Stretchable Light-emitting Diode Research Articles

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics.

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Structural and Material-Based Approaches for the Fabrication of Stretchable Light-Emitting Diodes.

Stretchable displays, capable of freely transforming their shapes, have received significant attention as alternatives to conventional rigid displays, and they are anticipated to provide new opportunities in various human-friendly electronics applications. As a core component of stretchable displays, high-performance stretchable light-emitting diodes (LEDs) have recently emerged. The approaches to fabricate stretchable LEDs are broadly categorized into two groups, namely "structural" and "material-based" approaches, based on the mechanisms to tolerate strain. While structural approaches rely on specially designed geometries to dissipate applied strain, material-based approaches mainly focus on replacing conventional rigid components of LEDs to soft and stretchable materials. Here, we review the latest studies on the fabrication of stretchable LEDs, which is accomplished through these distinctive strategies. First, we introduce representative device designs for efficient strain distribution, encompassing island-bridge structures, wavy buckling, and kirigami-/origami-based structures. For the material-based approaches, we discuss the latest studies for intrinsically stretchable (is-) electronic/optoelectronic materials, including the formation of conductive nanocomposite and polymeric blending with various additives. The review also provides examples of is-LEDs, focusing on their luminous performance and stretchability. We conclude this review with a brief outlook on future technologies.

Ultrastretchable Electrically Self-Healing Conductors Based on Silver Nanowire/Liquid Metal Microcapsule Nanocomposites.

Stretchable conductive nanocomposites are essential for deformable electronic devices. These conductors currently face significant limitations, such as insufficient deformability, significant resistance changes upon stretching, and drifted properties during cyclic deformations. To tackle these challenges, we present an electrically self-healing and ultrastretchable conductor in the form of bilayer silver nanowire/liquid metal microcapsule nanocomposites. These nanocomposites utilize silver nanowires to establish their initial excellent conductivity. When the silver nanowire networks crack during stretching, the microcapsules are ruptured to release the encased liquid metal for recovering the electrical properties. This self-healing capability allows the nanocomposite to achieve ultrahigh stretchability for both uniaxial and biaxial strains, minor changes in resistance during stretching, and stable resistance after repetitive deformations. The conductors have been used to create skin-attachable electronic patches and stretchable light-emitting diode arrays with enhanced robustness. These developments provide a bioinspired strategy to enhance the performance and durability of conductive nanocomposites.

Maskless Fabrication of Highly Conductive and Ultrastretchable Liquid Metal Features through Selective Laser Activation.

In the rising field of stretchable electronics, liquid metals are ideal candidate conductors with metallic conductivity and intrinsic deformability. The complex patterning methods of liquid metal features have limited their widespread applications. In this study, we report a maskless fabrication approach for the facile and scalable patterning of liquid metal conductors on an elastomer substrate. Laser-activated patterns are employed as versatile templates to define arbitrary liquid metal patterns. The as-prepared liquid metal features show an excellent conductivity of 3.72 × 104 S/cm, a high resolution of 70 μm, ultrahigh stretchability of up to 1000% strain, and electromechanical durability. The practical suitability of liquid metal conductors is demonstrated by fabricating a stretchable light-emitting diode (LED) matrix and a smart sensing glove. The maskless fabrication technique introduced here allows versatile patterning of liquid metal conductors with affordable costs, which may stimulate a broad range of applications in stretchable electronic devices and systems.

