The excellent performance of molybdenum disulfide as a 2D semiconductor material is that they are very flexible. Electrons can move quickly in such semiconductors. At the same time, these semiconductors are transparent because they are only about one atom thick. 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 molybdenum disulfide (MoS2) and the source and drain of the transistor are The resistance will be too high, making this excellent material unusable. Now South Korean engineers have found a way to apply molybdenum disulfide transistors to flexible OLED displays. They use this transistor in thickness A simple 6×6 dot matrix is formed on a plastic sheet of only 7 μm. This plastic sheet can be applied to human skin. This simple plastic sheet display is very flexible and bends with a bending radius of less than 1 cm. It will not be damaged.
Jong-Hyun Ahn, a flexible electronics specialist 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 cm 2 /V-sec (cm2/Vs). The semiconductors that make up the display backplane are systems for switching and lighting pixels. They The required carrier mobility must be able to drive enough current to operate these pixels and also meet the video bit rate requirements. 'For traditional LCD displays, their backplanes can use lower carrier mobility. The amorphous silicon is made. 'Ahn said that the electron mobility of this material is about 1cm2/V-s. But OLED displays require higher carrier mobility. OLEDs including LG and Samsung Display manufacturers use higher-mobility materials such as polysilicon (>10 cm2/V-sec) and oxide semiconductors. However, 'These materials are hard and brittle.' Ahn said. They can be bent to a certain degree. Degree, but not repetitive bending.
A molybdenum disulfide transistor is sandwiched by two layers of aluminum oxide (Al2O3) in both up and down directions. This device has a high mobility, and high mobility is critical for the pixel current delivery of the OLED display. To make ultra-thin, flexible OLED displays, Ahn and his team needed to release molybdenum disulfide from the transistors that “grab” it. Ahn said: “The contact resistance between molybdenum disulfide and the transistor electrodes is very high, high. The resistance reduces the carrier mobility of molybdenum disulfide transistors. ' The key to solving this problem is to realize that 2D semiconductors are very susceptible to the surrounding materials. Unlike the common method of placing transistors on a silicon oxide surface, Ahn's team The materials used are very smooth and easy to control. They sandwich the transistors in two layers of insulating aluminum oxide. The contact surfaces of aluminum oxide and molybdenum disulfide increase the electrons in the semiconductor, similar to the doping chemistry in the silicon material. Substances make it a semiconductor phenomenon. This enhancement overcomes the problem of high contact resistance and improves charge carrier mobility. In addition, smooth dielectric materials There will be no spots that could trap the charge, further increasing the mobility to 17 to 20 cm 2 /V-sec.
Ahn and his team next hope to make a smartwatch or smartphone-sized flexible screen. They reported this invention to the Science Advances journal this week.