Solvent Additive-Assisted Anisotropic Assembly and Enhanced Charge Transport of π-Conjugated Polymer Thin Films

Advances in the Use of Conducting Polymers for Healthcare Monitoring

Conducting polymers (CPs) are an innovative class of materials recognized for their high flexibility and biocompatibility, making them an ideal choice for health monitoring applications that require flexibility. They are active in their design. Advances in fabrication technology allow the incorporation of CPs at various levels, by combining diverse CPs monomers with metal particles, 2D materials, carbon nanomaterials, and copolymers through the process of polymerization and mixing. This method produces materials with unique physicochemical properties and is highly customizable. In particular, the development of CPs with expanded surface area and high conductivity has significantly improved the performance of the sensors, providing high sensitivity and flexibility and expanding the range of available options. However, due to the morphological diversity of new materials and thus the variety of characteristics that can be synthesized by combining CPs and other types of functionalities, choosing the right combination for a sensor application is difficult but becomes important. This review focuses on classifying the role of CP and highlights recent advances in sensor design, especially in the field of healthcare monitoring. It also synthesizes the sensing mechanisms and evaluates the performance of CPs on electrochemical surfaces and in the sensor design. Furthermore, the applications that can be revolutionized by CPs will be discussed in detail.

Enhanced Nitric Oxide Sensing Performance of Conjugated Polymer Films through Incorporation of Graphitic Carbon Nitride

Organic field-effect transistor (OFET) gas sensors based on conjugated polymer films have recently attracted considerable attention for use in environmental monitoring applications. However, the existing devices are limited by their poor sensing performance for gas analytes. This drawback is attributed to the low charge transport in and the limited charge-analyte interaction of the conjugated polymers. Herein, we demonstrate that the incorporation of graphitic carbon nitride (g-C₃N₄) into the conjugated polymer matrix can improve the sensing performance of OFET gas sensors. Moreover, the effect of graphitic carbon nitride (g-C₃N₄) on the gas sensing properties of OFET sensors based on poly(3-hexylthiophene) (P3HT), a conjugated polymer, was systematically investigated by changing the concentration of the g-C₃N₄ in the P3HT/g-C₃N₄ composite films. The obtained films were applied in OFET to detect NO gas at room temperature. In terms of the results, first, the P3HT/g-C₃N₄ composite films containing 10 wt.% g-C₃N₄ exhibited a maximum charge carrier mobility of ~1.1 × 10^-1 cm² V^-1 S^-1, which was approximately five times higher than that of pristine P3HT films. The fabricated P3HT/g-C₃N₄ composite film based OFET sensors presented significantly enhanced NO gas sensing characteristics compared to those of the bare P3HT sensor. In particular, the sensors based on the P3HT/g-C₃N₄ (90/10) composite films exhibited the best sensing performance relative to that of the bare P3HT sensor when exposed to 10 ppm NO gas: responsivity = 40.6 vs. 18.1%, response time = 129 vs. 142 s, and recovery time = 148 vs. 162 s. These results demonstrate the enormous promise of g-C₃N₄ as a gas sensing material that can be hybridized with conjugated polymers to efficiently detect gas analytes.

Analysis of the Solvent Effects on the Crystal Growth of Peripherally Chlorinated Boron Subphthalocyanines

We report the characterization of the crystal/nano-structures for hexachloro-boron subphthalocyanines (Cl6BsubPc) grown via a variety of solvent diffusion methods and also with having axial substitutions variants: chloride; phenoxy; naphthoxy and...

