Flexible Displays Research Articles

P‐13.2: Bending Performance Analysis of Flexible OLED Screens

OLEDs have become a promising candidate for flexible displays. It is obvious that the requirements on the mechanical property of a flexible OLED screen are higher than common OLED displays. However, with a stacked structure and a complicated property of optically clear adhesive (OCA), it is very difficult to theoretically compute or experimentally measure the stress and strain distributions on each layer during the bending process. The paper employs finite element analysis to evaluate the bending property of a flexible OLED screen. For common U‐shaped bending, the maximum stress increases rapidly as the bending radius decreases. For small bending radius, the water‐drop‐shaped mode presents higher structure reliability than U‐shaped bending.

P‐6.4: Measurement for optical performance of flexible display based on imaging spectrometer

With the development of display technology, several new displays have attracted much attention such as OLED display, QLED display, Laser display and Mini/MicroLED display. Standard measurement methods for optical performance of flat display have been established. However, it's not easy for traditional luminance meter or colorimeter to deal with flexible display evaluation because of its viewing angle dependence which results in spectrum shift and color change. In this paper, we simulate color measurement accuracy by colorimeter and compare the results with spectrum measurement device. We analyze the influence of display's viewing angle and spectrum on the measurement accuracy. The measurement accuracy improves obviously with spectrum measurement device under different condition. In addition, we carry out spectrum and color measurement based on our imaging spectrometer which can measure multiple viewing angle at the same time. Compared with traditional spectrometer, the measurement efficiency is improved significantly without adjustment setting. The imaging spectrometer can be used for multiple viewing angle measurement and is suitable for optical performance measurement of flexible display.

P‐13.4: Creep Analysis of Flexible AMOLED Screen During Static Bending

In daily life, at different temperatures, flexible AMOLED screens are prone to devise damage and adhesive layer peeling during bending. The mainstream way to solve this problem is to simulate the stress of the display layer and the strain of the cover film at the optimal temperature of the flexible screen bending. In this paper, the bending simulation model of the AMOLED screen was established, and the film layers at different temperatures were analyzed. After studying the creep of different coatings, the creep behavior of the cover film at the optimum temperature was analyzed to fit the viscoelastic constitutive parameters. The results show that the slope of the stress‐strain curve of the cover film increases significantly at 20 ℃, and the probability of mesh deformation increases by 8% compared with 60 ℃. In addition, the creep strain of each film layer of the AMOLED module at room temperature was compared. It shows that the creep strain of the OCA layer colloid is larger than that of other film layers and the creep time of the cover present a deep influence on the position of the stress neutral‐layer of the display layer.

P‐13.6: The investigation on high‐temperature color deviation of flexible AMOLED display

As a new type of display technology, organic light‐emitting diode display technology (OLED) is widely used in vehicle, tablet, industrial control medical, mobile phone, wearable and other fields, and it is described as the future mainstream display technology after the liquid crystal display technology. However, when the AMOLED display is working for a long time, the display screen will heat up, the temperature of the screen will rise. And as the temperature rises, the performance of the TFT and OLED devices will be affected. The investigation on high temperature color deviation is studied in detail, we compared and analyzed the change of CIE‐x and CIE‐y color point, I‐V characteristic curve, efficiency and recombination center at different temperatures. The results show that the recombination center of OLED device changes with the temperature increasing, leads to the changes of the efficiency. The difference of red, green and blue efficiency leads to the color deviation, and the green pixels is the most sensitive to the change of current due to its highest efficiency, which is easier to show greenish phenomenon.

16.3: A Method to Improve The Reliability of T/G IGZO AMOLED Flex Panel

For large OLEDs, the amorphous InGaZnO (a‐IGZO) thin film transistor is considered as the next generation of flexible backplane technology due to its high mobility, good uniformity, low off‐state current and low process temperature. However, the reliability of a‐IGZO TFT with OLED flexible encapsulation material is inferior at high temperature, which is speculated that the downward diffusion of H in flexible encapsulation materials leads to the threshold voltage drifting to negative. We successfully prepared a self‐aligned top‐gate IGZO thin film transistor backplane for flexible AMOLED display with a layer of metal oxide (MOX) that can effectively block H. The backplane has excellent electrical performance and reliability, which can meet the requirements of flexible display devices.

