Applications In Flexible Devices Research Articles

Tailoring the optical properties of dye–polymer composite films based on polyvinyl alcohol doped with phenol red toward flexible optoelectronic applications

The composite films of polyvinyl alcohol (PVA) incorporating varying concentrations of phenol red (PR) dye were fabricated using the solution casting method. Structural characteristics of the prepared films were explored through XRD and FTIR spectroscopy analyses. The inclusion of PR dye within the PVA matrix induced cross-linking, enhancing the amorphous nature of the doped PVA samples, as evidenced by XRD patterns. In the presence of PR dopants, the FTIR peaks for PVA exhibited altered intensities and increased width, indicating physical interactions between the functional groups of PVA and PR dopants. The impact of PR additives on the optical properties of PVA was investigated across the spectral range of 190–2500 nm. The PVA/phenol red composite demonstrated enhanced UV blocking in the 190–400 nm wavelength range, rendering it suitable for applications such as UV notch filters, including laser blocking filters. The indirect and direct optical band-gaps of PVA polymer films were reduced from 5.20 eV and 5.78 eV to 4.30 eV and 5.28 eV, respectively, with an increase in the PR filling ratio from 0 wt% to 1.8 wt%. The dispersion region of the refractive index was analyzed using the single oscillator model (Wemple-DiDomenico). Calculation and discussion of values such as dispersion and oscillation energies (Ed and Eo), permittivity at infinite frequency (ε ∞), and lattice contribution to the dielectric function (ε L) of PVA/PR composite films were conducted. These advancements position PVA/PR composite films for potential applications in flexible optoelectronic devices, including light-emitting diodes and photodetectors.

Flexible Ultrasonic‐Induced Wireless Energy Conversion Technology in Implantable Biomedicine

AbstractNumerous associated challenges arise with the advancement and continuous improvement of implantable medical devices, such as the frequent requirement for battery replacement. The flexible ultrasonic‐induced wireless energy conversion (UWEC) technology has garnered significant interest due to its superior energy transmission efficiency, safety, and biocompatibility compared to traditional electromagnetic wireless energy conversion technology. This paper provides a comprehensive review of the recent advancements in two types of flexible UWEC technologies, triboelectric nanogenerators, and piezoelectret nanogenerators, with a focus on their applications in implantable medical treatments. First, the energy conversion principles and general materials of flexible UWEC are presented. Then, the research and application of flexible UWEC devices are introduced in detail based on their medical functions. At the end of this review, perspectives and opportunities are discussed.

Self‐healing waterborne polyurethane elastomers based on synergistic hydrogen, disulfide and borate bonding for composite conductors

AbstractAs a functional material, polyurethane elastomers have found different applications in a variety of fields such as flexible electronics and smart coatings, but due to the damage during use, they are often endowed with self‐healing properties. However, conventional intrinsically self‐healing polyurethanes often don't balance excellent mechanical and self‐healing properties, so it has become crucial to prepare polyurethane elastomers with excellent self‐healing properties and high mechanical properties. In this study, a self‐healing waterborne polyurethane elastomers (IP‐DTDAx‐BAy‐WPU‐n) was synthesized based on 4, 4‐diaminodiphenyl sulfide (DTDA) boronic acid (BA). The disulfide bond has high motility at low temperatures and induces molecular‐level healing at the damaged interface, contributing to good repair under mild conditions. The borate ester bond limits intermolecular chain slippage and improves the mechanical properties as a dynamic cross‐linking point, while its dynamic reversible effect can synergistically improve the self‐healing ability. The results show that the synthesized elastomer films have good mechanical properties (strength 23.21 MPa, elongation at break 1259.08%, toughness 83.28 MJ m−3), good elastic recovery and self‐healing properties (93%). Furthermore, a composite conductors (IP‐DTDAx‐BAy‐WPU/AgNWs) with self‐healing properties was prepared by compositing AgNWs nanowires with polyurethane elastomers, which laid the foundation for its application in flexible electronic devices and wearable sensing devices.

Flexible Aluminum-Air Battery Based on High-Performance Three-Dimensional Dual-Network PVA/KC/KOH Composite Gel Polymer Electrolyte.

