Fully Conjugated Ladder Polymers and Novel Dopants for N-Type Organic Thermoelectrics

Organic thermoelectric materials are systems capable of converting thermal differences into voltage differences. Within a thermoelectric device, the transition between thermal gradient and electric potential results from the diffusion of charge carriers towards the hot side and away from the cold side. This phenomenon is identified as the Seebeck effect, and its coefficient quantifies how rapidly a material responds to a perturbation. The thermoelectric efficiency is directly related to a high electrical conductivity. In contrast, a large thermal conductivity translates in a volatile temperature gradient, which minimizes the voltage difference and limits the diffusion of charge carriers. For these reasons, semiconducting organic polymers are excellent candidates for non-toxic, greener and more versatile thermoelectric applications. To further improve their efficiency, additional charge carriers are injected into these devices via doping. In organic semiconductors, charge carriers move from the frontier orbitals of the dopant towards the undoped species. Therefore, a greater energy difference between the frontier orbitals of the two species not only guarantees a higher doping efficiency, but enhances the overall thermoelectric performance. In this work, we investigate fully conjugated ladder polymers as n-dopable thermoelectric candidates. Ladder polymers have been shown to perform well as n-type materials in thermoelectric devices, not only on account of their high electrical conductivity, but also their low lying LUMOs rendering them susceptible to reduction by conventional n-dopants. However, they tend to be insoluble and difficult to process. We address these shortcomings by designing and synthesizing novel ladder polymers with solubilizing groups that do not sacrifice performance. Unlike previous reports, we are employing slightly electron donating linkers to avoid overly unreactive monomers which are known to impede the polymerization. In addition, we are incorporating highly electron deficient functionalities within the polymer backbone to further lower the frontier orbitals and improve the doping efficiency. Finally, there is a paucity of n-type dopants and the mechanism of the best-performing dopants remains unclear. We are investigating a class of dopants with readily tunable oxidation potentials that minimally affect their structure, steric bulk and polarity to investigate the role of oxidation potentials without altering other relevant variables. With a deeper understanding of doping mechanisms and with highly electron deficient fully-conjugated ladder polymers, our group points towards record performances in the field of n-type organic thermoelectrics.