Flexible Transparent Heaters Research Articles

Cohesively improved conductivity, transparency, and stability of Ag NW flexible transparent conductive thin films by covering Al2O3 layer

Developing feasible Ag nanowire (NW) transparent conductive thin films (TCFs) with good conductivity, transparency, mechanical durability, strong adhesion, and high stability is a great challenge. Herein, novel TCFs composed of Ag NWs/Al2O3 on polyethylene terephthalate (PET) substrates with synchronously improved conductivity, transparency, mechanical durability, adhesion, and stability, are prepared. The corresponding values (before coating Al2O3: 13.0 Ω/sq. at 86.1 %, after coating Al2O3: 11.7 Ω/sq. at 87.7 %) indicate that the deposition of Al2O3 enhances the transparency and conductivity of Ag NW networks. Moreover, the resistance does not change significantly after 50 taping cycles and 2000 bending cycles with a bending radius of 5.0 mm, indicating the strong adhesion and good mechanical flexibility of Al2O3/Ag NWs composites. In addition, Al2O3/Ag NWs composites possess excellent stability to resist strong oxidizing, hot and humid environments. As a proof of concept, the flexible transparent heater prepared by Al2O3/Ag NWs composites is successfully demonstrated, verifying the practicability.

Stretchable and Transparent Heaters Based on Hydrophobic Ionogels with Superior Moisture Insensitivity.

Flexible and stretchable transparent heaters (THs) have been widely used in various applications, including deicing and defogging of flexible screens as well as thermotherapy pads. Ionic THs based on ionogels have emerged as a promising alternative to conventional electronic THs due to their unique advantages in terms of transparency-conductance conflict, uniform heating, and interfacial adhesion. However, the commonly used hydrophilic ionogels inevitably introduce a moisture-sensitive issue. In this work, we present a stretchable and transparent hydrophobic ionogel-based heater that utilizes ionic current-induced Joule heating under high-frequency alternating current. This ionogel-based TH exhibits exceptional multifunctional properties with low hysteresis, a fracture strain of 840%, transmittance of 93%, conductivity of 0.062 S m-1, temperature resistance up to 165 °C, voltage resistance up to 120 V, heating rate of 0.1 °C s-1, steady-state temperature at 115 °C, and uniform heating even when bent or stretched (up to 200%). Furthermore, it maintains its heating performance when it is directly exposed to water. This hydrophobic ionogel-based TH expands the range of materials available for ionic THs and paves the way for their practical applications.

Ultrahigh-Temperature-Tolerant NiO@Ag NW-Based Transparent Heater with High Stability, Durability, Ultraflexibility, and Quick Thermal Response

High-performance electrical heaters with high transmittance, fast response speed, superior heat resistance, ultraflexibility, high working temperature, and sufficient stabilities are highly desirable for application in heating systems in areas such as healthcare and aerospace. Herein, we demonstrate an ultrahigh-temperature-tolerant flexible transparent heater (FTH) consisting of a NiO@Ag core–shell nanowire (NiO@Ag NW) network and a colorless polyimide (cPI) film. NiO nanoshells are formed uniformly on the surface of Ag NWs through the solution process, which enhances the stability of Ag NWs. The heating behaviors and transmittance can be controlled by the NiO@Ag NW spin-coating cycle. The NiO@Ag NW-based FTHs show a fast thermal response of ∼15 s, a high transmittance of 86.2% in the visible region, and a large thermal figure of merit of 30.7 m2·°C/W under optimal preparation conditions. The saturation temperature of FTHs has barely changed after 20 taping tests and 1000 bending cycles at a bending radius of 0.5 mm. Particularly, the heating temperature of FTHs can reach 266 °C in the air under an applied voltage of 9 V. Moreover, the FTHs possess sufficient heating reliability, stability, and repeatability during the long-term and repeated heating cycles. In addition, the FTHs are resistant to a number of harsh environments.