High-resolution maskless patterning of AgNWs based on adhesion enhancement of a printed conductive polymer

Facile high-resolution patterning of silver nanowires (AgNWs) is a key enabling technology for expanding their usage to flexible and stretchable electronics applications. Although various patterning methods have been proposed, only high-cost and complex mask-based methods have demonstrated potential micrometer-scale pattern-ability, which have no design freedom of changing patterns in a facile manner. Herein, we suggest a maskless high-resolution patterning method by combining an adhesive underlayer and inkjet-printed conductive polymer patterns. AgNWs located between them are tightly bonded and thus are resistant to a solvent-based rinsing process. AgNWs under the conductive polymer patterns remain while uncovered AgNWs are removed during the rinsing process. Our approach allows easy patterning of AgNWs on various substrates including glass, plastic, and stretchable elastomer. The excellent mechanical properties of AgNW interconnects are demonstrated in a stretchable light-emitting diode array. By introducing an electrohydrodynamic printing method, high-resolution AgNW patterns with linewidths below 20 µm are fabricated, which are used for a heater based on transparent grid AgNW electrodes. Based on our results, we believe that our method paves the way for a convenient patterning process of high-resolution electrode circuits with a high degree of freedom even on a deformable platform.

A Biaxially Stretchable and Washable LED Display Enabled by a Wavy-Structured Metal Grid

In this work, we fabricated a biaxially stretchable light-emitting diode (LED) display based on a wavy structured-metal electrode grid packaged together with LED chips. The micro-electromechanical systems (MEMS) fabrication process was utilized to fabricate the wavy-structured metal connections and rigid islands. The resulting LED display can be readily bent, rolled, stretched and even washed with no degradation of its image quality. Here at least 1000 stretching cycles at a 10% strain can be tolerated. This work demonstrated the potential of inorganic materials as stretchable displays and MEMS compatible processes which can be used as stretchable electronics fabrication processes. [2022-0029]

Graphene‐Based Intrinsically Stretchable 2D‐Contact Electrodes for Highly Efficient Organic Light‐Emitting Diodes (Adv. Mater. 31/2022)

Organic Light-Emitting Diodes Intrinsically stretchable light-emitting diodes (ISOLEDs) are becoming increasingly important for wearable electronics. In article number 2203040, Han-Young Woo, Tae-Woo Lee, and co-workers report the design of highly efficient ISOLEDs that use a graphene-based 2D-contact stretchable electrode. As a benefit of a newly designed conjugated polyelectrolyte with efficient electron injection capability, the ISOLED yields an unprecedently high current efficiency of 20.3 cd A−1.

High-brightness all-polymer stretchable LED with charge-trapping dilution.

Next-generation light-emitting displays on skin should be soft, stretchable and bright1-7. Previously reported stretchable light-emitting devices were mostly basedon inorganic nanomaterials, such as light-emitting capacitors, quantum dots or perovskites6-11. They either require high operating voltage or have limited stretchability and brightness, resolution or robustness under strain. On the other hand, intrinsically stretchable polymer materials hold the promise of good strain tolerance12,13. However, realizing high brightness remains a grand challenge for intrinsically stretchable light-emitting diodes. Here we report a material design strategy and fabrication processes to achieve stretchable all-polymer-based light-emitting diodes with high brightness (about 7,450 candela per square metre), current efficiency (about 5.3 candela per ampere) and stretchability (about 100per cent strain). We fabricate stretchable all-polymer light-emitting diodes coloured red, green and blue, achieving both on-skin wireless powering and real-time displaying of pulse signals. This work signifies a considerable advancement towards high-performance stretchable displays.

Materials and design strategies for stretchable electroluminescent devices.

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

Ultrasensitive wearable sensor with novel hybrid structures of silver nanowires and carbon nanotubes in fluoroelastomer: Multi-directional sensing for human health monitoring and stretchable electronics

Today, wearable sensors are being developed for personalized human health monitoring for physical motion detection and physiological signal detection, and well-being. These wearable devices need to be extremely flexible, lightweight, and stretchable to be compatible with the contours of the human body, and mechanically and dynamically stable. Their functionality can be assured by simultaneously increasing conductivity, stretchability, and sensitivity for accurate and fast response. The challenge in fabricating stretchable sensors with these functionalities is embracing current wearable fashion. In this work, for the first time, a new 3D hybrid nanonetwork structures of high aspect ratio silver nanowires (AgNW) and carbon nanotubes (CNT) in fluoroelastomer FKM are developed. An optimized solution mixing (SM) and layer by layer (LBL) assembly technique, and LBL drop-coating process are employed to establish the hierarchical and shell-like nanostructures. The sensor films are ultra-thin and show high conductivity up to 2 × 105 S. m−1, high stretchability of up to 300%, and a maximum sensitivity (gage factor) of up to 2.5 × 107. The multifunctional sensor with hybrid nanonetwork shows high performance in various wearable electronic applications including human motion detection and stretchable light-emitting diode (LED) circuits. These ultra-sensitive materials show capability for multidirectional sensing by special configurations for each human body part movement, and small movements for human heartbeat monitoring.

