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Akanksha Menon1 Rylan Wolfe1 Seth Marder1 John Reynolds1 Shannon Yee1

1, Georgia Institute of Technology, Atlanta, Georgia, United States

Over 60% of available waste heat is at temperatures below 250 °C. Low cost and large scale thermoelectric devices based on conducting polymers can capture this heat and convert it directly into electricity. Organic materials are suitable for low grade thermal energy harvesting as they can be synthesized from abundant elements and can be processed from solution using low cost fabrication techniques. Furthermore, their low thermal conductivity and flexible nature enable new device architectures and applications. Despite these advantages, polymer-based thermoelectric devices have not made an impact and this is largely attributed to the lack of high performance n-type polymers (power factor > 10 µW/m-K2 ), To address this, we investigate metallo-organic polymers (also known as coordination polymers) as a suitable class of ntype polymers that are electrically conducting and maintain their stability in air. First, we present the synthesis, characterization and thermoelectric properties of n-type poly(nickel-ethenetetrathiolate) or NiETT. By modifying the reaction conditions and film post-treatment by annealing, the thermoelectric properties can be simultaneously enhanced to obtain a high performing n-type polymer that maintains its stability under ambient conditions. Specifically, to address challenges with the reproducibility of air-oxidized NiETTs, a chemical oxidant is used to systematically tune the thermoelectric properties. Furthermore, the observations and insight gained from the ETT study are extended to other metallo-organic polymers such as the poly(nickel-tetrathiooxalate) or NiTTO. This is another coordination polymer that is easier to control synthetically as it is polymerized electrochemically. Herein, we present for the first time the thermoelectric properties of NiTTO films in a PVDF matrix, as well as its thermoelectric properties with varying counterions. Finally, temperature-dependent thermoelectric property measurements reveal that these polymers show semiconducting behavior that is consistent with thermally-activated hopping transport. The development of these two high performance and air stable n-type materials enables their application in realistic devices for thermoelectric energy harvesting.

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