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Perovskite solar cells (PSCs) offer unique advantage of enabling lightweight and flexible power sources compared to the commercially available rigid wafer-based silicon solar cells, while their power conversion efficiencies are now nearly comparable. To ensure high performance, flexibility and lightweight characteristics in flexible perovskite solar cells and modules, it is essential to develop electrodes that are highly conductive, highly transparent, flexible and thin. In this review, we first highlight important technical metric, figure of merits (FoM), that reflects not only the optical and electrical properties of the transparent electrodes but also their mechanical flexibility. Then, we review recent advancements in transparent conductive electrodes for flexible perovskite solar cells, focusing on various materials and designs including conducting polymers, metal nanowires and meshes, carbon-based nanomaterials, transparent conducting oxides and other novel materials. Lastly, we discuss the challenges and future prospects toward commercialization of flexible perovskite solar modules. [ABSTRACT FROM AUTHOR]

Flexible optoelectronics, based on non-planar substrates, hold promise for diverse applications such as wearables, health monitors, and displays due to their cost-effective manufacturing methods. Despite the superior properties of metal oxides, the challenge of processing them at high temperatures incompatible with plastic substrates necessitates innovative annealing approaches. Photonic curing, which delivers microsecond to millisecond broadband (200–1500 nm) light pulses on a sample, emerges as a viable solution. Depending on the optical properties, the targeted film absorbs the radiant energy resulting in rapid heating while the transparent substrate absorbs a minimal amount of light and remains at ambient temperature. The light intensity can be high, but since the light pulse is short, the total energy absorbed by the sample remains low and will not damage the plastic substrate. This perspective explores the innovative application of photonic curing to fabricate flexible metal oxide optoelectronics, including thin-film transistors, metal–insulator–metal devices, solar cells, transparent conductors, and Li batteries, emphasizing the conversion of sol–gel precursors to metal oxides. However, this technique was initially developed for sintering metal nanoparticles to conductive patterns and poses intriguing challenges in explaining its mechanism for metal oxide conversion, especially considering the limited absorption of visible light by most sol–gel precursors. The review delves into UV-induced photochemistry, common flexible metal-oxide optoelectronic components, and non-intuitive distinctions between photonic curing and thermal annealing. By elucidating the distinctive role of photonic curing in overcoming temperature-related challenges and advancing the fabrication of flexible metal oxide optoelectronics, this perspective offers valuable insights that could shape the future of flexible optoelectronics. [ABSTRACT FROM AUTHOR]

Abstract The quest for mechanoluminescence (ML) in zinc sulfide (ZnS) spans more than a century, initially sparked by observations of natural minerals. There has been a resurgence in research into ML materials in recent decades, driven by advances in optoelectronic technologies and a deeper understanding of their luminescent properties under mechanical stress. ZnS, in particular, has garnered attention owing to its remarkable ability to sustain luminescence after more than 100,000 mechanical stimulations, positioning it as a standout candidate for optoelectronic applications. In contrast to conventional photoluminescent and electroluminescent light sources, ZnS composite elastomers have emerged as flexible, stretchable self‐powered light sources with considerable practical implications. This review introduces the development history, ML mechanisms, prototype ML devices, ZnS‐based ML material preparation methods, and their diverse applications spanning environmental mechanical‐to‐optical energy conversion, E‐signatures, anti‐counterfeiting, wearable information sensing devices, advanced battery‐free displays, biomedical imaging, and optical fiber sensors for human–computer interactions, among others. By integrating insights from ML‐optics, mechanics, and flexible optoelectronics, and by summarizing pertinent perspectives on current scientific challenges, application technology hurdles, and potential solutions for emerging scientific frontiers, this review aims to furnish fundamental guidance and conceptual frameworks for the design, advancement, and cutting‐edge application of novel mechanoluminescent materials.

Marrying nanostructures with thin films is a recent trend in flexible optoelectronics to improve light trapping and mechanical stability. Radial junction (RJ) a‐Si:H thin film optoelectronics, directly deposited upon the vapor‐liquid‐solid (VLS) grown silicon nanowires (SiNWs), can pave the way to achieving excellent flexibility and a high power‐to‐weight ratio (PTWR). Thanks to their vertical‐standing geometry, SiNWs framework can enhance the mechanical stability of RJ units, by keeping the working region far away from the stress‐rich substrate surface. Using low‐melting‐point metals as catalysts, the SiNW array can be fabricated at a low temperature down to 350 °C in a PECVD system, making it possible to directly construct robust RJ units on flexible, low‐cost substrates, such as Al foil. In this review, the recent progress of the flexible RJ a‐Si:H thin film solar cells/photodetectors that are directly fabricated on the surface of low‐cost flexible substrates is reviewed. Moreover, these RJ flexible optoelectronics are demonstrated for use in wearable and real‐time health monitoring, and as cardiac pacemakers for heart stimulation. This RJ technology highlights a promising potential to establish high‐performance, low‐cost, mechanically stable flexible optoelectronic for wearable, portable applications. [ABSTRACT FROM AUTHOR]

By virtue of its narrow pulse width and high peak power, the femtosecond pulsed laser can achieve high-precision material modification, material additive or subtractive, and other forms of processing. With additional good material adaptability and process compatibility, femtosecond laser-induced application has achieved significant progress in flexible electronics in recent years. These advancements in the femtosecond laser fabrication of flexible electronic devices are comprehensively summarized here. This review first briefly introduces the physical mechanism and characteristics of the femtosecond laser fabrication of various electronic microdevices. It then focuses on effective methods of improving processing efficiency, resolution, and size. It further highlights the typical progress of applications, including flexible energy storage devices, nanogenerators, flexible sensors, and detectors, etc. Finally, it discusses the development tendency of ultrashort pulse laser processing. This review should facilitate the precision manufacturing of flexible electronics using a femtosecond laser. [ABSTRACT FROM AUTHOR]

