Air-Stable n-Type Polymers for Thermoelectric Applications

Abstract

Polymer-based thermoelectrics have the potential for waste heat recovery and energy harvesting from temperatures less than 200°C. These devices can be fabricated via printing processes that are low cost from materials that are lightweight and flexible, with applications ranging from self-powered sensors to wearable electronics. In the past decade, significant progress has been made with high performing p-type thermoelectrics such as PEDOT-derivatives, while n-type polymers have lagged behind due to their air sensitivity and low electrical conductivity. In this work, we optimize the thermoelectric properties of metallo-organic polymers with nickel as the metal center. We report the synthesis, characterization and thermoelectric properties of poly(nickel-ethenetetrathiolate) and poly(nickel-tetrathiooxalate) that are intrinsically electrically conducting. We modify the extent of oxidation and study the role of different oxidants and counterions on tuning the electrical conductivity. Interestingly, the Seebeck coefficient (thermopower) remains largely unchanged over the course of oxidation indicating that this may be a viable technique to decouple thermopower and electrical conductivity in such materials. The temperature dependent properties for these polymers show semi-conducting behavior that is consistent with hopping transport. Film measurements by blending with an inert matrix indicate that these polymers maintain their stability in air, making them suitable for energy harvesting devices. To enhance the Seebeck coefficient, we blend the metallo-organic polymers with other materials to form composites. This enables optimization of the power factor to realize high performance n-type polymer thermoelectrics.