The Woodruff School

SUMMARY

An abundance of heat is rejected to the environment owing to efficiency limitations of existing heat engines. Thermoelectric generators, comprising p- and n-type semiconductors, can recover this waste heat and convert it directly into electricity. Given that 60% of waste heat is at temperatures below 250°C, low cost and large scale thermoelectric devices are required to economically capture the heat. In this regard, conducting polymers constitute a suitable class of materials for low grade thermal energy harvesting as they are abundant and can be processed from solution. Furthermore, their low thermal conductivity and flexibility enables new device architectures. Despite these advantages, polymer-based devices have not made an impact. This is largely attributed to two factors: (i) lack of high performance n-type polymers, and (ii) low power outputs owing to poor device design. To address this, I have investigated metallo-organic polymers as n-type materials that are electrically conducting and stable in air. Specifically, I will discuss the synthesis, characterization and thermoelectric properties of Poly(nickel-ethenetetrathiolate) or NiETT. By modifying reaction conditions and performing film post-treatment by annealing, thermoelectric properties can be enhanced to obtain a high performing n-type polymer. As a parallel effort to developing materials, there is a need for new device designs that leverage the benefits of conducting polymers. In this regard, I have developed two new device architectures tailored for polymers. The first is a radial design based on characteristic thermal length scales for polymers that enables a 10x improvement in power density compared to conventional flat plate devices. This architecture uses heat spreading that eliminates the need for active cooling, while also reducing electrical contact resistance thereby enabling higher power densities. The second design is a closed-packed layout for printing high fill factor devices. By using fractal space-filling curves as interconnect patterns, the thermoelectric device can be load matched to a variety of applications, thereby eliminating the need for power convertors. These developments could enable low cost thermoelectric applications beyond waste heat recovery, such as self-powered sensors and wearable electronics.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Akanksha Krishnakumar Menon

TIME:	Monday, March 26, 2018, 2:00 p.m.

PLACE:	MRDC Building, 4211

TITLE:	Development of Organic Thermoelectric Materials and Devices for Energy Harvesting

COMMITTEE:	Dr. Shannon Yee, Chair (ME) Dr. Asegun Henry (ME) Dr. Samuel Graham (ME) Dr. Bernard Kippelen (ECE) Dr. John Reynolds (Chemistry)

SUMMARY An abundance of heat is rejected to the environment owing to efficiency limitations of existing heat engines. Thermoelectric generators, comprising p- and n-type semiconductors, can recover this waste heat and convert it directly into electricity. Given that 60% of waste heat is at temperatures below 250°C, low cost and large scale thermoelectric devices are required to economically capture the heat. In this regard, conducting polymers constitute a suitable class of materials for low grade thermal energy harvesting as they are abundant and can be processed from solution. Furthermore, their low thermal conductivity and flexibility enables new device architectures. Despite these advantages, polymer-based devices have not made an impact. This is largely attributed to two factors: (i) lack of high performance n-type polymers, and (ii) low power outputs owing to poor device design. To address this, I have investigated metallo-organic polymers as n-type materials that are electrically conducting and stable in air. Specifically, I will discuss the synthesis, characterization and thermoelectric properties of Poly(nickel-ethenetetrathiolate) or NiETT. By modifying reaction conditions and performing film post-treatment by annealing, thermoelectric properties can be enhanced to obtain a high performing n-type polymer. As a parallel effort to developing materials, there is a need for new device designs that leverage the benefits of conducting polymers. In this regard, I have developed two new device architectures tailored for polymers. The first is a radial design based on characteristic thermal length scales for polymers that enables a 10x improvement in power density compared to conventional flat plate devices. This architecture uses heat spreading that eliminates the need for active cooling, while also reducing electrical contact resistance thereby enabling higher power densities. The second design is a closed-packed layout for printing high fill factor devices. By using fractal space-filling curves as interconnect patterns, the thermoelectric device can be load matched to a variety of applications, thereby eliminating the need for power convertors. These developments could enable low cost thermoelectric applications beyond waste heat recovery, such as self-powered sensors and wearable electronics.