Flexible Transparent Devices Research Articles

Hydrogel Based UV Photodetector Based on Polarization of Free Water Molecules

AbstractConductive hydrogels retain the intrinsic softness and elasticity that are hallmark features of traditional hydrogels, and they also assimilate the electrical properties of the embedded conductive materials. Conventional epitaxial growth techniques for heterojunctions require precise lattice matching between adjacent materials, significantly limiting technological advancements in photodetector innovation. The integration of conductive hydrogels with semiconductors to fabricate photodetectors presents a feasible solution that bypasses the traditional lattice matching constraints inherent in the epitaxial growth of heterojunction semiconductor photodetectors. Herein, device integration is enhanced by developing a transparent conductive hydrogel, positioned between graphene and gallium nitride, which effectively mitigates the fluidity issues of the polar liquid complicating device encapsulation. A comprehensive analysis of the photodetection characteristics of the graphene/conductive hydrogel/gallium nitride heterostructure is conducted, thereby establishing its structural framework. Flexible transparent devices are further prepared based on PEDOT/Alg (Ca2+) and demonstrate their potential application in 360° omnidirectional photodetection. This work introduces a novel approach for the enhanced integration of hydrogel‐based spectrally selective and flexible transparent photodetectors.

Structure and Thermomechanical Properties of Polyvinylidene Fluoride Film with Transparent Indium Tin Oxide Electrodes

This paper is devoted to the study of the structure and thermomechanical properties of PVDF-based ferroelectric polymer film. Transparent electrically conductive ITO coatings are applied to both sides of such a film. In this case, such material acquires additional functional properties due to piezoelectric and pyroelectric effects, forming, in fact, a full-fledged flexible transparent device, which, for example, will emit a sound when an acoustic signal is applied, and under various external influences can generate an electrical signal. The use of such structures is associated with the influence of various external influences on them: thermomechanical loads associated with mechanical deformations and temperature effects during operation, or when applying conductive layers to the film. The article presents structure investigation and its change during high-temperature annealing using IR spectroscopy and comparative results of testing a PVDF film before and after deposition of ITO layers for uniaxial stretching, its dynamic mechanical analysis, DSC, as well as measurements of the transparency and piezoelectric properties of such structure. It is shown that the temperature-time mode of deposition of ITO layers has little effect on the thermal and mechanical properties of PVDF films, taking into account their work in the elastic region, slightly reducing the piezoelectric properties. At the same time, the possibility of chemical interactions at the polymer-ITO interface is shown.

Transparent UHF RFID tags based on CVD-grown graphene films

Ultra-high frequency (UHF) radio frequency identification (RFID) tags need to be attached or embedded to objects in various environments to achieve non-contact automatic identification. Graphene shows unique electrical and optical properties, which makes it become a promising material for radio frequency devices. In this paper, the transparent UHF RFID tags were fabricated based on graphene films with different number of stacked layers prepared by chemical vapor deposition (CVD). Through structural design, parameter optimization and experimental measurements, the reading distance of the transparent RFID tags was tested and compared. As the graphene film stacked layers increased, the reading distance of graphene-based RFID tags was farther. The UHF RFID tag based on the CVD-grown graphene with the light transmittance of 88% reached the maximum reading distance of 2.78 m in the frequency range of 860–960 MHz. In addition, the reading distance of graphene-based RFID tags at different bending angles and cycles was measured. The results reveal transparent graphene-based RFID tags have good flexibility and stability and can be used in flexible transparent devices.

All inorganic and transparent ITO/boehmite/ITO structure by one-step synthesis method for flexible memristor

In this work, an ITO/boehmite layer/ITO-stacked structure was fabricated at low temperature for transparent flexible memristor. Typical and reproducible bipolar switching behavior was easily obtained under applied bias. The resistive switching behaviors with satisfied electrical reliability such as retention, endurance and mechanical characteristics were all exhibited at different bending angles. The switching mechanism was considered to be related to the movement of H+ in boehmite layer. This low temperature processed, all inorganic and transparent flexible device provides a feasible way to develop new type of information devices and wearable electronics.

