Substrates For Flexible Electronics Research Articles

Recent Progress and Challenges in Paper-Based Microsupercapacitors for Flexible Electronics: A Comprehensive Review.

Recent advances in paper-based microsupercapacitors (p-MSCs) have attracted significant attention due to their potential as substrates for flexible electronics. This review summarizes progress in the field of p-MSCs, discussing their challenges and prospects. It covers various aspects, including the fundamental characteristics of paper, the modification of paper with functional materials, and different methods for device fabrication. The review critically analyzes recent advancements, materials, and fabrication techniques for p-MSCs, exploring their potential applications and benefits, such as flexibility, cost-effectiveness, and sustainability. Additionally, this review highlights gaps in current research, guiding future investigations and innovations in the field. It provides an overview of the current state of p-MSCs and offers valuable insights for researchers and professionals in the field. The critical analysis and discussion presented herein offer a roadmap for the future development of p-MSCs and their potential impact on the domain of flexible electronics.

Toward Sustainable Electronics: Exploiting the Potential of a Biodegradable Cellulose Blend for Photolithographic Processes and Eco‐Friendly Devices

Flexible electronics has emerged as a promising field for the development of electronic devices with applications in wearables, biomedical sensors, and edible electronics. Biomaterials play a crucial role in fabricating flexible substrates, and the utilization of polymer blends offers exciting possibilities for tuning mechanical and chemical properties. This paper highlights the potential of a novel polymer blend based on ethyl cellulose (EC) and hydroxypropyl cellulose (HPC) in the fabrication of substrates for flexible electronics. By blending the two cellulose ethers, it is possible to tune the mechanical and chemical properties of the final substrate, tailored to meet specific requirements. To exploit such innovative green substrates for photolithographic processes, their stability, and processability is extensively investigated. The feasibility of photolithographic processes on such biodegradable and edible substrates is demonstrated by fabricating both resistive and capacitive sensors through standard photolithographic processes, presenting a breakthrough in terms of applicability. The utilization of such biomaterials holds tremendous potential for driving technological advancements in various fields. These materials pave the way for innovative devices catering to diverse applications, from agriculture to food and biomedicine. Importantly, they also promote a sustainable approach for their fabrication, laying the foundation for an environment‐aware future of technological progress.

Partially fluorinated metallic surface constructing robust, thermally conductive and electrically insulating polymer composite

Metallic fillers have shown limited potential for developing advanced polymer substrates for flexible electronics due to their electrical conductivity and poor interfacial interaction with polymers, despite their excellent thermal conductivity (λ). Here, metallic aluminum powders (APs) after surface modification were selected as thermally conductive fillers to modify polyimide (PI) films. We constructed a partially fluorinated alumina layer (PFAL) on the APs surface by employing direct fluorination of F2/N2 mixture. It was demonstrated that the new core-shell structure equipped the fluorinated APs (FAPs) with much better dispersibility and stronger interaction with polyamide acids (PAAs) and PIs bulk, in contrast to APs. Even when the loading of FAPs was up to 50 wt%, the tensile strength of PI/FAPs still remained at a high value of 87.2 MPa. Moreover, the in-plane λ (λ∥) and out-of-plane λ (λ⊥) at room temperature reached 16.83 W/(m·K) and 1.42 W/(m·K), respectively. Both theoretical simulations and experiments demonstrated that the positive effect was due to the strong interaction between the electron-deficient imide ring in PI macromolecules and the electro-negative F atom in the PFAL shell. Overall, this study provides a promising strategy for enhancing the interfacial interaction between metallic fillers and polymers for the development of high-performance flexible electronic devices.

Low-temperature growth of MoS2 on polymer and thin glass substrates for flexible electronics.

Recent advances in two-dimensional semiconductors, particularly molybdenum disulfide (MoS2), have enabled the fabrication of flexible electronic devices with outstanding mechanical flexibility. Previous approaches typically involved the synthesis of MoS2 on a rigid substrate at a high temperature followed by the transfer to a flexible substrate onto which the device is fabricated. A recurring drawback with this methodology is the fact that flexible substrates have a lower melting temperature than the MoS2 growth process, and that the transfer process degrades the electronic properties of MoS2. Here we report a strategy for directly synthesizing high-quality and high-crystallinity MoS2 monolayers on polymers and ultrathin glass substrates (thickness ~30 µm) at ~150 °C using metal-organic chemical vapour deposition. By avoiding the transfer process, the MoS2 quality is preserved. On flexible field-effect transistors, we achieve a mobility of 9.1 cm2 V-1 s-1 and a positive threshold voltage of +5 V, which is essential for reducing device power consumption. Moreover, under bending conditions, our logic circuits exhibit stable operation while phototransistors can detect light over a wide range of wavelengths from 405 nm to 904 nm.

