Controlling Energetic and Structural Disorder of Doped Conjugated Polymers for High-Performance Organic Thermoelectrics

Molecular doping is a fascinating technology that can easily increase the electrical conductivity of conjugated polymers. It is actively being explored as a fundamental approach for developing thermoelectric materials and flexible bio-integrated electronics. A variety of dopants, from inorganic salts to molecular dopants and Lewis acids, along with innovative doping strategies, have successfully pushed the electrical conductivity of conjugated polymers to approximately ~1000 S/cm. Despite these noteworthy outcomes, inherent limitations hinder further progress in this field. Conjugated polymers lack a three-dimensional crystal structure and contain structural imperfections, leading to a substantial energetic disorder with a broad distribution of the density of states in their energy landscape. Moreover, the incorporation of dopants introduces structural disorder in crystalline ordering, localizing charge carriers to just a few monomer units. This energetic and structural disorder acts as a significant hurdle to advancing doped conjugated polymers. In this talk, I will present a chemically doped conjugated polymer, indacenodithiophene-co-benzothiadiazole (IDTBT), which is able to maintain low energetic disorder due to the highly planar backbone chain structure. I will thoroughly introduce the relationship between the exceptional charge transport properties revealed by the Kang-Snyder model and narrow-band model, and the polymer chain structure inspected through Raman spectroscopy and DFT calculations. Also, I will discuss a doping strategy aimed at reducing dopant-induced disorder in highly doped conjugated polymers, with a particular focus on a promising Lewis acid dopant, tris(pentafluorophenyl)borane (BCF). The improvements in charge transport properties that result from this strategy will be substantiated through an analysis of structural disorder, energetic disorder, and charge carrier localization, including quantitative investigation using coherence length, paracrystallinity, and paramagnetic susceptibility. Lastly, I will provide a brief overview of the high thermoelectric performance achieved in our research and present the potential of thermoelectric materials based on doped conjugated polymers.