Photoelectrochemical (PEC)-type photodetectors are gaining significant interest as neoteric self-powered devices, owing to their high spectral responsivity and quick photoresponse. However, the underlying leakage issue makes the aqueous electrolyte contained inside PEC-type photodetectors unsuitable for manufacturing flexible optoelectronics. In this study, we report a flexible, self-powered PEC-type photodetector based on a cellulose-based hydrogel. Using a cellulose-based hydrogel electrolyte as the electron-transferring medium and two-dimensional Bi2O2Se nanosheet-coated indium tin oxide glass as the working electrode, the photodetector exhibits excellent self-powered capability. The hydrogel-based PEC photodetector exhibits a broadband photoresponse in the range of 365–850 nm. Moreover, under the condition of an illumination wavelength of 365 nm at a bias voltage of 0.6 V, the photodetector exhibits a responsivity of 0.68 mA/W, a detectivity of 2.44 × 108 Jones, and a fast response time of 85/103 ms, which are comparable to those of photodetectors based on traditional aqueous electrolytes. Additionally, the hydrogel-based PEC photodetectors show good flexibility and stability, maintaining more than 70.3% of their photocurrent density even after 100 cycles of bending at 120°, and no obvious photocurrent decay is observed after storage for 1 week. Based on these results, cellulose-based hydrogel electrolytes have significant potential for future applications in self-powered photodetectors. [ABSTRACT FROM AUTHOR]

Due to the novel physicochemical properties of polymeric nanocomposites (PNCs), a remarkable role of PNCs has been accomplished in plenty of flexible optoelectronic applications. Solution cast procedure has been performed to prepare different contents (0.1, 0.5, 1.0, 5.0, and 10 wt%) of Ag1.6Cu0.2S (ACS) incorporated in PVP/PVA blend. Fourier transforms infrared and scanning electron microscope measurements were carried out to investigate the influence of ACS on PVP/PVA structure and morphology. UV–visible spectrophotometry technique was applied to examine PVP/PVA optical properties modifications. Spectroscopic analysis reveals that direct/indirect optical bandgap of PVP/PVA host shrinks from 5.21/4.95 eV (PVP/PVA0) to 4.87/4.54 eV (PVP/PVA10). Moreover, obvious enrichments in the whole linear/nonlinear optical parameters are achieved by ACS doping. For example, refractive index and dielectric constants of PVP/PVA are increased from 1.69 and 2.87 to 1.91 and 3.63 (PVP/PVA10). While two orders of magnetite enhancement in nonlinear third‐order susceptibility are achieved via 10% ACS doping. The electrical performance of ACS PNCs has been explored, which discloses remarkable enrichments in PVP/PVA DC‐electrical conductivity (σDC) and activation energy. The obtained structural, spectroscopic and electrical investigations prove that ACS PNCs could play an efficient role in plenty of optoelectronic applications. Highlights: Solution casting was used to prepare PVP/PVA loaded with Ag/CuS (ACS) NPs.FT‐IR analysis reveals successful incorporation of ACS in PVP/PVA structure.Enrichments in linear/nonlinear optical parameters are achieved by ACS doping.Electrical performance of ACS PNCs has greatly been enhanced.ACS PNCs could play an efficient role in plenty of optoelectronic applications. [ABSTRACT FROM AUTHOR]

Flexible photodetectors (FPDs), which provide excellent advantages such as great wearability, portability, even implantability, have attracted tremendous interest in wearable healthcare monitoring, bendable imaging sensors, portable optical communication, etc. Recently, organic and perovskite‐based photoactive materials have been considered promising candidates for FPDs due to their superior optoelectronic performance, appealing mechanical flexibility, low‐temperature solution processability, and cost‐effectiveness. In this review, the milestone progress of organic, perovskite, and organic‐perovskite hybrid‐based FPDs and their applications in artificial intelligence are summarized. First, a brief introduction of device configurations, working principles, and key parameters for FPDs are presented. Subsequently, functional materials in FPDs, especially flexible photoactive materials of organic, perovskite, and organic‐perovskite hybrids, are summarized and analyzed. More importantly, representative applications of FPDs are summarized, including wearable healthcare monitoring, imaging sensors, and optical communication. The outlook and challenges for the field are proposed at the end. The purpose of this review is to not only elaborate on the fundamental design principles of FPDs, but also serve as a roadmap for next‐generation artificial intelligence sensing technologies. [ABSTRACT FROM AUTHOR]

Abstract Flexible photodetectors (FPDs), which provide excellent advantages such as great wearability, portability, even implantability, have attracted tremendous interest in wearable healthcare monitoring, bendable imaging sensors, portable optical communication, etc. Recently, organic and perovskite‐based photoactive materials have been considered promising candidates for FPDs due to their superior optoelectronic performance, appealing mechanical flexibility, low‐temperature solution processability, and cost‐effectiveness. In this review, the milestone progress of organic, perovskite, and organic‐perovskite hybrid‐based FPDs and their applications in artificial intelligence are summarized. First, a brief introduction of device configurations, working principles, and key parameters for FPDs are presented. Subsequently, functional materials in FPDs, especially flexible photoactive materials of organic, perovskite, and organic‐perovskite hybrids, are summarized and analyzed. More importantly, representative applications of FPDs are summarized, including wearable healthcare monitoring, imaging sensors, and optical communication. The outlook and challenges for the field are proposed at the end. The purpose of this review is to not only elaborate on the fundamental design principles of FPDs, but also serve as a roadmap for next‐generation artificial intelligence sensing technologies.