With a large theoretical capacity and high energy density, aluminum-air batteries are a promising energy storage device. However, the rigid structure and liquid electrolyte of a traditional aluminum-air battery limit its application potential in the field of flexible electronics, and the irreversible corrosion of its anode greatly reduces the battery life. To solve the above problems, a PVA/KC/KOH (2 M) composite gel polymer electrolyte (GPE) with a three-dimensional dual-network structure consisting of polyvinyl alcohol (PVA), kappa-carrageenan (KC), and potassium hydroxide was prepared in this paper by a simple two-step method and applied in aluminum-air batteries. At room temperature, the ionic conductivity of the PVA/KC/KOH (2 M) composite GPE was found to be up to 6.50 × 10-3 S cm-1. By utilizing this composite GPE, a single flexible aluminum-air battery was assembled and achieved a maximum discharge voltage of 1.2 V at 5 mA cm-2, with discharge time exceeding 3 h. Moreover, the single flexible aluminum-air battery maintains good electrochemical performance under various deformation modes, and the output voltage of the battery remains at about 99% after 300 cycles. The construction of flexible aluminum-air batteries based on a three-dimensional dual-network PVA/KC/KOH composite GPE provides excellent safety and high-multiplication capabilities for aluminum-air batteries, making them potential candidates for various flexible device applications.

Photoresponsive Janus microfibers array film with tunable electrical anisotropy

A new kind of photoresponsive film is produced by integrating photoconductive material into the type-II anisotropic conductive film. This film consists of orderly arranged Janus microfibers, with each Janus microfiber is comprised of a conductive region and an insulating region. Copper phthalocyanine, as the photoconductive material, is introduced into the conductive regions of these Janus microfibers, and thus the electrical conduction of the conductive regions can be enhanced by applying appropriate external light irradiation, while the insulating regions are basically unaffected. Based on this property, the electrical conduction along with the length direction of the Janus microfibers in the film is adjustable by light, the perpendicular direction along the film surface remains constantly insulating, and thus the electrical anisotropy of the resulting Janus microfibers array film can be tuned under light irradiation. The products have potential applications in flexible electronic devices and control systems.

Aggregation induced strong photoluminescence at room temperature in large-area C8BTBT thin films

Study of optical properties of 2,7-dioctyl[1]benzothieno[2,3-b]benzothiophene (C8BTBT) is promising for applications in low-cost and flexible optoelectronic devices. Here, we systematically investigated the thickness dependent structural and photophysical properties of large-area C8BTBT films. The large-area C8BTBT thin films of different thickness below 80 nm were prepared by off-centre spin coating method. Grazing incidence X-ray diffraction measurements confirm the highly crystalline nature of the thin films and reveal that crystal coherence length increases with increase of film thickness. The film morphology and its thickness were studied using atomic force microscopy. Further, optical absorption and emission behaviors of the thin films were analyzed. The results show strong ultraviolet (UV) emissions at room temperature. The photoluminescence behaviour of higher-thickness films differs from that of lower-thickness films. The thickness dependent UV emission spectra are explained in terms of defects and aggregation behavior of C8BTBT thin films. We determine the impact of thickness and aggregation (J- and H-type) on emission behaviors of the C8BTBT thin films. Our findings open a new avenue for the future optimization and prospective applications in photonics based on two-dimensional organic semiconductors.

Transparent and flexible thermoelectric thin films based on copper sulfides

As a promising thermoelectric material, CuS has attracted significant attention due to its high conductivity, abundance of elements, and eco-friendliness. However, the study on CuS-based thermoelectric thin films is still lacking, impeding the advancement of CuS-based thermoelectric devices. Herein, high-quality CuS thin films have been fabricated through a facile vulcanization process. The effects of vulcanization temperature and film thickness on the thermoelectric properties of CuS thin films have been investigated. An optimal high power factor of 73.25 μW/m−1 K−2 is found at 400 K for a 20 nm-thick sample, and the optical transmittance is over 80% in the visible light spectrum. Meanwhile, excellent flexibility of the CuS thin films has been demonstrated. These results demonstrate the high promise of CuS thin films for transparent and flexible thermoelectric device applications.