Facile one-pot synthesis of rod-coil bio-block copolymers and uncovering their role in forming the efficient stretchable touch-responsive light emitting diodes

One pot synthesized bio derived and polyfluorene-based (PF-based) conjugated block copolymers exhibited high mechanical flex-stretch endurance and could effectively suppress non-radiative recombination through insulating block present in the conjugated block contributes to higher exciton binding energy. They showed potential for fabricating highly durable wearable stretchable nano and microelectronic devices. • Facile synthesis of PF-based BCPs addresses the limitations in the synthetic field. • High PLQY value can be induced by coil block due to high EBE. • The LED with PF-BCPs exhibits 6 times EQE greater than one with PF homopolymer. • The stretchable LED shows high flexibility (strain 150%) and endurance (300 cycles). • The touch-responsive LEDs exhibits excellent stability and durability. Bio-derived optoelectronic material is captivating and sustainable research as it reduces the environmental toxicity and comforting the wearable aspects. Our research involves the synthesis of series of bio-derived polyfluorene- block -poly( δ -decanolactone) (PF- b -PDL) conjugated block copolymers through smart one-pot procedure that involves simple purification for fabricating touch-responsive light-emitting diode (LED) devices. Compared with PF homopolymer, the block copolymers exhibit higher photoluminescence quantum yields and higher exciton binding energies. PF 18 - b -PDL 13 specifically exhibits external quantum efficiency (EQE %) (~6 times higher than PF homopolymer). Moreover, because of coily PDL block inducing a highly stable bound state in block copolymer generating the increment in PL lifetime and exciton binding energies than the homopolymer. Furthermore, the diblock copolymers device exhibits fully solution processability, higher carrier recombination efficiency, flex-stretch stability, good structural integrity and mechanical endurance highlighting the brighter potential of our bio-derived block copolymers for fabricating highly durable wearable stretchable nano and microelectronic devices.

Intrinsically stretchable organic light-emitting diodes.

Soft and conformable optoelectronic devices for wearable and implantable electronics require mechanical stretchability. However, very few researches have been done for intrinsically stretchable light-emitting diodes. Here, we present an intrinsically stretchable organic light-emitting diode, whose constituent materials are all highly stretchable. The resulting intrinsically stretchable organic light-emitting diode can emit light when exposed to strains as large as 80%. The turn-on voltage is as low as 8 V, and the maximum luminance, which is a summation of the luminance values from both the anode and cathode sides, is 4400 cd m-2 It can also survive repeated stretching cycles up to 200 times, and small stretching to 50% is shown to substantially enhance its light-emitting efficiency.

Novel stretchable light‐emitting diodes based on conjugated‐rod block elastic‐coil copolymers

In this study, we successfully fabricated an intrinsically stretchable light‐emitting diode (LED) by using a new conjugated rod–coil block copolymer, poly[2,7‐(9,9‐dioctylfluorene)]‐block‐poly(isoprene) (PF‐b‐PI). ( PF conjugated rods and PI coils exhibited fluorescent and intrinsically stretchable properties, respectively. Copolymers with three different rod/coil ratios, namely PF18K‐b‐PI5K, PF12K‐b‐PI5K and PF12K‐b‐PI10K, were utilized for elastic LED demonstration. Notably, PF18K‐b‐PI5K and PF12K‐b‐PI5K LEDs can work under a strain of 140% because of suitable ratios of rod/coil lengths. Finally, a stretchable PF12K‐b‐PI5K LED was measured with a low voltage of 2 V and with a brightness of 70 cd m−2 at 10 V. This LED was subjected to 100 cycles of luminance–strain testing under 50% strain and it maintained a stable level of brightness. These results demonstrated that the luminescent PF12K‐b‐PI5K rod–coil block copolymers exhibit exceptional stretchability and high stability; these copolymers can be used in stretchable LEDs for a wide range of applications. © 2020 Society of Chemical Industry