I–III–VI Quantum Dots and Derivatives: Design, Synthesis, and Properties for Light-Emitting Diodes

Quantum dots (QDs) are important frontier luminescent materials for future technology in flexible ultrahigh-definition display, optical information internet, and bioimaging due to their outstanding luminescence efficiency and high color purity. I-III-VI QDs and derivatives demonstrate characteristics of composition-dependent band gap, full visible light coverage, high efficiency, excellent stability, and nontoxicity, and hence are expected to be ideal candidates for environmentally friendly materials replacing traditional Cd and Pb-based QDs. In particular, their compositional flexibility is highly conducive to precise control energy band structure and microstructure. Furthermore, the quantum dot light-emitting diodes (QLEDs) exhibits superior prospects in monochrome display and white illumination. This review summarizes the recent progress of I-III-VI QDs and their application in LEDs. First, the luminescence mechanism is illustrated based on their electronic-band structural characteristics. Second, focusing on the latest progress of I-III-VI QDs, the preparation mechanism, and the regulation of photophysical properties, the corresponding application progress particularly in light-emitting diodes is summarized as well. Finally, we provide perspectives on the overall current status and challenges propose performance improvement strategies in promoting the evolution of QDs and QLEDs, indicating the future directions in this field.

Full-color micro-LED display with photo-patternable and highly ambient-stable perovskite quantum dot/siloxane composite as color conversion layers

In this paper, we successfully fabricated color conversion layers (CCLs) for full-color-mico-LED display using a perovskite quantum dot (PQD)/siloxane composite by ligand exchanged PQD with silane composite followed by surface activation by an addition of halide-anion containing salt. Due to this surface activation, it was possible to construct the PQD surface with a silane ligand using a non-polar organic solvent that does not damage the PQD. As a result, the ligand-exchanged PQD with a silane compound exhibited high dispersibility in the siloxane matrix and excellent atmospheric stability due to sol–gel condensation. Based on highly ambient stable PQD/siloxane composite based CCLs, full-color micro-LED display has a 1 mm pixel pitch, about 25.4 pixels per inch (PPI) resolution was achieved. In addition, due to the thin thickness of the black matrix to prevent blue light interference, the possibility of a flexible display that can be operated without damage even with a bending radius of 5 mm was demonstrated.

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.

High glass transition temperature and ultra‐low thermal expansion coefficient polyimide films containing rigid pyridine and bisbenzoxazole units

AbstractPolyimide (PI) films with extremely high heat‐resisting and dimensional stability are ideal substrate materials for flexible organic light‐emitting diodes. In this study, three diamines containing rigid bisbenzoxazole structures, 2,2′‐p‐phenylenebis(5‐aminobenzoxazole) (phDBOA), 2,2′‐m‐pyridylenebis(5‐aminobenzoxazole), and 2,2′‐p‐pyridylenebis(5‐aminobenzoxazole) (p‐PDBOA), were polymerized with 3,3′,4,4′‐biphenyl tetracarboxylic dianhydride and pyromellitic dianhydride (PMDA) separately with traditional two‐step approach to prepare a set of PI films. Introduction of bisbenzoxazole improves stiffness of molecular chain, whereas that of pyridine increases in‐plane orientation and molecular chain tend to be densely stacked. Among prepared films, PI‐6 (PMDA/p‐PDBOA) shows the most excellent mechanical properties, with an ultra‐high glass transition temperature of >450°C, an ultra‐low thermal expansion coefficient of <5 ppm K−1 (50–450°C), a tensile strength of 259 MPa, and a modulus of 8.3 GPa. Given these excellent properties, PI‐6 could be applied in flexible display.

Selective Purity Modulation of Semiconducting Single-Walled Carbon Nanotube Networks for High-Performance Thin-Film Transistors

Single-walled carbon nanotube (SWCNT) random networks have become strong candidates for next-generation electronics due to their exceptional mechanical, electrical, and optical properties. However, metallic nanotubes in networks generally incur a trade-off between the charge carrier mobility and on/off ratio, limiting the performance of SWCNT-based devices. Therefore, various methods to increase the purity of semiconducting nanotubes in entire random networks have been reported, but this direction has faced other issues, such as nanotube shortening, higher cost, and higher energy. Here, we introduce SWCNT random network-based thin-film transistors (SWCNT TFTs) with a varying purity profile of semiconducting SWCNTs across the channel, exploiting the superior mobility of metallic SWCNTs by partially tuning the semiconducting SWCNT purity and developing a novel perspective on metallic nanotubes in semiconductor channels. Based on the high-precision drop-on-demand capability of inkjet printing and various concentrations of semiconducting SWCNT ink, we form selectively patterned channel regions with different semiconducting SWCNT purities. The metallic nanotube-dominant region drastically increases the carrier density with a minimized Schottky barrier, while high-purity semiconducting regions at the channel boundaries effectively block off-state leakage through carrier depletion. As a result, the SWCNT TFTs with selectively patterned metallic nanotube regions show superior carrier mobility (75.50 cm2 V–1 s–1) and channel width normalized on-current (34.33 nA μm–1) without compromising the on/off ratio (1.62 × 107). To show the feasibility of our device in high-performance electronics, we demonstrate all-inkjet-printed flexible display driving circuits with two transistors that enable low-power, high-performance operation in display applications.

