Roll-to-Roll processable organic thin film transistor based circuits and their sensory application

Thesis

This thesis is trying to explore a possible routine to manufacture flexible sensor via vacuum-based processes. It starts with an exploration of a route to manufacture Organic Thin Film Transistors (OTFTs) and then a series of stabilization methods are discussed to reduce the probability of short circuit, current leakage and improve the performance of OTFTs, i.e., increasing mobility and decreasing threshold voltage. In addition, a possible mechanism of bias stress is investigated with a model and the relevant evolution of trapping state and trapping energy is plotted over time and distance from the dielectric/semiconductor interface. Next, a short-time bias scheme is demonstrated to reduce error of bias stress during strain tests. A first example of a strain sensor is based on the architecture of floating gate OTFTs, i.e., a PVDF coated floating gate that transfers the signal of charge imbalance from the ferroelectric effect to transistors, and then the OTFT will output the amplified signal. The advantage of floating gate structure is the separation of the transistor and sensor part to avoid interference between transistor and sensing reaction and giving the option for the transistor to be encapsulated from the environment. However, the bias voltage is a key parameter to control the floating gate sensor, with the sensitivity of the device dependant on it, i.e., it is very difficult to obtain both a sensitive signal at same time as one unaffected by bias stress. In order to obtain a sensitive and stable signal, a series of OTFT-based functional circuits are developed and a transimpedance converter is fabricated to convert nano-scale input current to a readable voltage output (0 – 3V). After encapsulation, the linear converting relationship is maintained and the mechanical flexibility is not affected. Finally, a series of sensors are built with the OTFT-based signal conditioning circuit to detect arterial pulse from human body, pH, hydrogen peroxide and glucose from solution. Conclusively, this thesis demonstrates a new architecture of OTFT-based sensing system, and that can work as a versatile platform to integrate with different functionalized sensing electrodes to process signals to a readable level and visualized with smartphone via Bluetooth microcontroller.

Authors: Zhang, K

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: roll to roll; healthcare application; sensors; organic thin film transistor; flexible electronics
• Subjects: Thin film circuits