SWNT Anchored with Carboxylated Polythiophene "Links" on High-Capacity Li-Ion Battery Anode Materials.

Conjugated polymers possessing polar functionalities were shown to effectively anchor single-walled carbon nanotubes (SWNTs) to the surface of high-capacity anode materials and enable the formation of electrical networks. Specifically, poly[3-(potassium-4-butanoate) thiophene] (PPBT) served as a bridge between SWNT networks and various anode materials, including monodispersed Fe3O4 spheres (sFe3O4) and silicon nanoparticles (Si NPs). The PPBT π-conjugated backbone and carboxylate (COO-) substituted alkyl side chains, respectively, attracted the SWNT π-electron surface and chemically interacted with active material surface hydroxyl (-OH) species to form a carboxylate bond. Beneficially, this architecture effectively captured cracked/pulverized particles that typically form as a result of repeated active material volume changes that occur during charging and discharging. Thus, changes in electrode thickness were suppressed substantially, stable SEI layers were formed, electrode resistance was reduced, and enhanced electrode kinetics was observed. Together, these factors led to excellent electrochemical performance.

Thermally Switchable Liquid Crystals Based on Cellulose Nanocrystals with Patchy Polymer Grafts.

A thermally "switchable" liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). "Patchy" polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the "patchy" PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the "brush" PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the "patchy" PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.

Nucleation, Growth, and Alignment of Poly(3-hexylthiophene) Nanofibers for High-Performance OFETs.

Conjugated semiconducting polymers have been the subject of intense study for over two decades with promising advances toward a printable electronics manufacturing ecosystem. These materials will deliver functional electronic devices that are lightweight, flexible, large-area, and cost-effective, with applications ranging from biomedical sensors to solar cells. Synthesis of novel molecules has led to significant improvements in charge carrier mobility, a defining electrical performance metric for many applications. However, the solution processing and thin film deposition of conjugated polymers must also be properly controlled to obtain reproducible device performance. This has led to an abundance of research on the process-structure-property relationships governing the microstructural evolution of the model semicrystalline poly(3-hexylthiophene) (P3HT) as applied to organic field effect transistor (OFET) fabrication. What followed was the production of an expansive body of work on the crystallization, self-assembly, and charge transport behavior of this semiflexible polymer whose strong π-π stacking interactions allow for highly creative methods of structural control, including the modulation of solvent and solution properties, flow-induced crystallization and alignment techniques, structural templating, and solid-state thermal and mechanical processing. This Account relates recent progress in the microstructural control of P3HT thin films through the nucleation, growth, and alignment of P3HT nanofibers. Solution-based nanofiber formation allows one to develop structural order prior to thin film deposition, mitigating the need for intricate deposition processes and enabling the use of batch and continuous chemical processing steps. Fiber growth is framed as a traditional crystallization problem, with the balance between nucleation and growth rates determining the fiber size and ultimately the distribution of grain boundaries in the solid state. Control of nucleation can be accomplished through a sonication-based seeding procedure, while growth can be modulated through supersaturation control via the tuning of solvent quality, the use of UV irradiation or through aging. These principles carry over to the flow-induced growth of P3HT nanofibers in a continuous microfluidic processing system, leading to thin films with significantly enhanced mobility. Further gains can be made by promoting long-range polymer chain alignment, achieved by depositing nanofibers through shear-based coating methods that promote high fiber packing density and alignment. All of these developments in processing were carried out on a standard OFET platform, enabling us to generalize quantitative structure-property relationships from structural data sources such as UV-vis, AFM, and GIWAXS. It is shown that a linear correlation exists between mobility and the in-plane orientational order of nanofibers, as extracted from AFM images using advanced computer vision software developed by our group. Herein, we discuss data-driven approaches to the determination of process-structure-property relationships, as well as the transferability of structural control strategies for P3HT to other conjugated polymer systems and applications.

Patterning Bubbles by the Stick-Slip Motion of the Advancing Triple Phase Line on Nanostructures.

