Elayne M. Thomas

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

The orientational correlation length of domains in a semiconducting polymer controls its thermoelectric performance. The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with FnTCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m−1 K−2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m−1 K−2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.

Photocrosslinking polymeric ionic liquids via anthracene cycloaddition for organic electronics

Polymeric ionic liquids (i.e., PILs) are single ion-conducting materials that exhibit the thermal and electrochemical stability of ionic liquids and the mechanical properties of polymers. Although PILs are exciting for a variety of applications in energy conversion and storage, the tradeoff between mechanics and ion transport remains an important limitation in materials design. Herein, a photocrosslinkable PIL based on the cycloaddition reaction of anthracene is converted from a viscous liquid into a soft solid without detrimental effects on the bulk ionic conductivity. The independent control of mechanical- and ion-conducting properties results from negligible changes in polymer segmental dynamics (i.e., glass transition temperature) upon crosslinking. This was demonstrated for both a polymer (i.e., N = 279) and its corresponding oligomer (i.e., N = 10). The ease of processability facilitated by the presented molecular design is illustrated by both patterning the PIL into μm-sized features, and incorporating it as a dielectric in thin-film transistors for low-voltage operation independent of device fabrication geometry.