Bio-based substrate for flexible electronics - application to a 2.45 GHz wearable patch antenna

In this paper, a bio-based and biocompatible polymer, Cellulose Laurate (CL), is proposed for flexible radio-frequency (RF) electronics. The synthesis of CL films together with their characterizations (chemical, thermal, mechanical and dielectric) are presented. The results obtained allow considering this material for RF flexible applications as a possible alternative to petrosourced substrates. Therefore, CL has been used to fabricate a flexible patch antenna that operates in an industrial, scientific and medical (ISM) frequency band. The central frequency selected is 2.45 GHz. The antenna fabrication process is based on the combination of laser structuring and the use of copper adhesive tape. Measurements of the antenna reflection coefficient and radiation patterns show that CL is a good candidate as a RF substrate. Furthermore, it is demonstrated that the antenna performance is only slightly impacted under bending conditions.

Ti3 C2 TX MXene Ink Direct Writing Flexible Sensors for Disposable Paper Toys.

Flexible electronics have gained great attention in recent years owing to their promising applications in biomedicine, sustainable energy, human-machine interaction, and toys for children. Paper mainly produced from cellulose fibers is attractive substrate for flexible electronics because it is biodegradable, foldable, tailorable, and light-weight. Inspired by daily handwriting, the rapid prototyping of sensing devices with arbitrary patterns can be achieved by directly drawing conductive inks on flat or curved paper surfaces; this provides huge freedom for children to design and integrate "do-it-yourself (DIY)" electronic toys. Herein, viscous and additive-free ink made from Ti3 C2 TX MXene sediment is employed to prepare disposable paper electronics through a simple ball pen drawing. The as-drawn paper sensors possess hierarchical microstructures with interweaving nanosheets, nanoflakes, and nanoparticles, therefore exhibiting superior mechanosensing performances to those based on single/fewer-layer MXene nanosheets. As proof-of-concept applications, several popular children's games are implemented by the MXene-based paper sensors, including "You say, I guess," "Emotional expression," "Rock-Paper-Scissors," "Arm wrestling," "Throwing game," "Carrot squat," and "Grab the cup," as well as a DIY smart whisker for a cartoon mouse. Moreover, MXene-based paper sensors are safe and disposable, free from producing any e-waste and hazard to the environment.

Unidirectional Nanopore Dehydration Induces a Highly Stretchable and Mechanically Robust Silk Fibroin Membrane Dominated by Type II β-Turns.

Aqueous silk fibroin solution is dehydrated by evaporation into a water-soluble cast film (SFME) with poor mechanical properties but becomes by unidirectional nanopore dehydration (UND) into silk fibroin membrane (SFMU) with water-stable and good mechanical robustness. The thickness and tensile force of the SFMU are almost twice those of the MeOH-annealed SFME. The UND-based SFMU has a tensile strength of 15.82 MPa, an elongation of 665.23%, and a type II β-turn (Silk I) that accounts for 30.75% of the crystal structure. Mouse L-929 cells adhere, grow, and proliferate well on it. The UND temperature can be used to tune the secondary structure, mechanical properties, and biodegradability. UND induced the oriented arrangement of the silk molecules, which led to the formation of the SFMU dominated with Silk I structure. The silk metamaterial by controllable UND technology has great potential in medical biomaterials, biomimetic materials, sustained drug release, and flexible electronic substrates.

