Flexible Displays Research Articles

Guest Editorial: Materials & Devices for Advanced Flexible Sensors

The papers in this special section focus on materials and devices for flexible sensors. Presents recent advances in skin electronics, touch sensors for flexible display, near-infrared spectroscopy (NIRS) and organic electrochemical transistors (OECT), respectively. Three research article introduces new methods in flexible pressure sensing array, ammonia sensors and charge plasma junctionless tunnel field effect transistor (CP JLTFET), respectively.

Flexible micro-LED display and its application in Gbps multi-channel visible light communication

A flexible full-color micro-LED display with high mechanical robustness was fabricated by printing quantum dots (QDs) on a blue micro-LED array using standard photolithography. The red and green colors yielded from QDs exhibit a better color gamut than conventional color filters. The light conversion efficiency was enhanced by adding TiO2 nanoparticles to the QD-photoresist composite. This full-color micro-LED display was successfully mounted on various unusual substrates such as curved glass, fabrics, and human skin, enabling diverse optoelectronic applications. In addition, wireless multi-channel visible light communication (VLC) based on the wavelength-division-multiplexing orthogonal-frequency-division-multiplexing (WDM-OFDM) technique was demonstrated using a QD-based color micro-LED panel. A high data transmission rate of 1.9 Gbps was successfully obtained owing to the high electrical–optical modulation bandwidth of the QD-based micro-LED panel.

Flexible Displays with Multimode Reversible Switching between Transparent and Colorful States

AbstractIntegrating responsive photonic crystals (RPCs) and quantum dots (QDs) effectively to realize multimode dynamic color regulation is of great significance for promoting advances in color display science. Herein, a new type of flexible dynamic color display with multimode reversible switching between transparent and colorful states based on the combination of RPCs and CdTe QDs is designed. This unique system is fabricated by encapsulating a thin layer of ordered arrays of SiO2@CdTe@SiO2 (SQS) colloids with poly(dimethylsiloxane) (PDMS), which generates an elastic film with sandwiched structure of PDMS/SQS/PDMS. Owing to the similarity in the optical properties of PDMS and SQS, the display is highly transparent in the normal state. Moreover, the resultant flexible display exhibits obvious change in color by mechanical stimuli or UV exposure. Finally, the ability to display joints motion amplitudes of human body as well as the applications in advanced security is demonstrated.

Metal halide perovskite nanocrystal with fluorescence enhancement in wet and acidic environment

Metal halide perovskite nanocrystals have excellent prospects in solid-state lighting, flexible display, sensing, and other optoelectronic applications due to their superb photoelectric properties (high quantum efficiency and fast carrier diffusion speed). However, they are susceptible to environmental factors such as ultraviolet radiation and polar solvents. Especially in a water environment, poor stability seriously restricts the application of perovskite optical devices. In this paper, a kind of porous polymer (ethylene-vinyl acetate copolymer, EVA) and perovskite nanocrystals were mixed to form perovskite nanocrystals-EVA composites. The obtained composites were aged in water for 21 days, and the fluorescence enhancement was 170 times. Even in a strongly acidic environment, the fluorescence enhancement was 91 times. The information encryption mode of the composite material can effectively encrypt information in a wet environment due to the change of light wavelength, making a white light-emitting diode (w-LED). This metal halide perovskite nanocrystals-EVA composite will unleash a wide variety of solid-state lighting and flexible display possibilities that are limited by the use of existing methods in humid and strongly acidic environments.

From Kirigami to Hydrogels: A Tutorial on Designing Conformally Transformable Surfaces

Abstract Elastic surfaces that morph between multiple geometrical configurations are of significant engineering value, with applications ranging from the deployment of space-based photovoltaic arrays, the erection of temporary shelters, and the realization of flexible display systems, to understanding the encapsulation and release of viral RNAs. In general, ensuring that a shape with a planar rest configuration can deploy into a target three-dimensional (3D) shape is a nontrivial problem. Moreover, it is difficult to physically realize the local deformations necessary to achieve such global transformation. Here, we give a tutorial on applying conformal mapping to rationalize the geometrical deformation of several microstructure designs. A conformal map is a function that locally preserves angles and shapes but not lengths: some regions are scaled (enlarged or shrunk) more than others. To transform a planar surface to 3D, we implement uniform local scalings as mechanical deformations. Numerous natural and architected material systems exhibit such behavior, including kirigami, origami, hydrogel, linkage mechanisms, and fabric membranes. The design and fabrication of conformally transformable surfaces is a transdisciplinary challenge involving insights from advanced manufacturing, computational design, material science, and mechanics. By recognizing that many material systems exhibit isotropic deformation, we hope to inspire researchers to adopt conformal mapping in designing next-generation surface-based engineering systems.