High-stability transparent flexible energy storage based on PbZrO3/muscovite heterostructure

Antiferroelectric materials for dielectric energy storage with fast charging-discharging rate is an important research direction. In this study, to build a platform for the potential application in flexible transparent devices, a combination of the muscovite substrate and the antiferroelectric PbZrO 3 (PZO) is studied as a model system. The growth of PZO is first optimized on rigid substrates and then transferred to muscovite with the form of epitaxial and polycrystalline films. The energy storage performance with robust electrical and mechanical stability is systematically demonstrated. High energy densities of 46~52 J/cm 3 were obtained; Compared with the epitaxial PZO, the polycrystalline PZO shows an increase of efficiency by 28% and possesses higher heat resistance. Moreover, fabricated on a transparent indium tin oxide electrode, the PZO heterostructure exhibits excellent energy performance and an optical transmittance of up to 70–80%. Through this study, a paradigm for reliable flexible transparent fast charging-discharging energy storage element is developed. • To build a platform for the potential application in flexible transparent devices, a combination of the muscovite substrate and the antiferroelectric PbZrO 3 (PZO) is studied as a model system. • The energy storage performance with robust electrical and mechanical stability is systematically demonstrated. High energy densities of 46~52 J/cm 3 were obtained. • Compared with the epitaxial PZO, the polycrystalline PZO shows an increase of efficiency by 28%. • Moreover, fabricated on a transparent indium tin oxide electrode, the PZO heterostructure exhibits excellent energy performance and an optical transmittance of up to 70–80%. • Through this study, a paradigm for reliable flexible transparent fast charging-discharging energy storage element is developed.

Single-Crystalline Silicon Frameworks: A New Platform for Transparent Flexible Optoelectronics.

Single-crystalline silicon (sc-Si) is the dominant semiconductor material for the modern electronics industry. Despite their excellent photoelectric and electronic properties, the rigidity, brittleness, and nontransparency of commonly used silicon wafers limit their application in transparent flexible optoelectronics. In this study, a new type of Si microstructure, named single-crystalline Si frameworks (sc-SiFs), is developed, through a combination of wet-etching and microfabrication technologies. The sc-SiFs are self-supported, flexible, lightweight, tailorable, and highly transparent. They can withstand a small bending radius of less than 0.5mm and have a transparency of up to 96% in all wavelength ranges, owing to the hollowed-out framework structures. Thus, the sc-SiFs provide a new platform for high-performance transparent flexible optoelectronics. Taking transparent flexible photodetectors (TFPDs) as an example, substrate-free and self-driven TFPDs are achieved based on the sc-SiFs. The devices exhibit superior performance compared to other reported TFPDs and reveal the great potential for integrated optoelectronic applications. The development of sc-SiFs paves the way toward the fabrication of high-performance transparent flexible devices for a host of applications, including e-skins, the Internet of Things, transparent flexible displays, and artificialvisualcortexes.

Highly robust, transparent, and conductive films based on AgNW-C nanowires for flexible smart windows

Flexible transparent conductive films (FTCFs) have attracted tremendous concern because it is a key component of next generation electronics and optoelectronic devices. Silver nanowires (AgNWs) random networks have become the most promising candidates for FTCFs, with high transmittance, low sheet resistance, and excellent mechanical flexibility. However, there are many barriers to the application of FTCFs based on AgNWs, such as easy oxidization and weak adhesion to the substrates. Here, we developed a facile approach to fabricate AgNW-C core–shell nanowires derived from the carbonization of glucose by a one-pot solvothermal method. A uniform amorphous carbon shell coating improves the chemical stability of AgNWs and the hydroxyls on the surface of the nanowires enhance the adhesion to the substrate. The AgNW-C/poly(ethylene terephthalate) (PET) transparent conductive film shows excellent robustness when subjected to bending (ΔR/R0 ≈ 2.8% after bending 2000 times). Besides, the conductivity of AgNW-C/PET film remains relatively stable after 100 peeling-off cycles by a 3 M tape test. A flexible electrochromic device (FCD) has also been constructed based on the AgNW-C/PET film, which shows promising stability and mechanical flexibility due to the remarkable electrochemical stability and mechanical strength of the AgNW-C/PET films. The results present the significant potential applications of the flexible transparent device based on AgNW-C/PET film for many fields in the future.