The Impact of the Azo-Chromophore Sort on the Features of the Supramolecular Azopolyimide Films Desired to Be Used as Substrates for Flexible Electronics

High-performance supramolecular polyimide systems were synthesized via a simple and innovative approach using two types of azo-chromophores, leading to concomitant special properties: high thermostability, the ability to be processed in the form of films with high flexibility, adequate morphological features, and good structuring capacity via phase mask ultraviolet (UV) laser irradiation, induced by the presence of the azo groups (-N=N-). The dimension and the anisotropy degree of the micro/nano patterns obtained on the surface of the flexible films (determined by atomic force microscopy) depend on the azo-dye type used in the supramolecular azopolyimide synthesis, which were higher when the azo-chromophore containing a -cyano group (-C≡N) was used. The molecular dynamics method, an excellent tool for an in-depth examination of the intermolecular interactions, was used to explain the morphological aspects. Energetic, dynamic and structural parameters were calculated for the two systems containing azo-chromophores, as well as for the pristine polymer system. It was highlighted that the van der Waals forces make a major contribution to the intermolecular interactions. The results from the combination of the dynamic analysis and the concentration profile explain the better mobility of the polyimide chains with a maximum content of azo groups in the cis configuration compared to the other systems. Taking all these data into account, the surfaces of the films can be tuned as required for the proposed applications, namely as substrates for flexible electronis.

Shape morphing of plastic films

Three-dimensional (3D) architectures have qualitatively expanded the functions of materials and flexible electronics. However, current fabrication techniques for devices constrain their substrates to 2D geometries and current post-shape transformation strategies are limited to heterogenous or responsive materials and are not amenable to free-standing inert plastic films such as polyethylene terephthalate (PET) and polyimide (PI), which are vital substrates for flexible electronics. Here, we realize the shape morphing of homogeneous plastic films for various free-standing 3D frameworks from their 2D precursors by introducing a general strategy based on programming the plastic strain in films under peeling. By modulating the peeling parameters, previously inaccessible free-standing 3D geometries ranging from millimeter to micrometer were predicted theoretically and obtained experimentally. This strategy is applicable to most materials capable of plastic deformation, including polymers, metals, and composite materials, and can even enable 4D transformation with responsive plastic films. Enhanced performance of 3D circuits and piezoelectric systems demonstrates the enormous potential of peeling-induced shape morphing for 3D devices.

Optimization of Spraying Process via Response Surface Method for Fabrication of Cellulose Nanofiber (CNF) Film

Cellulose nanofiber (CNF) is a sustainable bionanomaterial which has fibril’s width varying from 5 nm to ~73 nm with an average length of 8 μm. It can be used as a base material for various functional materials such as barrier, flexible electronic substrates, and membrane etc. Though several methods such as solvent casting and vacuum filtration are available for the production of CNF film in laboratory scale, the major constraints are film formation time, shrinkage on the film, and poor uniformity. Spraying CNF suspension is one of the emerging methods which forms the film rapidly. The present investigation deals with the optimization of critical parameters such as CNF suspension concentration, velocity of the conveyor, and spray distance involved in the spraying process via central composite design (CCD) in the response surface methodology (RSM). The influence of these parameters on the basis weight and thickness of the CNF film was evaluated from the linear models. It concludes that the CNF suspension concentration is a strong parameter for controlling the basis weight and the thickness of the CNF film. The developed linear models were validated with experimental data confirming that it was a good fit. Given this correspondence, these models may be used for scaling up the spraying process for the fast production of CNF film.

Near 90% Transparent ITO-Based Flexible Electrode with Double-Sided Antireflection Layers for Highly Efficient Flexible Optoelectronic Devices.

As a widely used substrate for flexible electronics, indium-tin oxide-based polymer electrodes (polymer-ITO electrodes) exhibit poorly visible light transmittance of less than 80%. The inferior transmittance for polymer-ITO electrodes severely limits the performance improvement of polymer-ITO based electronics. Here, a conceptually different approach of the double-sided antireflection coatings (DARCs) strategy is proposed to modulate both the air-polymer substrate interface and ITO-air interface refractive index gradient, to synergistically improve the transmittance of polymer-ITO electrodes. On the basis of SiO2 nanoparticles antireflection layer on polymer substrate, a polymer-metal oxide composite antireflection film is fabricated on the ITO side. Resultantly, the transmittance of ITO-based flexible electrodes is successfully improved from 76.8% to 89.8%, which is the highest transmittance among the reported ITO-based flexible electrodes. Furthermore, the photoluminescence emission intensity of luminescent materials enveloped with the DARCs electrodes increases by 74% over that with reference electrodes, demonstrating the DARCs antireflection strategy can efficiently improve the performance of flexible optoelectronic devices. With DARCs electrode, the flexible perovskite solar cells exhibit an enhanced efficiency from 18.80% to 20.85%.