Tuning the Ferromagnetic Resonance Frequency of Microstructured Permalloy Film on Flexible Substrate

The importance of flexible ferromagnetic films in the application of flexible spintronic devices and microwave magnetic devices underscores the necessity for an in‐depth understanding of the dynamic magnetic properties of these films. In particular, it is crucial to comprehend the regulation of the ferromagnetic resonance (FMR) frequency of flexible ferromagnetic films. This work outlines the preparation of periodic permalloy microstrip arrays with stripe domain on flexible PET films and applies them to linearly tune the FMR. The high‐frequency optical branch (5.17 GHz) and low‐frequency acoustic branch (2.89 GHz) are observed in the direction perpendicular (x‐direction) and parallel (y‐direction) to the microstrip, respectively. The Young's modulus mismatch between the PET film and permalloy layer leads to the directional distribution of local tension. This results in the enhancement of the Ht (219.4 Oe) required for the in‐plane uniform precession on the PET film, compared to that on the Si substrate (181.6 Oe). By adjusting the width and thickness of the permalloy microstrip, Ht can be adjusted linearly on the PET film. This flexible ferromagnetic film with linear regulation of FMR frequency presents a new option for the future development of flexible microwave detectors and filters.

Multiparametric AFM insights into electron transport mechanisms in biomemristors

Biomemristors have attracted significant attention due to their applications in biodegradable and flexible electronic devices, and their prospective functions in information storage, neural synapses, and neuromorphic computation. Hence, understanding biomaterials' resistive switching (RS) mechanism is essential to design novel biomemristors with enhanced performance. In the last decade, various atomic force microscopy (AFM) modes, such as Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (cAFM), in situ/operando visualize the electron transport behaviors within biomemristors at the nanoscale. This review provides a comprehensive discussion of multiparametric AFM techniques to characterize biomemristors and inspire new ideas in developing next-generation consumer electronics. We also summarize the progress of revealing the internal mechanism of biomemristors with functional AFM to visualize and understand the switching behavior. Finally, the opportunities and challenges of further utilizing multiparametric AFM to explore the electron transport mechanisms in biomemristors are presented.

Effect of mild mechanical stresses on device physics of slot-die coated flexible perovskite solar cells

Halide perovskites are a promising class of novel photoactive materials for their application in flexible photovoltaic devices. Tolerance and stability of devices to various mechanical stresses are important aspects of this technology for its effective and long-term operation. Currently, this aspect has not been fully researched and addressed by the research community. Hence, this work is aimed at studying the effect of mild mechanical stresses (MMSs) induced by convex bending on the behaviour of ITO/ZnO/MAPbI3/Spiro-OMeTAD/Au slot-die-coated flexible perovskite solar cells (FPSCs) and investigating the underlying mechanisms leading to performance degradation in devices. The carrier generation, recombination, and extraction processes within FPSCs are investigated before and after the application of MMSs. The obtained results suggest that the photovoltaic performance of the FPSCs deteriorates by up to 50% after convex bending, due to increased non-radiative recombination losses. The results obtained in this work provide some valuable insights into the dynamics of recombination processes in FPSCs exposed to MMSs and can be used for developing new strategies for improving the mechanical stability of devices.

3D printed dual network Cross-Linked hydrogel electrolytes for high area capacity flexible zinc ion Micro-Batteries

Hydrogel electrolytes have wide applications in flexible zinc ion micro-batteries (FZIMBs) due to their excellent mechanical flexibility, biocompatibility, and ionic conductivity. However, traditional templating methods are limited to fabricating two-dimensional and simple three-dimensional structures, making them insufficient for preparing complex structures in FZIMBs. Here, we report a printable hydrogel electrolyte ink with excellent rheological properties. By utilizing 3D printing technology, we achieve the preparation of a custom-shaped, double-network crosslinked polyacrylamide-hydroxypropyl methylcellulose hydrogel electrolyte (PHHE). The PHHE exhibits high ionic conductivity (31.72 mS cm−1), good cycling stability, and mechanical flexibility. Furthermore, we constructed FZIMBs with high areal capacities (6.45 mAh cm−2 at 0.5 mA cm−2) and good mechanical flexibility by leveraging PHHE and 3D printing technology. Integrating FZIMBs with a pressure sensing component, we establish a sensing interaction system, demonstrating practical applications of flexible wearable devices. This work presents a novel method for the preparation of customized hydrogel electrolytes and highlights the significant potential of FZIMBs in practical applications.

