Solution‐state doping‐assisted molecular ordering and enhanced thermoelectric properties of an amorphous polymer

Abstract

Molecular doping of conjugated polymers is the performance-determining step in the fabrication of organic thermoelectric devices. Although strongly oxidizing salt-type dopants effectively produce charge carriers in polymer chains, their poor solubility in the processing solvents leads to undesired precipitation and complicates the film fabrication process. Thus, it is important to develop a conjugated polymer that can be readily doped in a mixed solution using organic dopants. In this study, we synthesized amorphous but highly electron-rich polymers (PCT1 and PCT2) by combining 4H-cyclopenta[2,1-b:3,4-b']dithiophene and thieno[3,4-b]thiophene moieties, whose full-names are poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-(2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate 4,6-diyl)] (PCTs). The PCT polymers were heavily doped with the organic dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), and the resulting PCT:F4TCNQ complex formed an integer charge transfer complex and promoted the intermolecular ordering of PCT polymers. Notably, the PCT:F4TCNQ complex exhibited unique thermoelectric behaviors, with significantly increased power factors even in the presence of large quantities of F4TCNQ, reaching values 3.2-fold higher than those achieved with FeCl3 sequential doping. Moreover, the higher-molecular-weight polymer, PCT2, exhibited more efficient molecular ordering and charge carrier transport than PCT1, which are crucial for enhancing the thermoelectric properties.