UC Berkeley

Thermoelectrics can convert thermal energy directly into electrical energy or, alternatively, induce solid-state heating or cooling under electrical bias. As a result, these devices hold potential contributions in waste heat recovery and thermal management applications. Due to high material and processing costs, the applications of traditional bulk thermoelectrics, which have been around for over half a century, have been constrained to niche markets. Organic materials are becoming an appealing option for thermoelectrics with molecular design of such materials enabling tunability of electronic transport. Building efficient thermoelectric architectures requires complementary p-type (hole transporting) and n-type (electron transporting) components. While several high performance hole-transporting organic systems have been developed, a scarcity of stable n-type doping strategies compatible with facile processing has been a major impediment to the advancement of n-type organic electronics, and thermoelectric studies of organic n-type systems are scarce.

To develop design rules for improved n-type organic materials, we have used the perylene diimide (PDI) core as an n-type organic model system. The PDI core is air-stable and amenable to synthetic modification, thereby facilitating tunability of both molecular structure and electronic properties. Use of trimethylammonium functionalization with hydroxide counterions, tethered to the core by alkyl spacers, enables both water solubility and self-doping in PDI variants. By complementing thermoelectric characterization of these variants with insight on the electronic and structural property changes from optical spectroscopy, magnetic resonance studies, and X-ray chemical and structural characterization techniques, we show that a chemical transformation in the charged end groups upon thin film drying is critical to the underlying mechanism that enables charge carrier generation in these self-doping materials in the solid-state. Doping using tethered functionality is highly generalizable to other n-type small molecule systems of interest, including naphthalene diimides, diketopyrrolopyrroles, and PCBM. Structural modifications of the side chains can also be used to improve thin film thermoelectric properties. We show that changing the length of the alkyl spacer between the charged end groups and the PDI core dramatically increases electrical properties through a morphology-induced enhancement in the thin film effective electron mobility. The top derivatives in our study demonstrated the highest power factors reported for solution-processed films of n-type small molecules. We also show that broader system tunability and an enhancement in performance can be achieved through counteranion-mediated dopant activation and by complementing the inherent solid-state self-doping of these materials with chemical doping in solution.

The findings in this dissertation help shape promising molecular design strategies for future enhancements in the thermoelectric performance of n-type materials. Furthermore, the potential for coupling these n-type system advances with complementary p-type organic materials brings the development of fully solution-processable organic thermoelectric modules and their integration into applications, currently inaccessible by traditional, rigid inorganics, closer to reality.