The stick-slip motion of the triple phase contact line (TCL) has wide applications in inkjet printing, surface coatings, functional material assembly, and device fabrication. Here, for the first time, we report that on an alumina substrate with nanostructures, the stick-slip motion of the advancing TCL during spreading of an emulsion droplet can serve as an effective nanopatterning process. Air enclosed in the substrate nanostructures can be exchanged with liquid during the "stick" phase, resulting in the formation of bubbles arranged in a ring pattern. The process takes place in two stages: rings of air form first and then, as the volume of air increases, they separate into air bubbles as a result of the Plateau Rayleigh instability. During the first stage, the rings form due to the stick-slip of the advancing TCL and are ascribed to hydrogen-bonding interactions. Ultimate bubble size is dependent on the substrate pore dimensions. The process was simulated using finite-element analysis to elucidate the mechanism associated with subsequent bubble formation. The simulations corroborate well with the experimental results. This stick-slip motion of the advancing TCL provides new insights into the phenomena associated with droplet spreading and wetting, and the ability to control the formation of patterned bubbles will be promising in applications ranging from microfluidics to printing of functional materials and devices based on bubble templates and applications requiring submerged hydrophobic surface.

Amplified Photon Upconversion by Photonic Shell of Cholesteric Liquid Crystals.

As an effective platform to exploit triplet-triplet-annihilation-based photon upconversion (TTA-UC), microcapsules composed of a fluidic UC core and photonic shell are microfluidically prepared using a triple emulsion as the template. The photonic shell consists of cholesteric liquid crystals (CLCs) with a periodic helical structure, exhibiting a photonic band gap. Combined with planar anchoring at the boundaries, the shell serves as a resonance cavity for TTA-UC emission and enables spectral tuning of the UC under low-power-density excitation. The CLC shell can be stabilized by introducing a polymerizable mesogen in the LC host. Because of the microcapsule spherical symmetry, spontaneous emission of the delayed fluorescence is omnidirectionally amplified at the edge of the stop band. These results demonstrate the range of opportunities provided by TTA-UC systems for the future design of low-threshold photonic devices.

Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates.

The resolution of inkjet printing technology is determined by wetting and evaporation processes after the jet drop contacts the substrate. Here, the wetting of different picoliter solubilized polymer droplets jetting onto one-end-closed porous alumina was investigated. The selected polymers are commonly used in inkjet ink. The synergistic effects of the hierarchical structure and substrate surface modification were used to control the behavior of polymer-based ink drops. A model that invokes the effect of surface tension was applied to calculate the amount of polymer solution penetrating into the pores. The calculation corroborates experimental observations and shows that the volume of polymer solution in the pores increases with an increase in pore radius and depth, resulting in less solution remaining on the substrate surface. The structure of the porous substrate coupled with intrinsic polymer properties and surface modifications all contribute to the resolution that can be achieved via inkjet printing.

Enhanced Alignment of Water-Soluble Polythiophene Using Cellulose Nanocrystals as a Liquid Crystal Template.

Cellulose nanocrystals (CNCs) are bioderived, rodlike particles that form a chiral nematic liquid crystal (LC) in water. In this work, CNCs were used to induce long-range order in a semiconducting polymer, poly[3-(potassium-4-butanoate) thiophene-2,5-diyl] (PPBT). When mixed with CNCs, it was found that PPBT was incorporated into the liquid crystal "template" to form ordered structures with highly birefringent domains, as observed under polarized light. We show that the π-π interactions between polymer chains, which contribute considerably to the energetics of the semiconducting system, are directly influenced by the presence and packing of the liquid crystal phase. Upon increasing the concentration of CNCs from the isotropic to chiral nematic regime, we observe a bathochromic shift in the UV-vis spectra and the emergence of the 0-0 vibrational peak, suggesting enhanced π-π stacking leading to chain coplanarization. Furthermore, the chiral nature of the PPBT/CNC mixture was evidenced by a negative peak in circular dichroism (CD) spectroscopy, promoting the notion that the polymer chains followed the helicoidal twist of the chiral nematic liquid crystal host. At high temperatures, the peak height ratios and overall intensities of the UV-vis and CD spectra associated with PPBT decreased as the chiral nematic pitch grew larger in size.