Dielectric, structural and mechanical properties of thermally aged biaxially oriented polymeric substrates for flexible electronics

Two materials, polyethylene naphthalate (PEN) and polyethylene terephthalate (PET), which are often used as flexible substrates for electronics, were exposed to accelerated thermal aging at temperatures of 190 °C and 210 °C for 5000 and 2000 h, respectively. Changes in their thermal, dielectric, and mechanical properties were investigated. The results revealed that dielectric losses increase slower for PET than for PEN at lower aging temperatures. In contrast, at higher aging temperatures, similar behavior was observed for both materials. During aging, the dissipation factors of PET and PEN increased by one order of magnitude over those of the original states. The dielectric frequency-temperature characteristics obtained by broadband dielectric spectroscopy (BDS) shifted and indicated changes in the individual dielectric relaxation mechanisms (i.e., α, β, β*). Discoloration induced by thermal aging and quantified by ultraviolet–visible spectroscopy by the peak at 453 nm was detected primarily in the spectrum of PEN. The mechanical tensile strength of PET deteriorated considerably (10 times) after only a few hundred hours, whereas PEN was stable after even longer exposure times. It was found that all of the changes mentioned above are primarily related to the change in the crystalline-to-amorphous phase ratio, as demonstrated by differential scanning calorimetry (DSC) and supported by density measurements.

Synergistically Improved Oxygen Barrier Properties of Polyethylene Terephthalate by Combining “Active” and “Passive” Barrier Techniques

AbstractThe relatively poor oxygen barrier properties of polyethylene terephthalate (PET) limit its high‐end use in flexible electronic substrates. In this work, the oxygen barrier properties of PET films are highly improved by biaxially stretching PET films containing oxygen scavengers. The “active” oxygen scavengers result in a higher crystallinity of biaxially stretched PET films, improving the “passive” barrier properties. Biaxial stretching processing increases the surface area of oxygen scavengers’ domain, which consumes the oxygen molecules more effectively, thus improving the “active” barrier properties. More importantly, the enhanced “passive” barrier path prolongs the residence time of oxygen molecules in the films, allowing the oxygen scavenges to have enough time to sufficiently react with oxygen molecules. As a result, the oxygen permeability coefficient of biaxially stretching PET films containing oxygen scavengers is as low as 14.5792 cc mil m−2 day−1 0.1 MPa−1, synergistically improved by a factor of 11.99 compared to pristine PET films.

Interfacial damage in flexible electronics with collagen substrate: effect of environmental conditions

Stretchable polymers such as polyethylene terephthalate, polyether sulfone etc. are widely used as substrates for flexible electronics. Though these materials are clinically approved and safe to employ for in-vitro applications, still, using collagen as an alternative would be beneficial thanks to its biocompatibility, biodegradability, non-toxicity and flexibility. Recently, some researchers were successful in fabricating sensors with collagen substrates; however, the softer collagen base attached to the stiffer metallic layers might create a mismatch in the interfacial properties and affect the device’s performance. Therefore, it is essential to investigate the mechanical behaviour of collagen-metallic interfaces. It was observed that at lower strain levels (1.25%) the interfacial delamination between the collagen-and metallic layers is negligible while at higher strain (5%), a considerable delamination was noticed. The effect of environment should also be considered as the stress generated for both metallic layers and collagen substrates differed by two orders of magnitude compared to dry ones.

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.

Regulating Ag Wettability via Modulating Surface Stoichiometry of ZnO Substrates for Flexible Electronics

AbstractA novel and highly efficient methodology to regulate (enhance or suppress) the Volmer–Weber 3D growth mode of ultra‐thin (<10 nm) Ag layers by modulating the surface stoichiometry of ZnO substrates prior to Ag deposition is presented. Relative to pristine ZnO layers, oxygen‐deficient surface states formed by preferential removal of surface oxygen atoms remarkably improve Ag layer wettability, whereas oxygen‐excessive surface states formed by oxygen atom incorporation strongly facilitate Ag agglomeration. The dissimilar nucleation and coalescence dynamics are elucidated via combined molecular dynamics and force‐bias Monte Carlo simulations. The improved wettability results in significantly lower sheet resistance in the ultra‐thin (6–10 nm) Ag layers, for example, 6.03 Ωsq−1 at 8 nm, than the previously reported values from numerous other approaches in the equal thickness range. When this unique methodology is applied to ZnO/Ag/ZnO transparent electrodes, simultaneous improvement in electrical conductivity and visible transparency is realized, with a resultant Haacke figure of merit value of 0.139 Ω−1 that is >50% higher than the best reported value for an identically structured electrode. We select transparent heating devices as a model system to confirm that the superior optoelectronic properties are highly sustainable under simultaneous and severe electrical, mechanical, and thermal stresses.

