A subparts per million-sensitive nitrogen dioxide (NO2) sensing circuit with improved humid air stability was realized incorporating UV-ozone treatment on a poly(bisdodecylquaterthiophene)/polystyrene blend film. The circuit consisted of a pair of organic field-effect transistors (OFETs) in series, one OFET with and one without this treatment. In contrast to most previous OFET sensors, the readout was obtained from the voltage Vout at a point between the OFETs. The circuit showed a low detection limit (200 ppb) toward NO2 and greatly reduced the voltage drift in the humid environment compared to the current drift of the circuit or the individual OFETs because of the balance of conductance drifts on either side of the readout point, which differs from the existing OFET-based sensors. By using Vout as the detection parameter, the sensitivity of the circuit approaches 25 and 400% for NO2 concentrations of 200 ppb and 20 ppm, respectively. Moreover, the Vout is substantial enough to be easily measured by a voltmeter, which could remove the need for complex equipment (semiconductor analyzer system) for the sensing test. We thus demonstrate a simplified approach to stabilized OFET circuits that could be used in printable, flexible, or wearable sensors.
Keywords: humidity-stable sensors; nitrogen dioxide sensor; organic field-effect transistor; sensing circuits; thiophene polymer.