Thermoelectric Performances of n-Doped Ladder-Type Conjugated Polymers Using Various Viologen Radical Cations

Understanding the dopant-polymer interaction is of interest to the conducting polymer research community due to its influence on charge transport properties and also thermoelectric performance. However, studies on such interactions are often complicated by the change in polymer morphology upon the addition of dopants. Here, we utilized sequential solution doping of a ladder-type poly(benzimidazobenzophenanthrolinedione) (BBL) via viologen radical cation salts. The strong interchain interaction in BBL prevents the infiltration of the viologen radical cations into individual BBL fibrils, thus minimizing the disruption of the polymer morphology. By changing the N-substitution (benzyl or hexyl) and counteranion (chloride or iodide) of the viologen radical cations, the dopant-polymer interaction in such a system was studied. Our results suggest that the anion-( )pi (radical) interaction between the counteranion and BBL (BBL-) is detrimental to the charge transport properties and thermoelectric performance of this system. This anion-pi (radical) interaction between the counteranion and BBL (BBL-) is governed by the solution state of the viologen radical cation (tightly bound anion or dimerized with loosely bound anion) during the doping process, which is controlled by the bulkiness of the N-substitution and the Lewis basicity of the counteranion. As a result, BBL doped with benzyl viologen monochloride showed the highest conductivities with reasonably high Seebeck coefficients while BBL doped with hexyl viologen monochloride showed inferior conductivities and Seebeck coefficients.

Automated summary

In this study, sequential solution doping of a ladder-type poly(BBL) with different viologen radical cations was conducted to investigate the dopant-polymer interaction. It was found that the anion-π (radical) interaction between the counteranion and BBL negatively affected the charge transport properties and thermoelectric performance. The solution state of the viologen radical cation during the doping process, controlled by N-substitution bulkiness and counteranion Lewis basicity, played a crucial role in determining the conductivity and Seebeck coefficients of the doped BBL.