Domed Silica Microcylinders Coated with Oleophilic Polypeptides and Their Behavior in Lyotropic Cholesteric Liquid Crystals of the Same Polypeptide.

Liquid crystals can organize dispersed particles into useful and exotic structures. In the case of lyotropic cholesteric polypeptide liquid crystals, polypeptide-coated particles are appealing because the surface chemistry matches that of the polymeric mesogen, which permits a tighter focus on factors such as extended particle shape. The colloidal particles developed here consist of a magnetic and fluorescent cylindrically symmetric silica core with one rounded, almost hemispherical end. Functionalized with helical poly(γ-stearyl-l-glutamate) (PSLG), the particles were dispersed at different concentrations in cholesteric liquid crystals (ChLC) of the same polymer in tetrahydrofuran (THF). Defects introduced by the particles to the director field of the bulk PSLG/THF host led to a variety of phases. In fresh mixtures, the cholesteric mesophase of the PSLG matrix was distorted, as reflected in the absence of the characteristic fingerprint pattern. Over time, the fingerprint pattern returned, more quickly when the concentration of the PSLG-coated particles was low. At low particle concentration the particles were "guided" by the PSLG liquid crystal to organize into patterns similar to that of the re-formed bulk chiral nematic phase. When their concentration increased, the well-dispersed PSLG-coated particles seemed to map onto the distortions in the bulk host's local director field. The particles located near the glass vial-ChLC interfaces were stacked lengthwise into architectures with apparent two-dimensional hexagonal symmetry. The size of these "crystalline" structures increased with particle concentration. They displayed remarkable stability toward an external magnetic field; hydrophobic interactions between the PSLG polymers in the shell and those in the bulk LC matrix may be responsible. The results show that bio-inspired LCs can assemble suitable colloidal particles into soft crystalline structures.

Exciton Bimolecular Annihilation Dynamics in Push-Pull Semiconductor Polymers.

Exciton-exciton annihilation is a ubiquitous nonlinear dynamic phenomenon in materials hosting Frenkel excitons. In this work, we investigate the nonlinear exciton dynamics of an electron push-pull conjugated polymer by fluence-dependent transient absorption and excitation-correlation photoluminescence spectroscopy, where we can quantitatively show the latter to be a more selective probe of the nonlinear dynamics. Simulations based on a time-independent exciton annihilation model show a decreasing trend for the extracted annihilation rates with excitation fluence. Further investigation of the fluence-dependent transients suggests that the exciton-exciton annihilation bimolecular rates are not constant in time, displaying a t-1/2 time dependence, which we rationalize as reflective of one-dimensional exciton diffusion, with a diffusion length estimated to be 9 ± 2 nm. In addition, exciton annihilation gives rise to a long-lived species that recombines on a nanosecond time scale. Our conclusions shed broad light onto nonlinear exciton dynamics in push-pull conjugated polymers.

Unveiling Multiquantum Excitonic Correlations in Push-Pull Polymer Semiconductors.

Bound and unbound Frenkel-exciton pairs are essential transient precursors for a variety of photophysical and biochemical processes. In this work, we identify bound and unbound Frenkel-exciton complexes in an electron push-pull polymer semiconductor using coherent two-dimensional spectroscopy. We find that the dominant A0-1 peak of the absorption vibronic progression is accompanied by a subpeak, each dressed by distinct vibrational modes. By considering the Liouville pathways within a two-exciton model, the imbalanced cross-peaks in one-quantum rephasing and nonrephasing spectra can be accounted for by the presence of pure biexcitons. The two-quantum nonrephasing spectra provide direct evidence for unbound exciton pairs and biexcitons with dominantly attractive force. In addition, the spectral features of unbound exciton pairs show mixed absorptive and dispersive character, implying many-body interactions within the correlated Frenkel-exciton pairs. Our work offers novel perspectives on the Frenkel-exciton complexes in semiconductor polymers.