Self-assembled Growth of SnO2 Nanoshells on Copper Nanowires for Stable and Transparent Conductors

Despite a century of research, the inability to design flexible transparent electrodes with low cost and high stability has been the greatest obstacle in realizing flexible electronics. Herein, we report the growth of self-assembled SnO2 nanoshells on thin copper nanowire (Cu NW) networks using a simple and effective solution method. In addition, the SnO2–Cu NW flexible films exhibit an extremely low sheet resistance of ∼13.6 Ω/sq, with ∼83.5% optical transparency. In harsh environments (such as strong oxidation, heat, and humidity conditions), the films demonstrate excellent resistance stability, which can compete with the resistance stability of noble silver nanowire films. Particularly, the films are robust and able to withstand a high temperature of 210 °C without any deterioration of performance. Employing the outstanding stability of SnO2–Cu NWs, we also demonstrate the macroscopic use of a transparent conductor in a flexible transparent heater.

Low-temperature polyol synthesis of millimeter-scale-length silver nanowires enabled by high concentration of Fe3+ for flexible transparent heaters

Silver nanowires (AgNWs) emerge as a promising candidate for flexible electronic devices. AgNWs with high aspect ratios improve the optoelectronic properties of transparent conductive electrodes. However, syntheses conducted at a high temperature usually produce AgNWs either too short or too thick. Here, we report a one-pot low-temperature polyol method using a high concentration of Fe(NO3)3 (up to 160 mM). Fe3+ was found to accelerate AgNWs growth by five times, possibly reduced to Fe2+ and functioning as an oxygen scavenger to preserve Ag seeds, allowing the synthesis to be conducted at as low as 70 °C, which was about 100 °C lower compared with a common polyol method. Low temperature suppressed excessive nucleation, producing AgNWs longer than 1100 μm. AgNWs were dispersed in an acidic solution and purified with a non-pressure filtration method. transparent conductive electrodes fabricated with long AgNWs on polyethylene terephthalate exhibited a sheet resistance of 7.8 Ω/sq at a transmittance of 95.0% without heat treatment. Transparent heaters of AgNWs on polyethylene terephthalate and polydimethylsiloxane substrates were heated to 79.1 °C at 3 V and 174.1 °C at 8 V power supply, respectively, and their application was demonstrated in a quick defogging process. The simple synthesis method of ultralong AgNWs may expand their applications in high-performance flexible transparent electronic devices.

Microscale hybrid 3D printed ultrahigh aspect ratio embedded silver mesh for flexible transparent electrodes

Increasing the aspect ratio (AR) of embedded metal meshes (EMMs) is an effective approach to solve the “trade-off” effect of flexible transparent electrodes (FTEs). However, achieving low-cost and green manufacturing of ultrahigh AR EMM still faces great challenges. Here, we report a novel solution for the ultrahigh AR EMMs by microscale hybrid printing nano silver paste. An improved printed nozzle is introduced to facilitate the stable electric-filed-driven 3D printing, then the imprinting process is used to achieve an EMM with AR of up to 20.1, which is the highest AR ever reported. The fabricated FTE exhibits unprecedented photoelectric performance with a figure of merit of up to 88,194 and excellent optoelectronic properties after rigorous mechanical and environmental stability tests. Its practical application was also demonstrated to be feasible through flexible transparent heater and electromagnetic shielding. The proposed method provides a promising strategy to fabricate FTEs with ultrahigh AR EMM.

Chemically Welding Silver Nanowires toward Transferable and Flexible Transparent Electrodes in Heaters and Double-Sided Perovskite Solar Cells

Silver nanowires (AgNWs) are important materials for flexible transparent electrodes (FTEs). However, the loose stacking of nanowire junctions greatly affects the electric conductivity across adjacent nanowires. Soldering can effectively reduce the wire-wire contact resistance of AgNWs by epitaxially depositing nanosolders at the junctions, but the process normally needs to be performed with high energy consumption. In this work, we proposed a simple room-temperature method to achieve precise welding of junctions by adjusting the wettability of the soldered precursor solution on the surfaces of AgNWs. The nanoscale welding at nanowire cross junctions forms efficient conductive networks. Furthermore, reduced graphene oxide (rGO) was used to improve the stability of FTEs by wrapping the rGO around the AgNW surface. The obtained FTE shows a figure-of-merit (FoM) of up to 439.3 (6.5 Ω/sq at a transmittance of 88%) and has significant bending stability and environmental and acidic stability. A flexible transparent heater was successfully constructed, which could reach up to 160 °C within a short response time (43 s) and exhibit excellent switching stability. When laminating this FTE onto half perovskite solar cells as the top electrodes, the obtained double-side devices achieved power conversion efficiencies as high as 16.15% and 13.91% from each side, pointing out a convenient method for fabricating double-sided photovoltaic devices.

