In Situ Studies of the Swelling by an Electrolyte in Electrochemical Doping of Ethylene Glycol-Substituted Polythiophene.

Organic mixed ionic electronic conductors (OMIECs) have the potential to enable diverse new technologies, ranging from biosensors to flexible energy storage devices and neuromorphic computing platforms. However, a study of these materials in their operating state, which convolves both passive and potential-driven solvent, cation, and anion ingress, is extremely difficult, inhibiting rational material design. In this report, we present a novel approach to the in situ studies of the electrochemical switching of a prototypical OMIEC based on oligoethylene glycol (oEG) substitution of semicrystalline regioregular polythiophene via grazing-incidence X-ray scattering. By studying the crystal lattice both dry and in contact with the electrolyte while maintaining potential control, we can directly observe the evolution of the crystalline domains and their relationship to film performance in an electrochemically gated transistor. Despite the oEG side-chain enabling bulk electrolyte uptake, we find that the crystalline regions are relatively hydrophobic, exhibiting little (less than one water per thiophene) swelling of the undoped polymer, suggesting that the amorphous regions dominate the reported passive swelling behavior. With applied potential, we observe that the π-π separation in the crystals contracts while the lamella spacing increases in a balanced fashion, resulting in a negligible change in the crystal volume. The potential-induced changes in the crystal structure do not clearly correlate to the electrical performance of the film as an organic electrochemical transistor, suggesting that the transistor performance is strongly influenced by the amorphous regions of the film.

Room-Temperature Postannealing Reduction via Aqueous Sodium Borohydride and Composition Optimization of Fully Solution-Processed Indium Tin Oxide Films.

Solution-processed transparent conductive oxides offer the advantages of low-cost, high-throughput fabrication of electronic devices compared to the specific requirements of vacuum deposition techniques. However, adapting the current state of the art to ink deposition calls for optimization of the precursor ink composition and the postdeposition process. Solution processing of indium tin oxide films can be accomplished at reduced temperatures (250-400 °C) by annealing soluble precursor metal salts together with a fuel/oxidizer, causing an exothermic reaction with elevated local temperatures. Following layer-by-layer cycles of deposition and annealing, a postprocessing step is required via heating (300 °C) under a 5% H2 reducing atmosphere. To address the discrepancy between the versatility of ink deposition and the limitations of controlled atmosphere postprocessing, here we investigate the effects of postprocess dipping in aqueous sodium borohydride at room temperature as an alternative, which allows for a completely solution-based process from ink to film. In addition to postprocessing, the solution composition was also optimized by removing the fuel additive and by adjusting the In/Sn content. Indium tin oxide (ITO) films were spin-coated and annealed in air at 250, 300, and 400 °C and characterized by UV/vis spectroscopy to obtain optical transmittance, atomic force microscopy to obtain film thickness and surface morphology, and a Hall effect system for electrical parameters. Additional data from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) indicate that crystallinity is affected by the reducing environment. Results revealed an order-of-magnitude improvement of the Haacke figure of merit (FOM) from 4.3 × 10-4 Ω-1, 382 Ω/□ sheet resistance (Rs), and 84% transmittance (%T) for the traditional 9:1 In/Sn precursor ink with fuel additive followed by 300 °C of 5% H2-furnace post-treatment compared to that of the optimized fully solution-processed 8.5:1.5 In/Sn ink without fuel followed by an ambient air at 25 °C dipping in aqueous sodium borohydride, leading to 3.0 × 10-3 Ω-1 FOM, 84.5 Ω/□ Rs, and 87%T including the glass substrate.