Flexible Display Devices Research Articles

Analysis of fracture mechanisms of ultrathin glass for flexible cover window under pen drop conditions

AbstractThe fracture mechanism of a chemically strengthened ultrathin glass (UTG) for the cover window of flexible display devices was investigated under pen loading conditions for the first time to better understand the UTG impact resistance characteristics. High‐speed camera analysis, fracture fragment analysis, and finite element modeling were employed to investigate the fracture mechanism of the UTG under pen drop conditions. A pen‐on‐plate (POP) test was also employed to examine the fracture characteristics under static loading condition, and its results were compared with those of the pen drop test (PDT) to find a correlation between the static and dynamic loading conditions. The correlation found between the POP and PDT tests indicates that a quantitative estimation of contact displacement under PDT can be obtained by measuring the first ring crack size, to more easily determine the impact resistance characteristics of UTG.

74‐2: Invited Paper: Highly Reliable Dielectric Interlayers for Flexible Displays and e‐Paper

With the strong increase of mobile data‐communication needs, mobile devices become more important than ever. To be able to provide the users the best experience, display sizes will need to increase while portability needs to be maintained. Flexible, foldable and rollable displays are serving these emerging user needs. This challenges every layer in the display, requiring the display panel to become thinner and thinner while maintaining robustness and reliability. This paper will talk about highly reliable photo patternable interlayers in TFT stacks and touch sensor layers based on inorganic‐organic hybrid polymers. A novel solution, that improves process‐ability for flexible display devices. In addition, hybrid polymer‐based films provide good process durability, higher transparency, and higher environmental durability. Therefore, these new formulations are a key enabler for foldable and rollable OLED and e‐paper displays.

TiO2 doped polymer dispersed and stabilised liquid crystal smart film with high contrast ratio, low driving voltage and short response time

ABSTRACT Due to the switchability between transparent and light-scattering states, polymer-dispersed liquid crystals (PDLCs) are widely used as smart windows, flexible display devices, projectors and other devices. In this work, we propose a new composite system containing TiO2 nanoparticles (NPs), liquid crystals and polymer that have low driving voltage, high contrast ratio (CR) and quick response ability. Distinguishing from the traditional PDLC system, the coexisting structure of vertically oriented polymer fibres (VOPFs) inside the porous polymer matrix was constructed and the TiO2 nanoparticle (NP) after functionalisation was introduced. It was found that the introduction of an appropriate amount of functionalised TiO2 NPs can effectively improve the electro-optical properties of the composite system. This new system provides a new idea for solving the compatibility problem of inorganic particles and organic matter.

Bendable optical diffuser constructed by interfacial hydrogen-bonding polymer complexation

Bendable optical diffuser is an indispensable photon management element in various flexible display devices. Interfacial hydrogen bonding complexation of poly(vinylpyrrolidone) (PVPON) and polymer complex nanoparticles (PCNP) can produce an effective optical diffuser layer on rigid quartz or glass substrate. However, the layer deposited on the flexible poly(ethylene terephthalate) (PET) substrate will rupture during the drying and dehydration process. Here, we propose a strategy to toughen the PVPON/PCNP layer, preventing its break on PET substrate when hygroscopic condition changed. The resulting layer maintains the high performance of optical diffusion. More than 10,000 times bending fatigue testing confirms that the toughened optical diffusion layer has better bending resistance than the commercially available PET/PMMA diffuser used in liquid crystal display (LCD).

