Facile Photo-cross-linking System for Polymeric Gate Dielectric Materials toward Solution-Processed Organic Field-Effect Transistors: Role of a Cross-linker in Various Polymer Types

Surface engineering of high-k polymeric dielectric layers with a fluorinated organic crosslinker for use in flexible-platform electronics

High-k polymeric layers were prepared by combining various functional groups and were applied as gate dielectrics for practical organic field-effect transistors (OFETs). Crosslinking of the polymeric layers through UV-assisted organic azide fluorine-based crosslinkers induced dramatic improvements in the electrical performance of the OFET, such as field-effect mobility and bias-stress stability. Our synthesis and manufacturing method can be a useful technique for ensuring device operation stability and electrical property enhancement. With this analysis, we further applied our polymer-dielectric OFETs to flexible-platform-based electronic components, including unit OFETs and simple logic devices (NOT, NAND, and NOR gates). The outcomes of this research and development suggest a suitable method for the low-cost mass production of large-area flexible and printable devices, using a printing-based approach to replace current processes.

Optimization of fluorinated phenyl azides as universal photocrosslinkers for semiconducting polymers

Fluorinated phenyl azides (FPA) enable photo-structuring of π-conjugated polymer films for electronic device applications. Despite their potential, FPAs have faced limitations regarding their crosslinking efficiency, and more importantly, their impact on critical semiconductor properties, such as charge-carrier mobility. Here, we report that azide photolysis and photocrosslinking can achieve unity quantum efficiencies for specific FPAs. This suggests preferential nitrene insertion into unactivated C‒H bonds over benzazirine and ketenimine reactions, which we attribute to rapid interconversion between the initially formed hot states. Furthermore, we establish a structure‒activity relationship for carrier mobility quenching. The binding affinity of FPA crosslinker to polymer π-stacks governs its propensity for mobility quenching in both PM6 and PBDB-T used as model conjugated polymers. This binding affinity can be suppressed by FPA ring substitution, but varies in a non-trivial way with π-stack order. Utilizing the optimal FPA, photocrosslinking enables the fabrication of morphology-stabilized, acceptor-infiltrated donor polymer networks (that is, PBDB-T: ITIC and PM6: Y6) for solar cells. Our findings demonstrate the exceptional potential of the FPA photochemistry and offer a promising approach to address the challenges of modelling realistic molecular interactions in complex polymer morphologies, moving beyond the limitations of Flory‒Huggins mean field theory.

Controlling Fluorination Density of Soluble Polyimide Gate Dielectrics and its Influence on Organic Crystal Growth and Device Operational Stability

Fluorinated polyimides (PIs) are among the most promising candidates for gate dielectric materials in organic electronic devices because of their solution processability and outstanding chemical, mechanical, and thermal stabilities. Additionally, fluorine (F) substitution improves the electrical properties of PI thin films, such as enhanced dielectric properties and reduced surface trap densities. However, the relationship between the fluorination density of PIs and crystal growth modes of vacuum-deposited conjugated molecules on PI thin films, which is directly related to the lateral charge transport along the PI-organic semiconductor interface, has not been systematically studied. Herein, five different soluble PIs with different F densities were synthesized, and the correlation between fluorination and thin-film properties was systematically investigated. Not only were their dielectric properties modulated, but the growth modes of the organic molecules deposited on the PI thin films also changed with increasing surface F density. This phenomenon was observed by both surface and crystallographic analyses, which resulted in extremely high operational stability of field-effect transistors and the successful fabrication of organic complementary circuits. We believe that the correlation between PI backbone fluorination and its thin-film properties will provide practical insights into the material design based on controlled molecular directed surface assembly on fluorinated polymer dielectrics.