Polyvinyl alcohol (PVA) is considered as one of the most attractive polymers due to its unique physical and chemical characteristics. This work aims to enhance the optical and electrical performance of PVA/MWCNTs blend via doping with Cu/ZnS nanoparticles (NPs) for flexible eco-friendly applications. The optical and electrical performance of MWCNTs/PVA blend has been enhanced via doping with Cu/ZnS nanoparticles (NPs) for flexible optoelectronics. Undoped and Cu/ZnS doped MWCNTs/PVA polymeric nanocomposites (PNCs) films were equipped by casting method. The structure and optical characteristics were explored using Fourier transform infrared (FT-IR) and UV–visible–NIR spectrophotometery. The electrical properties as a function of temperature were studied using a four-probe stage and a Keithley sourcemeter. FT-IR measurements reveal clear variations in the structure of the PNCs due to Cu/ZnS doping. The impact of Cu/ZnS content on the major optical constants of MWCNTs/PVA has been investigated. The direct/indirect optical bandgap (Egdir./Egindir) decreases from 5.40 and 4.92 eV (undoped MWCNTs/PVA) to 4.77 eV and 4.24 eV (5 wt% PNCs). Wemple-DiDomenico (WDD) and Sellmeier oscillator models were used to investigate the dispersion constants. The influence of Cu/ZnS content on electrical conductivity (σdc) and activation energy has been explored. Great enhancement of the optical and electrical constants of MWCNTs/PVA is achieved via doping with Cu/ZnS NPs. The obtained results nominate Cu/ZnS PNCs for applications in flexible optoelectronics. [ABSTRACT FROM AUTHOR]

Mechanoluminescence, a smart luminescence phenomenon in which light energy is directly produced by a mechanical force, has recently received significant attention because of its important applications in fields such as visible strain sensing and structural health monitoring. Up to present, hundreds of inorganic and organic mechanoluminescent smart materials have been discovered and studied. Among them, strontium‐aluminate‐based materials are an important class of inorganic mechanoluminescent materials for fundamental research and practical applications attributed to their extremely low force/pressure threshold of mechanoluminescence, efficient photoluminescence, persistent afterglow, and a relatively low synthesis cost. This paper presents a systematic and comprehensive review of strontium‐aluminate‐based luminescent materials' mechanoluminescence phenomena, mechanisms, material synthesis techniques, and related applications. Besides of summarizing the early and the latest research on this material system, an outlook is provided on its environmental, energy issue and future applications in smart wearable devices, advanced energy‐saving lighting and displays. [ABSTRACT FROM AUTHOR]

Polysiloxanes and materials based on them (silicone materials) are of great interest in optoelectronics due to their high flexibility, good film-forming ability, and optical transparency. According to the literature, polysiloxanes are suggested to be very promising in the field of optoelectronics and could be employed in the composition of liquid crystal devices, computer memory drives organic light emitting diodes (OLED), and organic photovoltaic devices, including dye synthesized solar cells (DSSC). Polysiloxanes are also a promising material for novel optoectronic devices, such as LEDs based on arrays of III–V nanowires (NWs). In this review, we analyze the currently existing types of silicone materials and their main properties, which are used in optoelectronic device development. [ABSTRACT FROM AUTHOR]

The rapid advancement of flexible optoelectronic devices, such as light-emitting diodes, solar cells, and electrochromic devices, necessitates the development of high-performance flexible transparent electrodes (TEs). Dielectric/metal/dielectric (DMD)-type TEs are promising alternatives to conventional indium tin oxide (ITO) due to the high electrical conductance, excellent visible transparency, and sufficient mechanical flexibility. However, the tradeoff between electrical conductance and visible transparency poses a challenge to performance enhancement. This study introduces an Ar-ion-mediated interface modification method to address this tradeoff by dual smoothing of dielectric/metal interfaces in TiO x /Cu/ZnO TEs. Implementing this dual smoothing methodology significantly enhances both electrical conductance and visible light transmittance, achieving a Haacke figure of merit 200% higher than that of an unmodified otherwise identical structure. The highest figure of merit is 0.113 Ω -1 , a record high for Cu-thin-layer-based DMD TEs, far surpassing ITO electrode values. Further, the enhanced optoelectronic performance remains highly durable under severe and simultaneous electrical, thermal, and mechanical stresses, showcasing the potential for significant advances in flexible optoelectronics.

Abstract Mechanoluminescence, a smart luminescence phenomenon in which light energy is directly produced by a mechanical force, has recently received significant attention because of its important applications in fields such as visible strain sensing and structural health monitoring. Up to present, hundreds of inorganic and organic mechanoluminescent smart materials have been discovered and studied. Among them, strontium‐aluminate‐based materials are an important class of inorganic mechanoluminescent materials for fundamental research and practical applications attributed to their extremely low force/pressure threshold of mechanoluminescence, efficient photoluminescence, persistent afterglow, and a relatively low synthesis cost. This paper presents a systematic and comprehensive review of strontium‐aluminate‐based luminescent materials’ mechanoluminescence phenomena, mechanisms, material synthesis techniques, and related applications. Besides of summarizing the early and the latest research on this material system, an outlook is provided on its environmental, energy issue and future applications in smart wearable devices, advanced energy‐saving lighting and displays.

2D organic molecular crystals (2DOMCs) are promising materials for the fabrication of high‐performance optoelectronic devices. However, the growth of organic molecules into 2DOMCs remains a challenge because of the difficulties in controlling their self‐assembly with a preferential orientation in solution‐process crystallization. Herein, fullerene is chosen as a model molecule to develop a supramolecular gel crystallization approach to grow large‐area 2DOMCs by controlling the perfect arrangement on the {220} crystal plane with the assistance of a gelated solvent. In this case, the gel networks provide tuneable confined spaces to control the crystallization kinetics toward the growth of dominant crystal faces by their inhibiting motions of solvent or solute molecules to enable the growth of perfect crystals at appropriate nucleation rates. As a result, a large‐area fullerene 2DOMC is produced successfully and its corresponding device on a flexible substrate exhibits excellent bendable properties and ultra‐high weak light detection ability (2.9 × 1011 Jones) at a 10 V bias upon irradiation with 450 nm incident light. Moreover, its photoelectric properties remain unchanged after 200 cycles of bending at angles of 45, 90, and 180°. These results can be extended to the growth of other 2DOMCs for potentially fabricating advanced organic (opto)electronics. [ABSTRACT FROM AUTHOR]

