Morphology and Charge Transport Properties of Conducting Polymers for Field-Effect Transistors and Lithium Battery Electrodes

Shrayesh Patel from UC Santa Barbara

Semiconducting (conjugated) polymers are highly attractive for various organic electronic and energy storage and conversion applications due to tunable electronic properties, solution processibility, and mechanical flexibility.  One of the challenges with the application of semiconducting polymers has been low charge carrier mobility (<1 cm2V-1s-1) relative to conventional inorganic semiconductors.  The charge carrier mobility is highly dependent on the structure, nanoscale morphology, and long-range order of the polymer thin film (thickness <100 nm).  One route to improve device mobility of thin-film transistors is through preferential alignment of the polymer chains between the source and the drain contacts.  For the first part of the talk, I will report on the structural and morphological characterization of a highly aligned semiconducting polymer (cyclopental dithiophene and thiadiazole pyridine copolymer, CDT-PT) for thin-film field-effect transistors. The characterization techniques include a combination of transmission electron microscopy (TEM), grazing incidence wide angle X-ray scattering (GIWAXS), and near edge x-ray absorption fine structure (NEXAFS) spectroscopy.  This characterization will provide further insight on the significance of alignment for enhanced charge carrier mobility.  For the second part of the talk, I will present my work on conducting copolymers for lithium battery electrodes.  A traditional porous lithium battery electrode consists of a redox-active material (e.g. LiFePO4), carbon black for electronic conduction, and non-conductive binder (e.g. PVDF) that holds the particles in place.  The pores are backfilled with liquid organic electrolyte for ionic conduction.  A new electrode design was developed using a diblock copolymer consisting of semiconducting poly(3-hexylthiophene) (P3HT) for electronic transport and poly(ethylene oxide) (PEO) and LiTFSI mixture for lithium ion transport.  This copolymer serves both as a binder and as an electronic and ionic charge transport medium essential for enabling redox reactions.  Also, the conductive polymeric binder eliminates the flammable liquid electrolyte; thus developing safer batteries consisting of only solid materials.  By taking advantage of the semiconducting nature of P3HT, we can uniquely control the electronic charge carrier mobility in the electrode as function of potential, which is not possible for traditional carbon-containing electrodes. Specifically, I will present results pertaining to (i) lab-scale battery performance, (ii) bulk charge transport properties of the conductive polymeric binder, and (iii) the effect of the potential-dependent mobility of P3HT on battery performance.