General Observation of an Insulator-Metal Transition in Polymer Electrochemical Transistors

One of the main fascinations behind the transport physics in polymers is the observation of a metallic state. The presence of a finite density of states at the Fermi level has been shown through multiple transport signatures, such as a finite electrical conductivity at low temperatures approaching absolute zero,[1] a Pauli contribution to the paramagnetic susceptibility,[2] a plasma oscillation of the nearly free electron gas,[1] and temperature-dependence of the Seebeck coefficient obeying Mott formula.[3, 4] While the earliest reports of these observations date back to the 1970s, up until now metallicity has been reported mostly for polymers with low degree of structural disorder and relatively high conductivity (<i>e.g.,</i> around 1000 S/cm in specially prepared polyaniline[1] and FeCl₃ ion-exchange doped PBTTT[3], and 600 S/cm in AsF₅doped <i>cis</i>-polyacetylene[5]).<br/><br/>In this study, we demonstrate the generality of the insulator-to-metal transition in a wide range of polymers having very different microstructures and electrical conductivities. Experimental access to an extended regime of carrier densities is made possible in an electrochemical transistor device architecture, which has recently been showed to be as effective[6] as the recently developed ion-exchange doping technique.[6, 7] We show that a clear metallic temperature-dependence of Seebeck coefficient is also present in representative polymers with low conductivity and crystallinity. Independent confirmation of the presence of the metallic states are provided through photoemission and electron paramagnetic resonance spectroscopies. We argue that metallicity is accessible in semiconducting polymers in general, as evidenced by a similar evolution of the Seebeck coefficient with temperature and carrier density, provided that a high degree of carrier density could be achieved to shift the Fermi level into the delocalized states of the energy band.<br/><br/>[1] Lee, K.<i> et. al.,</i> Metallic transport in polyaniline, <i>Nature</i> 441, 65-68 (2006).<br/>[2] Kang, K. <i>et. al.,</i> 2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion, <i>Nat. Mater</i> 15, 896-902 (2016).<br/>[3] Huang, Y. <i>et. al.</i>, Design of experiment optimization of aligned polymer thermoelectrics doped by ion-exchange, <i>Appl. Phys. Lett.</i> 119, 11903 (2021).<br/>[4] Watanabe, S. <i>et. al.</i>, Validity of the Mott formula and the origin of thermopower in pi-conjugated semicrystalline polymers, <i>Phys. Rev. B</i> 100, 241201(R) (2019).<br/>[5] Chiang, C. K. <i>et. al.</i>, Electrical Conductivity in Doped Polyacetylene, <i>Phys. Rev. Lett.</i> 39, 1098 (1977).<br/>[6] Jacobs, I. E. <i>et. al.</i>, High-Efficiency Ion-Exchange Doping of Conducting Polymers, <i>Adv. Mater</i>, 2102988 (2021).<br/>[7] Yamashita, Y. <i>et. al.</i>, Efficient molecular doping of polymeric semiconductors driven by anion-exchange, <i>Nature</i> 572, 634-638 (2019).