Engineering counter-ion-induced disorder of a highly doped conjugated polymer for high thermoelectric performance

Disorder of doped conjugated polymers, which affects how effectively charge carriers are transported, is an important factor to be controlled to achieve high thermoelectric (TE) performance. However, experimentally controlling the disorder in highly doped polymers is difficult because of the limitations of dopant engineering. Here, by using a counter-ion exchange method, we systematically control the counter-ion-induced disorder in a highly doped state and analyze how the disorder changes the TE transport properties in poly(3,4-ethylenedioxythiophene) (PEDOT). Multi-cyano-functionalized counter-ions, which exhibit different Coulombic attraction with PEDOT, change the structural and energetic disorder in PEDOT. These changes in the disorder are evaluated with respect to several qualities of PEDOT: crystalline ordering, density of states (DOS), and polaron behaviors. Decreasing the counter-ion-induced localization of charge carriers in PEDOT increases the planarity of the PEDOT chains and narrows the DOS of PEDOT. In addition, an analysis of the number of unpaired polarons and the Curie susceptibility shows quantitatively how the charge carriers are localized by the counter-ion-induced disorder in PEDOT. These changes result in opposite behaviors of the electrical conductivity and the Seebeck coefficient in PEDOT in response to the extent of disorder and thereby produce a high figure of merit ZT of 0.21 with a remarkable power factor in the PEDOT film with the lowest degree of disorder.