Enhanced stretchability towards a flexible and wearable reflective display coating using chalcogenide phase change materials.

Reflective color switching coatings based on chalcogenide phase change material (PCM) is becoming one of the most promising display technologies in the future. However, it is still a challenge to kindly control the stress and enhance the stretchability for flexible display coatings. Here, we report crack-reduced reflective color coatings on a flexible substrate by using buckling structure to regulate the distribution of vacancies in PCM. It significantly suppresses the formation of cracks and improves the robustness of optical and electrical properties during stretching of the display device, which opens the doors of opportunity for phase change display applications in a wide range of flexible and wearable display fields.

Integrated Self-Capacitance On-cell Touch Panel in Flexible OLED Display

Integrated Self-Capacitance On-cell Touch Panel in Flexible OLED Display

Highly Conductive and Compliant Silver Nanowire Nanocomposites by Direct Spray Deposition

The silver nanowire (Ag NW)/elastomer nanocomposite represents a prototypical form of a compliant conductor for flexible and stretchable electronic devices. The widespread implementations are currently hindered by the complicated procedures to effectively disperse Ag NWs into elastomer matrices. In this study, we report a facile and scalable coating process to create Ag NW nanocomposites on various flexible/stretchable substrates. As-synthesized Ag NWs from the high-yield polyol-reduction approach are homogeneously dispersed into a variety of dilute elastomer solutions, thereby enabling direct spray deposition into highly compliant conductors. The as-prepared nanocomposite exhibits excellent conductivity (∼11,000 S/cm) and high deformability to 100% strain. The stable electrical properties are largely retained under repetitive mechanical manipulations including stretching, bending, and folding. The patterned features of conductive nanocomposites are conveniently accessed using shadow masks or selective laser ablation. The practical suitability is demonstrated by the successful implementations of a stretchable sensing patch and a flexible light-emitting diode display.

Elastic Fluorescent Protein-Based Down-Converting Optical Films for Flexible Display.

Protein-based material design provides great advantages to developing smart biomaterials with tunable structures and desired functions. They have been widely used in many biomedical applications including tissue engineering and drug delivery. However, protein-based materials are not yet widely used in optoelectronic materials despite their excellent optical and tunable mechanical properties. Here, we synthesized engineered fluorescent proteins (FPs) fused with elastic protein for the development of optoelectrical down-converting optical filters for flexible display materials. We synthesized sequence-specific FPs to tune blue, green, yellow, and red colors and fused them with elastic protein to tune mechanical properties. We fabricated flexible self-supporting film materials and characterized mechanical properties and down-converting optical properties. We also fabricated a hybrid light-emitting diode (LED) to down convert blue to desired green, red, and white colors. Furthermore, we constructed a flexible white LED using organic LED as a flexible substrate. Our modular synthesis approach of tunable bio-optoelectrical material approaches will be useful to design future biocompatible and flexible display materials and technologies.

Photochromic and mechanochromic cotton fabric for flexible rewritable media based on acrylate latex with spiropyran cross-linker

Rewritable materials play a significant role in reducing paper production and consumption, information storage and anticounterfeiting, etc. Rewritable textiles have attained much attention due to the flexibility, easy availability and functionality. How to develop a photochromic and mechanochromic dual-stimuli responsive fabric as a flexible rewritable media or display substrate is still rarely reported. In the present work, we present the rational design and synthesis of stimuli-responsive acrylate latex via emulsion polymerization with a spiropyran cross-linker. Spiropyran was chemically bonded into the polymer network of latex particles. Then cotton fabrics were spraying coated by the latex with different composition and add-on percentage. Writing without ink was carried out to investigate the photochromic and mechanochromic properties of the functionalized cotton fabrics. The functionalized cotton fabric still could change color under writing without ink after soaping for 50 times and light irradiation for 20 cycles. Our developed flexible cotton fabric can be used as a long-term, efficient, and stable rewritable media or flexible display substrate, even in the harsh environment such as washing.

