Structural and Dynamic Disorder, Not Ionic Trapping, Controls Charge Transport in Highly Doped Conducting Polymers

The last decade has seen renewed interest in the fundamentals of doping processes in semiconducting polymers.[1] Molecular doping has been the most studied approach, in large part because films are easy to fabricate and show high conductivities. However, molecular doping is subject to several limitations: 1. disorder due morphological disruption from dopant ions; 2. limited charge densities due to insufficiently strong dopants; 3. reduced doping efficiency from charge transfer complexation; and 4. poor stability resulting from the inherent reversibility of charge transfer. These difficulties largely stem from the dual role molecular dopants must play. The neutral dopant first acts as a redox agent, while the product of this redox reaction—the ionized dopant—then acts as a compensating ion. A recently proposed method based on ion exchange allows for replacement of the dopant counter-ion with arbitrary ion from an electrolyte solution.[2,3] This separates the redox and charge compensation steps, and enables the fabrication of doped films with a range of different counterions. The improved microstructural control this process allows enables us for the first time to systematically address a longstanding but still poorly understood question: what limits the electrical conductivity at the high doping levels relevant to organic thermoelectrics? Is it the formation of charge carrier traps in the Coulomb potentials of the counterions, or is it the structural disorder in the polymer lattice?<br/>Here[4] we present a simple framework for understanding ion exchange doping and evaluate a wide range of charge transfer dopants and electrolytes. From this systematic study, we identify a set of experimental conditions which allows for extremely high doping levels of approaching 1 charge per monomer and nearly 100% ion exchange efficiency in several different polymer classes. In each of these degenerately doped polymer systems, we achieve record or near-record conductivities: &gt;1200 S/cm for PBTTT, 220 S/cm for P3HT, 80 S/cm for DPP-BTz, and 15 S/cm for IDTBT. Most critically, we show that in this regime conductivity is poorly correlated with ionic size, but strongly correlated with paracrystalline disorder. This observation, backed by a detailed electronic structure model that incorporates ion-hole and hole-hole interactions and a carefully parameterized model of disorder, indicates that trapping by dopant ions is negligible, and hence that the doping efficiency in these systems is near 100%. Our results imply that in this high carrier density regime, relevant to thermoelectric devices, maximizing crystalline order is the most critical factor in improving conductivity. This realization, along with the greatly improved control enabled by ion exchange, provides a clear path forward for the design of polymer thermoelectrics.<br/>1. Jacobs, I. E., Moulé, A. J., Adv. Mater. 2017, 29, 1703063<br/>2. Yamashita, Y. et al. Nature 2019, 572, 634–638<br/>3. Jacobs, I. E., Adv. Mater. 2021, 2102988<br/>4. Jacobs, I. E., et al. arXiv:2101.01714