Raising Dielectric Permittivity Mitigates Dopant‐Induced Disorder in Conjugated Polymers

Abstract

Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density-of-states (DOS). The authors examine the effects of dopant-induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them. Their simulations, based on a modified Gaussian disorder model with Miller-Abrahams hopping rates, show that dopant-induced broadening of the DOS negatively impacts the Seebeck coefficient versus electrical conductivity trade-off curve. Increasing the dielectric permittivity of the polymer mitigates dopant-carrier Coulomb interactions and improves charge transport, evidenced by simultaneous increases in conductivity and the Seebeck coefficient. They verified this increase experimentally in iodine-doped P3HT and P3HT blended with barium titanate (BaTiO 3 ) nanoparticles. The addition of 2% w/w BaTiO 3 nanoparticles increased conductivity and Seebeck across a broad range of doping, resulting in a fourfold increase in power factor. Thus, these results show a promising path forward to reduce the dopant-charge carrier Coulomb interactions and mitigate their adverse impact on charge transport.

Delocalization Enhances Conductivity at High Doping Concentrations

“…Also, since the transport energy is largely constant, it is beneficial to have a large DOS at the Fermi level, which for example has been argued to be achievable by molecules with a large dielectric constant. [25,60] Finally, as a design rule, the conductivity at constant charge carrier concentration c = n/N i can be improved by decreasing the intermolecular distance a NN . Although this would (quadratically) decrease the mobility prefactor μ 0 , the overall conductivity prefactor qμ 0 n would (linearly) increase due to the (cubic) increase of N i and, at constant c, n with a NN .…”

Dopant-Induced Energetic Disorder in Conjugated Polymers: Determinant Roles of Polymer–Dopant Distance and Composite Electronic Structures

“…We doped these polymers by exposing the films to iodine vapors in a sealed chamber and then measured the Seebeck coefficient (α) and electrical conductivity (σ) simultaneously as the films dedoped, as previously published. 10,14 In Figure 1, we show α−σ curves for RRa P3HT, RR P3HT, and their blends. At the highest level of doping, 100% RR P3HT film showed the highest conductivity, whereas 100% RRa P3HT showed the lowest conductivity.…”

“…Once the DOS of the doped polymer is computed, the hopping of carriers between localized sites is determined from Miller-Abrahams rates. Further details of the implementation are given in our recent work. , Figure S1 shows the error associated with the simulated curves. To capture transport in films with multiple domains, we simulated an additional DOS in RR P3HT with an offset of 0.1 eV between the DOSes that represent the amorphous and ordered regions.…”

“…Further details of the implementation are given in our recent work. 14,28 Figure S1 shows the error associated with the simulated curves. To capture transport in films with multiple domains, we simulated an additional DOS in RR P3HT with an offset of 0.1 eV between the DOSes that The Journal of Physical Chemistry C represent the amorphous and ordered regions.…”

Organic Materials for Electronic and Thermoelectric Applications

“…Those highly dopable polymers are reaching extremely high conductivities compared to polymers with similar chemical structure. Some dopants, such as dodecaborane-based ones [56], electrostatically shield counterions from carriers by effectively intercalating into the lamellae, far from polarons on the polymer backbone. Thus, the importance of choosing the right dopant [9] for a certain host is crucial to obtaining good conductivity.…”