NO₂-Affinitive Conjugated Polymer for Selective Sub-Parts-Per-Billion NO₂ Detection in a Field-Effect Transistor Sensor

Conjugated polymers (CPs) have provided versatile semiconducting implements for the development of soft electronic devices. When three CPs with the same conjugated framework but different side chains were adopted in the field-effect transistor (FET) sensor for NO2 detection, the response to NO2 showed an opposite tendency to the charge carrier mobility of each CP. Morphological and structural characterizations revealed that the flexible glycol side chain enhances NO2 affinity as well as prevents the formation of lamellar stacking of the CP chains, thereby providing routes for the facile diffusion of NO2. Additionally, theoretical calculations for CP–NO2 complex formation at the molecular level support the relatively low energy barrier for inter-chain transition of NO2 between the glycol-based conjugated frameworks, which implies the spontaneous internal diffusion of NO2 to the semiconductor–dielectric interface in the FET-based sensor. As a result, the CP with a NO2-affinitive morphology exhibited an exceptional sensitivity of 13.8%/ppb upon NO2 (100 ppb) exposure for 50 s and provided excellent selectivity to the FET-based sensor toward other environmentally abundant harmful gases, such as SO2, CO2, and NH3. In particular, the theoretic limit of detection reached down to 0.24 ppb, which is the lowest value ever reported for organic FET-based NO2 gas sensors.