Control of the Electrostatic Association between a Highly Doped Conjugated Polymer and a Counter-Ion for High Thermoelectric Performance

DoctorOrganic thermoelectric generators (TEGs) requires high carrier density for high TE performance; therefore, doping is an essential step to fabricate the TE device. In highly doped states of conjugated polymers, the transport characteristics of charge carriers are dominantly determined by the electrostatic interaction between the host polymer and the counter-ions. Therefore, to obtain high TE performance, it is important to systematically control counter-ion-induced effect on charge transport. However, the limitation of dopant engineering in highly doped states restricts the systematic control of the electrostatic interaction, so the improvement of TE performance by engineered electrostatic association has not been achieved. Herein, by using counter-ion exchange methods, this thesis is dedicated to understanding the effect of counter-ion-induced electrostatic association on charge transport in a highly doped conjugated polymer. Engineering the physical properties of introduced counter-ions determines the optimized molecular conformation and the degrees of disorder for high TE performance. In Chapter 2, by exchanging the existing dopants to alkylsulfonate anions with different physical properties, controlled electrostatic interaction of poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate (PEDOT:PSS) leads to a positive deviation from the trade-off relation between the electrical conductivity and the Seebeck coefficient. An increase in carrier density by doping leads to inverse behavior between two salient TE parameters: the Seebeck coefficient and the electrical conductivity, which restricts the improvement of TE performance. However, controlling electrostatic interaction between the conducting polymer and the counter-ion can play an important role in TE charge transport to overcome the limitation. Herein, I systematically investigate the effect of the electrostatic interaction between conducting polymers and counter-ions on the TE properties. By adding small-molecule anionic dopants, alkylsulfonate anions, with different physical properties, I systematically change the electrostatic interaction between PEDOT and PSS. Upon reduction of the electrostatic interaction, considerable changes in the film morphology, chain conformation, and crystalline ordering are observed and analyzed, all of which strongly affect the TE charge transport. Decreased electrostatic interaction between PEDOT and PSS changes the film morphology of PEDOT:PSS from a pancake-like conformation to a fibrous conformation, and I verify that this change originated in a solution state. The decreased interaction also increase the planarity in PEDOT, resulting in a high proportion of quinoid structure and improved molecular ordering of PEDOT:PSS with increased π–π packing density. Furthermore, the weaken electrostatic interaction of PEDOT:PSS in and around the added anions leads to variation of the oxidation level of PEDOT with varying alkyl chain length. As a result, optimizing the interaction enhanced the TE power factor (= 700.2 μW·m−1·K−2) and figure of merit ZT (= 0.25) by increasing both electrical conductivity and the Seebeck coefficient. These results deviate from the empirical relation among the TE parameters of electrical conductivity, Seebeck coefficient, and the power factor and suggest that the electrostatic interaction between the conducting polymer and the dopant should be considered an important factor as the dopant amount. In Chapter 3, counter-ion-induced disorder in a highly doped conjugated polymer is systematically engineered to obtain high TE performance. The structural and energetic disorder of the polymers is a critical factor that influences charge transport in doped conjugated polymers because it determines how effectively charge-carrier pathways are secured and how the charge carriers are delocalized along the polymers. However, the degrees of disorder in a highly doped state of conjugated polymers is governed by the Coulomb interaction between a charge carrier and a counter-ion. Accordingly, experimentally controlling the disorder in highly doped conjugated polymers has been difficult because of the limitations of dopant engineering. Herein, by using a counter-ion exchange method, I systematically engineer the counter-ion-induced disorder in a highly doped state and analyze how the disorder changes the TE transport properties in PEDOT. Multi-cyano-functionalized counter-ions, which exhibit different Coulombic attraction with PEDOT, affect the structural and energetic disorder in PEDOT. The changes in the disorder are evaluated in several characteristics of PEDOT: crystalline ordering, density of states (DOS), and polaron behaviors. Strong localization of charge carriers in PEDOT with a high degree of structural disorder is accompanied by decrease in planarity of PEDOT chains with a low proportion of a quinoid structure. Furthermore, an increase in counter-ion-induced localization of charge carriers generates an increase in energetic disorder with broadened DOS in PEDOT. A decrease in the number of unpaired polarons and the degrees of localization of polarons also show a decrease in degrees of disorder in PEDOT. These changes result in opposite behaviors of the electrical conductivity and the Seebeck coefficient in PEDOT in response to the degree of disorder and thereby yield a high figure of merit ZT (= 0.21) with a remarkable power factor (= 630.6 μW·m−1·K−2) in the PEDOT film which has the lowest degree of disorder