Electrical, optical and mechanical properties of monolayer MoTe2 for applications in wearable optoelectronic devices

Transition metal dichalcogenides (TMDs) have excellent optical and mechanical properties and have potential application value in wearable optoelectronic response devices. MoTe2, a representative material of TMDs, is studied by first-principles calculation in this paper. The results show that the MoTe2 monolayer has a direct band gap of 1.110eV, which has a strong light absorption capacity and can produce a high concentration of photogenerated charge carriers after light absorption. The material is soft and exhibits the unique mechanical properties of layered materials. The effects of biaxial strain and defects on the properties of the materials were analyzed. The results show that the biaxial compression strain can enhance the light absorption curve of the material, enhance the light absorption of the photogenerated carrier, and expand the range of its energy distribution. The tensile strain decreases the value of the photon absorption curve and decreases the range of energy distribution of photogenerated carriers. The Mo vacancy defect increases the absorption curve value in the low energy region and broadens the optical response range of the material. The two types of vacancy defects both induce a ‘discrete’ distribution of photogenerated carriers. The Mo vacancy significantly affects the elastic modulus and anisotropy properties of the material, resulting in the material changing from ductile to brittle. When Mo vacancy is added, the spatial distribution of the elastic modulus of the material also changes greatly. Therefore, MoTe2 has potential application in flexible optoelectronic devices, and its performance can be controlled by strains and defects.

Femtosecond laser precision machining of carbon film based on aramid paper substrate

Carbon film has many applications in flexible electronic devices (FED) due to its low cost and high conductivity. However, it is difficult to remove carbon film from the substrate without damaging it. This research explores the use of femtosecond laser, which has minimal thermal effect and non-contact processing, for carbon film ablation. The main objectives are to determine the ablation threshold of carbon film and aramid paper, to optimize the laser ablation parameters and scanning path, and to analyze the ablation mechanism and quality. The results show that the optimal laser fluence is between 0.22 J/cm2 and 1.45 J/cm2, and the optimal scanning method is grid scanning method. The laser ablation achieves a removal depth of 9.4 μm and a minimum Sa of 3.0 μm. Raman spectroscopy confirms the complete removal of carbon film and the transformation of graphite to amorphous carbon. The ablation process is mainly governed by combustion and evaporation. A resistance strain gauge is fabricated using laser ablation to demonstrate its potential in carbon film FED manufacturing.

High mobility and hysteresis free InGaSnO thin film transistors by co-sputtering via low temperature post annealing process

In this work, InGaSnO thin film transistors (TFTs) with high mobility (>50 cm2/Vs) were prepared by a simple co-sputtering process at a low temperature of 150 ℃. In the co-sputtering process of InSnO and Ga2O3, the composition of InGaSnO can be reasonably controlled by adjusting the sputtering power in the radio frequency plasma, which significantly affects the electrical performance and stability of InGaSnO TFTs. The highly overlapping 5 s electron cloud of In and Sn and the high binding energy of Sn-O lay the foundation for the high mobility of InGaSnO TFTs. The defects related to oxygen in the channel are suppressed by the reasonable control of the composition of Ga in the multicomponent oxides. Meanwhile, the interface defects between the gate dielectric and the active layer are reduced, and a dense metal-oxygen-metal network is formed which is conducive to electron transmission. We have achieved an InGaSnO TFTs with mobility of 58.7 cm2/Vs and threshold voltage of -1.33 V. The stability under positive and negative bias stress is also significantly improved. In general, a simple co-sputtering process is entirely feasible to realize high-mobility oxide TFTs. The control of the process temperature below 150 ℃ also extends its application in flexible electronic devices.

