Flow Effects on the Controlled Growth of Nanostructured Networks at Microcapillary Walls for Applications in Continuous Flow Reactions

Low-cost microfluidic devices are desirable for many chemical processes; however, access to robust, inert, and appropriately structured materials for the inner channel wall is severely limited. Here, the shear force within confined microchannels was tuned through control of reactant solution fluid-flow and shown to dramatically impact nano- through microstructure growth. Combined use of experimental results and simulations allowed controlled growth of 3D networked Zn­(OH)F nanostructures with uniform pore distributions and large fluid contact areas on inner microchannel walls. These attributes facilitated subsequent preparation of uniformly distributed Pd and PdPt networks with high structural and chemical stability using a facile, in situ conversion method. The advantageous properties of the microchannel based catalytic system were demonstrated using microwave-assisted continuous-flow coupling as a representative reaction. High conversion rates and good recyclability were obtained. Controlling materials nanostructure via fluid-flow-enhanced growth affords a general strategy to optimize the structure of an inner microchannel wall for desired attributes. The approach provides a promising pathway toward versatile, high-performance, and low-cost microfluidic devices for continuous-flow chemical processes

Visible-Light-Excitable Sky-Blue TADF-Type Organic Afterglow

Blue afterglow emitters contribute one of the three primary afterglow colors and can be used to generate other afterglow colors via energy transfer. Statistics shows that although there are examples of blue afterglow emitters with high afterglow efficiency and long afterglow lifetime, such emitters require ultraviolet-B excitation to switch on their afterglow properties. Here, we fabricate visible-light-excitable sky-blue thermally activated delayed fluorescent (TADF)-type organic afterglow materials via a dopant-matrix strategy, where TADF represents thermally activated delayed fluorescence. Luminescent difluoroboron β-diketonate (BF2bdk) dopants were synthesized by our cascade reaction. Upon doping into rigid organic matrices, the resultant BF2bdk-matrix materials exhibit sky-blue ambient afterglow, which can be excited by ultraviolet-A, 405 nm purple, or 420 nm blue light. The materials possess afterglow lifetimes of several hundred milliseconds and an afterglow efficiency of up to 72.9%. Because of the singlet nature, the sky-blue TADF afterglow emitters can serve as efficient donors for energy transfer to produce afterglow materials with other colors

Toward Precision Control of Nanofiber Orientation in Conjugated Polymer Thin Films: Impact on Charge Transport

The deposition of conjugated polymers is typically subject to chain-entanglement effects, which can severely hinder chain unfolding, alignment, and π–π stacking during rapid solution-coating processes. Here, long-range ordering and highly aligned poly­(3-hexylthiophene) (P3HT) thin films were demonstrated by preprocessing the polymer solution with ultraviolet (UV) irradiation/solution aging and then depositing via the blade-coating method, which is compatible with roll-to-roll printing processes. The surface morphologies and optical anisotropy of deposited films revealed that the degree of chain alignment was greatly improved with increased levels of polymer assembly that can be precisely controlled by solution-aging time. The correlations between oriented nanofibrillar structures and their charge-transport anisotropy were further systematically investigated by blade coating pretreated solutions parallel and perpendicular to the direction of the source and drain electrodes. Interestingly, charge transport across the well-aligned P3HT nanofibers was more efficient than along the long-axis of nanofibrillar structures owing to enhanced intramolecular charge transport and tie-chains. The facile and scalable solution-coating method investigated here suggests an effective approach to induce anisotropic crystalline structures, which are readily obtained by directly controlling their intrinsic solution properties without the need for extrinsic techniques such as surface templating or shearing blade patterning

Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation

Solution shearing has attracted great interest for the fabrication of robust and reliable, high performance organic electronic devices, owing to applicability of the method to large area and continuous fabrication, as well as its propensity to enhance semiconductor charge transport characteristics. To date, effects of the design of the blade shear features (especially the microfluidic shear design) and the prospect of synergistically combining the shear approach with an alternate process strategy have not been investigated. Here, a generic thin film fabrication concept that enhanced conjugated polymer intermolecular alignment and aggregation, improved orientation (both nanoscale and long-range), and narrowed the π–π stacking distance is demonstrated for the first time. The impact of the design of shearing blade microfluidic channels and synergistic effects of fluid shearing design with concomitant irradiation strategies were demonstrated, enabling fabrication of polymer-based devices with requisite morphologies for a range of applications