Graphene shows great potential for flexible optoelectronic devices owing to its unique 2D structure and excellent electronic, optical, and mechanical properties. Chemical vapor deposition (CVD) is the most promising method for fabricating large‐area and high‐quality graphene films at an acceptable cost; therefore enormous efforts have been attempted to investigate the flexible optoelectronic devices based on CVD‐grown graphene. Here, recent advances and significant development of CVD‐grown graphene towards flexible optoelectronics including photodetectors, organic solar cells, and light‐emitting diodes are reviewed. Insight into the challenges of improvement of optoelectronic properties, work function tuning, as well as interfacial control of CVD‐grown graphene for high‐performance devices is provided. In particular, the availability to fabricate large‐area devices on the flexible substrates is discussed, which is crucial to drive the practical use of CVD‐grown graphene for future wearable optoelectronics. [ABSTRACT FROM AUTHOR]

InSe, a member of the layered materials family, is a superior electronic and optical material which retains a direct bandgap feature from the bulk to atomically thin few-layers and high electronic mobility down to a single layer limit. We, for the first time, exploit strain to drastically modify the bandgap of two-dimensional (2D) InSe nanoflakes. We demonstrated that we could decrease the bandgap of a few-layer InSe flake by 160 meV through applying an in-plane uniaxial tensile strain to 1.06% and increase the bandgap by 79 meV through applying an in-plane uniaxial compressive strain to 0.62%, as evidenced by photoluminescence (PL) spectroscopy. The large reversible bandgap change of ∼239 meV arises from a large bandgap change rate (bandgap strain coefficient) of few-layer InSe in response to strain, ∼154 meV/% for uniaxial tensile strain and ∼140 meV/% for uniaxial compressive strain, representing the most pronounced uniaxial strain-induced bandgap strain coefficient experimentally reported in 2D materials. We developed a theoretical understanding of the strain-induced bandgap change through first-principles DFT and GW calculations. We also confirmed the bandgap change by photoconductivity measurements using excitation light with different photon energies. The highly tunable bandgap of InSe in the infrared regime should enable a wide range of applications, including electro-mechanical, piezoelectric and optoelectronic devices.

Abstract 2D organic molecular crystals (2DOMCs) are promising materials for the fabrication of high‐performance optoelectronic devices. However, the growth of organic molecules into 2DOMCs remains a challenge because of the difficulties in controlling their self‐assembly with a preferential orientation in solution‐process crystallization. Herein, fullerene is chosen as a model molecule to develop a supramolecular gel crystallization approach to grow large‐area 2DOMCs by controlling the perfect arrangement on the {220} crystal plane with the assistance of a gelated solvent. In this case, the gel networks provide tuneable confined spaces to control the crystallization kinetics toward the growth of dominant crystal faces by their inhibiting motions of solvent or solute molecules to enable the growth of perfect crystals at appropriate nucleation rates. As a result, a large‐area fullerene 2DOMC is produced successfully and its corresponding device on a flexible substrate exhibits excellent bendable properties and ultra‐high weak light detection ability (2.9 × 1011 Jones) at a 10 V bias upon irradiation with 450 nm incident light. Moreover, its photoelectric properties remain unchanged after 200 cycles of bending at angles of 45, 90, and 180°. These results can be extended to the growth of other 2DOMCs for potentially fabricating advanced organic (opto)electronics.

Top-emitting OLEDs (TOLEDs) represent a promising technology for the development of next-generation flexible and rollable displays, thanks to their improved light outcoupling and their compatibility with opaque substrates. Metal thin films are the most used electrodes for the manufacturing of TOLEDs, but they show poor resistance to mechanical deformation, which compromises the long-term durability of flexible devices. This paper reports the exploitation of a dielectric mirror (DBR) based on seven pairs of TiO2 and SiO2 combined with a polymeric electrode as an alternative to the bottom metal electrode in flexible TOLEDs. The DBR showed a maximum reflectivity of 99.9% at about 550 nm, and a stop-band width of about 200 nm. The reflectivity remained unchanged after bending and treatment with water and solvents. Green TOLED devices were fabricated on top of DBRs, and demonstrated good stability in terms of electro-optical and colorimetric characteristics, according to varying viewing angles. These results demonstrate that the combination of the flexible DBR with the polymeric anode is an interesting strategy for improving the durability of flexible TOLEDs for display applications, implemented on different kinds of free-standing ultra-thin substrates.

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries, including healthcare and wellness, electronic skins, the automotive industry, and foldable or rollable displays. Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface. Two-dimensional van der Waals materials (2D VdWMs) have exceptional multifunctional properties, including large specific area, dangling-bond-free interface, plane-to-plane van der Waals interactions, and excellent mechanical, electrical, and optical properties. Thus, 2D VdWMs have considerable application potential in functional intelligent flexible devices. To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures, new designs and configurations of electronics and optoelectronics have emerged. However, these new designs and configurations do not consider lattice mismatch and process incompatibility issues. In this review, we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly. Moreover, we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Metal–halide perovskites have recently shown tremendous progress in flexible photodetector applications owing to their great optical and electronic properties. However, apart from charge generating material, the high performance device requires a reasonable choice of electrodes for efficient carrier management as well. For example, the widespread use of gold and silver electrodes often results in perovskite device degradation while being expensive. Here, low‐cost and chemically inert single‐walled carbon nanotube (SWCNT) thin films are employed as electrodes to create highly responsive and flexible photodetector based on cesium lead tribromide (CsPbBr3) microcrystals. Direct growth of the perovskite on SWCNT forms excellent contact between the components leading to the state‐of‐the‐art responsivity for flexible perovskite photodetectors 1321 A W−1 at 5 V and under illumination intensity 1 mW cm−2 at 505 nm wavelength. The advanced properties of SWCNT films realized on a flexible substrate allow for robust operation over 104 cycles of device bending along with all parameters stability at ambient conditions for at least 1.5 months. The proposed design reveals the potential of SWCNT thin film electrodes for high performance perovskite flexible devices. [ABSTRACT FROM AUTHOR]

