiao Tung University in Taiwan, organic light-emitting-diode (OLED) technology had begun to draw increasing attention as the next-generation display platform (as well as a potential source for general illumination). We believed that among existing display technologies, active-matrix organic light-emitting diodes (AMOLEDs) had the strongest potential.

Fig. 1: The paper authors among these NCTU OLED lab group members are Chang-Yen Wu (second from right), Shang-Yu Su (fourth from right), Meng-Huan Ho (center), and Prof. Chin H. (Fred) Chen (third from left).

At the same time, amorphous-oxide TFTs have attracted much attention and are seen as the next-generation TFT backplane for AMOLEDs. They appear to have neither instability issues in terms of mobility nor a sub-threshold gate-voltage swing, and they exhibit large carrier mobility. Moreover, oxide TFTs can be deposited at much lower temperatures, which, in principle, makes possible the mass production of AMOLEDs on flexible plastic substrates. However, oxide-TFTs can only be used to fabricate n-channel TFTs. For conventional OLEDs, the bottom anode can only be fabricated at the source end of the driving oxide TFT, which invariably impacts the stability of the source voltage that depends on the voltage drop across the OLED materials. The most direct way of solving this problem is to use an inverted-type OLED (IOLED) for n-channel TFTs because it provides a bottom cathode that can be connected to the drain end of the n-channel TFT through which the current circuit of the TFT can be decoupled from the resistive loss of the OLED materials. Accordingly, the research and development of IOLEDs have become increasingly important and timely with regard to the realization of oxide TFTs with n-channel-driven large-panel AMOLEDs. Figure 2 shows a diagram of an inverted-type OLED integrated with an oxide TFT.

Fig. 2: A diagram of an inverted-type OLED integrated with an oxide TFT.

Lab Work

Typical OLEDs possess a transparent ITO electrode with high work function as the anode. For inverted bottom-emitting OLED (IBOLED) devices, the ITO has to be inverted to function as a cathode. This creates some problems because it is difficult to inject electrons from ITO into the organic layer, due to their severe energy level mismatch. This mismatch, in turn, causes the drive voltage to rise sharply and the efficiency to fall off.

The team members' advisor, Professor Chin H. (Fred) Chen, thought that electron injection could be one of the key factors in developing IBOLEDs and encouraged his students to improve the carrier injection and device performance of flexible IOLEDs. For this project, we chose plastic polyethersulphone (PES) as our flexible substrate because it has a higher glass-transition temperature than most of the other commercially available flexible substrates.

Su was responsible for sputtering ITO on the PES substrate. Ho provided a general n-i-p IBOLED structure to overcome the carrier-injection barrier. One day, Wu came up with a synergy idea to intentionally create a microcavity within the IBOLEDs, which is accomplished by inserting a thin semi-transparent silver (Ag) layer between the ITO and the n-doped layer.

By using this concept in our OLED design, we found that the inserted thin Ag layer not only improved the electron injection but also enhanced the device's normal efficiency and color saturation through the cavity structure between a high-reflection back electrode and a semi-transparent metallic ground contact. By using these flexible IBOLEDs along with the synergistic microcavity effect, we were able to achieve maximum efficiencies that were 1.5 times higher than those of conventional OLEDs, representing more than a 20% improvement over an IBOLED without using Ag thin film.

We therefore demonstrated that both power efficiency and color saturation in an IBOLED on a flexible PES substrate can be enhanced by inserting a semi-transparent metal-assisted electron-injection layer between ITO and the n-doped ETL. This created a beneficial microcavity effect, which could be exploited to enhance color saturation without impacting its electrical properties. We believe that among existing display technologies, AMOLEDs have the best potential to become the "ultimate display" solution, due to their fast motion-picture response time, vivid color, high contrast, and super-slim lightweight nature. We expect that the technology of flexible AMOLEDs will further mature in the near future and that flexible AMOLED products will be seen in the marketplace soon.

On behalf of the Organic Light Emitting Diode Technology Research Laboratory, we deeply appreciate the selection of our paper by the JSID Awards Committee. This award gives us great encouragement with regard to the further development of our advanced research.

Chang-Yen Wu and Shang-Yu Su received their masters' degrees from the Institute of Electro-Optical Engineering and the Display Institute, respectively, at National Chiao Tung University (NCTU), Taiwan, in 2009. Meng-Huan Ho received his Ph.D. degree from the Department of Applied Chemistry at NCTU in 2010. •