Simple Recycling of Ag+, Surfactant, and Solvent for Repeatable Synthesis of Silver Nanowires for Applications as Flexible Transparent Heaters

Silver nanowires (AgNWs) have shown great potential in various optical and electronic applications, especially in flexible and wearable devices. Polyol method is now a mainstream strategy to synthesize AgNWs. However, unreacted reagents including silver salt, surfactant, and solvent are usually discarded after the collection of products, resulting in a huge waste as well as a burden to the environment. In this work, we demonstrate a simple recycle method to reuse these reagents based on a two-step direct centrifugation protocol. Larger nanowires are separated, while remaining small nanowires and nanoparticles provide nucleation sites and likely function as catalysts to achieve a three-times faster nanowire growth. The cyclic synthesis can maintain at least five consecutive rounds to produce AgNWs with similar size and yield and more rounds under suitable adjustment of Ag+ supply. Transparent conductive films based on AgNWs grown from recycled reagents show excellent and consistent optoelectronic performance, namely, a sheet resistance of 141.3 Ω/sq at a transmittance of 99.7% and 9.48 Ω/sq at 92.9%. A transparent heater with a PDMS/AgNWs/PET structure exhibits a resistance of 8.6 Ω at a total transmittance of 76.5% and is heated to 75.8 °C at a 3.5 V power supply. A fast-defrosting process is demonstrated. The recycling method significantly reduces the cost of AgNW production and may widen their application in high-performance flexible devices.

Near-Zero Transmittance Loss, Highly Durable, Flexible, Transparent Electrode with an Ultrathin Ag Nanoporous Structure

Recent advances in state-of-the-art optoelectrical and photovoltaic devices have necessitated further improvements in the efficiency of transparent electrodes by simultaneously minimizing electrical and optical losses, which remain technically challenging to achieve. In this study, we developed a strategy for constructing a highly efficient transparent electrode that retains near-perfect optical transparency at tunable wavelengths in the visible spectral range and has excellent electrical reliability against heavy processing loading. The optoelectrical features were achieved by integrating an ultrathin Ag nanoporous layer and a substoichiometric electron-transferring SiOx film in a multilayered oxide/metal/oxide configuration. Complete sealing of the Ag nanoporous geometry by a subsequently coated SiOx film facilitates near-zero optical transmittance loss (<1%, averaged at 0.7%) over a wide spectral range of 500–700 nm at a near-bulk resistivity of 7.2 × 10–8 Ω m while ensuring extreme durability against chemical corrosion, electrical/thermal degradation, and mechanical deformation. The unexpected performance of such a defective Ag-layered geometry, which has rarely been considered in the design of transparent electrodes, refutes conventional approaches that emphasize the implementation of a completely continuous layered geometry of Ag to optimize the optical and electrical performances. These results provide an alternative solution for maximizing the performance and durability of Ag-based transparent electrodes that can be synthesized on either glass or polymer substrates for various optoelectrical applications, including flexible transparent Joule heaters.

Stable Flexible Transparent Electrodes for Localized Heating of Lab‐on‐a‐Chip Devices

AbstractIn situ biological observations require stable, accurate, and local temperature control of specimen. Several heating elements are coupled with microfluidic systems, but few of them are transparent to visible light and therefore compatible with microscopic observation. Traditional transparent electrodes, such as indium tin oxide, still suffer from high fabrication cost and brittleness, which is not fully compatible to emerging microfluidic devices. Here, a lightweight, low‐cost, flexible transparent heater based on percolating silver nanowire networks, protected with a transparent zinc oxide film, for the in situ monitoring of biological experiments is proposed. Using the fluorescence of dyes bound to double‐stranded DNA to monitor its temperature in situ, it is demonstrated that such nanocomposites allow rapid and reproducible heating under low applied voltage. Furthermore, selective heating is achieved in different zones of the same microchannel or for adjacent microchannels of the chip heating at different temperatures, with a single transparent heater and bias.