Effects of Solvent Vapor Annealing on Morphology and Charge Transport of Poly(3-hexylthiophene) (P3HT) Films Incorporated with Preformed P3HT Nanowires

We systematically studied the influence of solvent vapor annealing on the molecular ordering, morphologies, and charge transport properties of poly(3-hexylthiophene) (P3HT) thin films embedded with preformed crystalline P3HT nanowires (NWs). Solvent vapor annealing (SVA) with chloroform (CF) was found to profoundly impact on the structural and morphological changes, and thus on the charge transport characteristics, of the P3HT-NW-embedded P3HT films. With increased annealing time, the density of crystalline P3HT NWs was increased within the resultant films, and also intra- and intermolecular interactions of the corresponding films were significantly improved. As a result, the P3HT-NW-embedded P3HT films annealed with CF vapor for 20 min resulted in a maximized charge carrier mobility of ~0.102 cm² V⁻¹ s⁻¹, which is higher than that of pristine P3HT films by 4.4-fold (μ = ~0.023 cm² V⁻¹ s⁻¹).

Conjugated polymer/paraffin blends for organic field-effect transistors with high environmental stability

Improving the environmental stability of conjugated polymers remains a fundamental challenge that limits their widespread adoption and commercial application in electronic and photonic devices. Although paraffin can have excellent barrier properties against moisture in ambient air, the use of conjugated polymer/paraffin blends to fabricate organic field-effect transistors (OFETs) with high environmental stability has not been attempted. Here, we demonstrate that conjugated polymer/paraffin blends can greatly enhance the environmental stability of OFETs. Compared to conventional systems such as poly(3-hexylthiophene) (P3HT)/polystyrene and P3HT/polydimethylsiloxane blends, P3HT/paraffin blends exhibit superior environmental stability after 30 days of exposure to the ambient atmosphere. Furthermore, the conjugated polymer/paraffin blends provide stable electronic properties under severe mechanical deformation [a strain (ε) of ∼150%], overcoming a critical challenge arising from the use of fragile crystalline conjugated polymer films for flexible and stretchable electronic devices. In comparison with a conventional spin-coating method, a shear-coating technique provides enhanced molecular ordering and alignment, resulting in improved charge carrier mobility in the blend film OFETs. In particular, shearing in the evaporation regime improves the molecular ordering and alignment of the blend films more than shearing in the Landau-Levich regime. Interestingly, the environmental stability of the sheared blend films varies depending on the shear speed. Specifically, OFETs based on blend films sheared at 0.5 and 6.0-10.0 mm s^-1 exhibit excellent environmental stability, maintaining 80% of their initial mobility after 30 days of exposure to air. In contrast, the environmental stability of the OFETs decreases considerably when the blend films are sheared at 1.0-4.0 mm s^-1; the mobility decreases to as low as ∼20% of the initial value.

Morphology and Electronic Properties of Semiconducting Polymer and Branched Polyethylene Blends

A new strategy for influencing the solid-state morphology of conjugated polymers was developed through physical blending with a low-molecular-weight branched polyethylene. This nontoxic and low-boiling-point additive was blended with a high-charge-mobility diketopyrrolopyrrole-based conjugated polymer, and a detailed investigation of the new blended materials was performed by various characterization tools, including X-ray diffraction, UV-vis spectroscopy, and atomic force microscopy. Interestingly, the branched additive was shown to reduce the crystallinity of the conjugated polymer while promoting aggregation and phase separation in the solid state. Upon thermal removal of the olefinic additive, the thin films maintained a lower crystallinity and aggregated morphology in comparison to a nonblended polymer. The semiconducting performance of the new branched polyethylene/conjugated polymer blends was also investigated in organic field-effect transistors, which showed a stable charge mobility of around 0.3 cm² V^-1 s^-1 without thermal annealing, independent of the blending ratio. Furthermore, using the new polyethylene-based additive, the concentration of a conjugated polymer required for the fabrication of organic field-effect transistor devices was reduced down to 0.05 wt %, without affecting charge transport, which represents a significant improvement compared to usual concentrations used for solution deposition. Our results demonstrate that the physical blending of a conjugated polymer with nontoxic, low-molecular-weight branched polyethylene is a promising strategy for the modification and fine-tuning of the solid-state morphology of conjugated polymers without sacrificing their charge-transport properties, thus creating new opportunities for the large-scale processing of organic semiconductors.