Large‐area fabrication and stacking of various nanometer‐thick functional layers from solutions is essentially important for the construction of flexible thin‐film optoelectronic devices, but very challenging. The existing fabrication methods suffer from either non‐uniformity caused by the coffee‐ring effect or serious solution waste (excess of 90% for spin coating), and are hard to scale up and create stacks. Here, it is shown that centrifugal casting is a universal, scalable, and efficient method to fabricate uniform nanometer‐thick films and their stacks of various materials. The coffee‐ring effect is effectively suppressed, the solution utilization ratio is higher than ≈61%, and the films/stacks show a smooth surface/high‐quality interface. Using this method, flexible quantum dot light‐emitting diode displays with uniform luminance in a large lighting area of ≈115 cm2 that have not been achieved even on rigid substrates by the existing methods, are realized. This efficient and low‐cost solution processing method paves a way for large‐area fabrication of various flexible thin‐film optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this review, the primary focus is the recent advances in the development of freestanding inorganic crystalline semiconductors and their manipulation technology for flexible optoelectronic applications. We firstly cover the details of the growth and processing techniques of freestanding inorganic crystalline semiconductors in various dimensions and their material property under strain condition. Finally, fabrication processes and opto-electrical properties of flexible optoelectronics are introduced. Future research directions are also discussed, including further enhancement of device performance, building more types of optoelectronic devices on flexible substrates, and process integration for the advanced optoelectronic circuits and systems.

In recent years, flexible UV photodetectors (PDs) with complex environmental adaptability and great wearability have attracted the attention of researchers worldwide. Wide bandgap inorganic semiconductor materials with excellent optoelectronic properties and mechanical stability are key functional materials for UV PD devices. However, the high temperature processing and inherent brittleness limit the further application of high-quality inorganic semiconductors in the field of flexible optoelectronics. In this work, we develop a specific flip-chip bonding fabrication technique that utilizes high-temperature treated inorganic semiconductor materials for high-performance flexible UV detection devices. Leveraging this technique, a 7 × 7 pixel flexible UV photodetector array (UV-FPDA) device based on a vertical architecture Mg-doped ZnO/NiO (Mg:ZnO/NiO) heterojunction transistor is built. The UV-FPDAs exhibit a high responsivity of 75.8 A/W and an outstanding detectivity of 8.5 × 10 12 Jones. Besides, the UV-FPDAs also demonstrate excellent bending stability. Furthermore, the photoresponse characteristics of each pixel are trained and learned by an artificial neural network to achieve clear imaging of UV light information. Our results provide a new pathway for the application of inorganic semiconductors in the field of high-performance flexible UV photodetection.

The optoelectronic signatures of free-standing few-atomic-layer black phosphorus nanoflakes are analyzed by in situ transmission electron microscopy (TEM). As compared to other 2D materials, the band gap of black phosphorus (BP) is related directly to multiple thicknesses and can be tuned by nanoflake thickness and strain. The photocurrent measurements with the TEM show a stable response to infrared light illumination and change of nanoflakes band gap with deformation while pressing them between two electrodes assembled in the microscope. The photocurrent spectra of an 8- and a 6-layer BP nanoflake samples are comparatively measured. Density functional theory (DFT) calculations are performed to identify the band structure changes of BP during deformations. The results should help to find the best pathways for BP smart band gap engineering via tuning the number of material atomic layers and programmed deformations to promote future optoelectronic applications., (© 2023 The Authors. Small published by Wiley‐VCH GmbH.)

Flexible optoelectronics have fully demonstrated their transformative roles in various fields, but their fabrication and application have been limited by complex processes. Liquid metals (LMs) are promising to be ideal raw materials for making flexible optoelectronics due to their extraordinary fluidity and printability. Herein, we propose a painting-modifying strategy based on solution processability for directly printing out fluorescent flexible optoelectronics from LMs via surface modification. The LMs of eGaIn, which were obtained by the mixing of gallium with indium metal spheres, were used as ink to paint high-finesse patterns on flexible substrates. Through introducing surface modification of LMs, the gallium atom on the surface of the LMs was directly transformed into the composite fluorescent functional layers of GaO(OH) and GaN after being modified with an ammonia aqueous solution. Owing to painting, this strategy is not limited by any curved surfaces, shapes, or facilities and has excellent adaptability. Particularly, the fluorescent layers were obtained through a spontaneous, instantaneous, and solution-processable process that is environmentally friendly, easy to administrate, recyclable, and adjustable. The present finding breaks through the limitations of LMs in making flexible optoelectronics and provides strategies for addressing severe challenges facing existing materials and flexible optoelectronics. This method is expected to be very useful for fabricating flexible lights, transformable displays, intelligent anticounterfeiting devices, skin-inspired optoelectronics, and chameleon-biomimetic soft robots in the coming time.

Many organisms move directly toward light for prey hunting or navigation, which is called phototaxis. Mimicking this behavior in robots is crucially important in the energy industry and environmental exploration. However, the phototaxis robots with rigid bodies and sensors still face challenges in adapting to unstructured environments, and the soft phototaxis robots often have high requirements for light sources with limited locomotion performance. Here, we report a 3.5 g soft microrobot that can perceive the azimuth angle of light sources and exhibit rapid phototaxis locomotion autonomously enabled by three-dimensional flexible optoelectronics and compliant shape memory alloy (SMA) actuators. The optoelectronics is assembled from a planar patterned flexible circuit with miniature photodetectors, introducing the self-occlusion to light, resulting in high sensing ability (error < 3.5°) compared with the planar counterpart. The actuator produces a straightening motion driven by an SMA wire and is then returned to a curled shape by a prestretched elastomer layer. The actuator exhibits rapid actuation within 0.1 s, a significant degree of deformation (curvature change of ∼87 m -1 ) and a blocking force of ∼0.4 N, which is 68 times its own weight. Finally, we demonstrated the robot is capable of autonomously crawling toward a moving light source in a hybrid aquatic-terrestrial environment without human intervention. We envision that our microrobot could be widely used in autonomous light tracking applications.

