Alternating current electroluminescence devices: recent advances and functional applications

Wearable smart devices and visualisation sensors based on alternating current electroluminescence (ACEL) have received considerable attention in recent years. Due to the unique properties of ACEL devices, such as high mechanical strength, adaptability to complex environments, and no need for energy level matching, ACEL is suitable for multifunctional applications and visualisation sensing platforms. This review comprehensively outlines the latest developments in ACEL devices, starting with an analysis of the mechanism, classification, and optimisation strategies of ACEL. It introduces the functional applications of ACEL in multicolour displays, high-durability displays, stretchable and wearable displays, and autonomous function displays. Particularly, it emphasises the research progress of ACEL in sensory displays under interactive conditions such as liquid sensing, environmental factor sensing, kinetic energy sensing, and biosensing. Finally, it forecasts the challenges and new opportunities faced by future functional and interactive ACEL devices in fields such as artificial intelligence, smart robotics, and human-computer interaction.

Bright Bifacial White-Light Illumination by Highly Deformable Electroluminescent Devices Based on Transparent Ionic-Hydrogel Electrodes and Quantum-Dot Color Conversion

Deformable alternating-current electroluminescent (ACEL) devices are of increasing interest because of their potential to drive innovation in soft optoelectronics. Despite the research focus on efficient white ACEL devices, achieving deformable devices with high luminance remains difficult. In this study, this challenge is addressed by fabricating white ACEL devices using color-conversion materials, transparent and durable hydrogel electrodes, and high-k nanoparticles. The incorporation of quantum dots enables the highly efficient generation of red and green light through the color conversion of blue electroluminescence. Although the ionic-hydrogel electrode provides high toughness, excellent light transmittance, and superior conductivity, the luminance of the device is remarkably enhanced by the incorporation of a high-k dielectric, BaTiO3. The fabricated ACEL device uniformly emits very bright white light (489 cd m-2) with a high color-rendering index (91) from both the top and bottom. The soft and tough characteristics of the device allow seamless operation in various deformed states, including bending, twisting, and stretching up to 400%, providing a promising platform for applications in a wide array of soft optoelectronics.

Alternating Current Electroluminescence for Human-Interactive Sensing Displays

The development of stimuli-interactive displays based on alternating current (AC)-driven electroluminescence (EL) is of great interest, owing to their simple device architectures suitable for wearable applications requiring resilient mechanical flexibility and stretchability. AC-EL displays can serve as emerging platforms for various human-interactive sensing displays (HISDs) where human information is electrically detected and directly visualized using EL, promoting the development of the interaction of human-machine technologies. This review provides a holistic overview of the latest developments in AC-EL displays with an emphasis on their applications for HISDs. AC-EL displays based on exciton recombination or impact excitations of hot electrons are classified into four representative groups depending upon their device architecture: 1) displays without insulating layers, 2) displays with single insulating layers, 3) displays with double insulating layers, and 4) displays with EL materials embedded in an insulating matrix. State-of-the-art AC HISDs are discussed. Furthermore, emerging stimuli-interactive AC-EL displays are described, followed by a discussion of scientific and engineering challenges and perspectives for future stimuli-interactive AC-EL displays serving as photo-electronic human-machine interfaces.

CuInS2 Quantum Dot and Polydimethylsiloxane Nanocomposites for All-Optical Ultrasound and Photoacoustic Imaging

Dual-modality imaging employing complementary modalities, such as all-optical ultrasound and photoacoustic imaging, is emerging as a well-suited technique for guiding minimally invasive surgical procedures. Quantum dots are a promising material for use in these dual-modality imaging devices as they can provide wavelength-selective optical absorption. The first quantum dot nanocomposite engineered for co-registered laser-generated ultrasound and photoacoustic imaging is presented. The nanocomposites developed, comprising CuInS2 quantum dots and medical-grade polydimethylsiloxane (CIS-PDMS), are applied onto the distal ends of miniature optical fibers. The films exhibit wavelength-selective optical properties, with high optical absorption (> 90%) at 532 nm for ultrasound generation, and low optical absorption (< 5%) at near-infrared wavelengths greater than 700 nm. Under pulsed laser irradiation, the CIS-PDMS films generate ultrasound with pressures exceeding 3.5 MPa, with a corresponding bandwidth of 18 MHz. An ultrasound transducer is fabricated by pairing the coated optical fiber with a Fabry-Pérot (FP) fiber optic sensor. The wavelength-selective nature of the film is exploited to enable co-registered all-optical ultrasound and photoacoustic imaging of an ink-filled tube phantom. This work demonstrates the potential for quantum dots as wavelength-selective absorbers for all-optical ultrasound generation.

Self-Integratable, Healable, and Stretchable Electroluminescent Device Fabricated via Dynamic Urea Bonds Equipped in Polyurethane

Reversible bonding between polymer chains has been used primarily to induce self-healing of damaged polymers. Inspired by the dynamic nature of such bonding, we have developed a polyurethane equipped with dynamic urea bonds (PEDUB) that has high strength sufficient to make it be freestanding and have a healing capability and self-bonding property. This allowed subsequent heterogeneous multicomponent device integration of electrodes/substrate and light-emitting pixels into a light-emitting device. We first used the PEDUB to individually fabricate a highly stretchable electrode containing Ag nanowires and stretchable composites with ZnS-based particles. They were successfully assembled into a stretchable, waterproof electroluminescent (EL) device even under mild conditions (60 °C for 10 min) owing to the reversible exchange of urea bonds and low glass transition temperature of PEDUB. The assembled device with an AC-driven EL architecture retained excellent EL characteristics even after stretching, submersion in water, and cutting owing to the robust solid-state bonding interfaces induced by the dynamic urea bonds. Consequently, various shapes of the illuminating elastomer and an illuminated picture were realized for the first time using the mosaic-like assembly method. This first demonstration of multicomponent assembly paves the way for future stretchable multifunctional devices.

Patternable and Widely Colour-Tunable Elastomer-Based Electroluminescent Devices

We demonstrate wide colour tunability of polydimethylsiloxane-based alternating-current-driven electroluminescent devices with intrinsically stretchable characteristics achieved by simply modulating the electrical frequency. By employing both a screen-printed emitting layer and frequency-dependent colour tuning of ZnS:Cu-based phosphors, we demonstrate various coloured patterned images in a single device. We also show enhanced colour-tuning performance by mixing multi-colour phosphors, which results in a broad range of available coordinates in colour space. We believe that our demonstrated method could be used for manipulating broader colour expression as well as in various applications involving stretchable devices.