Ryan Schofield

Abstract:

Advanced fibers enable the fabrication of structures and composites for applications reliant on lightweight, oxidation resistant, mechanically strong, and electrically insulating materials, e.g. in all forms of land, air, and space transportation and in applications within extreme environments. Hexagonal boron nitride (h-BN) fibers harness these advantages, and in addition, offer ultra-high-strength-to-weight ratio and low density. Yet, existing precursors for polymer-derived BN fibers are limited to insoluble and air/moisture sensitive polyborazylenes, hindering fiber production at scale. In this contribution, we report a reliable, controllable, and scalable synthesis methodology for producing pure micro- and nano-h-BN fibers, offering a competitive alternative to NASA’s energy-intensive h-BN nanotubes production. The single-source precursor, N-methyl polyaminoborane (PMeAB), plays a pivotal role in this process. The catalytic, and scalable, synthesis of PMeAB with controlled molecular weights (Mw = 110,500–290,500 g·mol−1) enables the production of h-BN fibers by electrospinning method and thermolysis under ammonia. PMeAB molecular weight and concentration were identified as key factors dictating the viscosity and surface tension, and thus influencing the overall spinnability of the PMeAB solution. We reveal that the subsequent formation of a cross-linked intermediate during PMeAB thermolysis is essential to retain the fibrous morphology during the conversion to h-BN fibers. Comprehensive characterization demonstrated the purity and homogeneity of the h-BN fibers, with ~ 97 at.% of B and N contents combined throughout the fiber body. This newly disclosed route to h-BN fibers offers a route to potentially valuable multifunctional filler material for advanced lightweight composites suitable for applications in extreme environments.

Abstract:

Nanofibrous active layers offer hierarchical control over molecular structure, and the size and distribution of electron donor:acceptor domains, beyond conventional organic photovoltaic architectures. This structure is created by forming donor pathways via electrospinning nanofibers of semiconducting polymer, then infiltrating with an electron acceptor. Electrospinning induces chain and crystallite alignment, resulting in enhanced light‐harvesting and charge transport. Here, the charge transport capabilities are predicted, and charge separation and dynamics are evaluated in these active layers, to assess their photovoltaic potential. Through X‐ray and electron diffraction, the fiber nanostructure is elucidated, with uniaxial elongation of the electrospinning jet aligning the polymer backbones within crystallites orthogonal to the fiber axis, and amorphous chains parallel. It is revealed that this structure forms when anisotropic crystallites, pre‐assembled in solution, become oriented along the fiber– a configuration with high charge transport potential. Competitive dissociation of excitons formed in the photoactive nanofibers is recorded, with 95%+ photoluminescence quenching upon electron acceptor introduction. Transient absorption studies reveal that silver nanoparticle addition to the fibers improves charge generation and/or lifetimes. 1 ns post‐excitation, the plasmonic architecture contains 45% more polarons, per exciton formed, than the bulk heterojunction. Therefore, enhanced exciton populations may be successfully translated into additional charge carriers.

Abstract:

Electrospinning has been applied to produce ceramic fibers using sol gel-based spinning solutions consisting of ceramic precursors, a solvent, and a polymer to control the viscosity of the solution. However, the addition of polymers to the spinning solution makes the process more complex, increases the processing time, and results in porous mechanically weak ceramic fibers. Herein, we develop a coelectrospinning technique, where a nonspinnable sol (<10 mPa s) consisting of only the ceramic precursor(s) and solvent(s) is encapsulated inside a polymeric shell, forming core-shell precursor fibers that are further calcined into ceramic fibers with reduced porosity, decreased surface defects, uniform crystal packing, and controlled diameters. We demonstrate the versatility of this method by applying it to a series of nonspinnable sols and creating high-quality ceramic fibers containing TiO₂, ZrO₂, SiO₂, and Al₂O₃. The polycrystalline TiO₂ fibers possess excellent flexibility and a high Young's modulus reaching 54.3 MPa, solving the extreme brittleness problem of the previously reported TiO₂ fibers. The single-component ZrO₂ fibers exhibit a Young's modulus and toughness of 130.5 MPa and 11.9 KJ/m³, respectively, significantly superior to the counterparts prepared by conventional sol-gel electrospinning. We also report the creation of ceramic fibers in micro- and nanospring morphologies and examine the formation mechanisms using thermomechanical simulations. The fiber assemblies constructed by the helical fibers exhibit a density-normalized toughness of 3.5-5 times that of the straight fibers due to improved fracture strain. This work expands the selection of the electrospinning solution and enables the development of ceramic fibers with more attractive properties.