Flexible transparent Ag nanowire/UV-curable resin heaters with ultra-flexibility, high transparency, quick thermal response, and mechanical reliability

The non-fast thermal response and unstable mechanical properties of flexible transparent heaters (FTHs) make it difficult for them to meet the requirements for wearable electronics. In this paper, the influences of heater thickness and resistance of Ag nanowire networks, on the thermal response, optical and mechanical flexible properties are investigated systematically. The relationships between the mechanisms behind the thermal response (bending behaviors) and the thickness, which is important for enhancing the thermal and flexible performances of FTH, are explored. The optimized Ag NW/UV-curable resin -based FTHs, with a thickness of 2 µm and sheet resistance of 9.9 Ω/□, exhibit a quick thermal response time within 12 s, a high optical transmittance of 86.4% and superior mechanical properties. The resistance of the FTH shows almost no change after bending 1000 times at a curvature radius of 0.5 mm. The fast-heating characteristics associated with its transparency and flexibility make the Ag NW/UV-curable resin -based FTHs a potential candidate in wearable electronics.

Highly sandwich-structured silver nanowire hybrid transparent conductive films for flexible transparent heater applications

Hybridization with other materials has been an effective method to improve the photoelectric performance and stability of silver nanowire (AgNW) based transparent conductive films (TCFs). In the work, hybrid TCFs with a sandwiched structure composed of MXene, AgNW, and graphene were proposed. MXene sheets in the bottom acted as the intermediate layer between substrates and AgNWs could significantly improve the adhesion, graphene could be used as protective layer, and both filled the AgNW network to improve conductivity and flatness. As a result, the TCF displayed good photoelectric performance (14.4 Ω/sq with 87.5% transmittance at 550 nm), low surface roughness, enhanced adhesion and stability. Furthermore, transparent heaters (THs) were fabricated with the MXene/AgNW/graphene hybrid TCFs. The THs exhibited uniform heating distribution, fast thermal response, good repeatability and long-term working stability. Therefore, flexible THs proposed are expected to be widely used in defogging systems, smart windows, and other flexible electronic devices.

High-Temperature Flexible Transparent Heater for Rapid Thermal Annealing of Thin Films

Rapid heating is highly sought-after for compositional control in the sintering-annealing process of materials containing volatile elements. Although several innovative approaches have been reported, suitable processes for the rapid annealing of thin films are still very rare. Herein, we develop a high-temperature rapid heating method for thin-film preparation through employing an indium tin oxide (ITO)/mica-based flexible transparent heater (FTH). The ITO/mica FTH shows an extremely fast thermal response (2 s), high saturation temperature (&gt;830 \ifmmode^\circ\else\textdegree\fi{}C), and high heating-cooling rates (&gt;${10}^{4}$ \ifmmode^\circ\else\textdegree\fi{}C ${\mathrm{min}}^{\ensuremath{-}1}$). Thermodynamic analysis reveals that the excellent heating performance can be largely attributed to the high heat-transfer coefficient and the low thickness of the mica substrates. The remarkable potential of our ITO/mica FTH for practical use is unambiguously evident by the demonstration of water heating and rapid thermal annealing of ferroelectric oxide thin films.

Simultaneous Enhancement in Visible Transparency and Electrical Conductivity via the Physicochemical Alterations of Ultrathin-Silver-Film-Based Transparent Electrodes.

