Ahn and his team then wanted to make a smart watch or a smartphone-sized flexible screen.
Molybdenum disulfide has a very good performance as a 2D semiconductor material, that is, they are easily bent. 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 for making flexible OLED displays. However, when manufacturers attempt 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 the app. Now, Korean engineers have found a way to apply molybdenum disulfide transistors to flexible OLED displays. They used this transistor to form a simple 6 x 6 dot matrix on a plastic sheet measuring only 7 microns thick. This piece of plastic can be applied to human skin. This simple plastic sheet display is very soft and can be bent without bending at 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 they need to tackle. This property 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 illuminating pixels. The required carrier mobility must be able to drive enough current to operate these pixels, as well as the video bit rate. "For traditional LCD screens, their backsheets can be made from amorphous silicon with lower carrier mobility," Ahn said. The material has an electron mobility of about 1 cm2/V-sec. But OLED displays require higher carrier mobility. OLED display manufacturers, including LG and Samsung, use higher mobility materials such as polysilicon (>10 cm 2 /V-sec) and oxide semiconductors. However, "these materials are hard and brittle," Ahn said. They can be bent to a certain extent, but they cannot be bent repeatedly.
A molybdenum disulfide transistor is sandwiched by two layers of aluminum oxide (Al2O3) from the upper and lower directions. This device has high mobility, and high mobility is critical for delivering current to the pixels of an OLED display. To make an ultra-thin flexible OLED display, Ahn and his team needed to release molybdenum disulfide from the transistor that "caught" 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 is to recognize that 2D semiconductors are very susceptible to surrounding materials. . Unlike the usual means of placing transistors on the surface of silicon oxide, Ahn's team uses materials that are very smooth and easy to control. They sandwiched the transistor in two layers of insulating aluminum oxide. The interface between the aluminum oxide and the molybdenum disulfide increases the electrons in the semiconductor, similar to the phenomenon of doping chemicals into the silicon material to make it a semiconductor. This enhancement overcomes the problem of high contact resistance and improves charge carrier mobility. In addition, the smooth dielectric material does not create spots that can trap the charge, further increasing the mobility to 17 to 20 square centimeters per volt-second.
They reported the invention to the journal Science Advances this week.