Recently, significant efforts have been directed at overcoming the limitations of conventional rigid optoelectronic devices, particularly their poor mechanical stability under bending, folding, and stretching deformations. One of major approaches for rendering optoelectronic devices mechanically deformable is to replace the conventional electronic/optoelectronic materials with functional nanomaterials or organic materials that are intrinsically flexible/stretchable. Further, advanced device designs and unconventional fabrication methods have also contributed to the development of soft optoelectronic devices. Accordingly, new devices such as bio-inspired curved image sensors, wearable light emitting devices, and deformable bio-integrated optoelectronic devices have been developed. In this review, recent progress in the development of soft optoelectronic materials and devices is outlined. First, various materials such as nanomaterials, organic materials, and their hybrids that are suitable for developing deformable photodetectors, are presented. Then, the nanomaterials and organic/polymeric materials that are applicable in deformable light-emitting diodes are described. Finally, representative system-level applications of flexible and stretchable photodetectors and light-emitting diodes are reviewed, and future prospects are discussed. [ABSTRACT FROM AUTHOR]

Crystalline or amorphous metal oxides are widely used in various optoelectronic devices as key components, such as transparent conductive electrodes, dielectrics or semiconducting active layers for thin‐film transistor (TFT) backplanes in large‐area displays, photovoltaics, and light‐emitting diodes. Although crystalline inorganic materials demonstrate outstanding optoelectronic performance, owing to their wide bandgaps, large conductivities, and high carrier mobilities, their inherent brittleness makes them vulnerable to mechanical stress, thereby limiting the use of metal‐oxide films in emerging flexible electronic applications. In this study, stress‐diffusive organic–inorganic hybrid superlattice nanostructures are developed to overcome the mechanical limitation of crystalline oxides and to provide high mechanical stability to metal‐oxide semiconductors. In particular, hybrid transparent superlattice electrodes based on crystalline indium–tin oxide exhibit high electrical conductivities of up to 555 S cm–1 (resistance variation < 3%) and effectively reduce the mechanical stress on the inorganic layer (up to 10 000 bending cycles with a radius of 1 mm). Furthermore, to ensure the viability of the hybrid superlattice flexible electronics, all solution‐processed superlattice crystalline indium–gallium‐oxide TFTs are implemented on a thin (≈5 µm) polyimide substrate, providing highly robust and excellent electrical performance (average mobility of 7.6 cm2 V–1 s–1). [ABSTRACT FROM AUTHOR]

Luminescent inorganic photochromics have great potential applications in 3D optical memory due to their excellent thermal stability and chemical resistance. Nevertheless, constructing inorganic photochromics with a high luminescence switching contrast remains challenging for optical information storage. Understanding inherent relationships between microenvironment changes, photophysical material properties, and rational control of their luminescence properties are crucial to address these issues. This paper proposes an effective strategy to significantly improve the luminescence switching contrast by more than 300% by site‐selective occupancy engineering on BaMgSiO4:Eu2+ photochromic materials. An effective energy transfer process is established between luminescent and photochromic units, which may provide new insights for designing high‐performance optical storage systems. The fabricated flexible films exhibit excellent waterproof, flexibility, and stretchability characteristics, accompanied by reversible color switching and larger color contrast. Optical data storage can be not only visually encoded and decoded on the flexible films by 405 and 532 nm LED excitation, respectively. Writing, erasing, and reading strategies for flexible films are particularly suitable for complex application environments in optical storage and rewritable flexible devices. [ABSTRACT FROM AUTHOR]

Top-emitting OLEDs (TOLEDs) represent a promising technology for the development of next-generation flexible and rollable displays, thanks to their improved light outcoupling and their compatibility with opaque substrates. Metal thin films are the most used electrodes for the manufacturing of TOLEDs, but they show poor resistance to mechanical deformation, which compromises the long-term durability of flexible devices. This paper reports the exploitation of a dielectric mirror (DBR) based on seven pairs of TiO2 and SiO2 combined with a polymeric electrode as an alternative to the bottom metal electrode in flexible TOLEDs. The DBR showed a maximum reflectivity of 99.9% at about 550 nm, and a stop-band width of about 200 nm. The reflectivity remained unchanged after bending and treatment with water and solvents. Green TOLED devices were fabricated on top of DBRs, and demonstrated good stability in terms of electro-optical and colorimetric characteristics, according to varying viewing angles. These results demonstrate that the combination of the flexible DBR with the polymeric anode is an interesting strategy for improving the durability of flexible TOLEDs for display applications, implemented on different kinds of free-standing ultra-thin substrates. [ABSTRACT FROM AUTHOR]

Full-Stokes polarization detection, with high integration and portability, offers an efficient path toward next-gen multi-information optoelectronic systems. Nevertheless, current techniques relying on optical filters create rigid and bulky configurations, limiting practicality. Here, a flexible, filter-less full-Stokes polarimeter featuring a uniaxial-oriented chiral perovskite film is first reported. It is found that, the strategic manipulation of the surfactant-mediated Marangoni effect during blade coating, is crucial for guiding an equilibrious mass transport to achieve oriented crystallization. Through this approach, the obtained uniaxial-oriented chiral perovskite films inherently possess anisotropy and chirality, and thereby with desired sensitivity to both linearly polarized light and circularly polarized light vectors. The uniaxial-oriented crystalline structure also improves photodetection, achieving a specific detectivity of 5.23 × 10 13  Jones, surpassing non-oriented devices by 10×. The as-fabricated flexible polarimeters enable accurate capture of full-Stokes polarization without optical filters, exhibiting slight detection errors for the Stokes parameters: ΔS 1  = 9.2%, ΔS 2  = 8.6%, and ΔS 3  = 6.5%, approaching the detection accuracy of optics-filter polarimeters. This proof of concept also demonstrates applications in matrix polarization imaging., (© 2024 Wiley‐VCH GmbH.)

Quasi-one-dimensional nanoribbons (NRs) are promising candidate materials for flexible optoelectronics with high device density. The optical properties of nanomaterials are strongly influenced by excitons, and it is important for flexible devices to explore how excitons behave under strain. In this work, we investigate the electronic and excitonic properties of bent black phosphorus (BP) NRs via density-functional theory and many-body Green's function methods. We find that inhomogeneous strain fields affect not only the first excitonic energy and thus the optical gap, but also the spatial distribution of the excitons. In particular, excitonic funnels can occur in bent black phosphorus nanoribbons (BPNRs), which drive higher-energy excitons toward lower energy locations. The funnels can be effectively tuned by the intensity and sign (compressed to tensile) of the strain field. Another critical factor is anisotropy: the funnel effect is much more pronounced in armchair BPNRs than in zigzag BPNRs. In addition, we find that the excitonic funnel effect in two-dimensional monolayer BP differs from that in multilayer BP films.

