Applications In Flexible Devices Research Articles

P(AMPS‐co‐PEGMA)‐doped and PVDF‐HFP‐enhanced stretchable polymer electrolytes: application as flexible electrochromic devices

AbstractPolymer electrolytes are being considered as an effective solution for electrochemical devices to replace currently used organic liquid electrolytes due to their excellent mechanical properties and security. Herein, we prepared stretchable polymer electrolytes (SPEs) doped with poly[(2‐acrylamido‐2‐methylpropanesulfonic acid)‐co‐poly(ethylene glycol) dimethacrylate] (P(AMPS‐co‐PEGMA)) and enhanced with poly[(vinylidene fluoride)‐co‐hexafluoropropylene] (PVDF‐HFP), through UV in situ polymerization. The SPEs showed good mechanical performance with an elongation of more than 40% and ionic conductivity reaching the order of 10−3 S cm−1. In terms of application, flexible electrochromic devices based on the UV in situ‐prepared SPEs were assembled, the color switching performance being still good even after 400 switching cycles and 1000 bending cycles, which indicated that the SPEs could have excellent prospects in application for flexible devices. © 2023 Society of Industrial Chemistry.

Mask‐Based Separator with Sustained‐Release LiNO3 as Dendrite Growth Barrier for Potassium Metal Battery

AbstractSeparators with low‐cost, superior mechanical properties, such as nanofiber materials, are considered a viable option for solving the dendritic problem in alkali metal batteries. Consuming the dendrites rather than blocking them underpins the long‐life stable cycle performance of the cell. Herein, a five‐layer structural separator (LiNO3@PVDF@mask) is presented, in which a mask is used as a framework and the polyvinylidene fluoride (PVDF) layer loaded with LiNO3 can generate a passivation layer by reacting with K dendrites. In the case of potassium metal batteries, for example, the LiNO3@PVDF@mask separator has excellent mechanical properties that can effectively cope with the hazards caused by the K dendrite. In addition, sustained‐release LiNO3 can react with penetrated K dendrites to form inactive substances like KNO3 and K2O, blocking further dendrite growth. Importantly, the LiNO3@PVDF@mask separator uses a low‐cost abundant mask and possesses excellent wettability of electrolyte with a reduced amount of liquid electrolyte, enabling one to further iron out cost problems. This study opens up a new direction for research and contributes to practical applications of flexible devices for rechargeable alkaline metal batteries.

A Centimeter-Scale Type-II Weyl Semimetal for Flexible and Fast Ultra-Broadband Photodetection from Ultraviolet to Sub-Millimeter Wave Regime.

Flexible photodetectors with ultra-broadband sensitivities, fast response, and high responsivity are crucial for wearable applications. Recently, van der Waals (vdW) Weyl semimetals have gained much attention due to their unique electronic band structure, making them an ideal material platform for developing broadband photodetectors from ultraviolet (UV) to the terahertz (THz) regime. However, large-area synthesis of vdW semimetals on a flexible substrate is still a challenge, limiting their application in flexible devices. In this study, centimeter-scale type-II vdW Weyl semimetal, Td -MoTe2 films, are grown on a flexible mica substrate by molecular beam epitaxy. A self-powered and flexible photodetector without an antenna demonstrated an outstanding ability to detect electromagnetic radiation from UV to sub-millimeter (SMM) wave at room temperature, with a fast response time of ≈20 µs, a responsivity of 0.53mA W-1 (at 2.52 THz), and a noise-equivalent power (NEP) of 2.65 nW Hz-0.5 (at 2.52 THz). The flexible photodetectors are also used to image shielded items with high resolution at 2.52 THz. These results can pave the way for developing flexible and wearable optoelectronic devices using direct-grown large-area vdW semimetals.