Emerging near-unity internal quantum efficiency and color purity from red-emitting phosphors for warm white LED with enhanced color rendition

A robust and bright red emitter is recognized as a key enabler for high-quality warm white light-emitting diode (w-LED). Herein, triple molybdate scheelite-type NaCaLa1-x(MoO4)3:xEu3+ (NCLM:xEu3+) phosphors derived from NaCaLa(MoO4)3 host by the equivalent-valent substitution strategy are successfully synthesized and systematically studied. Impressively, the NCLM:Eu3+ phosphors (λem = 615 nm) can be effectively stimulated by 395 and 466 nm light, exhibiting an ultra-high color purity and quantum efficiency close to 100%. The structure-related emission characteristics were analyzed based on the Judd-Oflet theory. The packaged white LED device used tri-color phosphors displays a superior color rendering index of 86.0 and low correlated color temperature of 2797 K. Moreover, the NCLM-based polydimethylsiloxane film maintains an impressive luminescent performance when it is folded or bent, demonstrating the potential applications in the field of flexible display and other light-emitting devices.

Strain mediated light emission using heterojunctions of all-inorganic mixed-halide perovskite nanocrystals via piezo-phototronic effect

Utilizing the superior luminescence properties and high color purity of inorganic perovskite nanocrystals (NCs), we report the fabrication of color-saturated CsPbBr 3−x I x (x = 0–3)/ZnO heterojunctions based white light emitting diodes (LEDs) on a flexible platform. Here, bandgap engineered mixed halide perovskite NCs have been prepared by solution phase synthesis technique that was spin coated on sputtered ZnO film, acting as an electron injector layer as well as an efficient luminescent material. Broad (~ 350–700 nm) electroluminescence (EL) emission features covering the near UV and entire visible region of the electromagnetic spectrum have been successfully achieved at an ambient (humidity ~ 50–55%) condition with comparatively lower forward bias (V th ~ 2 V). Finally, we demonstrate the significant enhancement (up to ~ 75%) of EL intensity by applying an external stress owing to the piezoelectric properties of ZnO films. The modulation of energy band alignment in CsPbBr 3 /ZnO heterojunction due to the local strain induced positive piezo-potential in ZnO leads to an enhanced radiative recombination efficiency in CsPbBr 3 domain, resulting in superior EL emission characteristics. The combinatorial fascinating optical properties of inorganic perovskites and piezo-phototronic effect of ZnO provide a promising platform to fabricate highly luminescent LEDs that may find perspective applications in high-resolution flexible display devices and wearable electronics. • Band gap engineered mixed halide inorganic perovskite NCs (CsPbBr 3−x I x (x = 0–3)) by solution phase synthesis technique. • Perovskite/ZnO p-n junction based moisture resistant broad white LEDs under fairly lower forward bias of ~ 2V. • Successful tuning of EL emission by increasing the I - /Br - ratio of mixed halide perovskites through anion exchange reaction. • Enhancement of EL intensity up to ~ 75% by applying an external stress owing to the piezoelectric properties of ZnO films.

7: Active mura illuminating method based on area array light‐emitting devices

In non‐contact optical defect detection, we often encounter a problem that the reflection intensities from different positions in the detected target areas exceed the dynamic range of the camera response. To solve this problem, this paper focuses on the application of flexible display devices such as OLED, Mini LED and Micro LED as lighting sources. The intensity distribution of the reflected light of the detected product in the field of view of the camera is used as a feedback matrix to compensate the intensity distribution of the area array light source, which reduces the light intensity of high reflection area and improve the light intensity of low reflection area. As a result, one can avoid the image saturation of high reflection area and improve the image brightness of low reflection area. The defects in both high and low reflection area can be displayed clearly at the same time.

Novel electrochromic materials based on chalcogenoviologens for smart windows, E-price tag and flexible display with improved reversibility and stability

A series of electrochromic composites consisting of chalcogenoviologen-based ionic liquids ([C 6 EVC 6 ][TFSI] 2, E = S, Se, Te) and ferrocene-based ionic liquids ([FcC 11 ImC 1 ][TFSI]) is reported. Electrochromic devices (ECDs) filled with these composites were assembled, demonstrating multiple colors under different voltages, fast switching times, high color contrast ratio, good reversibility and cycle stability. ECDs working as smart windows on a module house could slow down the room temperature rising in hot weather through changing color. In addition, the eye-catching E-price tag and flexible display devices which can show information dynamically, were assembled successfully. This contribution provided a new kind of electrochromic composites-based smart device for intelligent life.