Ultrastretchable Helical Conductive Fibers Using Percolated Ag Nanoparticle Networks Encapsulated by Elastic Polymers with High Durability in Omnidirectional Deformations for Wearable Electronics

AbstractStretchable interconnects with invariable conductivity and complete elasticity, which return to their original shape without morphological hysteresis, are attractive for the development of stretchable electronics. In this study, a polydimethylsiloxane‐coated multifilament polyurethane‐based helical conductive fiber is developed. The stretchable helical fibers exhibit remarkable electrical performance under stretching, negligible electrical and mechanical hysteresis, and high electrical reliability under repetitive deformation (10 000 cycles of stretching with 100% strain). The resistance of the helical fibers barely increases until the applied strain reaches the critical strain, which is based on the helical diameter of each fiber. According to finite element analysis, uniform stress distribution is maintained in the helical fibers even under full stretching, owing to the fibers' true helix structure. In addition, the stretchable helical fibers have the ability to completely return to their original shapes even after being fully compressed in the vertical direction. Cylinder‐shaped connecting pieces made using 3D printing are designed for stable connection between the helical fibers and commercial components. A deformable light‐emitting diode (LED) array and biaxially stretchable LED display are fabricated using helical fibers. A skin‐mountable band‐type oximeter with helical fiber‐based electrodes is also fabricated and used to demonstrate real‐time detection of cardiac activities and analysis of brain activities.

Highly stretchable and contact-responsive light-emitting diodes based on MAPbBr3–PEO composite film

Highly stretchable, deformable, and contact-responsive light-emitting diodes (LEDs) were fabricated using a methylammonium lead tribromide (MAPbBr3)–poly(ethylene oxide) (PEO) composite film as light-emissive layer. The polymer matrix formed through the mixing of PEO induced stretchability to the composite films, which were mechanically robust. The stretchable LEDs emitted pure green light at 538 nm and exhibited outstanding performance of a low turn-on voltage of about 2.5 V, maximum luminance of 4257 cd m−2, and current efficiency of 1.77 cd A−1. Furthermore, the device can be stretched up to tensile strain of 70% without failure and can stably operate after 1000 repetitive stretching cycles with no significant degradation of the device performance.

Highly Stretchable Conductor by Self-Assembling and Mechanical Sintering of a 2D Liquid Metal on a 3D Polydopamine-Modified Polyurethane Sponge.

A highly stretchable conductor was fabricated through dip-coating a new liquid metal (LM) electric ink on a polydopamine (PDA)-modified three-dimensional (3D) polyurethane sponge (PUS) followed by mechanical sintering. The LM was first sonicated to nanodroplets to reduce the consumption of LM and then modified by 3-mercaptopropionic acid (LMNPS-MPA) to improve the interfacial adhesion between LM and PUS. The denser and even distribution of LMNPS-MPA self-assembling on PDA-treated PUS (PUS-PDA) was successfully prepared via hydrogen bonding interactions. Mechanical sintering of 3D PUS-PDA coated by a two-dimensional (2D) LM layer was then conducted to obtain a continuous conductive network. Comparing with those of the reported 3D conductors, the resulting PUS-PDA-LM composite conductor shows both high electrical conductivity (478 S cm-1) under a low LM consumption of 10 vol% and excellent conductivity stability with the relative resistance change, ΔR/R0, of 2% at 50% strain under stretching deformation. The as-prepared PUS-PDA-LM composites were then successfully applied as flexible and stretchable light-emitting diode (LED) arrays with excellent conductivity and conductivity stability at different deformations. We believe that the 3D stretchable PUS-PDA-LM conductor has many potential applications in flexible sensors, flexible circuits, rollable displays, etc.