Enhancing composite electrode performance: insights into interfacial interactions.

Propelled by the widespread adoption of portable electronic devices, electrochemical energy storage systems, particularly lithium-ion batteries (LIBs), have become ubiquitous in modern society. The electrode is the critical battery component, where intricate interactions between the materials govern both the energy output and the overall lifespan of the battery under operational conditions. However, the poor interfacial properties of traditional electrode materials fall short in meeting escalating performance demands. To facilitate the advent of next-generation lithium-ion batteries, attention must be devoted to the interfacial chemistry that dictates and modulates the various dynamic and transport processes across multiple length scales within the composite electrodes. Recent research has concentrated on systematically understanding the properties of distinct electrode components to engineer meticulously tailored electrode formulations. These are geared towards composite electrodes with heightened chemical stability, thermal robustness, enhanced local conductivities, and superior mechanical resilience. This review elucidates the latest advances in understanding the impact of interfacial interactions in achieving high-capacity, high-stability electrodes. Through comprehensive insights into the interfacial interactions between the various electrode components, we can create improved integrated systems that outperform those developed through empirical methods. In light of this, the adoption of a holistic approach to enhance the interactions among electrode materials becomes of paramount importance. This concerted effort ensures the attainment of heightened rate capability, facilitation of lithium-ion transport, and overall system stability throughout the entirety of the cyclic process.

Conjugated Polymer Process Ontology and Experimental Data Repository for Organic Field-Effect Transistors.

Polymer-based semiconductors and organic electronics encapsulate a significant research thrust for informatics-driven materials development. However, device measurements are described by a complex array of design and parameter choices, many of which are sparsely reported. For example, the mobility of a polymer-based organic field-effect transistor (OFET) may vary by several orders of magnitude for a given polymer as a plethora of parameters related to solution processing, interface design/surface treatment, thin-film deposition, postprocessing, and measurement settings have a profound effect on the value of the final measurement. Incomplete contextual, experimental details hamper the availability of reusable data applicable for data-driven optimization, modeling (e.g., machine learning), and analysis of new organic devices. To curate organic device databases that contain reproducible and findable, accessible, interoperable, and reusable (FAIR) experimental data records, data ontologies that fully describe sample provenance and process history are required. However, standards for generating such process ontologies are not widely adopted for experimental materials domains. In this work, we design and implement an object-relational database for storing experimental records of OFETs. A data structure is generated by drawing on an international standard for batch process control (ISA-88) to facilitate the design. We then mobilize these representative data records, curated from the literature and laboratory experiments, to enable data-driven learning of process-structure-property relationships. The work presented herein opens the door for the broader adoption of data management practices and design standards for both the organic electronics and the wider materials community.

Carboxyl-Alkyl Functionalized Conjugated Polyelectrolytes for High Performance Organic Electrochemical Transistors.

Contemporary design principles for organic mixed ionic electronic conductors (OMIECs) are mostly based on the ethylene glycol moiety, which may not be representative of the OMIEC class as a whole. Furthermore, glycolated polymers can be difficult to synthesize and process effectively. As an emerging alternative, we present a series of polythiophenes functionalized with a hybrid carboxyl-alkyl side chain. By variation of the alkyl spacer length, a comprehensive evaluation of both the impact of carboxylic acid functionalization and alkyl spacer length was conducted. COOH-functionalization endows the polymer with preferential intrinsic low-swelling behavior and water processability to yield solvent-resistant conjugated polyelectrolytes while retaining substantial electroactivity in aqueous environments. Advanced in situ techniques, including time-resolved spectroelectrochemistry and Raman spectroscopy, are used to interrogate the materials' microstructure, ionic-electronic coupling, and operational stability in devices. To compare these materials' performance to state-of-the-art technology for the design of OMIECs, we benchmarked the materials and demonstrated significant application potential in both planar and interdigitated organic electrochemical transistors (OECTs). The polythiophene bearing carboxyl-butyl side chains exhibits greater electrochemical performance and faster doping kinetics within the polymer series, with a record-high OECT performance among conjugated polyelectrolytes ([µC*]pOECT = 107 ± 4 F cm-1 V-1 s-1). The results provide an enhanced understanding of structure-property relationships for conjugated polyelectrolytes operating in aqueous media and expand the materials options for future OMIEC development. Further, this work demonstrates the potential for conjugated polymers bearing alkyl-COOH side chains as a path toward robust OMIEC designs that may facilitate further facile (bio)chemical functionalization for a range of (bio)sensing applications.