Functionalized MXene ink enables environmentally stable printed electronics

Establishing dependable, cost-effective electrical connections is vital for enhancing device performance and shrinking electronic circuits. MXenes, combining excellent electrical conductivity, high breakdown voltage, solution processability, and two-dimensional morphology, are promising candidates for contacts in microelectronics. However, their hydrophilic surfaces, which enable spontaneous environmental degradation and poor dispersion stability in organic solvents, have restricted certain electronic applications. Herein, electrohydrodynamic printing technique is used to fabricate fully solution-processed thin-film transistors with alkylated 3,4-dihydroxy-L-phenylalanine functionalized Ti3C2Tx (AD-MXene) as source, drain, and gate electrodes. The AD-MXene has excellent dispersion stability in ethanol, which is required for electrohydrodynamic printing, and maintains high electrical conductivity. It outperformed conventional vacuum-deposited Au and Al electrodes, providing thin-film transistors with good environmental stability due to its hydrophobicity. Further, thin-film transistors are integrated into logic gates and one-transistor-one-memory cells. This work, unveiling the ligand-functionalized MXenes' potential in printed electrical contacts, promotes environmentally robust MXene-based electronics (MXetronics).

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Solution-Processed Polymer Dielectric Interlayer for Low-Voltage, Unipolar n-Type Organic Field-Effect Transistors

The integration of organic electronic circuits into real-life applications compels the fulfillment of a range of requirements, among which the ideal operation at a low voltage with reduced power consumption is paramount. Moreover, these performance factors should be achieved via solution-based fabrication schemes in order to comply with the promise of cost- and energy-efficient manufacturing offered by an organic, printed electronic technology. Here, we propose a solution-based route for the fabrication of low-voltage organic transistors, encompassing ideal device operation at voltages below 5 V and exhibiting n-type unipolarization. This process is widely applicable to a variety of semiconducting and dielectric materials. We achieved this through the use of a photo-cross-linked, low-k dielectric interlayer, which is used to fabricate multilayer dielectric stacks with areal capacitances of up to 40 nF/cm2 and leakage currents below 1 nA/cm2. Because of the chosen azide-based cross-linker, the dielectric promotes n-type unipolarization of the transistors and demonstrated to be compatible with different classes of semiconductors, from conjugated polymers to carbon nanotubes and low-temperature metal oxides. Our results demonstrate a general applicability of our unipolarizing dielectric, facilitating the implementation of complementary circuitry of emerging technologies with reduced power consumption.

Amorphous Fluorinated Acrylate Polymer Dielectrics for Flexible Transistors and Logic Gates with High Operational Stability

Fluorinated amorphous polymeric gate-insulating materials for organic thin-film transistors (OTFTs) not only form hydrophobic surfaces but also significantly reduce traps at the interface between the organic semiconductor and gate insulator. Therefore, these polymeric materials can enhance the OTFT's operation stability. In this study, we synthesized a new polymeric insulating material series composed of acrylate and fluorinated functional groups (with different ratios) named MBHCa-F and used them as gate insulators for OTFTs and in other applications. The insulating features of the MBHCa-F polymers, including surface energy, surface atomic content properties, dielectric constant, and leakage current, were clearly analyzed with respect to the content of the fluorinated functional groups. At higher fluorine-based functional group content, the polymeric series exhibited higher fluorine-based contents at the surface and superior electrical properties, such as field-effect mobility and driving stability, at OTFTs. Therefore, we believe that this study provides a substantial method for synthesizing polymeric insulating materials to enhance the operational stability and electrical performance of OTFTs.

Combination of Polymer Gate Dielectric and Two-Dimensional Semiconductor for Emerging Field-Effect Transistors

Two-dimensional (2D) materials are considered attractive semiconducting layers for emerging field-effect transistors owing to their unique electronic and optoelectronic properties. Polymers have been utilized in combination with 2D semiconductors as gate dielectric layers in field-effect transistors (FETs). Despite their distinctive advantages, the applicability of polymer gate dielectric materials for 2D semiconductor FETs has rarely been discussed in a comprehensive manner. Therefore, this paper reviews recent progress relating to 2D semiconductor FETs based on a wide range of polymeric gate dielectric materials, including (1) solution-based polymer dielectrics, (2) vacuum-deposited polymer dielectrics, (3) ferroelectric polymers, and (4) ion gels. Exploiting appropriate materials and corresponding processes, polymer gate dielectrics have enhanced the performance of 2D semiconductor FETs and enabled the development of versatile device structures in energy-efficient ways. Furthermore, FET-based functional electronic devices, such as flash memory devices, photodetectors, ferroelectric memory devices, and flexible electronics, are highlighted in this review. This paper also outlines challenges and opportunities in order to help develop high-performance FETs based on 2D semiconductors and polymer gate dielectrics and realize their practical applications.