Flexible, biocompatible, degradable silk fibroin based display

Flexible electronic devices using natural biological protein materials have received extensive attention in recent years. However, it is rarely reported that biological protein materials are utilized in flexible display. Here we reported a novel epidermal e-ink display device in which the natural silk fibroin acts as an electrode layer. Through the mesoscopic doping of regenerated silk fibroin, the material overcomes the defects of high brittleness, poor water and temperature resistance, and poor chemical stability on the premise of retaining biocompatibility and biodegradability. This revolutionary technology of flexible display using bio-silk fibroin materials can be applied to the field of human electronic skin display since many parts of our human body can be used as display carriers, and the display technology on human body has made breakthroughs.

Review of the Versatile Patterning Methods of Ag Nanowire Electrodes

To use Ag nanowires for various industries, it is crucial to develop an appropriate patterning method. There are various types of patterning methods, but there has been no comprehensive review discussing and summarizing them. This review paper provides an overview of the various patterning techniques of Ag nanowire electrodes, including photolithography, nanoimprint lithography, inkjet printing, electrohydrodynamic jet printing, and other emerging methods. These transparent electrodes have received significant attention due to their high transparency, low sheet resistance, and flexibility, making them ideal for applications such as flexible electronics, touch screens, and solar cells. Each patterning technique has its benefits and limitations, and its suitability depends on specific application requirements. Photolithography is a well-established technique that can achieve high-resolution patterns, while nanoimprint lithography is a low-cost and versatile method for large-area patterning. Inkjet printing and E-jet printing provide the advantages of high throughput, precise control, and the ability to print on different substrates. Stencil printing, laser direct writing, and electrospinning are emerging techniques that showing high potential for patterning Ag nanowire electrodes. The choice of patterning technique ultimately depends on various factors, such as resolution requirements, cost, substrate compatibility, and throughput.

Dual-Phase Inspired Soft Electronic Sensors with Programmable and Tunable Mechanical Properties.

Wearable and stretchable sensors are important components to strictly monitor the behavior and health of humans and attract extensive attention. However, traditional sensors are designed with pure horseshoes or chiral metamaterials, which restrict the biological tissue engineer applications due to their narrow regulation ranges of the elastic modulus and the poorly adjustable Poisson's ratio. Inspired by the biological spiral microstructure, a dual-phase metamaterial (chiral-horseshoes) is designed and fabricated in this work, which possesses wide and programmable mechanical properties by tailoring the geometrical parameters. Experimental, numerical, and theoretical studies are conducted, which reveal that the designed microstructures can reproduce mechanical properties of most natural animals such as frogs, snakes, and rabbits skin. Furthermore, a flexible strain sensor with the gauge factor reaching 2 under 35% strain is fabricated, which indicates that the dual-phase metamaterials have a stable monitoring ability and can be potentially applied in the electronic skin. Finally, the flexible strain sensor is attached on the human skin, and it can successfully monitor the physiological behavior signals under various actions. In addition, the dual-phase metamaterial could combine with artificial intelligence algorithms to fabricate a flexible stretchable display. The dual-phase metamaterial with negative Poisson's ratio could decrease the lateral shrinkage and image distortion during the stretching process. This study offers a strategy for designing the flexible strain sensors with programmable, tunable mechanical properties, and the fabricated soft and high-precision wearable strain sensor can accurately monitor the skin signals under different human motions and potentially be applied for flexible display.