Flexible optoelectronics, as promising components hold shape‐adaptive features and dynamic strain response under strain engineering for various intelligent applications. 2D materials with atomically thin layers are ideal for flexible optoelectronics because of their high flexibility and strain sensitivity. However, how the strain affects the performance of 2D materials‐based flexible optoelectronics is confused due to their hypersensitive features to external strain changes. It is necessary to establish an evaluation system to comprehend the influence of the external strain on the intrinsic properties of 2D materials and the photoresponse performance of their flexible optoelectronics. Here, a focused review of strain engineering in 2D materials‐based flexible optoelectronics is provided. The first attention is on the mechanical properties and the strain‐engineered electronic properties of 2D semiconductors. An evaluation system with relatively comprehensive parameters in functionality and service capability is summarized to develop 2D materials‐based flexible optoelectronics in practical application. Based on the parameters, some strategies to improve the functionality and service capability are proposed. Finally, combining with strain engineering in future intelligence devices, the challenges and future perspective developing 2D materials‐based flexible optoelectronics are expounded. [ABSTRACT FROM AUTHOR]

Graphene has emerged as an attractive candidate for flexible transparent electrode (FTE) for a new generation of flexible optoelectronics. Despite tremendous potential and broad earlier interest, the promise of graphene FTE has been plagued by the intrinsic trade-off between electrical conductance and transparency with a figure of merit (σDC/σOp) considerably lower than that of the state-of-the-art ITO electrodes (σDC/σOp <123 for graphene vs. ∼240 for ITO). Here we report a synergistic electrical/optical modulation strategy to simultaneously boost the conductance and transparency. We show that a tetrakis(pentafluorophenyl)boric acid (HTB) coating can function as highly effective hole doping layer to increase the conductance of monolayer graphene by sevenfold and at the same time as an anti reflective layer to boost the visible transmittance to 98.8%. Such simultaneous improvement in conductance and transparency breaks previous limit in graphene FTEs and yields an unprecedented figure of merit (σDC/σOp ∼323) that rivals the best commercial ITO electrode. Using the tailored monolayer graphene as the flexible anode, we further demonstrate high-performance green organic light emitting diodes (OLEDs) with the maximum current, power and external quantum efficiencies (111.4 cd A−1, 124.9 lm W−1 and 29.7%) outperforming all comparable flexible OLEDs and surpassing that with standard rigid ITO by 43%. This study defines a straightforward pathway to tailor optoelectronic properties of monolayer graphene and to fully capture their potential as a generational FTE for flexible optoelectronics. [ABSTRACT FROM AUTHOR]

Nanoscale tellurium (Te) materials are promising for advanced optoelectronics owing to their many intriguing properties. However, the facile fabrication of highly uniform 1D Te nanostructures has been a challenge. Herein, an electrochemical exfoliation method is developed to prepare uniform Te nanorod (NRs), which can form stable and high concentration suspension in organic solvents. The morphology of the Te NRs can be well tuned by the electrochemical exfoliation condition. Flexible photodetectors based on Te NRs show an excellent responsivity of 6.1 A W−1. The detectivity achieves 1.2 × 1011 Jones, which is the highest among Te‐based photodetectors. Moreover, self‐powered photodetectors based on Te NRs are successfully fabricated, which exhibits a responsivity of 3 µA W−1. This new method paves the way for the applications of Te NRs in self‐powered flexible optoelectronics. [ABSTRACT FROM AUTHOR]

A path to further increase the power conversion efficiency of perovskite solar cells is maximizing sunlight absorption using patterned substrates with enhanced light‐trapping capabilities. However, to be competitive with traditional solar cells, especially in niche markets, low‐cost texturing methods have to be developed. Herein, a roll‐to‐roll hot embossing method to pattern flexible polyethylene terephthalate (PET) foils is presented as a proof of concept. The cylindrical mold for the embossing process is structured with periodic grooves by picosecond direct laser interference patterning (DLIP). The optical characterization of the textured PET shows a tenfold increase in the haze factor (diffuse‐to‐global transmittance ratio) compared with flat PET due to the high‐intensity diffracted beams. Flexible triple cation perovskite cells deposited onto these patterned substrates show on average an 8% (relative) higher efficiency than similar devices deposited on a reference flat PET substrate. This enhancement can be attributed to the increase in light trapping provided by the textured substrate. Finally, a cost analysis model shows that the additional cost of integrating the proposed hot embossing step into a perovskite solar module manufacturing facility will represent less than 0.35% of the initial fabrication cost. [ABSTRACT FROM AUTHOR]

In this review, the primary focus is the recent advances in the development of freestanding inorganic crystalline semiconductors and their manipulation technology for flexible optoelectronic applications. We firstly cover the details of the growth and processing techniques of freestanding inorganic crystalline semiconductors in various dimensions and their material property under strain condition. Finally, fabrication processes and opto-electrical properties of flexible optoelectronics are introduced. Future research directions are also discussed, including further enhancement of device performance, building more types of optoelectronic devices on flexible substrates, and process integration for the advanced optoelectronic circuits and systems. [ABSTRACT FROM AUTHOR]

Controlled growth of heterostructured nanowires and mechanisms of their formation have been actively studied during the last decades due to perspectives of their implementation. Here, we report on the self-catalyzed growth of axially heterostructured GaPN/GaP nanowires on Si(111) by plasma-assisted molecular beam epitaxy. Nanowire composition and structural properties were examined by means of Raman microspectroscopy and transmission electron microscopy. To study the optical properties of the synthesized nanoheterostructures, the nanowire array was embedded into the silicone rubber membrane and further released from the growth substrate. The reported approach allows us to study the nanowire optical properties avoiding the response from the parasitically grown island layer. Photoluminescence and Raman studies reveal different nitrogen content in nanowires and parasitic island layer. The effect is discussed in terms of the difference in vapor solid and vapor liquid solid growth mechanisms. Photoluminescence studies at low temperature (5K) demonstrate the transition to the quasi-direct gap in the nanowires typical for diluted nitrides with low N-content. The bright room temperature photoluminescent response demonstrates the potential application of nanowire/polymer matrix in flexible optoelectronic devices.

