Hydrogen sensors based on thin-film transistors

Abstract

Hydrogen has received increasing interest from scientists as it is one of the most attractive energy storage media. Since hydrogen is a flammable gas with potential danger due to its high diffusivity in air and low ignition energy, a variety of sensors have been developed for the reliable detection of H2 leakage. Recently, pentacene organic thin-film transistor (OTFT) has been drawing worldwide attention due to its potential applications in flexible and wearable electronics. Moreover, it has advantages such as good mechanical durability, low-voltage operation, easy fabrication and low cost. Therefore, the main purpose of this research is to realize a hydrogen sensor based on pentacene OTFT and optimize its performance. Another hydrogen sensor based on InGaZnO (IGZO) TFT is also proposed for comparison. Firstly, a novel hydrogen sensor based on pentacene OTFT is fabricated with Pd source/drain (S/D) electrodes as sensing medium. Upon hydrogen exposure, the device shows an obvious reduction in drain current because H dipoles accumulated at the electrode/pentacene interface create a potential barrier to oppose the flow of holes. A rapid, reversible and concentration-dependent response is observed without the need of heating. The detection limit reaches 200 ppm, which is far below the hydrogen explosion range. The response and recovery times for 14,000-ppm H2 are 12 s and 10 s respectively. Study on its working principle demonstrates that work-function decrease of source electrode, reduced carrier mobility and increased S/D series resistance are the reasons for the drain–current decrease upon hydrogen exposure. Then, various factors that affect the sensing performance of the sensor are studied, e.g. S/D-electrode thickness, material (Ni, Pt and Pd) and fabrication method (E-beam and sputtering), channel length, and working temperature. It is demonstrated that these factors not only affect the sensing performance of the sensor (including sensitivity and response/recovery time), but also the electrical characteristics of the OTFT itself. Next, the OTFT-based hydrogen senor is fabricated on adhesive vacuum tape for flexible applications. The sensor also realizes a rapid, reversible and concentration-dependent response upon hydrogen exposure. Measurements on a curved surface are performed to verify the flexibility of the sensor. When attached to a curved surface, the device shows normal transistor characteristics, which basically remain unchanged after one hour of tensile stress. Lastly, a hydrogen sensor based on IGZO TFT with Pd S/D electrodes as sensing medium is proposed. Upon hydrogen exposure, without the need of heating, a rapid, reversible and concentration-dependent response is realized. Its detection limit also reaches 200 ppm. Compared to its OTFT-based counterpart, the sensor shows an opposite response (drain–current increase) because the H dipoles accumulated at the source-electrode/IGZO interface create a potential barrier to assist the flow of electrons. Moreover, it possesses a lower sensitivity and requires longer response and recovery times because the IGZO film is less flexible than the pentacene film. However, the IGZO TFT sensor has higher thermal stability (due to the wider bandgap of IGZO) and longer life time, and thus great potential for detecting hydrogen leakage in harsh environments.