Nonhalogenated Solvent Processable and High-Density Photopatternable Polymer Semiconductors Enabled by Incorporating Hydroxyl Groups in the Side Chains.

Polymer semiconductors hold tremendous potential for applications in flexible devices, which is however hindered by the fact that they are usually processed by halogenated solvents rather than environmentally more friendly solvents. An effective strategy to boost the solubility of high-performance polymer semiconductors in nonhalogenated solvents such as tetrahydrofuran (THF) by appending hydroxyl groups in the side chains is herein presented. The results show that hydroxyl groups, which can be easily incorporated into the side chains, can significantly improve the solubility of typical p- and n-types as well as ambipolar polymer semiconductors in THF. Meanwhile, the thin films of these polymer semiconductors from the respective THF solutions show high charge mobilities. With THF as the processing and developing solvents these polymer semiconductors with hydroxyl groups in the side chains can be well photopatterned in the presence of the photo-crosslinker, and the charge mobilities of the patterned thin films are mostly maintained by comparing with those of the respective pristine thin films. Notably, THF is successfully utilized as the processing and developing solvent to achieve high-density photopatterning with ≈82000 device arrays cm-2 for polymer semiconductors in which hydroxyl groups are appended in the side chains.

Ultrastable and supersensitive conductive hydrogels conferred by “sodium alginate stencil” anchoring strategy

Although conductive hydrogels have been widely developed currently, their low sensitivity and poor stability severely limited their practical application in flexible wearable devices. Herein, a green “stencil” anchoring strategy was proposed in this study to engineer an ultra-stable and supersensitive hydrogel by virtue of polydopamine decorating sodium alginate molecular chains as “stencil” to anchor polyaniline as conductive component. The dispersion of polyaniline was significantly improved by the sodium alginate “stencil” in the conductive hydrogel. The developed conductive hydrogel exhibited outstanding properties that outperformed most conventional ones, including extraordinary sensitivity with a gauge factor of 38.2 and excellent stability with negligible shifting upon long-term cyclic stretching. Moreover, the conductive hydrogel displayed great self-adhesion and reliable self-healing performance endowed by its abundant catechol groups, hydrogen bondings and π-π stackings, respectively. Furthermore, the prepared hydrogel was also assembled as flexible strain and self-powered sensors, which displayed excellent sensing performance, indicating great potential in human-machine interactions, information transmission and road transportation.

Self-adhesive, ionic conductive, environmentally tolerant, and antibacterial phytic acid-reinforced zwitterionic hydrogels as flexible sensor applications

Ionic conductive hydrogels have attracted tremendous research interest for their intrinsic characteristics in the field of flexible sensors. However, the combination of multifunctionalities of high mechanical strength, satisfactory self-adhesiveness, ionic conductivity, anti-freezing, antibacterial properties, and biocompatibility remains a great challenge for the development of ionic conductive hydrogels. In this study, a novel ionic conductive hydrogel was developed by introducing highly conductive phytic acid (PA) to poly(sulfobetaine methacrylate) (polySBMA). PA molecules, as non-covalent crosslinkers and electrolytes in the polySBMA network, significantly improved the mechanical properties, self-adhesion, and electrical properties of the hydrogels. The optimal hydrogel displayed robust mechanical properties (tensile stress: 240.7 kPa), desirable adhesive strength (18.6 kPa to pig skin), and excellent ionic conductivity (2.44 S/m). Simultaneously, the presence of PA confers hydrogels with good antibacterial, anti-freezing properties and long-term environmental stability, and the hydrogels could remain soft even at -40 °C. Moreover, the hydrogel-based sensors showed high sensitivity (gauge factor up to 9.72), wide sensing range (≈400%), good stability, and accuracy for physical and physiological signals (tensile strain, pressure, fluid types and concentrations). This work has not only provided a simple strategy for fabricating tough and recoverable zwitterionic hydrogels, but also demonstrated the multifunctional properties of the zwitterionic hydrogels, which possess a great potential to fulfill flexible device applications.