Nanonet-/fiber-structured flexible ceramic membrane enabling dielectric energy storage

Ceramics are considered intrinsically brittle at macro scale due to the lack of slip mechanism and pre-existing defects, which greatly limits their potential applications in emerging fields including wearable electronic devices and flexible display. In this contribution, we developed BiFeO₃/SiO₂ dual-networks with exceptional flexibility through a coupled electronetting/electrospun method. The hybrid nanostructured networks endow the material with high tensile strength (2.7 MPa), excellent flexibility (80% recoverable deformation), and robust fatigue resistance performance (maintain flexibility after a 1000-cyclic compress test). After <i>in-situ</i> compounded with dielectric polymer via a layer-by-layer solution casting method, the resultant three-dimensional (3D) composite film exhibits a twice higher dielectric constant (<i>ε</i>_r) than polyether imide (PEI) film. More importantly, the breakdown strength of the 3D composite film is almost the same as that of the PEI film, resulting in an enhanced energy density of ~6.0 J/cm³ and a high efficiency of 80% at 4.58 MV/cm. The unique structure, combined with the excellent balance between mechanical and dielectric properties in flexible structures, is of critical significance to the design of flexible functional ceramics and broadening their applications in wearable electric devices.

Bilayer Metal Oxide Channel Thin Film Transistor With Flat Interface Based on Smooth Transparent Nanopaper Substrate

With the rapid development of electronic devices, a lot of electronic waste is generated yearly. They posed sereve environmental concerns. Integrating sustainable materials into electronic devices is one of promising solutions to achieve the balance between technological development and ecological protection. Cellulose nanopaper has emerged as a hot sustainable material for thin film transistors (TFT) in the flexible display. However, the performance of nanopaper TFT is generally poor. In this study, a transparent nanopaper with a root-mean-square surface roughness of 3.49 nm is prepared by spin coating cellulose nanofibrils on glass and applied as a substrate for metal oxide TFTs. Bilayer channel (IGZO/Al <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{2}}\text{O}_{{3}}$ </tex-math></inline-formula> ) TFTs with bottom gate structure are then deposited on transparent nanopaper by physical vapor deposition at room temperature. Consequently, the as-prepared nanopaper TFT not only exhibits a saturation mobility of as high as 21.98 cm2/(V·s), an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{\text {on}}/\text{I}_{\text {off}}$ </tex-math></inline-formula> ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${5.07}\times {10} ^{{6}}$ </tex-math></inline-formula> , and a subthreshold swing of 0.75 V/dec, but shows good bias stability. This work brings a step closer to green, sustainable, and inexpensive transparent paper electronics.

Facile fabrication of novel europium doped strontium yttrate (SrY2O4:Eu3+) electrospun nanofibers for flexible display applications

Novel europium doped strontium yttrate (SrY 2 O 4: Eu 3+ ) nanofibers have been firstly synthesized by electrospinning. SrY 2 O 4: Eu 3+ nanofibers have been systematically examined at different calcination temperatures. The TGA study reveals the thermal behaviour of SrY 2 O 4: Eu 3+ nanofibers which stabilises at 900⁰C. SrY 2 O 4: Eu 3+ nanofibers have an average diameter of 328 nm before calcination and 162 nm post calcination. X-ray diffraction (XRD) spectra of SrY 2 O 4: Eu 3+ nanofibers revealed the orthorhombic phase with Pnam space group. The ionic radii and charge compensation have significant effect on the XRD and photoluminescence (PL) spectra of SrY 2 O 4: Eu 3+ nanofibers. PL spectra was observed under 395 nm excitation and a strong emission line spotted at 612 nm is attributed to characteristic electric dipole transition 5 D 0 → 7 F 2 of Eu 3+ ion. Effect of dopant concentration on PL of host SrY 2 O 4: Eu 3+ nanofibers have also been explored. SrY 2 O 4 nanofibers doped with 10 mol% of Eu 3+ ions exhibit optimum emission intensity while concentration quenching is observed for higher dopant concentration. The SrY 2 O 4: Eu 3+ nanofibers show affirmative agreement with SrY 2 O 4: Eu 3+ bulk counterpart. The CIE coordinates for SrY 2 O 4: Eu 3+ nanofibers appear in red region with good colour purity. SrY 2 O 4: Eu 3+ nanofibers possess inherent luminescence property which makes it potential candidate for flexible display applications. • Fabrication of novel SrY 2 O 4: Eu 3+ nanofibers first time by electrospinning technique • SrY 2 O 4: Eu 3+ nanofibers manifest uniform continuity with an average diameter 162 nm • SrY 2 O 4: Eu 3+ nanofibers manifest remarkable emission in red region upon UV excitation • Ionic radii and charge compensation lead to substitution of Y 3+ ions by Eu 3+ ions • SrY 2 O 4: Eu 3+ nanofibers exhibit intrinsic luminescence property within fiber itself for flexible display application.

