Establishing Structure-Function Relationships for Soft Thermoelectric Materials Through Rational Design

Thermoelectric generators offer an exciting avenue for enhancing the efficiency of existing heat engines through the scavenging of waste heat. These direct heat to electricity converting devices even have the potential to power small electronic devices through body heat alone. Traditionally, thermoelectric materials are dominated by inorganic rare earth elements which, while achieving high degrees of performance, have limited the widespread deployment of thermoelectric generators due to prohibitive manufacturing costs, energy intensive processes and rigid device form factors. However, propelled by recent advances in soft electronic materials and the advent of personal electronic devices and the internet of things, there has been a surge of interest in developing carbon-based thermoelectric systems. Within this family of soft thermoelectrics, hybrid materials composed of organic and inorganic constituents have demonstrated interesting physical phenomenon and promising performance enhancements. While these materials are intriguing due to their potential for realizing novel transport physics, the non-linear interactions occurring at the organic-inorganic interface are not well understood. This makes the design of new cutting-edge hybrid materials more challenging. In order to establish more concrete design rules to help guide the development of next generation hybrid thermoelectric materials, we chose to begin by focusing on a well-studied hybrid model system composed of tellurium (Te) nanowire cores and a conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) shell. While this Te-PEDOT:PSS hybrid system has been extensively characterized, little has been done to systematically investigate the impacts of nanowire length and diameter as a knob for tuning thermoelectric performance. To pursue this, we utilized a synthetic procedure leveraging different molecular weights of polyvinyl pyrrolidone (PVP) to act as the structure directing and surface passivating ligand for the Te atoms during the nucleation process. Upon synthesizing the nanowires, a gentle ligand exchange process was used to transform the shell from insulating PVP to conductive PEDOT:PSS. We show that it is possible to tune Te nanowire dimensions by using different molecular weights of PVP and a 183% increase in thermoelectric performance can be achieved compared to prior literature reports of similarly sized Te-PEDOT:PSS nanowires. Interestingly, diameter appears to be the nanowire dimension most responsible for enhancing performance. To better understand this apparent diameter dependence, we chose to further simplify the model system to a single Te-PEDOT:PSS nanowire in order to be able to more effectively isolate transport phenomenon. We show that as the nanowire diameter is reduced, the electrical conductivity increases and the thermal conductivity decreases, while the Seebeck coefficient remains nearly constant. The origin of the decoupling of charge and heat transport lies in the fact that electrical transport occurs through the organic shell, while thermal transport is driven by the inorganic core. With this first ever experimental validation of electrical transport occurring predominantly within the organic shell, one clear avenue for enhancing thermoelectric transport is through modulation and control over shell morphology. Knowing this, we investigated alternative approaches for controlling organic polymer structure through heteroatom substitution. Poly(3-(3’,7’-dimethyloctyl) chalcogenophenes) (P3RX) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was chosen to study and the doping methodology, the heteroatom (X = Thiophene (T), Selenophene (Se), Tellurophene (Te)) and the extent of doping are systematically varied. By spectroscopically, structurally and thermoelectrically characterizing these P3RX polymers we find that heteroatom identity radically impacts degree of structural ordering. Furthermore, we find that structural ordering not only impacts electronic charge transport but also the success of dopant:polymer interactions. The findings detailed in this dissertation help to establish clear design rules upon which the next generation of cutting-edge hybrid materials can be based. In addition to the implications for enhancing thermoelectric transport in hybrid materials, the final study also aids in establishing a better understanding of how to leverage the most optimal organic polymer doping conditions, a topic highly relevant to the organic electronics community.

Advisor: Urban, Jeffrey
Wu, Junqiao