Poly(glycidyl azide) as Photo-Crosslinker for Polymers

Crosslinking polymers to form networks is a universal and routinely applied strategy to improve their stability and endow them with solvent resistance, adhesion properties, etc. However, the chemical crosslinking of common commercial polymers, especially for those without functional groups, cannot be achieved readily. In this study, we utilized low-molecular weight poly(glycidyl azide) (GAP) as polymeric crosslinkers to crosslink various commercial polymers via simple ultraviolet light irradiation. The azide groups were shown to decompose upon photo-irradiation and be converted to highly reactive nitrene species, which are able to insert into carbon-hydrogen bonds and thus crosslink the polymeric matrices. This strategy was demonstrated successfully in several commercial polymers. In particular, it was found that the crosslinking is highly localized, which could endow the polymeric matrices with a decent degree of crosslinking without significantly influencing other properties, suggesting a novel and robust method to crosslink polymeric materials.

Molecular Engineering of Printed Semiconducting Blends to Develop Organic Integrated Circuits: Crystallization, Charge Transport, and Device Application Analyses

Solution-based printing has contributed to the facile deposition of various types of materials, including the building blocks of printed electronics. In particular, solution-processable organic semiconductors (OSCs) are regarded as one of the most fascinating candidates for the fabrication of printed electronics. Herein, we report electrohydrodynamic (EHD) jet-printed p- and n-type OSCs, namely 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) and 6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14-tetraazapentacene (TIPS-TAP), and their use as single-OSC layers and as OSC mixed p-n layers to fabricate solution-processed p-, n-, and ambipolar-type organic field-effect transistors (OFETs). Use of the dragging mode of EHD jet printing, a process driven under a low electrostatic field with a short nozzle-to-substrate distance, was found to provide favorable conditions for growth of TIPS-PEN and TIPS-TAP crystals. In this way, the similar molecular structures of TIPS-PEN and TIPS-TAP yielded a homogeneous solid solution and showed ambipolar transport properties in OFETs. Therefore, the combination of single- and mixed-OSC layers enabled the preparation of various charge-transported devices from unit to integrated devices (NOT, NAND, NOR, and multivalued logic). Therefore, this fabrication technology can be useful for assisting in the production of OSC layers for practical applications in the near future.

Covalent Networking of a Conjugated-Polymer Photocatalyst to Promote Exciton Diffusion in the Aqueous Phase for Efficient Hydrogen Production

A conjugated polymer particle in an aqueous phase is covalently networked in 3D by crosslinking with azide groups, leading to significantly enhanced activity-a high photocatalytic H₂ evolution rate (11 024 µmol g^-1 h^-1 (λ > 420 nm)) and a high apparent quantum yield (up to 0.8%). The reaction between the photoactive azide and the alkyl chains of the conjugated polymer provides more intact intermolecular polymeric interactions in the colloidal state, thus preventing physical swelling and inhibiting the recombination of photoproduced carriers. The covalent network efficiently promotes exciton diffusion, which greatly facilitates charge separation and transfer. The azide photo-crosslinking also leads to more compact and better-packed nanoparticles in the aqueous phase and efficient transfer of excitons to the outer surface of the nanoparticles, where photocatalytic reactions occur. These results show that photo-crosslinking can suppress the adverse effects of alkyl chains which inhibit photocatalytic performance. Therefore, covalent crosslinking is a promising strategy for the development of solar and hydrogen energy.

Mass-Synthesized Solution-Processable Polyimide Gate Dielectrics for Electrically Stable Operating OFETs and Integrated Circuits

Polyimides (PIs) are widely utilized polymeric materials for high-temperature plastics, adhesives, dielectrics, nonlinear optical materials, flexible hard-coating films, and substrates for flexible electronics. PIs can be facilely mass-produced through factory methods, so the industrial application value is limitless. Herein, we synthesized a typical poly(amic acid) (PAA) precursor-based solution through an industrialized reactor for mass production and applied the prepared solution to form thin films of PI using thermal imidization. The deposited PI thin films were successfully applied as gate dielectrics for organic field-effect transistors (OFETs). The PI layers showed suitable characteristics for dielectrics, such as a smooth surface, low leakage current density, uniform dielectric constant (k) values regardless of frequency, and compatibility with organic semiconductors. Utilizing this PI layer, we were able to fabricate electrically stable operated OFETs, which exhibited a threshold voltage shift lower than 1 V under bias-stress conditions and a field-effect mobility of 4.29 cm² V^-1 s^-1. Moreover, integrated logic gates were manufactured using these well-operated OFETs and displayed suitable operation behavior.