Flexible transparent heteroepitaxial conducting oxide with mobility exceeding 100\u2009cm2\u2009V\u22121\u2009s\u22121 at room temperature

Flexible and transparent applications have become an emerging technology and have shifted to the forefront of materials science research in recent years. Transparent conductive oxide films have been applied for flat panel displays, solar cells, and transparent glass coatings. However, none of them can fulfill the requirements for advanced transparent flexible devices, such as high-frequency applications. Here, we present a promising technique for transparent flexible conducting oxide heteroepitaxial films: the direct fabrication of epitaxial molybdenum-doped indium oxide (IMO) thin films on a transparent flexible muscovite substrate. An n-type epitaxial IMO film is demonstrated with a mobility of 109 cm2 V−1 s−1, a figure of merit of 0.0976 Ω−1, a resistivity of 4.5 × 10−5 Ω cm and an average optical transmittance of 81.8% in the visible regime. This heteroepitaxial system not only exhibits excellent electrical and optical performance but also shows excellent mechanical durability. Our results illustrate that this is an outstanding way to fabricate transparent and flexible conducting elements for the evolution and expansion of next-generation smart devices.

Diffusion-activated high performance ZnSnO/Yb2O3 thin film transistors and application in low-voltage-operated logic circuits

The flourishing metal-oxide high-k dielectric materials have been regarded as the vital components of low voltage operated flexible transparent electronic devices. We herein report that ytterbium oxide (Yb2O3) and ZnSnO (ZTO) thin films were firstly integrated into ZTO-based thin film transistors (TFTs) with superior performance. Results have indicated that the 500 ℃-annealed ZTO/Yb2O3 TFTs possess the large saturation mobility of 9.1 cm2 V−1 S−1 and the high on/off current ratio of 2.15 × 107, which even surpass those of reported In-based TFTs. The deteriorative electrical properties in the aging process can be attributed to the carrier capture mechanism. However, the 460 ℃-processed TFTs demonstrate a tenfold increase in saturated mobility and an increase in on/off current ratio after 10 days aging. The inspiring electrical properties are attributed to the diffusion-activated carrier enhancement mechanism and electrons donor role of water molecular, which introduces a facile method to boost the device performance at lower processing temperatures. The neglected threshold voltage variations of 0.06 V and −0.2 V have been detected after bias stability experiments. The superior bias stability can be attributed to the charge delay effect induced by the continuous electric field. Meanwhile, the ultrahigh on/off current ratio of 1.1 × 107 and the recoverable transferring performance have verified the aging-activated mechanism. To confirm its potential application in digital circuits, a resistor-loaded inverter with gain of 5.6 has been constructed and good dynamic response behavior have been detected at a low voltage of 2 V. As a result, it can be concluded that the high temperature annealing TFTs need immediate encapsulation, while the performance of the lower temperature processing samples can be optimized after aging treatment, indicating the potential prospect in low power consumption large-scale flexible transparent devices.

Keggin and Dawson polyoxometalates as electrodes for flexible and transparent piezoelectric nanogenerators to efficiently utilize mechanical energy in the environment

In recent years, piezoelectric nanogenerators (PENGs) have been developed as a promising energy-harvesting electronic device. However, the electrodes of most PENGs devices are precious metals, thus increasing the production cost. Here, we propose a flexible transparent PENGs with polyoxometalates (POMs) as the electrodes; it can effectively utilize ambient mechanical energy to generate electricity. Five types of polyoxometalates with different structures and compositions are selected as the electrode materials for PENGs for the first time, and the output performance of different PENGs electrode devices is tested. The PENG device with (NH4)6P2Mo18O62 as the electrode can steadily provide a high electric output with an open-circuit voltage of 2.8 mV and a short-circuit current of 8.5 µA at the bending degree of 90°. At the same time, the transmission spectrum shows that the average visible transmittance (AVT) of PENG can reach 31%, thus outperforming the benchmark for window applications. Finally, the working mechanism, force analysis, repeatability, and stability of PENG are systematically evaluated. All the studies show that this flexible transparent device has potential application prospect in wearable electronic devices.