Highly crystalline and uniform conjugated polymer thin films by a water-based biphasic dip-coating technique minimizing the use of halogenated solvents for transistor applications

The commercialization of organic electronics will require minimizing the use of halogenated solvents used to solution-process organic semiconductors, which is a crucial step for large-area coating methods, such as the dip-coating method. Here, we report a novel biphasic dip-coating method which uses a water-based biphasic solution and produces a uniform, smooth, and crystalline conjugated polymer thin film in the presence of a solvent additive. We demonstrated that a solvent additive with a high boiling point and solubility parameter similar to that of the solution affected the solvent evaporation rate and improved the crystallinity of the dip-coated polymer thin film. The method used to add the solvent strongly influenced how the solvent additive diffused into the polymer solution, which affected the resulting film morphology. The crystallinity and morphology of the polymer films were correlated with the electrical characteristics, and the most crystalline film displayed a high hole field effect mobility of 0.0391 cm2 V-1 s-1 when processed from the solvent mixture without post-treatment. Our findings provide a direction for the development of reliable and promising organic thin film transistor technologies.

Controlled Self-Assembly of Conjugated Polymers via a Solvent Vapor Pre-Treatment for Use in Organic Field-Effect Transistors

A facile solution-processing strategy toward well-ordered one-dimensional nanostructures of conjugated polymers via a non-solvent vapor treatment was demonstrated, which resulted in enhancements to the charge transport characteristics of the polymers. The amount of crystalline poly(3-hexylthiophene) (P3HT) nanofibers was precisely controlled by simply varying the exposure time of solutions of P3HT solutions to non-solvent vapor. The effects of non-solvent vapor exposure on the molecular ordering and morphologies of the resultant P3HT films were systematically investigated using ultraviolet-visible (UV-vis) spectroscopy, polarized optical microscopy (POM), grazing incidence X-ray diffraction (GIXRD), and atomic force microscopy (AFM). The non-solvent vapor facilitates the π–π stacking in P3HT to minimize unfavorable interactions between the poor solvent molecules and P3HT chains. P3HT films deposited from the non-solvent vapor-treated P3HT solutions exhibited an approximately 5.6-fold improvement in charge carrier mobility as compared to that of pristine P3HT films (7.8 × 10⁻² cm² V⁻¹ s⁻¹ vs. 1.4 × 10⁻² cm² V⁻¹ s⁻¹). The robust and facile strategy presented herein would be applicable in various opto-electronics applications requiring precise control of the molecular assembly, such as organic photovoltaic cells, field-effect transistors, light-emitting diodes, and sensors.

Large-Scale Alignment of Polymer Semiconductor Nanowires for Efficient Charge Transport via Controlled Evaporation of Confined Fluids

Long-range alignment of conjugated polymers is as critical as polymer chain packing for achieving efficient charge transport in polymer thin films used in electronic and optoelectronic devices. Here, the present study reports a facile, scalable strategy that enables the deposition of macroscopically aligned polymer semiconductor nanowire (NW)-array films with highly enhanced charge carrier mobility, using a modified controlled evaporative self-assembly (MCESA) technique. Organic field-effect transistors (OFETs) based on highly oriented poly(3-hexylthiophene) (P3HT)-NW films exhibit more than 10-fold enhancement of carrier mobility, with the highest mobility of 0.13 cm² V^-1 s^-1, compared to the OFETs based on pristine P3HT films. Significantly, large-area aligned P3HT NW-films, which are deposited over 12 arrays of transistors on a 4 in. wafer by an MCESA coating, result in lower device performance variation (i.e., standard deviation ≈ ±0.0172 (16%) cm² V^-1 s^-1) as well as an excellent average device performance (i.e., average charge mobility ≈ 0.11 cm² V^-1 s^-1), compared to those obtained using the conventional CESA coating, overcoming a critical challenge in the field of OFETs.