Nitric oxide releasing polyvinyl alcohol and sodium alginate hydrogels as antibacterial and conductive strain sensors

Conductive hydrogels have promising applications in flexible electronic devices and artificial intelligence, which have attracted much attention in recent years. However, most conductive hydrogels have no antimicrobial activity, inevitably leading to microbial infections during utilization. In this work, a series of antibacterial and conductive polyvinyl alcohol and sodium alginate (PVA-SA) hydrogels were successfully developed with the incorporation of S-nitroso-N-acetyl-penicillamine (SNAP) and MXene through a freeze-thaw approach. Due to the reversibility of hydrogen bonding and electrostatic interactions, the resulting hydrogels had excellent mechanical properties. Specifically, the presence of MXene readily interrupted the crosslinked hydrogel network, but the best stretching can reach up to >300 %. Moreover, the impregnation of SNAP achieved the release of nitric oxide (NO) over several days under physiological conditions. Due to the release of NO, these composited hydrogels demonstrated high antibacterial activities (> 99 %) against both Gram-positive and negative S. aureus and E. coli bacteria. Notably, the excellent conductivity of MXene endowed the hydrogel with a sensitive, fast, and stable strain-sensing ability, to accurately monitor and distinguish subtle physiological activities of the human body including finger bending and pulse beating. These novel composited hydrogels are likely to have potential as strain-sensing materials in the field of biomedical flexible electronics.

Highly ductile, stable, and efficient organic photovoltaic blends enabled by polymerized ladder-type heteroheptacene-based small-molecule acceptors

Although many fully-conjugated polymerized small-molecule acceptors (PSMAs) with high photovoltaic performance have been reported recently, most of them exhibit high stiffness and are very brittle, which limits their applications in wearable and stretchable electronics. In this work, we show that PSMAs using ladder-type heteroheptacene-cored SMAs (M-series acceptors) as the main building blocks can achieve high mechanical ductility while maintaining high power conversion efficiencies (PCEs). To finely tune molecular stacking and miscibility of the resulting PSMAs (PQ-HD and PQ-OD), two pendant groups with different alkyl lengths (namely, 2-hexyldecyl and 2-octyldodecyl) are anchored on their backbones. With the shorter side-chains, a more intermixed bulk-heterojunction blend morphology as well as highly well-ordered molecular packing are achieved, leading to simultaneous enhancement of both photovoltaic and mechanical properties. The best-performing PSC based on PQ-HD with 2-hexyldecyl side-chains shows a high PCE of 13.36% and excellent mechanical stretchability with a crack-onset strain (COS) of 17.5%, both of which are superior to that based on PQ-OD with 2-octyldodecyl side-chains (PCE = 10.87% and COS = 12.8%). Moreover, the PQ-HD-based PSC also exhibits excellent thermal stability. These outstanding properties in combination with its low synthetic complexity shall make PQ-HD a promising candidate for future applications in flexible and wearable devices.

Transparent and Stretchable Electromagnetic Interference Shielding Film with Fence-like Aligned Silver Nanowire Conductive Network.

Flexible transparent conductive electrodes (TCEs) that can be used as electromagnetic interference (EMI) shielding materials have a great potential for use as electronic components in optical window and display applications. However, development of TCEs that display high shielding effectiveness (SE) and good stretchability for flexible electronic device applications has proven challenging. Herein, this study describes a stretchable polydimethylsiloxane (PDMS)/silver nanowire (AgNW) TCE with a fence-like aligned conductive network that is fabricated via pre-stretching method. The fence-like AgNW network endowed the PDMS/AgNW film with excellent optoelectronic properties, i.e., low sheet resistance of 7.68Ωsq-1 at 73.7% optical transmittance, thus causing an effective EMI SE of 32.2dB at X-band. More importantly, the fence-like aligned AgNW conductive network reveals a high stability toward tensile deformation, thus gives the PDMS/AgNW film stretch-stable conductivity and EMI shielding property in the strain range of 0-100%. Typically, the film can reserve ≈70% or 80% of its initial EMI SE when stretching at 100% strain or stretching/releasing (50% strain) for 128 cycles, respectively. Additionally, the film exhibits a low-voltage driven and stretchable Joule heating performance. With these overall performances, the PDMS/AgNW film should be well suited for use in flexible and stretchable optical electronic devices.

