Progress in Nickel-Coordinated Polymers as Intrinsically Conducting n-Type Thermoelectric Materials

Organic thermoelectrics have witnessed rapid development in the past decade for low temperature energy harvesting applications. While high-performance p-type polymers have been demonstrated, n-type materials have lagged behind due to the limited number of stable n-dopants and low doping efficiencies. Nickel-coordination polymers are a promising class of n-type polymers as they are conducting without extrinsic doping, thus overcoming a major challenge. However, advantages in thermoelectric properties are outweighed by a complicated synthesis and a poorly understood reaction mechanism that has resulted in a large variation in literature for the same material. This progress report provides a comprehensive and critical overview of syntheses and thermoelectric property optimization approaches for two coordination polymers, namely Ni-ethenetetrathiolate (NiETT) and Ni-tetrathiooxalate (NiTTO). In particular, material characterization and thin film fabrication techniques are discussed, and the importance of reporting statistically relevant thermoelectric properties is highlighted to ensure reproducibility among different groups. A short discussion on prototype devices based on NiETT is presented, and finally, directions for future development of these and other n-type metal-coordinated polymers are suggested.