sensors is governed significantly by the film morphology. [7][8][9] Therefore, strategic approaches to ameliorate the sensitivity of FET-based gas sensors have been centered on the morphological optimization of the OSC layer, such as increase of the diffusion or the contact area of the gas molecules and control of the OSC-dielectric interfacial properties. [10][11][12] In addition, ultrathin film and nano/micro-structured OSCs have been utilized as FET active layers, to improve the response rate of FET sensors to target analytes. [13] In particular, as crystallinity and molecular assembly of OSCs can be easily tuned by modifying the structure of the conjugated frameworks, various polymeric OSCs have been actively studied for FET-based gas sensors. [14][15][16] Although deconvoluting the individual effects of chemical structure and morphology on the detection sensitivity of FET-based gas sensors is challenging, development of effective structures that cause significant changes in carrier density through strong interaction with the analytes is inevitable to further improve the performance of such sensors.Nitrogen dioxide (NO 2 ) is a detrimental exhaust gas abundantly released into the atmosphere from industrial sources. It is toxic and, particularly dangerous gases because of its strong oxidizing power that makes it highly reactive. Moreover, since human respiration system can be fatally damaged when exposed to low concentrations of NO 2 in closed work-spaces, health and safety agencies strongly suggest a limit of exposure of 5 ppm of NO 2 for longer than 15 min. [17] Therefore, the precise, sensitive, and efficient detection of NO 2 is of the outmost importance for human health and disease prevention. In FET-based NO 2 sensors typically utilizing a p-type OSC as an active layer, the detection and sensitivity mechanisms generally follow hole carrier generation via charge transfer interaction with electron-accepting NO 2 . [18,19] This implies that the electron-donating properties of the p-type OSC are interconnected with the NO 2 detection ability of the FET sensors. The OSC with higher electron-donating ability would be beneficial to improve the detection sensitivity to NO 2 . In this contribution, three conjugated polymers (CPs, polymeric OSCs) are designed, which share the same indolocarbazole-based conjugated framework considered as known to a good electron donor. [20] In addition, to compare the effect of the morphology on NO 2 detection, different side chains were used to modulate Three conjugated polymers (CPs) containing indolocarbazole-based similar conjugated frameworks are designed to investigate the NO 2 detection capability of field-effect transistor (FET) sensors. The partly linear aliphatic side chain strengthens the crystallization tendency of the CPs, but the crystallinity is critically degraded when introducing an ethylene oxide (EO)modified side chain to the same conjugated framework. Although the highly crystalline morphology of the CP helps to achieve high hole mobility, the NO 2 response rate...

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To investigate the effect of a side chain on the electrical properties of a conjugated polymer (CP), we designed two different CPs containing alkyl and ethylene glycol (EG) derivatives as side chains on the same conjugated backbone with an electron donor-acceptor (D-A) type chain configuration. PTQ-T with an alkyl side chain showed typical p-type semiconducting properties, whereas PTQ-TEG with an EG-based side chain exhibited electrically conductive behavior. Both CPs generated radical species owing to their strong D-A type conjugated structure; however, the spin density was much greater in PTQ-TEG. X-ray photoelectron spectroscopy analysis revealed that the O atoms of the EG-based side chains in PTQ-TEG were intercalated with the conjugated backbone and increased the carrier density. Upon application to a field-effect transistor sensor for PTQ-T and resistive sensor for PTQ-TEG, PTQ-TEG exhibited a better NO2 detection capability with faster signal recovery characteristics than PTQ-T. Compared with the relatively rigid alkyl side chains of PTQ-T, the flexible EG-based side chains in PTQ-TEG have a higher potential to enlarge the free volume as well as improve NO2-affinity, which promotes the diffusion of NO2 in and out of the PTQ-TEG film, and ultimately resulting in better NO2 detection capabilities.

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