Research progress of self‐healing polymer materials for flexible electronic devices

AbstractSelf‐healing polymer materials have been a research hotspot in the field of smart materials since their invention. They have self‐diagnostic functions and are capable of self‐healing small cracks. By applying self‐healing materials to flexible electronic devices, the mechanical damage caused by bending, folding and scratching of these devices can be reduced. The reliability and service life of the devices could be improved accordingly. This paper provides a brief overview of self‐healing polymers and focuses on their applications in flexible electronic devices. In addition, this paper prospects the future development and challenges of self‐healing polymer materials.

Au-Ag NPs embedded Sago Starch-Sodium Alginate composites: An investigation of structural, thermal and dielectric properties for applications in flexible electronic devices

Flexible bio-friendly nanocomposite (NC) films based on Sago Starch-Sodium Alginate (SS-SA) blend embedded with Auc-Ags core–shell nanoparticles (NPs) were synthesized via solution casting approach. The appearance of XRD peaks corresponding to Ag NPs confirms the formation of Ag shell over Au core and also evidenced by HR-TEM, SAED pattern, FE-SEM. Thermogravimetric analysis (TGA) suggests enhanced thermal stability with addition of NPs in polymer blend. Dielectric spectroscopy reveals that, dielectric permittivity (ε') of polymer matrix increases from 18.8 (at 100 Hz) to 96.2 at room temperature (for 1.5 wt% Auc-Ags@SS-SA NC), and increases to 2227.4 at 353 K. This increase is attributed to the formation of nano-capacitors. Enhanced ac conductivity, low relaxation time and decrease in grain/grain boundary resistances with the addition of Auc-Ags NPs indicate enhanced charge transport. Enhanced dielectric permittivity and low tangent loss make these NC films a potential contender for charge storage in flexible electronic devices.

Highly Elastic, Self-Healing, Recyclable Interlocking Double-Network Liquid-Free Ionic Conductive Elastomers via Facile Fabrication for Wearable Strain Sensors.

Liquid-free ionic conductive elastomers (ICEs) are ideal materials for wearable strain sensors in increasingly flexible electronic devices. However, developing recyclable ICEs with high elasticity, self-healability, and recyclability is still a great challenge. In this study, we fabricated a series of novel ICEs by in situ polymerization of lipoic acid (LA) in poly(acrylic acid) (PAA) solution and cross-linking by coordination bonding and hydrogen bonding. One of the obtained dynamically cross-linked interlocking double-network ICEs, PLA-PAA4-1% ICE, showed excellent mechanical properties, with high elasticity (90%) and stretchability (610%), as well as rapid self-healability (mechanical self-healing within 2 h and electrical recovery within 0.3 s). The PLA-PAA4-1% ICE was used as a strain sensor and possessed excellent linear sensitivity and highly cyclic stability, effectively monitoring diverse human motions with both stretched and compressed deformations. Notably, the PLA-PAA4-1% ICE can be fully recycled and reused as a new strain sensor without any structure change or degradation in performance. This work provided a viable path to fabricate conductive materials by solving the two contradictions of high mechanical property and self-healability, and structure stability and recyclability. We believe that the superior overall performance and feasible fabrication make the developed PLA-PAA4-1% ICE hold great promise as a multifunctional strain sensor for practical applications in flexible wearable electronic devices and humanoid robotics.

"Salting out" in Hofmeister Effect Enhancing Mechanical and Electrochemical Performance of Amide-based Hydrogel Electrolytes for Flexible Zinc-Ion Battery.