Abstract:

Electrospun photoactive nanofibers hold significant potential for enhanced photon absorption and charge transport in organic photovoltaics. However, electrospinning conjugated polymers with fiber diameters comparable to exciton diffusion lengths for efficient dissociation, is difficult. Previously, spinning sub-100 nm poly(3-hexylthiophene) (P3HT) fibers has required the auxiliary polymer, poly(ethylene oxide) (PEO), and large antisolvent additions. Therefore, its success differs considerably across donor polymers, due to variable antisolvent addition limits before precipitation. Herein, plasmonic nanoparticle infusion into P3HT nanofibers is used to modulate viscosity and deliver a novel and unrivaled strategy to achieve reduced fiber diameters. Following PEO removal, the fibers measure 55 nm in diameter, 30% lower than any previous report – providing the shortest exciton diffusion pathways to the heterojunction upon electron acceptor infiltration. The nanoparticle-containing nanofibers present a 58% enhancement over their pristine thin-film counterparts. ~17% is ascribed to plasmonic effects, demonstrated in thin-films, and the remainder to along-fiber polymer chain alignment, introduced by electrospinning. The anisotropy of light absorbed when polarized parallel versus perpendicular to the fibers increases from 0.88 to 0.62, suggesting the diameter reduction improves the alignment, resulting in greater electrospinning-induced enhancements. Controlled by the electrospinning behavior of PEO, our platform may be adapted to contemporary donor-acceptor systems.

Abstract:

Achieving greater light-harvesting in thin photoactive layers remains critical to alleviating the absorption-extraction trade-off presented by organic photovoltaics (OPVs) - a flexible, lightweight, and low-cost alternative to silicon.

Herein, plasmonic nanoparticles are introduced to electrospinning solutions containing the electron donor polymer poly(3-hexylthiophene) (P3HT) and auxiliary polymer poly(ethylene oxide) (PEO), inducing viscosity/conductivity modulation and enabling the fabrication of record-low diameters of P3HT nanofibres with diameters of 55 nm, 30% lower than prior reports. Viscosity reduction is a time-dependent process allowing thinner fibres to be electrospun as the solution ages.

Electrospinning offers an impressive degree of chain alignment wherein anisotropic features orient along the fibre axis. For dissolved polymer, the chains are extended along the fibre. In crystalline regions, diffraction studies reveal the backbones are orthogonal to the axis. Crystallites assembled in solution before electrospinning grow in the π − π direction, and this anisotropy results in the observed backbone orientation.

Upon nanoparticle-induced diameter reductions, polymer alignment increases, providing optimal overlap of the transition dipole moment with the electric field vector of light, promoting photon-harvesting. Champion nanowebs provide up to 60% additional exciton generation versus a conventionally spin-coated thin-film.

A novel plasmon-active nanofibrous active layer is prepared by infiltration of the nanoweb with an electron acceptor. Photoluminescence quenching indicates that fibres offer competitive exciton dissociation levels of 95 – 97 %, whilst ultra-fast transient absorption confirms quenching translates into enhanced charge carrier populations with longer life-times. Compounded with absorption enhancements, ∼135% greater polaron populations were estimated 1 ns after photo-excitation in the presence of AgNPs prepared in-situ.

Proof-of-principle OPVs present efficiency approaching that of the bulk heterojunction, whilst AgNPs raise open-circuit voltage. Further work is required to achieve high-quality acceptor infiltration for effective carrier extraction. As the nanoparticle - PEO interaction dictates electrospinning, this platform is easily adapted to other contemporary semiconducting polymers.