CUHK Passions and Pursuits

27 Mobile and Malleable Miao Qian’s work on organic semiconductors T hanks to the development of organic semiconductors by chemists such as Prof. Miao Qian , we will likely soon benefit from products like mobile-phone screens that bend in our pockets and never crack, sensors that can alert inspectors to gas leaks, and wearable medical devices. Professor Miao and his research team have been experimenting with organic rather than inorganic materials in the fabrication of thin film transistors, a fundamental device in electronic circuits used for many applications. He sees plenty of applications for his work. The most common semiconductor at the moment is silicon, which is inorganic. But its production requires high temperatures and therefore lots of energy, with many steps necessary to turn sand into silicon. Organic semiconductors can be fabricated with a simple printing technique at room temperature. They have several other advantages. They are flexible, unlike their inorganic counterparts, and can produce devices that are both lightweight and very large. That would allow for malleable screens that would, for instance, allow a robotic hand to sense whatever it touches through an ‘eSkin.’ High-grade silicon has very rapid charge transport, which is necessary in an application such as the contents of a computer’s central-processing unit. But for a comparatively simple product such as a computer or television screen, low-grade silicon is sufficient, something an organic semiconductor can replace. Organic semiconductors are inherently more flexible than silicon. Professor Miao is also attempting to detect if gas is leaking from a device by using an organic semiconductor in a transistor that amplifies electrical signals, interacting with other molecules to provide feedback in the form of an alert. Professor Miao considers the creation of an n-type organic semiconductor based on nitrogen-containing pentacene to be the most significant discovery that he has made in the field so far. By experimenting with the chemical structures of organic semiconductors, he discovered that replacing carbon atoms in the well- known p-type semiconductor based on pentacene with four nitrogen atoms converted the material into a high-performance n-type semiconductor. The combination of p-type semiconductors, such as pentacene consisting only of carbon and hydrogen, and n-type semiconductors containing nitrogen atoms in the fabrication of electronic circuits allows easy fabrication of efficient circuits. ‘The development of n-type organic semiconductors is slower than that of p-type semiconductors,’ Professor Miao explained. The p-type pentacene contains 22 carbon atoms in its five-ring framework. Professor Miao and his team began experimenting with substituting various numbers of those atoms with nitrogen, replacing two atoms, then six, before settling on four as the best combination. ‘The speed of charge transport is the best for solution-processed n-type organic semiconductors,’ he explained. What’s more, the production begins with cheap starting materials, diaminobenzene and benzoquinone, which are both readily available. One of the key challenges now is figuring out how to create the organic semiconductors in a simple way. At first, Professor Miao’s best n-type semiconductor was processed in a high vacuum, which is expensive to achieve. ‘That is not good enough,’ Professor Miao said. He has since managed to produce the semiconductor film from a solution by surface modification, a simpler and cheaper method that was recently patented. He is working with a mechanical engineer to fabricate electronic circuits for large-area sensing applications. ‘We want to make the devices in a much cheaper, simpler way.’ c c A flexible OLED display (Source: Armin Kübelbeck, CC-BY-SA, Wikimedia Commons)