Hardness and Abrasion Resistance Characteristics of Poly(ethylene terephthalate) Films without and with Hard and Adhesive Coatings

In this study, we reports the first quantification details of the pencil hardness, eraser abrasion resistance and steel wool abrasion resistance of optically-transparent and biaxially-oriented poly(ethylene terephthalate) (PET) films in various thicknesses before and after hard coating (HC) and/or optically clear adhesive (OCA) coating. Surprisingly, the PET base films were determined to exhibit unrealistically very low hardness and abrasion resistances although their stress-strain behavior followed a ductile and tough plastic nature as a high performance engineering plastic: the pencil hardness was only ≤ 9B even at a low loading force, 0.100 kgf, and the onset loading force to cause scratches was far below 0.013 kgf in both eraser and steel wool abrasions. These collectively confirmed that, without surface protections, PET base films are not suitable for any advanced applications. Such the low hardness and poor abrasion resistances could be improved substantially by only thin HC coating of di(trimethylolpropane) tetraacrylate (HC-a, an ultraviolet curable acrylic resin). In addition, the OCA coating based on an acrylic copolymer adhesive provided adhesion function to the PET films, showing excellent adhesion strength onto chemically-toughened glass. However, the OCA layer causes cushioning effect because of its easy plastic deformation nature, making negative impact on the hardness and abrasion resistance performances of the PET film. Thus, the OCA coating should be thinned until retaining good adhesion to adherends. Overall, PET films coated with HC and/or OCA are suitable for advanced applications in various fields including smart phones, foldable phones, and flexible display devices.

Robust Nanofiber Films Prepared by Electro‐Microfluidic Spinning for Flexible Highly Stable Quantum‐Dot Displays

AbstractQuantum dot (QD)‐based liquid crystal displays (LCDs) are emerging as a new generation of LCDs due to their good performance. However, the QD fluorescent materials in LCDs are vulnerable to water and high temperatures, severely limiting their practical and long‐term use. Here, flexible and ultrastable QD‐based color‐converting films for LCD backlights are fabricated using robust poly(styrene‐methyl‐methacrylate‐acrylic acid) (poly(St‐MMA‐AA)) nanoparticle/polyamide 66 nanofiber (NPs@PA66) film with unique fiber–particle–fiber microstructure as protective substrate. Through an emerging strategy called electro‐microfluidic spinning technology (EMST), the nanofiber film not only exhibits excellent flexibility but also remarkably improves the mechanical property via the in situ particle‐mediated enhancement mechanism. An LCD backlight using the NPs@PA66 nanofiber film as QD loading substrate shows a wide color gamut of 116% and long‐term fluorescence stability under high temperature of 200 °C. More importantly, the fluorescence lifetime of NPs@PA66/QDs backlight reaches up to ≈64500 h, ≈22 times higher than that using encapsulated sandwiched polyethylene terephthalate (PET) QD film. These findings offer a promising method toward high‐strength nanofiber manufacturing, high‐stability flexible electronics and optoelectronic display devices.

