Thermoelectric devices have the unique ability to interconvert heat and electricity directly. Soft thermoelectric materials, including conjugated polymers and organic-inorganic hybrids, now demonstrate figures of merit approaching those of inorganic materials. These breakthroughs in materials development enable the design of thermoelectric devices that exhibit appropriate efficiencies for commercial use, while simultaneously leveraging the unique processing and mechanical advantages of soft materials. Such technology opens the door to a suite of new thermoelectric applications, including power generation for biomedical implants and the Internet of Things, or wearable heating and cooling devices. However, in order to realize deployment of such technologies, there is a fundamental need for deeper understanding of the complex transport physics underlying thermoelectric transport in soft materials.
The central focus of this dissertation is investigating the fundamental physical phenomena critical to carrier transport in hybrid organic-inorganic thermoelectric material. Due to the complex nature of this class of multiphase material, there remains a problematic lack of consensus in the field regarding transport in hybrid materials. The mechanisms of carrier transport, key physics responsible for high thermoelectric performance, and even how to model transport in these materials are all subjects of debate within the field. Here, I describe the design, synthesis, and characterization of a prototypical PEDOT:PSS-Te hybrid nanomaterial with the goal of performing careful study of the carrier physics and relevant molecular-scale phenomena in this material. A novel technique for patterning alloy nanophases is demonstrated, resulting in well-controlled PEDOT:PSS-Te-Cu1.75Te heterowires. The Te-Cu1.75Te energetics are well aligned to leverage the carrier filtering effects proposed in literature. Using a full suite of experimental, theoretical, and modeling tools, we reveal the key physics responsible for dictating carrier transport and thermoelectric properties in this material, testing each of the major hypothesis in the field. Contrary to popular belief in the field, it is revealed that energy filtering does not play a major role in the carrier transport and high thermoelectric performance of these materials; rather, organic structural effects at the hard-soft interface and interfacial charge transport emerge as the key phenomena underlying transport.
In a complementary study, I describe a platform approach for the synthesis of new solution-based, air stable n-type soft thermoelectrics. Using this approach, a composite perylene diimide-Te nanowire thermoelectric ink is prepared, demonstrating up to 20-fold enhancement over the individual components. The performance of these materials is competitive with the best-in-class for fully solution-processed, air stable n-type thermoelectric inks. We find experimental evidence linking reorganization of the perylene diimide molecules on the Te surface to enhanced electrical conductivity in the composite, further emphasizing the importance of structural effects in the organic phase to the overall thermoelectric properties of hybrid materials. Finally, leveraging the best materials from among the work in this dissertation, we demonstrate power generation in an all-ink flexible thermoelectric module with an innovative folded geometry.
The findings in this dissertation provide critical insight into the physics underlying carrier transport and high thermoelectric performance in hybrid organic-inorganic nanomaterials. This work highlights the importance of developing hybrid design strategies capable of leveraging molecular-level effects at the hard-soft interface. In furthering the field’s fundamental understanding of this material class, we drive progress towards the realization of flexible thermoelectric modules compatible with applications such as implantable medical devices, wearable technologies, and the Internet of Things.