Enhancing polymer thermoelectric performance using radical dopants

Abstract

Abstract Organic thermoelectric materials provide a means to reclaim, in part, potentially lost energy through a solid-state process that converts low-value thermal gradients (in the form of heat) to electricity; however, a new avenue towards improving the performance of these emerging thermoelectric materials must be brought to light before their widespread implementation becomes warranted. Here, we develop a blend of open-shell small molecules and closed-shell, conjugated polymers in order to evaluate how the chemical composition of the distinct open-shell, charge-neutral molecular dopants impacts the thermoelectric performance of a common hole-transporting (p-type) polymer semiconductor, poly(3-hexylthiophene) (P3HT). In doing so, we are able to increase the electrical conductivity of the P3HT composite both with and without affecting the oxidation state of the polymer. Specifically, the electrical conductivity of the conjugated polymer increases without changing the oxidation state of the P3HT when the preferentially-oxidized species 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) radical is incorporated into the hole-conducting thin film. Moreover, the inclusion of the preferentially-reduced galvinoxyl (GAV) radical also increases the electrical conductivity of the thin film; however, this small molecule dopant changes the oxidation state of the P3HT moiety. The ability to achieve the gains in the electrical conductivity of P3HT with the TEMPO radical in a manner that is independent of its oxidation state was confirmed using optical spectroscopy, cyclic voltammetry, and x-ray diffraction techniques. Importantly, this ability to enhance the electrical conductivity of P3HT in a manner that is independent of the oxidation state of the polymer provides a means to circumvent the oft-observed inverse relationship between the electrical conductivity and thermopower of the semiconducting polymer, and this combined effect allows for an ∼170-fold increase for the TEMPO-containing composites compared to a ∼70-fold increase for the GAV-containing composites. Thus, the described phenomena for charge-neutral radical dopants provides a set of critical design parameters for future polymer thermoelectric materials and composites, and it opens a new pathway towards high-performance organic thermoelectric devices.