Chain Conformation and Exciton Delocalization in a Push-Pull Conjugated Polymer.

Linear and nonlinear optical line shapes reveal details of excitonic structure in polymer semiconductors. We implement absorption, photoluminescence, and transient absorption spectroscopies in DPP-DTT, an electron push-pull copolymer, to explore the relationship between their spectral line shapes and chain conformation, deduced from resonance Raman spectroscopy and from ab initio calculations. The viscosity of precursor polymer solutions before film casting displays a transition that suggests gel formation above a critical concentration. Upon crossing this viscosity deflection concentration, the line shape analysis of the absorption spectra within a photophysical aggregate model reveals a gradual increase in interchain excitonic coupling. We also observe a red-shifted and line-narrowed steady-state photoluminescence spectrum along with increasing resonance Raman intensity in the stretching and torsional modes of the dithienothiophene unit, which suggests a longer exciton coherence length along the polymer-chain backbone. Furthermore, we observe a change of line shape in the photoinduced absorption component of the transient absorption spectrum. The derivative-like line shape may originate from two possibilities: a new excited-state absorption or Stark effect, both of which are consistent with the emergence of a high-energy shoulder as seen in both photoluminescence and absorption spectra. Therefore, we conclude that the exciton is more dispersed along the polymer chain backbone with increasing concentrations, leading to the hypothesis that polymer chain order is enhanced when the push-pull polymers are processed at higher concentrations. Thus, tuning the microscopic chain conformation by concentration would be another factor of interest when considering the polymer assembly pathways for pursuing large-area and high-performance organic optoelectronic devices.

Anisotropic Responsive Microgels Based on the Cholesteric Phase of Chitin Nanocrystals.

Anisotropic stimuli-responsive microgels based upon the cholesteric phase of chitin nanocrystals and N-isopropylacrylamide were designed and synthesized. The cholesteric structure was interrogated, and the texture was shown to directly influence the microgel shape and anisotropy. Changes in the microgel volume led to changes in the texture, where microgels comprising up to six bands exhibited a twisted bipolar texture, while those with greater volumes displayed a concentric-packing structure. As designed, the imprinted cholesteric phase induced an asymmetric response to temperature, leading to a change in shape and optical properties. Furthermore, the cholesteric structure is able to deform, facilitating transport into a small channel. Access to synthetic structures having a self-assembled twisted texture derived from cholesterics embedded within a polymer matrix will provide guidelines for designing biopolymer composites with programmable motion.

The Solution is the Solution: Data-Driven Elucidation of Solution-to-Device Feature Transfer for π-Conjugated Polymer Semiconductors.

The advent of data analytics techniques and materials informatics provides opportunities to accelerate the discovery and development of organic semiconductors for electronic devices. However, the development of engineering solutions is limited by the ability to control thin-film morphology in an immense parameter space. The combination of high-throughput experimentation (HTE) laboratory techniques and data analytics offers tremendous avenues to traverse the expansive domains of tunable variables offered by organic semiconductor thin films. This Perspective outlines the steps required to incorporate a comprehensive informatics methodology into the experimental development of polymer-based organic semiconductor technologies. The translation of solution processing and property metrics to thin-film behavior is crucial to inform efficient HTE for data collection and application of data-centric tools to construct new process-structure-property relationships. We argue that detailed investigation of the solution state prior to deposition in conjunction with thin-film characterization will yield a deeper understanding of the physicochemical mechanisms influencing performance in π-conjugated polymer electronics, with data-driven approaches offering predictive capabilities previously unattainable via traditional experimental means.