Thin Multilayer of Graphene as an Alternative to Indium Tin Oxide (ITO) in Touchscreens and Wearable Electronics

Along with achieving great technological advances, graphene has achieved a great amount of popularity and interest from the world of science because of its enhanced physical, mechanical and thermal properties. The research output on graphene and its applications in various fields will be more. This investigation focusses on the fundamental aspects of graphene, which are exploring the flexible touch screens such as Smart Phones, Ipad, Smart TV. The effect of these results of new graphene displays are discussed. With the aid of experimental techniques involving pre-surface treatment, adhesion promoters, graphene layer thickness and theoretical modelling using Comsol, results are achieved. Different parameters were studied such as pre-cleaning time of the glass, weight of graphene in the graphene-ethanol suspension, use of adhesion promoter such as PMMA. It was found that optimum parameters are longer cleaning times (15 minutes), lower graphene weights (<0.032 g) and use of PMMA. In the modelling, layer by layer embedding of graphene over a glass substrate was analysed. Different characteristics were studied such as optical transparency, adhesion and electrical resistivity. Modelling of graphene over glass gave the results that layer thickness needs to be optimized with lower layer thickness (<2 * 10−5 cm) being better.

Study on Printable Inorganic Alternating Current Electroluminescence Devices based on Different Electrode Structures

Traditional bottom light emitting devices are commonly limited by the light transmittance of the substrate and electrode, while the recent emerging inorganic alternating current electroluminescence (ACEL) devices, which manifest excellent application prospects in the field of flexible display are mainly prepared by directly mixing thermosetting resin and powder materials. Here, the electrode layer, dielectric layer and light-emitting layer based on different electrode structures, were printed in different orders and structures by the inking technology. Printable inorganic ACEL devices with bottom-emission structure (BES), top-emission structure (TES) and planar electrode structure (PES) were successfully obtained. The measurements of luminescent properties of printable ACEL with different electrode structures under different voltages by spectrometer indicated that BES-ACEL had the best brightness. Besides, TES-ACEL and PES-ACEL have the outstanding advantages of no substrate limitation and interaction characteristics, respectively. Therefore, this work opens up the new path for ACEL devices towards application in the fields of printing and packaging, interactive and intelligent display.

Effect of monovalent, divalent, and trivalent ions codoped with LaOCl: Eu3+ phosphors and its Judd-Ofelt analysis for display device applications

In this work, Lanthanum oxychloride LaOCl: Eu3+ (0–9 mol%) was synthesized using a conventional solid-state approach over the course of relatively low temperature (400 °C) and a shorter duration of 1 h. To investigate the phase purity of phosphors, crystallographic studies were performed using PXRD, which revealed the tetragonal structure with space group P4/nmm (No. 129). Morphological investigations were made using FESEM which unveils the agglomeration of phosphors. Photoluminescence spectra reveals that, the intensity of the 5D0→7F2 transition (618 nm), which increases with an increase in Eu3+ concentration (1–9 mol%), rises to a maximum of 7 mol% before quenching causes it to fall. The reason behind the quenching concentration of Eu3+ ions at 7 mol% was dipole-quadruple interaction which is determined using the Van Uitert equation. To improve luminescence efficiency metal ions like Li+, Na+, K+, Ca2+, and Bi3+ were exploited, among these LaOCl: Eu3+ co-doped with Li+ ion exhibit the highest PL intensity (39 times that of Eu3+). The Commission International de I'Eclairage (CIE) chromaticity coordinates of LaOCl: Eu3+ co-doped with Li+ phosphor is (0.590, 0.346) which is close to commercial red light-emitting phosphors. These phosphor materials were mixed with PVA and developed a flexible phosphor polymer composite, which can be used for flexible display device applications. Spectral parameters like J-O intensity parameters are used to analyze optical materials, these parameters were determined with the help of luminescence spectra.

Electrohydrodynamic Printing of PCL@CsPbBr3 Composite Fibers with High Luminescence for Flexible Displays

Metal halide perovskite quantum dots (PQDs) are widely used in the display field due to their excellent photoelectric properties, such as ultra-narrow half-peak widths and ultra-pure luminescence color purity. Inkjet printing, laser direct writing and electrospinning are all common methods for PQDs printing to prepare micropattern displays. In order to produce large-scale and high-resolution PQDs micropatterns, electrohydrodynamic (EHD) printing technology is capable of large-scale deposition of highly oriented nanofibers on rigid or pliable, flat or bent substrates with the advantages of real-time regulation and single control. Therefore, it has a lot of potential in the fabrication of pliable electronic devices for one-dimensional ordered light-emitting fibers. Polycaprolactone (PCL) as an EHD printing technology polymer material has the advantages of superior biocompatibility, a low melting point, saving energy and easy degradation. By synthesizing CsPbBr3 quantum dots (QDs) and PCL composite spinning stock solution, we used the self-built EHD printing platform to prepare the PCL@CsPbBr3 composite light-emitting optical fiber and realized the flexible display of high-resolution micropatterns in polydimethylsiloxane (PDMS) packaging. An x-ray diffractometer (XRD), scanning electron microscope (SEM) and photoluminescence (PL) were used to characterize and analyze the fiber’s morphology, phase and spectral characteristics. EHD printing technology may open up interesting possibilities for flexible display applications based on metal halide PQDs.