Effects of carbon concentration on high-hardness plasma-polymer-fluorocarbon film deposited by mid-range frequency sputtering

We propose a method for fabricating high-hardness plasma-polymer-fluorocarbon (PPFC) thin films with controllable optical and surface properties via manipulation of the target composition design and sputtering power density. The carbon/polytetrafluoroethylene (PTFE) composite polymeric material targets with the low electrical resistance were prepared by press-molding using a mechanically mixed powder of PTFE, carbon nanotubes, and graphite. The composite targets showed electrical sheet resistances of 0.1–100 Ω/sq. PPFC thin films were deposited by mid-range frequency (MF) sputtering at power densities within 0.62~4.92 W/cm2. The maximum surface hardness of the PPFC thin film was 4.75 GPa, which was 21.6 times higher than that of fluorocarbon thin film sputtered from PTFE under the same conditions. With the increase of the carbon concentration in the target, the carbon cross-linking density of the PPFC thin film increased but the fluorine concentration decreased. The concentration of fluorine in the PPFC thin films grew with increasing sputtering power density. The MF sputtered carbon-rich PPFC thin films are controllable with physical properties of optical transmittance, surface hardness and surface water repellency which could be applied as protective layers for transparent flexible devices.

Highly Stable Ni‐Based Flexible Transparent Conducting Panels Fabricated by Laser Digital Patterning

AbstractA novel simple laser digital patterning process to fabricate Ni‐based flexible transparent conducting panels using solution‐processed nonstoichiometric nickel oxide (NiOx) thin films and their applications for flexible transparent devices are reported in this study. A large‐scale synthesis route to produce NiOx nanoparticle (NP) ink is also demonstrated. A low‐power continuous‐wave laser irradiation photothermochemically reduces and sinters selected areas of a NiOx NP thin film to produce Ni electrode patterns. Owing to the innovative NiOx NP ink and substantially lowered applied laser power density, Ni conductors can be fabricated, for the first time to the best of the authors' knowledge, even on a polyethylene terephthalate substrate, which is known to have one of the lowest glass‐transition temperatures among polymers. The resultant Ni electrodes exhibit a high‐temperature oxidation resistance up to approximately 400 °C, and high corrosion resistance in tap water and even in seawater. Moreover, a superior mechanical stability of the Ni conductors is confirmed by tape‐pull, ultrasonic‐bath, bending/twisting, and cyclic bending (up to 10 000 cycles) tests. Finally, flexible transparent touch screen panels and electrical heaters are fabricated with mesh‐type Ni conductors to demonstrate possible applications.

Wafer-scale fabrication of microelectrode arrays on optically transparent polymer foils for the integration of flexible nanoscale devices

The integration of high-performance polymer substrates with added functionalities such as optical transparency and compatibility with micro-/nanolithography processes underpins future developments in flexible electronics and offers a myriad of applications such as displays, sensors, energy conversion etc. Recently, polyimide substrates such as Kapton® have emerged as an ideal choice for realizing high-performance, low-dimensional nanoelectronic platforms due to their high glass transition temperatures and chemical inertness. Flexible devices based on Kapton® polyimides, however, encounter fundamental challenges regarding the integration of optoelectronic platforms, due to their partly opaque optical characteristics. In this work, we realize a new photolithography process for wafer-scale patterning of metal microelectrode arrays (MEAs) on an optically transparent polyimide called Neopulim®. The newly established lithography protocol enabled the high throughput fabrication of flexible microchips with differently configured microelectrode arrays with uniform surface characteristics. The application potential of such optically transparent and flexible MEA chips is demonstrated by the realization of nanoscale devices based on two-dimensional graphene-based materials (GBMs) and one-dimensional zinc oxide nanowires (ZnO NWs). It is further shown that the use of Neopulim® with a high glass transition temperature (303 °C) characteristic withstands demanding device preparation steps such as use of thermal annealing and the self-assembly growth of NWs in a hot precursor solution, etc., opening a new chapter in the assembly and use of nanoscale optoelectronic platforms. The lithography procedure used to realize MEA chips and the fabrication of GBM and ZnO NW based optically transparent devices was characterized using state-of-the-art surface and electrical characterization tools. The devices were deployed as ion-sensitive field-effect transistors and as shown in this work, our devices provide an ideal platform for simultaneous optical and electrical readouts owing to high optical transparency. Scalable realization of optically transparent flexible devices at the nanoscale as presented in this work will open up new opportunities for next-generation platforms for optoelectronics, energy, sensors and biomedical applications.