Development of Composite-film-based Flexible Energy Harvester using Lead-free BCTZ Piezoelectric Nanomaterials

Composite-based piezoelectric devices are extensively studied to develop sustainable power supply and selfpowered devices owing to their excellent mechanical durability and output performance. In this study, we design a leadfree piezoelectric nanocomposite utilizing (Ba<sub>0.85</sub>Ca<sub>0.15</sub>)(Ti<sub>0.9</sub>Zr<sub>0.1</sub>)O<sub>3</sub> (BCTZ) nanomaterials for realizing highly flexible energy harvesters. To improve the output performance of the devices, we incorporate porous BCTZ nanowires (NWs) into the nanoparticle (NP)-based piezoelectric nanocomposite. BCTZ NPs and NWs are synthesized through the solidstate reaction and sol-gel-based electrospinning, respectively; subsequently, they are dispersed inside a polyimide matrix. The output performance of the energy harvesters is measured using an optimized measurement system during repetitive mechanical deformation by varying the composition of the NPs and NWs. A nanocomposite-based energy harvester with 4:1 weight ratio generates the maximum open-circuit voltage and short-circuit current of 0.83 V and 0.28 A, respectively. In this study, self-powered devices are constructed with enhanced output performance by using piezoelectric energy harvesting for application in flexible and wearable devices.

Effect of Hot Pressing on the Optical, Thermal, and Dielectric Properties of Solution Cast Prepared Nanocomposite Films Based on a Poly(Vinylidene Fluoride-co-Hexafluoropropylene) and Poly(Ethylene Oxide) Blend Matrix with Dispersed Zinc Oxide Nanoparticles

In our research described herein, the hot pressing treatment at 90 °C under 2 tons of pressure on solution cast polymer nanocomposite (PNC) films based on a host matrix of poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and poly(ethylene oxide) (PEO) with varying polymer blend compositions (20, 50, and 80 wt% P(VDF-HFP) or PEO) and 2 wt% zinc oxide (ZnO) nanofiller, was performed; the effects of this treatment and the composition variations on the optical, thermal, and dielectric properties was explored. The ultraviolet-visible wavelength range study revealed slightly reduced absorbance and a little increased energy band gap after the hot pressing of the nanocomposite films. Differential scanning calorimetry of the solution cast-hot pressed (SC-HP) PNC films explained the reduced thermal stability of the poly(ethylene oxide) crystallites and also the degree of crystallinity. The broadband dielectric measurements of these materials, at 27 °C, confirmed significant changes in various dielectric polarization processes, demonstrating a large redistribution of constituent interfaces by the hot press treatment. Additionally, the 90 °C hot pressed PNC film of equal polymer compositional ratio was further hot pressed at 130 °C and 160 °C, step by step, and characterized to examine the variations in dielectric properties. The properties of these thermally treated composite films are discussed concerning their use for flexible device applications.

Manipulating electron linking units and side chains for high-performance all-polymer photodetectors with stability over 5000 h

All-polymer photovoltaic devices have attracted extensive attention for their promising applications in various flexible devices. Although abundant polymer donors with superior properties are developed, new polymer acceptors are urgently required to optimize or update the performances of the devices. Herein, three novel polymer acceptors, namely PY-DTT, PY-DTP-C, and PY-DTP-Ph, based on dithienothiophene (DTT) and dithienopyrrole (DTP) with alkyl and aryl side chains as the linking units respectively are synthesized and employed in the all-polymer photodetectors (all-PPDs). The PY-DTP-Ph shows broader absorption, stronger intermolecular interaction, and more coplanar molecular conformation compared with the other two acceptors. Blending with donor PM6, the PY-DTP-Ph based all-PPDs exhibit maximum responsivity of 292.1 mA W−1 and specific detectivity of 5.4 × 1013 cm Hz1/2 W−1 at a bias of −0.1 V. Furthermore, the device achieves fast response speed (τr/τf = 1.2/21.8 μs) and a large linear dynamic range of 105.0 dB. Intriguingly, the photodetectors present extraordinary air stability for 5000 h, attributing to the ideal miscibility and morphology of the blend films. These results demonstrate close relationships between the electron linking units, side chains and the basic properties of polymer acceptors, which in turn affects the performances of the photodetector devices.