Semiliquid Metal Enabled Highly Conductive Wearable Electronics for Smart Fabrics.

Wearable electronics incorporating electronic components into commonly used fabrics can serve as new-generation personalized health-care systems for applications ranging from health-care monitoring to disease treatment. Conventional rigid materials including gold, silver, and copper generally require a complicated fabrication process to be sewn into clothes. At the same time, other high-stretchable nonmetal materials such as conductive polymers generally have a limitation of low electroconductivity, restricting their further applications. Recently, gallium-based liquid metals have exhibited superior advantages in flexible electronics and have presented valuable potential in creative printing technologies. Here, we proposed a novel wearable electronics prepared through roller printing technology based on the adhesion difference of semiliquid metal (Cu-EGaIn, eutectic gallium-indium mixed with copper microparticles) on cotton fabrics and polyvinyl acetate (PVAC) glue. Results have shown that the surface topography and chemical interaction of fabrics and PVAC glue determine the adhesion effect with the Cu-EGaIn mixture. The electric experiments have demonstrated the electromechanical stability of the fabricated lines on fabrics. Further, a series of smart fabrics were developed including an interactive circuit, stretchable light-emitting diode array, and thermal management device with advantages of easy operation, low cost, and large-area fabrication to show practical applications in the method. This strategy may play an important role in the design and fabrication of smart fabrics, contributing to the development of customized health-care systems.

Quantum Dot LEDs: Stretchable Organometal‐Halide‐Perovskite Quantum‐Dot Light‐Emitting Diodes (Adv. Mater. 22/2019)

A stretchable organometal-halide-perovskite quantum-dot light-emitting diode with both high efficiency and mechanical compliancy is reported by Hong-Bo Sun, Qibing Pei, and co-workers in article number 1807516. This work presents a new semiconductor material or microstructure toward the realization of stretchable light-emitting diodes.

Stretchable Organometal‐Halide‐Perovskite Quantum‐Dot Light‐Emitting Diodes

Stretchable light-emitting diodes (LEDs) and electroluminescent capacitors have been reported to potentially bring new opportunities to wearable electronics; however, these devices lack in efficiency and/or stretchability. Here, a stretchable organometal-halide-perovskite quantum-dot LED with both high efficiency and mechanical compliancy is demonstrated. The hybrid device employs an ultrathin (<3 µm) LED structure conformed on a surface-wrinkled elastomer substrate. Its luminescent efficiency is up to 9.2 cd A-1 , which is 70% higher than a control diode fabricated on the rigid indium tin oxide/glass substrate. Mechanical deformations up to 50% tensile strain do not induce significant loss of the electroluminescent property. The device can survive 1000 stretch-release cycles of 20% tensile strain with small fluctuations in electroluminescent performance.

Stretchable Light‐Emitting Diodes with Organometal‐Halide‐Perovskite–Polymer Composite Emitters

Intrinsically stretchable light-emitting diodes (LEDs) are demonstrated using organometal-halide-perovskite/polymer composite emitters. The polymer matrix serves as a microscale elastic connector for the rigid and brittle perovskite and induces stretchability to the composite emissive layers. The stretchable LEDs consist of poly(ethylene oxide)-modified poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as a transparent and stretchable anode, a perovskite/polymer composite emissive layer, and eutectic indium-gallium as the cathode. The devices exhibit a turn-on voltage of 2.4 V, and a maximum luminance intensity of 15 960 cd m-2 at 8.5 V. Such performance far exceeds all reported intrinsically stretchable LEDs based on electroluminescent polymers. The stretchable perovskite LEDs are mechanically robust and can be reversibly stretched up to 40% strain for 100 cycles without failure.