Highly transparent, all-oxide, heteroepitaxy ferroelectric thin film for flexible electronic devices

Transparent flexible electronics have captured tremendous attention as the next-generation of electronic devices. Recently, flexible devices for wearing are in high demand in practical applications under special circumstances, such as aeronautics and astronautics, which require highly-transparent and large-scale flexibility, as well as stability at elevated temperature. Here, highly transparent, epitaxial Pb(Zr0.1Ti0.9)O3 (PZT) ferroelectric thin films were fabricated on Sn-doped In2O3 (ITO) transparent electrodes on yttria-stabilized zirconia (YSZ) bufferred mica substrate using pulsed laser deposition. The structural, optical, mechanical, and electrical properties of these all-oxide heterostructures of the PZT/ITO/YSZ/mica were studied. The heteroepitaxy exhibit a high transmittance and an excellent remarkable mechanical properties with robust operation in bending cycle tests. Moreover, the electrical properties are examined to show the preservation of the primitive properties of PZT. Various bending tests are performed to show the tunability of functionalities and to confirm that the heterostructures retain the physical properties under repeated cycles at high temperature. These results demonstrated that these high-performance transparent flexible capacitors would open a route for the applications in transparent flexible devices under harsh conditions.

Development of polyimide films reinforced with boron nitride and boron nitride nanosheets for transparent flexible device applications

The benefits of reinforcing polyimide (PI) films with boron nitride (BN) particles and boron nitride nanosheets (BNNSs) were assessed with the aim of enhancing their thermal, optical, and mechanical properties for flexible device applications. BNNSs were prepared from BN particles using a liquid-phase exfoliation method assisted by an ultrasonic probe-type sonicator and centrifugator. PI-based composite films blended with BNNSs and BN particles were fabricated at various concentrations via mechanical stirring and spin coating. The transparency of the PI/BNNS composite films remained almost the same as that of pure PI films up to 3 wt.% whereas the transparency of the PI/BN composite films decreased with increasing concentration of the BN fillers at 550 nm. The thermal stability improved significantly with increasing concentrations of both BN and BNNS relative to that of pure PI films. The temperature for 5% weight loss of the PI/BNNS composite film was higher than that of the PI/BN composite film at the same filler concentration. The composite films with 2 wt.% BN or BNNS showed the lowest wear rate, and the PI/BNNS composite films showed more stable frictional behavior compared to the PI/BN composite films. In addition, bending tests showed that the PI/BNNS composite films exhibited excellent flexibility compared to the PI/BN composite films. Overall, the results indicate that the BNNS can be effectively used as a filler that can enhance the thermal and mechanical properties of polymer materials for flexible device applications.

High-Quality AZO/Au/AZO Sandwich Film with Ultralow Optical Loss and Resistivity for Transparent Flexible Electrodes.

Transparent flexible electrodes are in ever-growing demand for modern stretchable optoelectronic devices, such as display technologies, solar cells, and smart windows. Such sandwich-film-electrodes deposited on polymer substrates are unattainable because of the low quality of the films, inducing a relatively large optical loss and resistivity as well as a difficulty in elucidating the interference behavior of light. In this article, we report a high-quality AZO/Au/AZO sandwich film with excellent optoelectronic performance, e.g., an average transmittance of about 81.7% (including the substrate contribution) over the visible range, a sheet resistance of 5 Ω/sq, and a figure-of-merit (FoM) factor of ∼55.1. These values are well ahead of those previously reported for sandwich-film-electrodes. Additionally, the interference behaviors of light modulated by the coat and metal layers have been explored with the employment of transmittance spectra and numerical simulations. In particular, a heater device based on an AZO/Au/AZO sandwich film exhibits high performance such as short response time (∼5 s) and uniform temperature field. This work provides a deep insight into the improvement of the film quality of the sandwich electrodes and the design of high-performance transparent flexible devices by the application of a flexible substrate with an atomically smooth surface.