Paper and pseudo-paper based flexible membrane devices

<p indent=0mm>With unique advantages of thin, porous, flexible, flammable and affordable, paper has been widely used in our daily life. Early in the 19th century, paper was first applied in biochemical analysis. Since then, paper had been used as a unique analytical platform. Paper-based analysis devices are easy to use, cheap, portable and disposable, thus they are widely considered as platform for POCT (point-of-care testing) and on-site detection. Liquid can automatically transport in paper cellulose networks without external pumps or energy, thus do not require extra instruments. The large specific surface area of fiber networks are ideal places for storing testing bio-chemical substances. Paper is also flexible and easy to fold together to form multilayer substrates of 3D chips, make it suitable for 3D cell culture and building multifunctional microfluidic chips. Meanwhile, paper-based analysis devices also have restrictions as chaos porous structure of the fibers interferes the optical signals and nonspecific adsorbs the testing samples, leading a low sensitivity of detection based on colorimetric. Due to the roughness of the fibers, it is still difficult to process the paper with high precision. In order to solve this, new types of paper (pseudo-paper) have emerged. In addition to biochemical analysis, paper also shows great prospect in the field of microelectronics. Currently, paper-based materials have been widely used in fields of flexible electronic devices, for example electronic circuits and energy storage devices. Paper exhibits piezoelectric behavior and is porous, degradable, make it promising materials in fabricating flexible electronics, display, soft robots and energy storage devices. Recently, enormous paper-based and pseudo-paper devices have been reported by researchers in various fields. These devices focus on the construction of new cost-effective microfluidics and electronics. The combination of paper-based microfluidic devices and electronic devices promotes the development of artificial skin, soft robots and flexible electronics. Considerable breakthroughs of new highly integrated paper-based chips have been achieved. In order to summarize the modern developing of paper and pseudo-paper devices and inspire scientists in various research fields, we present here an overall review that focuses on the historical developments to recent advances and future prospects of paper-based and pseudo-paper devices. These aspects include the preparation methods, the manipulation of microfluidics and electrons and multiple applications based on such manipulation. We hope that this review will promote the development of paper devices in various research fields such as analytical chemistry, materials science and electronic science.

Transparent Pressure Sensor with High Linearity over a Wide Pressure Range for 3D Touch Screen Applications.

The demand for display technology is expected to increase with the continuous spread of portable electronics and with the expected emergence of flexible, wearable, and transparent display devices. A touch screen is a critical component in display technology that enables user interface operations, and the future generation of touch screens, the so-called 3D touch screens, is expected to be able to detect multiple levels of pressure. To enable 3D touch screens, transparent pressure sensors with high linearity over a working range that encompasses the pressure range of human touch (10-100 kPa) are required. In this work, a transparent and linear capacitive pressure sensor is reported with a transmittance over 85% and high linearity (R2 = 0.995) over 5-100 kPa of pressure. To render the sensor transparent, a microstructured "hard" elastomer layer was filled in with a refractive index matching a "soft" elastomer layer, through which light scattering was minimized. High linearity was attained from the sensor's unique architecture that increases the effective area of the capacitor with applied pressure. These attributes render our sensor highly suitable for future 3D touch screen applications.

Hysteresis-Free, High-Performance Polymer-Dielectric Organic Field-Effect Transistors Enabled by Supercritical Fluid.

Organic field-effect transistors (OFETs) are of the core units in organic electronic circuits, and the performance of OFETs replies critically on the properties of their dielectric layers. Owing to the intrinsic flexibility and natural compatibility with other organic components, organic polymers, such as poly(vinyl alcohol) (PVA), have emerged as highly interesting dielectric materials for OFETs. However, unsatisfactory issues, such as hysteresis, high subthreshold swing, and low effective carrier mobility, still considerably limit the practical applications of the polymer-dielectric OFETs for high-speed, low-voltage flexible organic circuits. This work develops a new approach of using supercritical CO2 fluid (SCCO2) treatment on PVA dielectrics to achieve remarkably high-performance polymer-dielectric OFETs. The SCCO2 treatment is able to completely eliminate the hysteresis in the transfer characteristics of OFETs, and it can also significantly reduce the device subthreshold slope to 0.25 V/dec and enhance the saturation regime carrier mobility to 30.2 cm2 V−1 s−1, of which both the numbers are remarkable for flexible polymer-dielectric OFETs. It is further demonstrated that, coupling with an organic light-emitting diode (OLED), the SCCO2-treated OFET is able to function very well under fast switching speed, which indicates that an excellent switching behavior of polymer-dielectric OFETs can be enabled by this SCCO2 approach. Considering the broad and essential applications of OFETs, we envision that this SCCO2 technology will have a very broad spectrum of applications for organic electronics, especially for high refresh rate and low-voltage flexible display devices.

