Ion Exchange Induced Efficient N‐Type Thermoelectrics in Solid‐State.

High‐performance n‐type solid‐state ionic thermoelectrics (SS i‐TEs) for low‐grade heat harvesting are highly desired and challenging. Here, the design and synthesis of an efficient n‐type mixed conductor via ion pair modulation is demonstrated, which consists of biguanide hydrochloride (MfmCl) and a poly(3,4‐ethylenedioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS) polymeric complex in a solid film. Theoretical calculations and nano/microstructure characterization reveal that the binding preference of ion pairs offers energetically favorable ion exchange in the matrix, which induces not only tightly bound Mfm PSS species but also favorable anion diffusion channels. Consequently, an enhanced ionic conductivity of 1.40 S m−1 with a record highest negative thermopower of −46.97 mV K−1 is achieved for the n‐type mixed conductor thus far. [ABSTRACT FROM AUTHOR]