Synthesis of silver nanowires towards the development the ultrasensitive AgNWs/SiNPLs hybrid photodetector and flexible transparent conductor

In this paper, we report the synthesis of silver nanowires (AgNWs) via polyol method towards the fabrication of low cost high sensitive hybrid photodetector. During AgNWs synthesis, NaCl was added to the reaction for controlling the free Ag+ ions concentration during the formation of initial Ag seeds. Field emission scanning electron microscopy results show that AgNWs of diameter ~ 50–80nm and length ~ 5–30µm can be achieved. UV–Visible absorption spectroscopy and energy dispersive spectroscopy results indicate the formation of AgNWs in highly pure phase. Hybrid detector was fabricated by depositing the AgNWs film on silicon nanopillar (SiNPLs) substrate. Fabricated hybrid AgNWs/SiNPLs photodetector exhibits ~ 10 times more sensitivity as compared to SiNPLs based detector. In addition, AgNWs/SiNPLs detector shows outstanding stability against 20 days exposure to ambient conditions. These results indicate that AgNWs can be utilized to fabricate the high performance one dimensional hybrid photodetectors. The effectiveness of AgNWs to make the transparent flexible devices has also been demonstrated. AgNWs film deposited on a flexible polyethylene terephthalate (PET) substrate revealed a maximum total transmittance and sheet resistance of ~ 94% and ~ 200/sq., respectively.

Quantification and analysis of Raman spectra of graphene materials

Graphene has received significant attention in recent years due to its outstanding electronic, mechanical, chemical and physical properties. Graphene materials can potentially be used in a variety of applications, such as functional nanocomposites, electrodes, flexible transparent devices and thin conductive films. This article focuses on the analysis of structural evolution and development of different of reduced graphene oxides (rGOs), and the results are compared with structural features of functionalised reduced graphene oxide and graphene. The aromatic disorder and irregularity of these materials influence their own properties; particularly, their electrical conductivity aspects were studied indirectly through Raman spectroscopy. The quantification of their Raman spectra and microstructural analysis were examined to assess the relationship between aromatic structures and electrical conduction mechanism. The results showed that aromaticity of GO changes under different chemical reduction treatments and hydroiodic acid reduction gave an electrical conductivity of 103.3 S cm−1 as highest amongst a number of rGOs produced. Moreover, the integrity of aromatic structure through different reduced graphene oxides changed quite significantly and the Raman results were able to correlate the electrical conductivity with their structural regularity.

Solution-processed fabrication of highly transparent mono- and tri-colored quantum dot-light-emitting diodes

Analogous to organic light-emitting diode (OLED), quantum dot-light-emitting diode (QLED) possesses a high eligibility with respect to device structure for the transformation to transparent device that may be pursued as a next-generation display. We report the fabrication of a series of highly transparent mono-colored blue, green, and red QLEDs with a standard architecture simply by replacing thermally evaporated Al with sputtered indium tin oxide (ITO) film as a top cathode. To alleviate the sputtering damage on the underlying electron transport layer while securing a reasonable sheet resistance of ITO film, a moderate sputtering power is judiciously chosen to attain high device performance. Fabrication of a transparent tri-colored white or full-color-capable QLED, comprising an emitting layer mixed with three primary colored QDs, is also demonstrated and further implemented on a flexible substrate of polyethylene naphthalate to additionally offer its feasibility toward transparent flexible device.

Human-Finger Electronics Based on Opposing Humidity-Resistance Responses in Carbon Nanofilms.

Carbon nanomaterials have excellent humidity sensing properties. Here, it is demonstrated that multiwalled carbon-nanotube (MWCNT)- and reduced-graphene-oxide (rGO)-based conductive films have opposite humidity/electrical resistance responses: MWCNTs increase their electrical resistance (positive response) and rGOs decrease their electrical resistance (negative response). The authors propose a new phenomenology that describes a "net"-like model for MWCNT films and a "scale"-like model for rGO films to explain these behaviors based on contributions from junction resistances (at interparticle junctions) and intrinsic resistances (of the particles). This phenomenology is accordingly validated via a series of experiments, which complement more classical models based on proton conductivity. To explore the practical applications of the converse humidity/resistance responses, a humidity-insensitive MWCNT/rGO hybrid conductive films is developed, which has the potential to greatly improve the stability of carbon-based electrical device to humidity. The authors further investigate the application of such films to human-finger electronics by fabricating transparent flexible devices consisting of a polyethylene terephthalate substrate equipped with an MWCNT/rGO pattern for gesture recognition, and MWCNT/rGO/MWCNT or rGO/MWCNT/rGO patterns for 3D noncontact sensing, which will be complementary to existing 3D touch technology.