Highly Stable and Conformable Double-Gated Organic Transistor—A Smart Approach for the Development of Flexible Circuits and Sensing Systems

Thin Film Transistors (TFTs) can be found in a wide variety of devices, and the necessity of having them flexible, but also to reduce the production costs, lead Organic Semiconductors to gain much interest over the past decades. Notably, this kind of materials offers different interesting characteristics, such as mechanical flexibility, solution processability, and allow the use of low temperature and large area fabrication techniques, making them cost efficient. As such, they have been employed in different scenarios, like circuitry or sensing. However, early aging effects and long-term stability due, mainly, to moisture and oxygen interaction with the active layer, represent a major drawback, strongly limiting their actual employment in real applications. Thus, novel strategies have been investigated for the development of new solutions that could increase the long-term stability of such devices. We propose in this work a simple approach that consists in the employment of a Double-Gate structure, which easily meets these requirements, being it a self-encapsulating one that protects the active layer from degradation. In this case, a first bottom gated OTFT was realized by using aluminium as gate electrode, a 180 nm thick parylene C layer as bottom gate dielectric, and gold for source and drain electrodes. Afterwards, a second 380 nm thick Parylene C film is deposited to act as top gate dielectric layer, and the device is completed by the deposition of a final top aluminium gate electrode. Different organic semiconductors have been tested and a comparison of their performance in the different configurations, Bottom Gated, Top Gated and Double Gated, will be presented. In this specific instance, it has been demonstrated that the employment of the double gated configuration generally gives rise to devices with better performances and a highly stable behaviour with negligible variations of the mobility and threshold voltage even after 6 months. Moreover, the employment of a second gate, allowed for a much finer control over some of the most meaningful electrical characteristics of the device. It has been verified that it is possible to modulate the transistor threshold voltage and its mobility by changing the two gate biases, thus leading to have more degrees of freedom in the adjustment of the final transistor performances, which can be a very important feature in the development of more performing and robust analog and logic circuits.
Additionally, such structure has also a very high potential for the development of smart flexible sensors. For instance, the top gate can be properly functionalised or interfaced with specific sensing elements/materials which could locally change the vertical field in the active channel when interacting with a specific external stimulus. We will report about some examples we have recently developed by using such approach.