Polarity Switch and Conductivity Enhancement of a Thermoelectric D–A Copolymer Enabled by Molecular Weight Control

Enriching the library of polarity-switchable conjugated polymers from p- to n-type is highly significant for organic thermoelectric materials, as high-performance n-type materials are in short supply compared with their complementary p-type counterparts. From the D–A copolymer of poly{3′,4′-dihexyl-(2,2’:5′,2″-terthiophene)-5,5″-diyl- alt -[4′,7′-di-2-(4-(2′-ethylhexyl))thienyl-(5′,6′-difluorobenzo[ c ][1′,2′,5′]thiadiazole)]-5,5-diyl} ( PTDT2fBT ), PTDT2fBT-L ( M n = 15.9 kDa) and PTDT2fBT-H ( M n = 29.7 kDa) with comparable polydispersity index (PDI) were obtained by Soxhlet fractionation sequentially with chloroform and toluene in this work. Upon doping by spin-coating FeCl 3 solution in acetonitrile, the Seebeck coefficients (α) switch from positive (p-type) to negative (n-type) when the concentration of FeCl 3 reaches 4 mg mL –1 and above for the PTDT2fBT-H films while remaining consistently positive for the PTDT2fBT-L films. The optimal electrical conductivity (σ) of the n-type PTDT2fBT-H film is nearly twice that of the PTDT2fBT-L film due to the higher doping efficiency and more favorable microstructure characteristics for charge transport within the PTDT2fBT-H film. The ability of PTDT2fBT-H to transition from the p-type to the n-type can be explained by its elevated HOMO level (−5.10 eV), which is 0.11 eV higher than that of PTDT2fBT-L . This energy difference enables the Fermi level of PTDT2fBT-H to cross the transport energy level downward upon doping. These findings suggest that regulating the HOMO level of a p-type D–A copolymer via M n control can facilitate the realization of its polarity switch and enhance their thermoelectric properties.