The influence of anion size on the thermoelectric properties and Seebeck coefficient inversion in PDPP-4T

Recent findings show that the Seebeck coefficient (S) can undergo a sign inversion at high doping concentrations in certain π-conjugated polymers. This work investigates how the counterion size influences whether S inversion occurs in electrochemically doped PDPP-4T. The results indicate that at high doping levels, PDPP-4T exhibits S inversion with the small counterion BF₄⁻, but not with the large counterion BArF⁻. Overall, the large BArF⁻ counterion leads to higher power factors in the most relevant range for thermoelectric devices, whereas the smaller BF₄⁻ counterion provides the possibility to alter the sign of S by varying the charge-carrier concentration.