With the development of flexible and wearable electronic devices, it is a new challenge for polymer hydrogel electrolytes to combine high mechanical flexibility and electrochemical performance into one membrane. In general, the high content of water in hydrogel electrolyte membranes always leads to poor mechanical strength, and limits their applications in flexible energy storage devices. In this work, based on the "salting out" phenomenon in Hofmeister effect, a kind of gelatin-based hydrogel electrolyte membrane is fabricated with high mechanical strength and ionic conductivity by soaking pre-gelated gelatin hydrogel in 2m ZnSO4 aqueous. Among various gelatin-based electrolyte membranes, the gelatin-ZnSO4 electrolyte membrane delivers the "salting out" property of Hofmeister effect, which improves both the mechanical strength and electrochemical performance of gelatin-based electrolyte membranes. The breaking strength reaches 1.5MPa. When applied to supercapacitors and zinc-ion batteries, it can sustain over 7500 and 9300 cycles for repeated charging and discharging processes. This study provides a very simple and universal method to prepare polymer hydrogel electrolytes with high strength, toughness, and stability, and its applications in flexible energy storage devices provide a new idea for the construction of secure and stable flexible and wearable electronic devices.

Localization of edge state in acoustic topological insulators by curvature of space

Topological insulators (TIs) with robust boundary states against perturbations and disorders have boosted intense research in classical systems. In general, two-dimensional (2D) TIs are designed on a flat surface with special boundary to manipulate the wave propagation. In this work, we design a 2D curved acoustic TI by perforation on a curved rigid plate to localize the edge state by means of the geometric potential effect, which provide a unique approach for manipulating waves. We experimentally demonstrate that the topological edge state in the bulk gap is modulated by the curvature of space into a localized mode, and the corresponding pressure distributions are confined at the position with the maximal curvature. Moreover, we experimentally verify the localized edge state is still topologically protected by introducing defects near the localized position. To understand the underlying mechanism for the localization of the topological edge state, a tight-binding model considering the geometric potential effect is proposed. The interaction between the geometrical curvature and topology in the system provides a novel scheme for manipulating and trapping wave propagation along the boundary of curved TIs, thereby offering potential applications in flexible devices.

Scalable novel lanthanide-ligand complex for robust flexible micro-supercapacitors

Herein we report on the excellent, supercapacitor performance and flexible device application of a robust, new polymeric metal-ligand complex, which is readily synthesized at room temperature via a straightforward, one-step protocol using the N4 donor, namely 3,3′-diaminobenzidine (DAB) and gadolinium(III) nitrate. Thus, the coordination polymer (Gd-DAB) complex synthesized in a facile and economically feasible synthesis route paves the way for realizing a new class of affordable, durable energy devices for storage applications. The results of complementary characterization techniques not only corroborated the proposed structure but also revealed a potential two-dimensional (2D)/sheet-like organization similar to the coordination polymer (COP). The electrochemical tests performed on the as-prepared sample, in the form of an electrode active component, showed brilliant supercapacitive characteristics with the capacitive retention exceeding over 100% even after 5000 cycles. The complex has been used as an active media for the fabrication of an interdigitated flexible and symmetric supercapacitor. The electrochemical storage device evaluation, under different bending angles (up to 180o) and twisting conditions at the current density of 10 μA/cm2 for 5000 cycles, revealed the capacitive retention of ∼50% after 5000 cycles.

The world of exotic crystals: Raman spectro-microscopy for probing local structure

In this article, we review briefly the recent progress on mechanically responsive molecular crystals which defy the common perception that “crystals are rigid”. Crystals which respond to different external stimuli are emerging fast and may find applications in miniature technologies, sensors, flexible devices, and soft-robotics. Here, we present some early examples of molecular crystals from our group, along with other examples of our choice, to emphasize on the use of Raman spectromicroscopy technique for characterizing crystals, polymorphic forms and strained crystals in the context of structure- property correlation and crystal engineering.

Toughening Thermoelectric Materials: From Mechanisms to Applications.

