Many-body charge transport physics of heavily doped polymer semiconductors

The charge transport physics of polymer semiconductors has been studied intensively since the discovery of electrical conduction in doped polyacetylene by Heeger,

MacDiarmid, and Shirakawa. While the charge transport physics in this class of materials at low carrier densities of less than 10¹⁸ to 10¹⁹ cm⁻³ has been relatively well established, the physics at much higher carrier densities of 10²⁰ to 10²¹ cm⁻³ remains poorly understood. In this transport regime there is on the order of one charge per molecular repeat unit, and naturally the transport is highly correlated because of the many-body Coulombic interactions between charges and dopant counter-ions, as well as between like charges.



In this study we investigate the transport physics of polymer semiconductors at such high carrier densities, which has been experimentally made possible through the use of organic electrochemical transistor and the recently developed ion-exchange doping technique. Through conductivity, Seebeck, and photoemission experiments we demonstrate that in a class of p-type donor-acceptor polymer it is possible to fully empty the highest occupied molecular orbital (HOMO), and to reversibly access the second highest occupied molecular orbital (HOMO-1). Across such wide range of doping levels, field-effect transfer measurements present evidence for the formation of a frozen Coulomb gap at the Fermi level. We discuss how these novel transport insights could be used to optimise the thermoelectric power factors.