Microfluidic Crystal Engineering of π‑Conjugated Polymers

Very few studies have reported oriented crystallization of conjugated polymers directly in solution. Here, solution crystallization of conjugated polymers in a microfluidic system is found to produce tightly π-stacked fibers with commensurate improved charge transport characteristics. For poly(3-hexylthiophene) (P3HT) films, processing under flow caused exciton bandwidth to decrease from 140 to 25 meV, π–π stacking distance to decrease from 3.93 to 3.72 Å and hole mobility to increase from an average of 0.013 to 0.16 cm² V^–1 s^–1, vs films spin-coated from pristine, untreated solutions. Variation of the flow rate affected thin-film structure and properties, with an intermediate flow rate of 0.25 m s^–1 yielding the optimal π–π stacking distance and mobility. The flow process included sequential cooling followed by low-dose ultraviolet irradiation that promoted growth of conjugated polymer fibers. Image analysis coupled with mechanistic interpretation supports the supposition that “tie chains” provide for charge transport pathways between nanoaggregated structures. The “microfluidic flow enhanced semiconducting polymer crystal engineering” was also successfully applied to a representative electron transport polymer and a nonhalogenated solvent. The process can be applied as a general strategy and is expected to facilitate the fabrication of high-performance electrically active polymer devices

Unipolar Electron Transport Polymers: A Thiazole Based All-Electron Acceptor Approach

Unipolar Electron Transport Polymers: A Thiazole Based All-Electron Acceptor Approac

Molecular Engineering of Nonhalogenated Solution-Processable Bithiazole-Based Electron-Transport Polymeric Semiconductors

The electron deficiency and trans-planar conformation of bithiazole is potentially beneficial for the electron-transport performance of organic semiconductors. However, the incorporation of bithiazole into polymers through a facile synthetic strategy remains a challenge. Herein, 2,2′-bithiazole was synthesized in one step and copolymerized with di­thienyl­diketo­pyrrolopyrrole to afford poly­(di­thienyl­diketo­pyrrolopyrrole-bithiazole), <b>PDBTz</b>. <b>PDBTz</b> exhibited electron mobility reaching 0.3 cm² V^–1 s^–1 in organic field-effect transistor (OFET) configuration; this contrasts with a recently discussed isoelectronic conjugated polymer comprising an electron-rich bithiophene and dithienyldiketopyrrolopyrrole, which displays merely hole-transport characteristics. This inversion of charge-carrier transport characteristics confirms the significant potential for bithiazole in the development of electron-transport semiconducting materials. Branched 5-decylheptacyl side chains were incorporated into <b>PDBTz</b> to enhance polymer solubility, particularly in nonhalogenated, more environmentally compatible solvents. <b>PDBTz</b> cast from a range of nonhalogenated solvents exhibited film morphologies and field-effect electron mobility similar to those cast from halogenated solvents

Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices

High-efficiency organic solar cells (OSCs) can be produced through optimization of component molecular design, coupled with interfacial engineering and control of active layer morphology. However, vertical stratification of the bulk-heterojunction (BHJ), a spontaneous activity that occurs during the drying process, remains an intricate problem yet to be solved. Routes toward regulating the vertical separation profile and evaluating the effects on the final device should be explored to further enhance the performance of OSCs. Herein, we establish a connection between the material surface energy, absorption, and vertical stratification, which can then be linked to photovoltaic conversion characteristics. Through assessing the performance of temporary, artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure, optimal vertical stratification can be achieved. Adjusting the surface energy offset between the substrate results in donor and acceptor stabilization of that stratified layer. Further, a trade-off between the photocurrent generated in the visible region and the amount of donor or acceptor in close proximity to the electrode was observed. Modification of the substrate surface energy was achieved using self-assembled small molecules (SASM), which, in turn, directly impacted the polymer donor to acceptor ratio at the interface. Using three different donor polymers in conjunction with two alternative acceptors in an inverted organic solar cell architecture, the concentration of polymer donor molecules at the ITO (indium tin oxide)/BHJ interface could be increased relative to the acceptor. Appropriate selection of SASM facilitated a synchronized enhancement in external quantum efficiency and power conversion efficiencies over 10.5%