(Plenary) Conjugated Polymers for Selective Chemical Sensing and Energy Conversion
Authors: Katz Howard E. 
Published: February 2020

The ability to control the carrier energy levels, functional group polarity, and film morphology make organic and polymeric semiconductors (OSCs) especially attractive for chemical sensing and for various energy interconversions. Their mechanical flexibility, low temperature processing, potential printability, capability of blending to form composites, predictable activity in various electronic circuit configurations, and use of common elements are additional attractive features. In this presentation, we focus on thiophene polymers and small conjugated molecular solids designed for maximum and selective responses for vapor sensing, with additional extensions to biosensing and thermoelectrics.

Over the past several years, we have reported the electronic signaling by OSCs of exposures to vapors of interest for environmental health and security, including ammonia and phosphonates. These responses were selective over common interferents such as solvents and water. In our most recent work on vapor sensors, we demonstrated the selective and quantitative detection of nitrogen dioxide (NO2), an important component of polluted air.

Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecyl thioquaterthiophene) (PQT12 and PQTS12, respectively), were used as active layers in organic field-effect transistors (OFETs) to detect NO2 at parts-per-million concentrations. Responses of several hundreds of percent current increases were obtained, among the highest sensitivities reported for an NO2-responsive device based on an organic semiconducting film. From measurements of cyclic voltammetry and the electronic characteristics, we found that the introduction of sulfurs into the side chains induces traps in films of the PQTS12 and decreases domain sizes, both of which could contribute to the higher sensitivity of PQTS12 to NO2 gas. The ratio of responses of PQTS12 and PQT12 is higher for exposures to lower concentrations, allowing us to distinguish responses to low concentrations for extended times from exposures to high concentrations from shorter times, important for monitoring of indoor air quality, and contributing to the selectivity of the detection over likely interferent vapors.

We used the same class of polymers for thermoelectric studies as well. Doping the polymer with sulfur in side chains (PQTS12) with the strong oxidant nitrosonium tetrafluroborate (NOBF4), we obtained a very high conductivity up to 350 S cm-1, which is the highest reported nonionic conductivity among films made from dopant-polymer solutions. We found the combination of efficient charge transfer, tighter π-π stacking and strong intermolecular coupling is responsible for the high conductivity. Furthermore, the high conductivity is stable in air without extrinsic ion contributions that are associated with the polymer most often used for high conductivity, poly(ethylenedithiothiophene) (PEDOT:PSS). The thermoelectric power factor compared favorably with prior reports for p-type polymers that were made by the alternative process of immersion of polymer films into dopant solutions, and fit the established models for thermoelectric polymers.

Our work on biosensors has emphasized the role of dielectric receptor layers and the interfaces of those layers with analyte media and the OSCs. We have investigated the perturbations of OSC currents in OFETs by protein biomarkers of brain injury, especially glial ﬁbrillary acidic protein (GFAP). We show that this protein, when bound to its antibody in a supporting polymer matrix on the OFET, causes an on-current change consistent with the charge that it delivers to the device surface, using p- and n-type OSCs. The use of an optimized hydrophobic bilayer on the OSC to protect it from the hydrophilic receptor layer, or the offset of the receptor layer from the OSC altogether, added to the device reliability. The incorporation of ion-complexing poly(ethylene glycol) (PEG) additives increased sensitivity.