Radical Polymers Alter the Carrier Properties of Semiconducting Carbon Nanotubes

Abstract

Radical polymers are composed of nonconjugated macromolecular backbones and pendant groups that bear stable open-shell sites; these functional macromolecules have been utilized in myriad electrochemical, optoelectronic, and thermoelectric devices to date. Here, we combine radical polymers with semiconducting single-walled carbon nanotubes (SWCNTs) to form composite materials using a scalable fabrication process. Importantly, we are able to tune the charge carrier characteristics of the SWCNTs by controlling the macromolecular architecture of the radical polymer used to form the composite. This is a critical handle as creating stable, electron-transporting (i.e., n-type) SWCNT thin films using scalable processes is a long-sought goal in the field. Specifically, hole-transporting (i.e., p-type) SWCNT thin films were deposited using a simple drop-casting methodology. Then, a radical polymer thin film was coated on top of the SWCNT thin film in order to create a composite system. Three different radical polymer chemistries were evaluated in terms of their ability to manipulate the optoelectronic properties of the composite systems relative to the pristine SWCNT thin films. Two of the three radical polymer chemistries, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) and poly(2,3-bis(2',2',6',6'-tetramethylpiperidinyl-N-oxyl-4'-oxycarbonyl)-5-norbornene) (PTNB), contained different macromolecular backbones but shared the same nitroxide radical open-shell chemistry. Instead, the third radical polymer, poly(2,6-ditert-butyl-4-((3,5-ditert-butyl-4-phenoxyl)(4-vinylphenyl)methylene)cyclohexa-2,5-dienone) (PGSt), was based upon the galvinoxyl styrene radical. Neither of the nitroxide-based radical polymers demonstrated any significant electrochemical interaction with the SWCNT thin films. Conversely, the addition of the galvinoxyl styrene-based radical polymer converted the SWCNT thin films from ones that were unipolar p-type materials to a system that allowed for both hole and electron transport (i.e., ambipolar behavior was observed) when the thin films were incorporated into organic field-effect transistor (OFET) test bed devices. In these ambipolar systems, the hole mobility and electron mobility values were determined to be 0.07 and 0.24 cm(2) V-1 s(-1), respectively. A combination of ultraviolet-visible-near-infrared (UV-vis-NIR) light absorption and Raman spectroscopy revealed the means by which this doping occurred. This provides for a foundational underpinning for the observed behavior, and this work also demonstrates a straightforward and scalable means by which to apply these functional open-shell macromolecules in flexible and printed electronic devices.