Stability in OTFT Gas Sensors
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Stability in OTFT gas sensors Josephine B. Lee1, Martin Heeney2, Steve Tierney2, Iain McCulloch2, Amanda Murphy3, Jinsong Liu3, Jean M.J. Fréchet3, and Vivek Subramanian1 1 Department of Electrical Engineering and Computer Sciences, University of California, Berkeley 144 MB Cory Hall, Berkeley, CA 94720 2 Merck Chemicals Ltd. NB-C, UK, Chilworth Science Park, University Parkway, Chilworth, Southampton, Hants, S016 7QD, UK 3 Department of Chemistry, University of California, Berkeley, CA 94720 ABSTRACT Organic thin film transistor (OTFT) gas sensors fabricated with an array of different active materials are monitored for shifts in performance characteristics during continuous operation over many days in both inert and ambient environments. We analyze the different patterns of degradation behavior observed in different materials and discuss approaches to decoupling a device’s sensor response from its stress response. INTRODUCTION OTFTs offer a promising route towards dramatically lowering the cost of electronic noses, ultimately enabling low-cost applications such as food packaging with integrated spoilage detection. The ability to tailor the chemical sensitivity of organic semiconductors through organic chemistry makes these materials well-suited for sensor array applications such as electronic noses. Moreover, the potential for solution deposition allows for low-cost integration of multiple sensing materials onto a single substrate and compatibility with cheap substrates such as plastics or foils. One of the greatest concerns in the field of organic electronics, however, is the long-term stability of these devices under continuous operation, both in inert and ambient environments. A host of phenomena may contribute to the shifting of electrical characteristics over time, including the creation of new trap states [1], charge trapping [2-4], the formation of bound hole pairs [5-6], migration of impurities [7], environmental doping [8-10], and oxidative degradation. Each of these processes affects the threshold voltage, charge mobility, and Ion/Ioff ratio differently. Organic electronics has attracted much interest lately for applications ranging from radio frequency identification (RFID) tags to display driver backplanes. However, for most OTFTs that have been demonstrated, the degradation of electrical parameters limits operational lifetime to below what these applications would require. Sensing applications are potentially more tolerant of device aging, since they rely on signal modulation rather than absolute signal values. For this same reason, though, it is essential that baseline shift in OTFTs be well characterized before they can be useful as sensors. EXPERIMENTAL DETAILS All devices used for the stability and sensing tests presented here were substrate-gated, bottomcontact OTFTs (Figure 1). Devices were fabricated on n+ silicon substrates with 95 nm thermally grown wet oxide as a dielectric. 50 nm thick gold pads, thermally evaporated with a chrome adhesion layer, served as source and drain contacts.
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