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

Abstract

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−π (radical) interaction between the counteranion and BBL (BBL − ) is detrimental to the charge transport properties and thermoelectric performance of this system. This anion−π (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.

Facile Synthesis of Solubilizing a Group-Free, Solution-Processable p-Type Ladder Conjugated Polymer and Its Thermoelectric Properties

“…It has displayed many promising results in the field of organic field-effect transistors (OFETs), 13,31 electrochemical transistors (OECTs) 10,32,33 and thermoelectrics (OTEs). 32,34,35 Motivated by this, we aim to develop new solution-processable ladder-type conjugated polymers by the use of new solvent system and control of reaction kinetics. Here, we report the synthesis and characterization of the first solubilizing group-free, solutionprocessable p-type ladder conjugated polymer, 6H-pyrrolo[3,2b:4,5-b′]bis [1,4]benzothiazine ladder (PBBTL) polymer (Figure 1) by the use of a 1:1 binary acid solvent system consisting of polyphosphoric acid (PPA) and phenylphosphonic acid (PhPO 3 H 2 ).…”

“…Benzimidazobenzophenanthrolinedione ladder (BBL) polymer (Figure ) is a n -type semiconducting ladder-type conjugated polymer and the only ladder-type conjugated polymer that has been extensively studied in electronic applications due to its good solubility and processability in methanesulfonic acid (MSA). It has displayed many promising results in the field of organic field-effect transistors (OFETs), , electrochemical transistors (OECTs) ,, and thermoelectrics (OTEs). ,, Motivated by this, we aim to develop new solution-processable ladder-type conjugated polymers by the use of new solvent system and control of reaction kinetics. Here, we report the synthesis and characterization of the first solubilizing group-free, solution-processable p -type ladder conjugated polymer, 6 H -pyrrolo­[3,2- b :4,5- b ′]­bis­[1,4]­benzothiazine ladder (PBBTL) polymer (Figure ) by the use of a 1:1 binary acid solvent system consisting of polyphosphoric acid (PPA) and phenylphosphonic acid (PhPO 3 H 2 ).…”

“…While the thermoelectric performances of FeCl 3 -doped PBBTL reported here is modest, we will like to emphasize that our results are preliminary as many of the processes involved in the device fabrication were not optimized. Also, different dopants and doping processes have shown to strongly affect the thermoelectric performance, as in the case for BBL. ,,,− …”

Polaron Delocalization Dependence of the Conductivity and the Seebeck Coefficient in Doped Conjugated Polymers

“…Together with their solution processability, good mechanical properties, and low toxicity, they make excellent ambient-temperature waste heat energy harvesters for wearable electronics. − To date, the majority of research efforts to achieve a higher power factor [PF, a product of conductivity (σ) and square of Seebeck coefficient (α)] of polymer-based thermoelectrics have been invested in improving the σ of doped conjugated polymers. Higher σ is generally achieved through polymer structural modification to achieve a higher mobility − or by introducing different dopants. ,− Alternatively, modifications of the polymer/dopant processing method such as addition of solvent additives, − solution shearing or rubbing, − and the sequential solution/vapor doping method ,, have been shown to increase the overall σ through better film crystallinity/alignment and minimized disruption. While the approaches above have been successful in understanding the structure–charge transport relationship in the doped polymers, they do not address how chemical structure influences the Seebeck coefficient in conjugated polymers.…”

P- and N-dopable ambipolar bulk heterojunction thermoelectrics based on ladder-type conjugated polymers

“…The thermoelectric properties of individual p-doped PBBTL and n-doped BBL have been investigated by us and other research groups previously. 18,[27][28][29][30][31] The thermoelectric performance of the BHJ film can be tuned not only by varying the doping time, but also by varying the p-and n-type polymer ratios. A full thermoelectric device based on a pair of p-and n-channel legs was fabricated using a 1 : 1 BHJ blend film doped at 1 s immersion for both p-and n-doping, respectively, demonstrating the simplicity of such a device architecture.…”