New Breakthrough In Flexible Semiconductors: Transmission Speed Is Enough To Drive OLED Pixels
Apr 28, 2018
Ahn and his team next hope to make a smart watch or smartphone-sized flexible screen.
One of the excellent properties of using molybdenum disulfide as a 2D semiconductor material is that they are easily bendable. Electrons can move quickly in such semiconductors. At the same time, such semiconductors are transparent because they are only about one atom thick. These features make them ideal materials for making flexible OLED displays. However, when manufacturers try to process molybdenum disulfide into transistors that control OLED pixels, the resistance between molybdenum disulfide (MoS2) and the source and drain of the transistor will be too high, making this excellent material impossible Get applied. Now, Korean engineers have found a way to apply molybdenum disulfide transistors to flexible OLED displays. They use this transistor to form a simple 6×6 dot matrix on a plastic sheet that is only 7 microns thick. This plastic sheet can be attached to human skin. This simple plastic sheet display is very soft and will not break when bent with a bend radius of less than 1 cm.
Jong-Hyun Ahn, a flexible electronics expert at Yonsei University in Seoul, explained that "carrier mobility" is the key performance that they need to tackle. This performance measures the rate at which charge passes through the semiconductor. For example, the material used to make most chips - crystalline silicon has a carrier mobility of 1400 square centimeters per volt-second (cm2/V-s). The semiconductors that make up the display backplane are systems for switching and lighting pixels. Their required carrier mobility must be able to drive enough current to operate these pixels and meet the video bit rate requirements. “For conventional LCD liquid crystal displays, their backplanes can be made of amorphous silicon with low carrier mobility.” Ahn said that the electron mobility of this material is about 1 square cm/V-sec. However, OLED displays require higher carrier mobility. OLED display manufacturers, including LG and Samsung, use higher mobility materials such as polysilicon (>10 square centimeters/volt-second) and oxide semiconductors. However, "These materials are hard and brittle," Ahn said. They can be bent to some extent, but they cannot be bent repeatedly.
A molybdenum disulfide transistor is sandwiched by two layers of aluminum oxide (Al2O3) from above and below. This kind of device has high mobility, and high mobility is very important for delivering current to the pixels of the OLED display. To make an ultra-thin, flexible OLED display, Ahn and his team needed to release molybdenum disulfide from the transistor that "grabbed" it. Ahn said: "The contact resistance between molybdenum disulfide and the transistor electrode is very high, and the high resistance will reduce the carrier mobility of the molybdenum disulfide transistor." The key to solving the problem lies in recognizing that 2D semiconductors are very vulnerable to the surrounding materials. . Unlike the common method of placing transistors on a silicon oxide surface, Ahn's team uses materials that are very smooth and easy to control. They sandwich the transistor in two layers of insulating aluminum oxide. The contact surfaces of Al2O3 and MoS2 increase the electrons in the semiconductor, similar to the doping of chemicals in the silicon material to make it a semiconductor phenomenon. This enhanced effect overcomes the problem of high contact resistance and increases charge carrier mobility. In addition, smooth dielectric materials do not generate spots that can trap charge, further increasing the mobility to 17 to 20 cm 2 /V-sec.
They reported this invention to the Science Advances journal this week.