Enhanced Electrical Properties of Platinum Spotted AZO/PET Thin Films Prepared by Sol–Gel Process

Recently, many researches on Al-doped ZnO (AZO) thin film based transparent conducting oxide (TCO) have been intensively investigated for the electronic and display device applications. In this study, AZO thin films with different thicknesses were deposited on polyethylene terephthalate (PET) substrates by sol-gel spin coating at a relatively low temperature. By optimizing the AZO thickness, maximum figure of merit (FOM) values were investigated and discussed. Commonly, PET substrates are used in the fabrication of flexible display devices. However, because of the low melting temperature of the PET substrate (~200 °C), AZO thin films spin-coated on PET substrates cannot be subjected to crystallization at high temperatures. Therefore, alternative advanced optical annealing method was considered to crystalize the AZO thin films on the PET substrates. In this experiment, optical annealing method will be proposed. To increase electrical conductivity, Platinum (Pt) dots were sprayed on the AZO sample to improve the electric conductivity. The Pt-spotted AZO thin films on flexible PET substrates prepared by sputtering exhibited high electrical conductivities and high optical transmittances. The 0.63 nm-thick Pt/AZO/PET film exhibited a transmittance of 80% in the 380-800 nm range and the 3.78 nm-thick Pt/AZO/PET film exhibited a resistivity of 5.61×10-4 Ώ·m. Notably, the 159 nm-thick Pt/AZO/PET film exhibited an FOM of 156. Moreover, the sheet resistances and transmittances of the prepared AZO/PET films were determined.

White-light-emitting flexible display devices based on double network hydrogels crosslinked by YAG:Ce phosphors

Stretchable and tough double network hydrogels with YAG:Ce phosphors as crosslinkers are synthesized for flexible white-light-emitting display devices.

In situ tensile fracturing of multilayer graphene nanosheets for their in-plane mechanical properties

The excellent mechanical properties of single- and few-layer graphene have been well-quantified and evidenced by computational methods and local indentation measurements. However, there are less experimental reports on the in-plane mechanical properties of multilayer graphene sheets, despite many practical applications in flexible electronic and energy devices (e.g. graphene flexible electronic display, battery, and storage devices) are actually based on these thicker nanosheets. Here, in-plane fracture behaviors of multilayer graphene nanosheets with thicknesses between ∼10 and 300 nm (∼10–1000 layers) are characterized and quantified by in situ scanning electron microscopy and transmission electron microscopy under tensile loading. We found that, generally, the fracture strengths of graphene nanosheets decrease as the thickness (or layers) increases; however, the fracture strain of thinner graphene sheets is less than that of thicker sheets. The fracture process of the thicker nanosheets includes the initial flattened stage, the stable elastic stage, and the rapid fracture with brittle characteristics, while the thinner nanosheets show obvious delamination between the atomic layers at fracture. This work provides critical experimental insights into the tensile fracture behavior of multilayer two-dimensional materials and a better understanding on their realistic mechanical performance for potential flexible device and composite applications.

28.4: Invited Paper: A Study on Stiffness Measurement Used for Flexible Display Devices

Flexible display technology has a great market prospect due to its light‐weight, flexibility, and robustness. However, how to characterize the mechanical deformation of flexible display devices has not been defined internationally. In this regard, we proposed that the bending stiffness can be used to characterize the mechanical deformation by testing flexible display devices with different structures at different bending angles in the international conference in May 2018. The experimental results indicated that the method for the bending stiffness measurement is well‐suitable for flexible display devices. We believe that the development of this kind of testing method would help establish the comprehensive characterizations of mechanical properties of flexible display devices and lay out the framework for the international standards.

