Development of Organic Semiconductors for Soft Electronics

Abstract

Organic semiconductor based soft electronic devices are demonstrated: (1) effects of conjugation length on charge transport in polymer semiconductors, (2) water-borne colloids of organic semiconductors, and (3) breath-figure molding of polymer semiconductors for sensors. To improve the charge carrier mobility of diketopyrrolopyrrole donor-acceptor copolymer semiconductors, the length of the donor building block is controlled using vinylene moieties, and its effects on crystalline structure and charge transport are systematically studied. We synthesize P29-DPP-TBT with two vinylene linkages between thiophene units and compare it with P29-DPP-TVT with single vinylene linkage. Density functional theory calculations predict enhanced backbone planarity of P29-DPP-TBT compared to P29-DPP-TVT, which can be related with the increased conjugation length of P29-DPP-TBT as proved by the increased free exciton bandwidth extracted from UV-vis absorption spectra and the wavenumber shift of the C–C peaks to higher values in Raman spectra. From two-dimensional grazing incident X-ray diffraction studies, it is turned out that the paracrystalline disorder is lower in P29-DPP-TBT than in P29-DPP-TVT. Near-edge X-ray absorption fine structure spectroscopy reveal that more edge-on structure of polymer backbone is formed in the case of P29-DPP-TBT. By measuring the temperature-dependence of the charge carrier mobilities, it is turned out that the activation energy for charge hopping is lower for P29-DPP-TBT than for P29-DPP-TVT. Collectively, these results imply that the substitution of extended π-conjugated donor moiety of polymeric semiconductors can yield a more planar backbone structure and thus enhanced intermolecular interaction which enables more perfect crystalline structure as well as enhanced charge transport behavior. A synthetic approach has demonstrated to enhance coalescence phenomenon during solidification of water-borne colloids so that thin, even, and continuous film morphology of polymer semiconductors can be realized. From theoretical study of complex colloids, it is shown that small-sized and uniform colloid particles are essential to minimize depletion contact energy between colloid particles and thus to enhance coalescence. Therefore, the newly synthesized polymer semiconductor is designed for better molecular affinity with surfactants, so that phase transfer of polymer semiconductors from organic phase to water phase can proceed more efficiently during mini-emulsion synthesis. This is achieved by substituting a Si atom to the branching C atom of the alkyl solubilizing group of a conventional donor-acceptor polymer semiconductor. Such a chemical modification increases the volumetric portion of hydrophobic alkyl chains and thus enables higher solu-bility as well as higher hydrophobicity, all of which are closely related with enhancing molecular affinity be-tween polymer semiconductor and surfactants. As a result, the performance of organic field-effect transistors fabricated from water-borne colloids can be improved to a level similar to the case of organic solvents. More importantly, the reproducibility of transistor performance is also greatly improved due to the small and uni-form water-borne colloidal particles. Strategically designed polymer semiconductor thin film morphology with both high responsivity to the specific gas analyte and high signal transport efficiency is reported to realize high-performance flexible NOx gas sensors. Breath-Figure (BF) molding of polymer semiconductors enables a finely defined degree of nano-porosity in polymer films with high reproducibility while maintaining a high charge carrier mobility characteristics of organic field effect transistors (OFETs). The optimized BF-OFET with a donor-acceptor copolymer exhibits a maximum responsivity of over 104%, sensitivity of 774%/ppm, and limit of detection (LOD) of 110 ppb against NO. When tested across at NO concentrations of 0.2–10 ppm, the BF-OFET gas sensor ex-hibits a response time of 100–300 s, which is suitable for safety purposes in practical applications. Further-more, BF-OFETs show a high reproducibility as confirmed by statistical analysis on 64 independently fabri-cated devices. Selectivity to NOx analytes is tested by comparing the sensing ability of BF-OFET to other re-ducing gases and volatile organic compounds. Finally, flexible BF-OFETs conjugated with plastic substrates are demonstrated and they exhibit a sensitivity of 500%/ppm and LOD of 215 ppb, with a responsivity degradation of only 14.2% after 10,000 bending cycle at 1% strain.