자극을 감지하는 센서와 신호를 시각화하는 디스플레이가 통합된 사용자 상호적 디스플레이

Wearable devices, which are currently widely used, are difficult to use because they are thick and hard. In order to solve this, a thin and flexible display is required. In addition, sensors which are combined with them respond to external stimuli, transmit signals to the device and display them. In this way, wearable devices are in a direction in which they interact in both ways rather than in a single way. Furthermore, instead of processing stimuli as signals and showing them on the display, devices that can react and represent changes in external stimuli immediately are being developed. As a step of process in information processing is reduced, the response time can be reduced and the device may be miniaturized and lightweight. In this paper, we will look at the display that responds to various types of external stimuli. Furthermore, we will introduce how a display can be utilized.

Polymerizable Surfactant Ligand for Stabilization and Film Formation of CsPbBr3 Nanocrystals.

Surfactant ligands are important in the synthesis of inorganic perovskite nanocrystals (NCs), not only for stabilizing NCs but also for surface defect passivation. A new polymerizable surfactant ligand with a multidentate l-cysteine head, a long oleoyl tail, and a polymerizable styrenyl group (NOSVC) is designed for the post-synthesis treatment and stabilization of colloidal CsPbBr3 NCs in this work. 1H nuclear magnetic resonance and X-ray photoelectron spectroscopy analysis show that the l-cysteine head has strong interactions with the NCs. The absolute photoluminescence quantum yields of the colloidal NCs are increased from 45.1% of the pristine NCs stabilized with oleic acid/oleyl amine to 91.8% after NOSVC treatment. NOSVC-stabilized CsPbBr3 colloidal NCs show enhanced stabilities when exposed in polar solvents. The NOSVC-stabilized CsPbBr3 NCs in a solid film state allow for a photopolymerization to be carried out with the assistance of a photoinitiator. The polymerized films of NOSVC-treated NCs exhibit significantly enhanced stability against thermal radiation, ultraviolet irradiation, and humidity. We also fabricated self-healing polymer films incorporating NOSVC-treated CsPbBr3 NCs as a green filter for a white light-emitting diode device. The green light-emitting films are very stable in humid environments, revealing the great application potential of NOSVC-treated CsPbBr3 NCs in flexible display and lighting devices.

Porous hydrogel scaffolds integrating Prussian Blue nanoparticles: A versatile strategy for electrochemical (bio)sensing

Hydrogels have emerged as promising porous soft materials from which to develop a wide range of biosensing platforms due to the possibility to easily engineer their properties and to effectively immobilize enzymes and nanomaterials in their matrix. Despite their attractive properties, the ability to form stable hydrogel films integrating well–dispersed catalytic nanomaterials and enzymes on electrode surfaces is still required to enable their implementation into electrochemical biosensors. Here, we report a facile approach to prepare hydrogel films embedding Prussian Blue nanoparticles (PBNPs) and different enzymes on electrode surfaces for electrochemical biosensing. A one-pot strategy was employed for the synthesis, under mild conditions, of PBNPs and the simultaneous immobilization of enzymes in a highly functionalized carboxymethyl cellulose matrix, yielding homogeneous hydrogel composites. Highly porous hydrogel films were successfully prepared by drop casting the hydrogel composites on the surface of flexible screen–printed electrodes (SPEs). Due to the high loading of well–dispersed PBNPs, the hydrogel films demonstrated good electrocatalytic activity for both the oxidation of NADH and the reduction of hydrogen peroxide resulting in high detection sensitivity. Two amperometric biosensors for the rapid detection of glucose and ethanol in serum were realized by employing hydrogel composites integrating glucose oxidase and alcohol dehydrogenase. By combining the unique features of hydrogels on flexible electrochemical strips, our approach holds great promise for the development of portable electrochemical (bio)sensors, which are easy to fabricate and versatile for the detection of a variety of analytes.