Stretchable fabric-based LiCoO[formula omitted], electrode for lithium ion batteries

There is a strong need for stretchable energy storage devices that can accommodate applications such as smart textiles, wearable sensors, flexible display devices, and biomedical implants. Here, we report the development and characterization of a stretchable fabric-based LiCoO2 electrode for lithium ion batteries. The performance of the fabric-based electrode is evaluated in a battery based on solid polymer nanocomposite electrolyte. The battery exhibits a capacity of 0.14 mAh cm−2 and a good cycling stability over 100 charge/discharge cycles. The mechanical properties of the stretchable fabric-based electrode are also evaluated including the tensile strength and % elongation. The electrical conductivity of the silver fabric-based LiCoO2 electrode is found to be higher than that of the rigid Al foil-based LiCoO2 electrode, and it is observed to increase during stretching. The stretchable fabric-based LiCoO2 cathode is a viable and promising candidate for the eventual integration in stretchable smart textile applications.

Energy transfer mechanism and tunable emissions from K3La(VO4)2:Dy3+/Eu3+ phosphors and soft-PDMS-based composite films for multifunctional applications

Novel Dy3+ and Eu3+ ions co-doped K3La(VO4)2 (KLVO) phosphors were synthesized by a facile citric acid-assisted sol-gel method. The crystal phases of the synthesized phosphor samples were identified by X-ray diffraction pattern after calcination process, which indicated the monoclinic phase patterns with a space group of P21/m(11). The luminescent behaviors of the self-activated KLVO and KLVO:Dy3+ and KLVO:Dy3+/Eu3+ phosphors were studied by the recorded photoluminescence excitation and emission spectra at room temperature. Under ultraviolet (UV) or near-UV excitations, the KLVO:Dy3+ phosphors exhibited characteristic emission properties. More importantly, different low concentrations of Eu3+ ions were co-doped into the optimized KLVO:Dy3+ phosphor to compensate its red component and the color-tunable emissions were noticed with an energy migration mechanism from Dy3+ to Eu3+ ions. Likewise, the thermal properties for the optimized co-doped material system were investigated, showing superior results for the practical application point of view. Furthermore, soft polydimethylsiloxane (s-PDMS)-based films were made for the first time utilizing the KLVO:5 mol%Dy3+/yEu3+(y = 0, 0.25, 0.5, 0.75, 1, 2 and 3 mol%) phosphors. Consequently, the obtained s-PDMS-based phosphor composite films demonstrated their suitability for the potential application of luminescent films, solid-state UV lightning devices and flexible display devices in optoelectronic industries.

App usage prediction for dual display device via two-phase sequence modeling

There have been many recent studies dealing with issues regarding app usage prediction. A number of smartphone manufacturers have recently released flexible dual display devices for multitasking purposes. In spite of many studies achieving higher levels of prediction accuracy, owing to certain practical limitations, they have not been effectively applied to real dual display devices. First, they trained their prediction model using only existing data collected from a single display device. Thus, they are unable to reflect the multitasking usage of dual display devices in their model prediction. In addition, previous studies have mostly been based on a Markov assumption, which needs to calculate the transition probability for all pairs of apps, which is difficult to calculate for newly installed apps. To address these limitations, we initially trained our prediction model using data collected from a single display during the first phase, and re-trained the prediction model using data collected from the prototype of a dual display device, which we developed along with LG Electronics, during the second phase. In addition, we applied an app prediction user interface to the prototype. Next, we applied a stacked long–short term memory architecture, a type of sequential deep learning architecture, to train the prediction model without calculating the transition probability. The experiment results show that our proposed method achieves an effective app usage prediction for dual display devices in a qualitative way, and outperforms all other approaches in quantitative experiments with top-3 and top-5 level accuracies. The reason for the higher performance of the proposed method is that it captures the real usage of a dual display and reflects multitasking use in its app usage prediction. In addition, we also verified the effects of a few variables such as the location, time, and two-phase training.