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Polymeric carbon nitride (PCN) is a stable semiconducting material with an intermediate band gap (2–3 eV), which is efficient for catalysis and optoelectronics. However, it is still a big challenge to synthesize large-area and transferrable PCN films for applications in solid state optoelectronics. In this work, by using nitrogen-doped graphene (NG) as a van der Waals epitaxial substrate, centimeter-size PCN films are synthesized via polymerization of melamine molecules. As-grown PCN/NG films can be then transferred onto other substrates (e.g. SiO 2 /Si wafers, quartz slides, polymer substrates). Structural characterization reveals a polymerized structure of PCN films with nitrogen-containing heterocycles. By stacking PCN/NG films with graphene films, it is possible to construct a photodetector responsive to near-UV and UV illumination under ambient conditions. The responsivities of the photodetector are 0.59 mA/W and ∼30 μA/W towards 365 nm lamp and 488 nm laser, respectively. Our PCN photodetectors also show fast response times (e.g. ∼0.29 s to 488 nm laser illumination). Furthermore, our PCN photodetector can be fabricated on polymer substrates. As-obtained flexible photodetectors can maintain its photo-response after 100 times bending. Our results clearly demonstrate the possibility of employing large-area carbon-based semiconductors to meet the increasing demands of wearable and portable electronics. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) nanomaterials have recently attracted considerable attention due to their promising applications in next-generation electronics and optoelectronics. In particular, the large-scale synthesis of high-quality 2D materials is an essential requirement for their practical applications. Herein, we demonstrate the wafer-scale synthesis of highly crystalline and homogeneous monolayer WS2 by an enhanced chemical vapor deposition (CVD) approach, in which precise control of the precursor vapor pressure can be effectively achieved in a multi-temperature zone horizontal furnace. In contrast to conventional synthesis methods, the obtained monolayer WS2 has excellent uniformity both in terms of crystallinity and morphology across the entire substrate wafer grown (e.g., 2 inches in diameter), as corroborated by the detailed characterization. When incorporated in typical rigid photodetectors, the monolayer WS2 leads to a respectable photodetection performance, with a responsivity of 0.52 mA/W, a detectivity of 4.9 × 109 Jones, and a fast response speed (< 560 μs). Moreover, once fabricated as flexible photodetectors on polyimide, the monolayer WS2 leads to a responsivity of up to 5 mA/W. Importantly, the photocurrent maintains 89% of its initial value even after 3,000 bending cycles. These results highlight the versatility of the present technique, which allows its applications in larger substrates, as well as the excellent mechanical flexibility and robustness of the CVD-grown, homogenous WS2 monolayers, which can promote the development of advanced flexible optoelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract: Piezo‐phototronic devices, where optoelectrical properties are directly influenced by mechanical stimuli, are highly desirable for applications in wearable devices and human–machine interfaces. Here, the piezoelectric and piezo‐phototronic properties of GaSe nanosheets, a layered metal‐monochalcogenide III–VI semiconductor with interesting piezoelectric, optical excitation, and semiconducting properties are investigated. A giant piezo‐phototronic response in GaSe is demonstrated for the first time. The out‐of‐plane local field due to band gap modulation drives the electrons (holes) to move toward the outer (inner) surface of wrinkles, which enhances electron–hole pair generation and the related photocurrent. Moreover, manual bending of GaSe reliably enhances the photocurrent by more than a factor of 50 at room temperature. This giant and linear piezo‐phototronic response combined with excellent stretchability suggests that GaSe is a valuable material for flexible optoelectronic‐mechanical applications. [ABSTRACT FROM AUTHOR]

The microstimulation of specific neural populations of the brain is one of the facile and reliable methods used in neuroscience for deduction of functional movement, complex behavior and even long-range connectivity. Recent advanced biomedical tools now employ flexible optoelectronic devices combined with optogenetic mouse models to induce high spatiotemporal modulation of specific brain activity. However, most current applications are limited to activation of small functional regions using blue-light driven channelrhodopsin. In this report, we introduce flexible AlGaInP vertical light-emitting diodes (VLEDs) for perturbation of specific functional areas of mouse cortex. Micro-scaled LEDs effectively compress the conductive balls dispersed in anisotropic conductive film (ACF) resulting red light emissions with high optical power density, capable of stimulating motor neurons deep below layer III from the brain surface. Selective operation of pulsed red light from f-VLEDs induces mouse body movements and synchronized electromyogram (EMG) signals. The expression of chrimson, red-shifted channelrhodopsin, enables red-light excitation of targeted functional area of motor cortex. This demonstration opens new opportunities for entire cortical mapping, to explore the connectivity between undefined motor areas in the mouse brain. [ABSTRACT FROM AUTHOR]

Curved image sensors inspired by animal and insect eyes have provided a new development direction in next-generation digital cameras. It is known that natural fish eyes afford an extremely wide field of view (FOV) imaging due to the geometrical properties of the spherical lens and hemispherical retina. However, its inherent drawbacks, such as the low off-axis illumination and the fabrication difficulty of a ‘dome-like’ hemispherical imager, limit the development of bio-inspired wide FOV cameras. Here, a new type of fisheye imaging system is introduced that has simple lens configurations with a curvilinear image surface, while maintaining high off-axis illumination and a wide FOV. Moreover, through comparisons with commercial conventional fisheye designs, it is determined that the volume and required number of optical elements of the proposed design is practical while capturing the fundamental optical performances. Detailed design guidelines for tailoring the proposed optic system are also discussed. [ABSTRACT FROM AUTHOR]