With the tendency of thermoelectric semiconductor devices towards miniaturization, integration, and flexibility, there is an urgent need to develop high-performance thermoelectric materials. Compared with the continuously enhanced thermoelectric properties of thermoelectric materials, the understanding of toughening mechanisms lags behind. Recent advances in thermoelectric materials with novel crystal structures show intrinsic ductility. In addition, some promising toughening strategies provide new opportunities for further improving the mechanical strength and ductility of thermoelectric materials. The synergistic mechanisms between microstructure-mechanical performances are expected to show a large set of potential applications in flexible thermoelectric devices. This review explores enlightening research into recent intrinsically ductile thermoelectric materials and promising toughening strategies of thermoelectric materials to elucidate their applications in the field of flexible thermoelectric devices.

Giant Humidity Effect of 2D Perovskite on Paper Substrate: Optoelectronic Performance and Mechanical Flexibility

AbstractFlexible optoelectronic devices have attracted enormous attention as an essential component in next‐generation wearable devices. To meet the trend of biocompatibility, flexibility and low cost, paper‐based optoelectronic devices have become compelling candidates. Moreover, 2D organic–inorganic metal halide perovskites have been extensively studied for the application in flexible optoelectronic devices owing to their long‐term moisture stability, optoelectronic tunability, and multiple quantum well (MQW) structures. Herein, the paper‐based flexible optoelectronic devices are designed by spin‐coating PEDOT:PSS and (iso‐BA)2MAn‐1PbnI3n+1 (n ≥ 1) on regular filter paper with responsivity of 2.62 mA W−1 and response time (τrise/τdecay) of 0.81/0.18 s at 35% relative humidity (RH). Furthermore, the responsivity may be increased by 250% within the humidity range from 35% to 90%, and remains above 92% of the initial value after measured for 1800 s at 60% RH. This work can pave the way for further studies about the humidity effect on perovskites for next‐generation green wearable electronics.

Smart MXene‐based bioelectronic devices as wearable health monitor for sensing human physiological signals

AbstractBiosafe wearable healthcare monitor has attracted significant attention owing to their applicability to wearable electronics. However, the narrow sensing range and poor response limit the application of flexible devices for comprehensive monitoring of human health‐related physiological signals (i.e., pulse diagnosis). Critical challenges remain in the development of biocompatible materials and the design of flexible bio‐integrated platforms for these purposes, targeting performance approaching those of conventional wafer‐based technologies and long‐term operational stability. In this context, this work presents a robust and flexible MXene/polydopamine (PDA)‐composite‐film‐based pressure sensor in a portable/wearable fashion, which establishes a unique intercalated spherical‐like PDA molecules structure, thereby resulting in excellent sensing performance. The MXene/PDA‐based pressure sensor has sensitivity of up to 138.8 kPa−1 in the pressure range of 0.18–6.20 kPa with fast response and recovery speed (t1 < 100 ms; t2< 50 ms). Associated embodiment involves real‐time precise measurements of a variety of health‐related physiological signals, ranging from wrist pulse, to finger motions, to vocalization and to facial expressions, with high sensitivity and accuracy. Studies on human subjects establish the clinical significance of these devices for future opportunities of health monitoring and intelligent control to predict and diagnose diseases.

Colorless polyimides derived from rigid trifluoromethyl-substituted triphenylenediamines

Colorless polyimides (CPIs) have wide applications in flexible photoelectric devices, thanks to their high transparency and heat-resistance. Herein, three rigid aromatic triphenylenediamine monomers containing varied number and position of trifluoromethyl (-CF3) substituents, i.e., c-3FTP, c-6FTP, and s-6FTP, were designed. The corresponding CPI films were fabricated by polycondensation with the commercial 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) or 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA). Due to the steric hindrance of –CF3 groups, three diamines all exhibit large torsion angles within 46.1°–87.1° between benzene rings determined via single crystal analysis. The structure-property analysis reveals that –CF3 substitution located on the central phenyl ring facilitates the high glass transition temperature (Tg), while more –CF3 substitutions are beneficial for high optical transparency. Among all the CPIs, the 6FDA/c-6FTP film exhibits the most remarkable balance in thermal, optical, and dielectric properties, including high thermal resistance (Tg = 385 °C, Td5% = 537 °C), excellent optical transparency (λcutoff = 350 nm, T450 = 84.5%, and b* = 1.44), low dielectric nature (Dk = 2.84, Df = 0.0053) at 10 GHz, and high surface hydrophobicity (θ = 100°, WH2O = 0.25%). This systematic study would complement the basic structure-property relationship of aromatic CPIs and inspire rational development of high-performance aromatic CPI materials toward applications in flexible display or 5G technology.

