Side-Chain-Assisted Transition of Conjugated Polymers from a Semiconductor to Conductor and Comparison of Their NO 2 Sensing Characteristics.

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 NO 2 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 NO 2 -affinity, which promotes the diffusion of NO 2 in and out of the PTQ-TEG film, and ultimately resulting in better NO 2 detection capabilities.