Electrohydrodynamic-Jet-Printed Cinnamate-Fluorinated Cross-Linked Polymeric Dielectrics for Flexible and Electrically Stable Operating Organic Thin-Film Transistors and Integrated Devices

Herein, printable polymer series containing different portions of cinnamate and perfluorinated phenyl functionalities, namely, polyperfluorostyrene-co-poly(vinylbenzyl cinnamates) (PFS-co-PVBCi (x:y)) copolymers, were synthesized and applied as gate dielectrics for organic thin-film transistors (OTFTs). The polymeric dielectrics were successfully printed via electrostatic force-assisted dispensing mode of electrohydrodynamic jet printing. The dielectric characteristics of the printed polymers, such as surface energy, dielectric constant, leakage current, atomic depth profiles, and deposited semiconducting layer characteristics, were clearly identified. In particular, the difference in driving stability of OTFTs according to the type of polymer was analyzed in detail and a possible mechanism was proposed. Results suggested that PFS-co-PVBCi (3:7) led to optimized consequences, yielding an almost negligible V_th shift under continuous bias stress. Through this, we successfully implemented flexible OTFT and logic devices using printed PFS-co-PVBCi (3:7) dielectrics with stable operation properties. Therefore, we believe that this study will facilitate the printing and synthesis of polymer dielectrics to produce printed and flexible OTFTs.

A molecular design approach towards elastic and multifunctional polymer electronics

Next-generation wearable electronics require enhanced mechanical robustness and device complexity. Besides previously reported softness and stretchability, desired merits for practical use include elasticity, solvent resistance, facile patternability and high charge carrier mobility. Here, we show a molecular design concept that simultaneously achieves all these targeted properties in both polymeric semiconductors and dielectrics, without compromising electrical performance. This is enabled by covalently-embedded in-situ rubber matrix (iRUM) formation through good mixing of iRUM precursors with polymer electronic materials, and finely-controlled composite film morphology built on azide crosslinking chemistry which leverages different reactivities with C-H and C=C bonds. The high covalent crosslinking density results in both superior elasticity and solvent resistance. When applied in stretchable transistors, the iRUM-semiconductor film retained its mobility after stretching to 100% strain, and exhibited record-high mobility retention of 1 cm2 V-1 s-1 after 1000 stretching-releasing cycles at 50% strain. The cycling life was stably extended to 5000 cycles, five times longer than all reported semiconductors. Furthermore, we fabricated elastic transistors via consecutively photo-patterning of the dielectric and semiconducting layers, demonstrating the potential of solution-processed multilayer device manufacturing. The iRUM represents a molecule-level design approach towards robust skin-inspired electronics.

Strategy for Selective Printing of Gate Insulators Customized for Practical Application in Organic Integrated Devices

Direct drawing techniques have contributed to the ease of patterning soft electronic materials, which are the building blocks of analog and digital integrated circuits. In parallel with the printing of semiconductors and electrodes, selective deposition of gate insulators (GI) is an equally important factor in simplifying the fabrication of integrated devices, such as NAND and NOR gates, and memory devices. This study demonstrates the fabrication of six types of printed GI layers (high/low-k polymer and organic-inorganic hybrid material), which are utilized as GIs in organic field-effect transistors (OFETs), using the electrostatic-force-assisted dispensing printing technique. The selective printing of GIs on the gate electrodes enables us to develop practical integrated devices that go beyond unit OFET devices, exhibiting robust switching performances, non-destructive operations, and high gain values. Moreover, the flexible integrated devices fabricated using this technique exhibit excellent operational behavior. Therefore, this facile fabrication technique can pave a new path for the production of practical integrated device arrays for next-generation devices.