Preparation and Physical Properties of Stretchable FeRh Films with Periodic Wrinkle Structure

AbstractStretchable magnetic materials are highly desirable for developing flexible high‐performance magnetoelectronic devices. However, it is still challenge to fabricate stretchable magnetic films with high growth temperature. In this work, metamagnetic FeRh films are grown on mica substrates at high temperature and then transferred onto the prestrained polydimethylsiloxane (PDMS). Due to the large modulus mismatch between FeRh films and PDMS, FeRh films with periodic wrinkle pattern is formed after releasing the prestrain, enabling a controllable stretchability up to a few tens of percent. Moreover, the meta‐magnetic phase transition temperature can be effectively controlled by the prestrain, with an increase of ≈4 K for every 10% increase in the prestrain. In case of the applied tensile strain less than the prestrain, the obtained flexible FeRh films can maintain stable performance after thousands of stretches, making it promising for applications in flexible magnetoelectronic devices.

Highly Sensitive Strain Sensor Fabricated by Direct Laser Writing on Lignin Paper with Strain Engineering

Paper‐based strain sensors (PSS) have broad prospects in disposable products due to their low cost and easy degradation as environmentally friendly materials. Herein, a strain sensor made of a laser‐induced carbonization electrode is created by direct laser writing with filter paper. The conductivity and gauge factor (GF) of this strain sensor are improved by adding lignin and applying strain engineering. This enables the sensor to simultaneously satisfy high sensitivity (GF ≈ 408 and 91) for weak tension and compression strain, respectively, and with long‐term reliability. The tensile strain GF factors of up to 201 are possible, even with a weak tensile strain of 0.00088%. Furthermore, the paper‐based sensor for monitoring physiological activities like finger gestures, pulsing, swallowing, and eye blinking is demonstrated. The facile fabrication and superior performance of PSS fabricated by direct laser writing with strain engineering may pave the way for promising applications of flexible, portable, and wearable electronic devices.

Defect Engineering in Earth‐Abundant Cu2ZnSnSe4 Absorber Using Efficient Alkali Doping for Flexible and Tandem Solar Cell Applications

To demonstrate flexible and tandem device applications, a low‐temperature Cu2ZnSnSe4 (CZTSe) deposition process, combined with efficient alkali doping, was developed. First, high‐quality CZTSe films were grown at 480 °C by a single co‐evaporation, which is applicable to polyimide (PI) substrate. Because of the alkali‐free substrate, Na and K alkali doping were systematically studied and optimized to precisely control the alkali distribution in CZTSe. The bulk defect density was significantly reduced by suppression of deep acceptor states after the (NaF + KF) PDTs. Through the low‐temperature deposition with (NaF + KF) PDTs, the CZTSe device on glass yields the best efficiency of 8.1% with an improved VOC deficit of 646 mV. The developed deposition technologies have been applied to PI. For the first time, we report the highest efficiency of 6.92% for flexible CZTSe solar cells on PI. Additionally, CZTSe devices were utilized as bottom cells to fabricate four‐terminal CZTSe/perovskite tandem cells because of a low bandgap of CZTSe (~1.0 eV) so that the tandem cell yielded an efficiency of 20%. The obtained results show that CZTSe solar cells prepared by a low‐temperature process with in‐situ alkali doping can be utilized for flexible thin‐film solar cells as well as tandem device applications.