High Performance Organic Thermoelectrics Enabled by Controlling Counter-Ion-Induced Disorder in Highly Doped Conjugated Polymers

The degree of structural and energetic disorder in conjugated polymers governs how effectively the charge carriers are transported as it impinges on the molecular geometry or π-orbital overlap. Thus, how the counter-ion-induced disorder affects the charge transport in highly doped conjugated polymer systems should be investigated to achieve high thermoelectric (TE) performance. However, experimentally controlling the disorder caused by the Coulomb interaction between the polymer and the dopant is difficult due to the limitations of dopant engineering. Herein, we systematically control the counter-ion-induced disorder and investigate the effect of disorder on the TE transport properties in poly(3,4-ethylenedioxythiophene) (PEDOT) by using anion exchange method. Multi-cyano-functionalized counter-ions, which exhibit different Coulomb interaction with PEDOT, change the structural and energetic disorder in PEDOT. To quantify the counter-ion-induced disorder, the degree of disorder is evaluated in several microscopic aspects: molecular ordering, density of states (DOS), and polaron behaviors. The low extent of counter-ion-induced disorder increases the planarity of the PEDOT chains and simultaneously narrows the DOS of PEDOT. In addition, an analysis of the number of unpaired polarons derived from the Curie susceptibility shows how the charge carriers are localized by the counter-ion-induced disorder. Consequently, these changes result in the opposite behaviors of the electrical conductivity and the Seebeck coefficient depending on the extent of disorder and thereby produce a remarkable power factor of 630.6 μW m^-1 K^-2 and figure-of-merit <i>ZT</i> of 0.21 with the lowest degree of disorder. This work would provide a comprehensive understanding of counter-ion-induced disorder effect on charge transport in terms of optimizing TE performance.