Solvent evaporation induced liquid crystalline phase in poly(3-hexylthiophene).

We report on the evolution of the chain orientation of a representative π-conjugated polymer, poly(3-hexylthiophene) (P3HT), during the solution-casting process, as monitored using polarized Raman spectroscopy. These measurements point to the formation of a liquid-crystalline phase of P3HT solutions within a specific time period during solvent evaporation, which leads to a conducting channel. These conclusions are based on the angular dependence of polarized Raman scattering peaks, the anisotropy in the fluorescence background signal, analysis of the scattering-peak shape, and direct observations of the three-phase contact line in an optical microscope under crossed polarizers. These results shed new light on the evolution of chain alignment and thus materials nanostructure, specifically in solution-processed P3HT and more generally in π-conjugated systems. They may further enable the design of improved materials and processes for this important class of polymers.

Integrated dynamic wet spinning of core-sheath hydrogel fibers for optical-to-brain/tissue communications.

Hydrogel optical light-guides have received substantial interest for applications such as deep-tissue biosensors, optogenetic stimulation and photomedicine due to their biocompatibility, (micro)structure control and tissue-like Young's modulus. However, despite recent developments, large-scale fabrication with a continuous synthetic methodology, which could produce core-sheath hydrogel fibers with the desired optical and mechanical properties suitable for deep-tissue applications, has yet to be achieved. In this study, we report a versatile concept of integrated light-triggered dynamic wet spinning capable of continuously producing core-sheath hydrogel optical fibers with tunable fiber diameters, and mechanical and optical propagation properties. Furthermore, this concept also exhibited versatility for various kinds of core-sheath functional fibers. The wet spinning synthetic procedure and fabrication process were optimized with the rational design of the core/sheath material interface compatibility [core = poly(ethylene glycol diacrylate-co-acrylamide); sheath = Ca-alginate], optical transparency, refractive index and spinning solution viscosity. The resulting hydrogel optical fibers exhibited desirable low optical attenuation (0.18 ± 0.01 dB cm-1 with 650 nm laser light), excellent biocompatibility and tissue-like Young's modulus (<2.60 MPa). The optical waveguide hydrogel fibers were successfully employed for deep-tissue cancer therapy and brain optogenetic stimulation, confirming that they could serve as an efficient versatile tool for diverse deep-tissue therapy and brain optogenetic applications.

Synergistic Effect of N,N-Dimethylformamide and Hydrochloric Acid on the Growth of MAPbI3 Perovskite Films for Solar Cells.

Perovskite solar cells have emerged as a promising next-generation electrical power generating tool. However, imperfections in perovskite films are one of the crucial factors preventing the commercialization of perovskite solar cells. Passivation has proven to be an effective strategy to reduce the density of defect states in perovskite crystals and inhibit ion migration. Although significant work on chloride ion and N,N-dimethylformamide (DMF) has shown that the additives are able to passivate different types of trap defects, systematic studies on the effects of DMF and HCl on perovskite crystallization when used in conjunction with each other are elusive. Here, we systematically investigated the synergistic effect of DMF and hydrochloric acid (HCl) on methylammonium (MA+)-based perovskite films with the two-step spin-coating method. As a Lewis base, DMF coordinates well with Pb2+ to facilitate a decrease in the number of defects, thereby improving the carrier separation and transport, while HCl improves the overall perovskite film morphology. Addition of 20 µL HCl/20 µL DMF to 10 mL of methylammonium iodide/isopropyl alcohol solution afforded ca. 500 nm thick perovskite films with no observable defects within the grains. The process allowed fabrication of devices with an active area of 0.16 cm2, which produced power conversion efficiencies up to 18.37% with minimal hysteresis.