A Stable PDLC Film with High Ageing Resistance from an Optimized System Containing Rigid Monomer.

With the switchability between transparent and light-scattering states, polymer-dispersed liquid crystals (PDLC) are widely used as smart windows, flexible display devices, projectors, and other devices. In outdoor applications, in addition to excellent electro-optical properties, there is also a high demand for film stability. In this work, a PDLC film with high mechanical strength and structural stability is prepared that can maintain stability at 80 °C for 2000 h. By choosing liquid crystals with a wide temperature range, adopting acrylate polymer monomers containing hydroxyl groups, and adjusting the polymer content, the PDLC film can work well from -20 °C to 80 °C. On this basis, the effects of the introduction of rigid monomers on the mechanical properties and electro-optical properties of PDLC films are investigated.

Impact of the Chemical Structure of Photoreactive Urethane (Meth)Acrylates with Various (Meth)Acrylate Groups and Built-In Diels-Alder Reaction Adducts on the UV-Curing Process and Self-Healing Properties.

A series of UV-curable urethane (meth)acrylates were obtained by copolymerization of the Diels-Alder adduct (HODA), isophorone diisocyanate, PEG1000, and various hydroxy (meth)acrylates. The aim of the present work was to determine the influence of the chemical structure of the introduced (meth)acrylic groups, i.e., hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate, on the UV-curing process and self-healing properties of cured coatings. The chemical structure of prepolymers was characterized by FTIR and NMR spectroscopy, whereas the UV-curing process was monitored in real time using FTIR and photo-DSC. In turn, the self-healing properties were characterized in relation to the thermally reversible mechanism, which was tested using the following methods: an FTIR spectroscope equipped with a heating attachment; DSC and TG apparatus; and an optical microscope equipped with a stage with programmable heating. The result of comprehensive research on the self-healing of photocurable coatings in the context of the presence of various photoreactive groups and the course of the curing process allows one to control the self-healing process by reducing the effective healing temperature. The self-healing properties, taken together with the fast UV curing of the coatings and excellent properties of cured coatings, make the material attractive for a variety of applications, in particular in cases where coatings are not repaired, e.g., for economic reasons or when it is not possible, such as in flexible electronic screens, car paint film, and aircraft interior finishes.

Strain insensitive flexible photodetector based on molybdenum ditelluride/molybdenum disulfide heterostructure

Flexible electronic and optoelectronic devices are highly desirable for various emerging applications, such as human-computer interfaces, wearable medical electronics, flexible display, etc. Layered two-dimensional (2D) material is one of the most promising types of materials to develop flexible devices due to its atomically thin thickness, which gives it excellent flexibility and mechanical endurance. However, the 2D material devices fabricated on flexible substrate inevitably suffer from mechanical deformation, which can severely affect device performances, resulting in function degradation and even failure. In this work, we propose a strain insensitive flexible photodetector based on MoS2/MoTe2 heterostructure on polyimide substrate, which provides a feasible approach to cancel unpredicted impacts of strain on the device performances. Specifically, the MoS2/MoTe2 heterostructure is deposited with 4 electrodes to form three independent devices of MoS2 FET, MoTe2 FET and MoS2/MoTe2 heterojunction. Among them, the MoS2/MoTe2 heterojunction is used as the photodetector, while the MoS2 FET is used as a strain gauge to calibrate the photo detection result. Such configuration is enabled by the Schottky barrier formed between the electrodes and the MoS2 flake, which leads to obvious and negligible photo response of MoS2/MoTe2 heterojunction and MoS2 FET, respectively, under low source-drain bias (ex. 10 mV). The experimental results show that the proposed mechanism can not only calibrate the photo response to cancel strain effect, but also successfully differentiate the wavelength (with fixed power) or power (with fixed wavelength) of light illumination.