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 FeCl3 solution in acetonitrile, the Seebeck coefficients (alpha) switch from positive (p-type) to negative (n-type) when the concentration of FeCl3 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 (sigma) 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.