Polaron Characteristics in Doped Conjugated Polymers

Doping semiconducting polymers enables a variety of applications and is important for improving the performance of many devices. The process of doping involves injecting or transferring charges to a host material. The injected charges are stabilized by the reorganization of nuclei and electronic polarization to form polarons. Each polaron charge is balanced by a counterion, either an ionized dopant or a guest ion, to ensure electrical neutrality. The counterion interacts with the polaron, thereby influencing the polymer optical and electronic response and ultimately device performance. Elucidating how the counterion impacts the polaron characteristics in doped conjugated polymers (CP’s) can provide an avenue for further optimizing performance. This work provides a detailed and comprehensive look at the characteristics of polarons in doped CP’s and the different ways these characteristics can be impacted by the counterion. Focusing on a hole-transport (p-type) polymer, pDPP-4T, we use first-principles calculations based on density functional theory to determine the ionization energy of single polymer chains for an excess charge sequentially stabilized by nuclei reorganization, electronic polarization and the counterion, relative to a non-stabilized charge. We also compute the associated polaron probability distributions. We observe that the resulting binding energies correlate with the polaron size and find that the counterion has the strongest stabilizing effect on the polaron. We also compute the optical response of the ionized chains and find that the position of the lowest energy polaron peak (P1) is increasingly blue shifted for more strongly bound polarons, with the polaron stabilized by the counterion exhibiting the highest energy P1 peak. The correlation between polaron binding and P1 peak position is explained using Koopman’s theorem and the energy level of the polaron orbital within the electronic band gap. The different quantitative assessments of the counterion impact on the polaron point to an adverse effect on electronic transport. We examine the effects of counterion size and position relative to the polaron center of charge and conclude that the counterion impact on the polaron can be reduced with larger polaron-ion distances. The use of larger counterions, which sit at longer distances from the polymer backbone, is one way of achieving this.