Low-temperature-processable amorphous-oxide-semiconductor-based phosphors for durable light-emitting diodes

In this study, we fabricated light-emitting diodes (LEDs) on glass substrates at a maximum process temperature of 200 °C using amorphous oxide semiconductor (AOS) materials as emission layers. Amorphous gallium oxide films doped with rare-earth elements (Eu, Pr, and Tb) were employed as AOS emission layers, and the LEDs emitted clear red, green, and pink luminescence upon direct-current application even in the ambient environment. Resonance photoelectron spectroscopy revealed the difference in the electronic structure of the films for each rare-earth dopant, suggesting different emission mechanisms, viz., electron–hole recombination and impact excitation. Although it is widely believed that amorphous materials are unsuitable for use as emission layers of LEDs because of their high concentrations of mid-gap states and defects, the developed rare-earth-doped AOS materials show good performance as emission layers. This study provides opportunities for the advancement of flexible display technologies operating in harsh environments.

Large-area flexible scintillator screen based on copper-based halides for sensitive and stable X-ray imaging

X-ray scintillators, which convert high-energy radiation into visible light signals, have been rapidly developed for their widespread use in medical, security, and commercial diagnostic technologies. However, the development of large-area flexible scintillator screens with excellent stability and scintillation performance for non-planar objects is relatively lagging behind. Here, we fabricated large-area flexible scintillator screens with excellent stability and scintillation properties by loading nanoscale Cs3Cu2I5 particles into an elastic polydimethylsiloxane (PDMS) matrix. Copper halide Cs3Cu2I5, with self-trapped exciton emission, exhibits intense blue emission with a large Stokes shift, which can effectively reduce the light self-absorption effect. The experimental results reveal that the PDMS matrix can effectively passivate the surface defects of Cs3Cu2I5 particles, improve the exciton binding energy and ensure effective exciton recombination, thereby achieving more efficient optical coupling efficiency. In addition, the Cs3Cu2I5@PDMS flexible film exhibits excellent thermal stability, photostability, and water stability, and the radiative luminescence intensity does not decay after a continuous irradiation for 100 min under high-dose X-rays. Large-area flexible scintillator screen with uniform luminescence achieves a high spatial resolution of 8.6 lp/mm and a high light yield of 46000 photons/MeV, and exhibits excellent linear response to X-ray dose rate and low detection limit of 96.54 nGyair/s. Moreover, the large-area flexible scintillator screen exhibits high-quality X-ray imaging for both planar and non-planar objects. Our results demonstrate that the stable and sensitive Cs3Cu2I5@PDMS scintillator screen may have promising application prospects in flexible X-ray imaging fields.

Decorated pyridine as hole transporting material (HTM) for solution-processed OLEDs

While organic light emitting diodes (OLEDs) technology is evolving at a fast pace, the development of flexible and full-color display panels with low power consumption provides superiority to these devices as eco-friendly lighting sources. Several strategies and methods have been attempted to improve the OLED device performance, especially in light-emitting materials. Even though, with significant advancements in efficiency, OLED technology demonstrates the feasibility to enhance further without saturation, thus attracting researchers to develop new materials with an effective device strategy. In this work, we report an integrated hole transporting material, 3,6-bis(4,6-diphenylpyridin-2-yl)-9-ethyl-9H-carbazole (CDP) with functionalization of carbazole unit with pyridine substituents. CDP adopted a moderately high triplet energy (2.61 eV), high thermal stability, and high hole transportability. Solution-processed CDP-based OLEDs were fabricated containing phosphorescent green emitter (Ir(PPy)3) and compared with conventional N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) based OLEDs. The CDP-based device exhibited a high performance with decreased layer thickness from ∼25 nm to ∼5 nm, with maximum external quantum efficiency, power efficiency, current efficiency, and brightness of 17.2 %, 66.8 lm/W, 64.0 cd/A, and 18,740 cd/m2, respectively which was superior to the NPB-based control device. Enhancement in device performance with reduced thickness of hole transport layer in CDP-based OLEDs distinctly demonstrates the effective potential of the designed material.

Highly Reliable Dielectric Interlayers for Flexible Displays and E‐paper

AbstractFlexible, foldable, and rollable displays aim to serve emerging user's needs.