High-Frequency Magnetoimpedance (MI) and Stress-MI in Amorphous Microwires with Different Anisotropies.

Magnetoimpedance (MI) in Co-based microwires with an amorphous and partially crystalline state was investigated at elevated frequencies (up to several GHz), with particular attention paid to the influence of tensile stress on the MI behavior, which is called stress-MI. Two mechanisms of MI sensitivity related to the DC magnetization re-orientation and AC permeability dispersion were discussed. Remarkable sensitivity of impedance changes with respect to applied tensile stress at GHz frequencies was obtained in partially crystalline wires subjected to current annealing. Increasing the annealing current enhanced the axial easy anisotropy of a magnetoelastic origin, which made it possible to increase the frequency of large stress-MI: for 90mA-annealed wire, the impedance at 2 GHz increased by about 300% when a stress of 450 MPa was applied. Potential applications included sensing elements in stretchable substrates for flexible electronics, wireless sensors, and tunable smart materials. For reliable microwave measurements, an improved SOLT (short-open-load-thru) calibration technique was developed that required specially designed strip cells as wire holders. The method made it possible to precisely measure the impedance characteristics of individual wires, which can be further employed to characterize the microwave scattering at wire inclusions used as composites fillers.

Mechanically enhanced healable and recyclable silicone with dynamic hindered urea bond for flexible electronics

A silicone-containing dynamic hindered urea bond for flexible electronic substrates is developed with enhanced mechanical strength and healing ability simultaneously.

Precise dot inkjet printing thought multifactorial statistical optimization of the piezoelectric actuator waveform

InkJet printing is a technique that in the last decades has attracted the attention of the scientific community because it allows producing customized patterns with a variety of inks and substrates, making it suitablemethod for the production of flexible circuits. However, the optimization of this technique is still problematic due to a large number of process variables. Among them the modification of the waveform parameters of the piezoelectric actuator that allows the ejections of the ink can be effective in tuning the printing quality. This work reports a study by a statistical method, namely Design of Experiments (DOE), to optimize the inkjet printing parameters for a nanoparticle-based silver (Ag) ink. This method showed the interplay of the waveform parameters into the definition of optimal drop reproducibility and the achievement of the optimal resolution. In particular, it is shown that mixed terms of the model have a statistical significance and therefore the proposed multifactorial approach provides a benefit in the optimization with respect to the more commonly used one-factor-at-a-time models (OFAT). For the first time, empirical equations have been reported within the corresponding 95% confidence intervals. These equations can be used as a tool to directly tune the properties of the printed dot by modifying the waveform parameters. As case study, we proposed the combinations of a commercial Ag nanoparticles ink with Kapton, a commonly used substrate for flexible electronics.

Fabrication of Transparent Cellulose Fiber Paper by Sequential Treatment of Cationic Cellulose Nanofibers and Polyvinylpyrrolidone

Paper-based material is receiving more and more attention as an alternative of plastics in flexible electronics. However, conventional paper made of cellulose fibers is opaque owing to its micron-sized void space among fibers. Herein, cellulose fiber paper was changed into transparent paper by sequentially coating cationic cellulose nanofibers (CNFs) and polyvinylpyrrolidone (PVP). The morphology, transparency, thermal and mechanical properties were analyzed. The results show that the coating of CNFs reduces the micron-sized void space in the cellulose fiber paper, favoring the further improvement on the transparency of paper by coating PVP. By optimizing the coating amount of CNFs and PVP, a transparent paper with a transmittance of 88.5 % at 550 nm is obtained. The as-prepared transparent paper also shows improved thermal stability, slightly increased tensile strength and significantly enhanced deformation resistance. It was a potential candidate of flexible electronic substrates.