A methodology for the simultaneous modulation of the chemical and physical states of an amorphous TiOx layer surface and its impact on the subsequent deposition of a polycrystalline Ag layer are presented. The smoothened TiOx layer surface comprising chemically altered, oxygen-deficient states serves as a nucleating platform for Ag deposition, facilitating a marked increase (∼75%) in the nucleation number density, which strongly enhances the wettability of ultrathin Ag layers. The physically smoothened TiOx/Ag interface further reduces the optical and electrical losses. When the proposed methodology is applied to TiOx/Ag/ZnO transparent conductive electrodes (TCEs), exceptional TCE properties are yielded owing to the simultaneous improvement in visible transparency and electrical conductivity; specifically, a record-high 0.22 Ω-1 Haacke figure of merit is realized. TCEs are prepared on flexible substrates to verify their applicability as stand-alone flexible transparent heaters and as integrated heaters within electrochromic devices to enhance color-switching reactions.

High-Performance Flexible Transparent Electrical Heater Based on a Robust Ordered Ag Nanowire Micromesh Conductor

High-performance electrical heaters with a rapid thermal response, homogeneous temperature distribution, and chemical and mechanical robustness are desirable for inevitable use in various areas such as defrosting windows and wearable thermotherapy. Herein, optimally engineered ordered Ag nanowire micromeshes are partially embedded in poly(ethylene terephthalate) matrix (Ag NMs/ePET) via a facile spray coating and transfer approach for flexible transparent conductor and electrical heater applications, which exhibit a slight sheet resistance variation of less than 5%. The resultant Ag NMs/ePET heater exhibits uniform heat distribution over the entire film and can achieve a saturation temperature of 134 °C with a rapid response time of 5 s under an input voltage of 6 V. In addition, owing to the large-diameter nanowire micromesh structure and the protective effect of PET encapsulation, the flexible transparent Ag NMs/ePET heater presents excellent mechanical and chemical robustness in extreme environments and still maintains a stable heating performance after 5000 repeated bending cycles (r = 5 mm) and exposure to a corrosive solution. This work lays the foundation for improving the electrical heater performance and provides a facile methodology to fabricate a high-performance flexible transparent conducting film for other innovative applications.

Engineering Silver Microgrids with a Boron Nitride Nanotube Interlayer for Highly Conductive and Flexible Transparent Heaters

AbstractThe use of flexible transparent conductive electrodes (TCEs) as printed heaters offers unique advantages where transparency is a necessary design feature. Many existing TCE materials however suffer from poor flexibility and require complex fabrication processes and thus are not commercially viable for such applications. The design and process optimization of screen‐printable silver metal microgrids over a layer of boron nitride nanotubes (BNNT) to produce highly conductive and mechanically robust transparent heaters with high transparency and low power requirements is reported. Square and hexagonal geometries are investigated alongside varying line width and pitch combinations to produce 16 different grid designs with optical transparency ranging between 65% and 89% and resistance values as low as 2.90 Ω sq−1. The BNNT thin film coating on polyethylene terephthalate substrates provides mechanical stability to the heater architecture by reducing the effects of deformation by up to 400%. The BNNT interlayer also contributes to an increase in thermal performance, achieving temperatures as high as 74.0 °C by initiating electrical sintering of the microgrids during heater operation which increases the Joule heating capacity of the features. Overall, this physical and material optimization provides a low‐cost, printable microgrid architecture for high performing and stable applications as TCE‐based heaters.

Microflow Manipulation by Velocity Field Gradient: Spontaneous Patterning of Silver Nanowires for Tailored Flexible Transparent Conductors

AbstractFlexible transparent conductors (TCs) are the fundamental components for emerging soft optoelectronics because of their excellent optical, electrical, and mechanical properties. These properties are attributed to reasonable alignment of conductive functional nanomaterials, especially 1D inorganic nanowires. Although various patterning technologies are proposed, patterning highly transparent conductors with satisfactory conductivity and flexibility in a facile, scalable, and versatile manner remains an open issue. Here, a directed self‐assembly strategy is presented for patterning silver nanowires (AgNWs) into flexible TCs with a cross‐linked network structure using microflow velocity‐field‐induced alignment at a liquid–solid interface. Under dual‐surface architectonics, the periodically varying internal microflow in the overcoated AgNW suspension facilitates the spontaneous alignment of the AgNWs on the designated regions in a layer‐by‐layer manner, yielding highly ordered AgNW TCs with an ultrahigh transmittance (98.2%), low sheet resistance (29.7 Ω sq−1), and prominent mechanical deformability. The proposed strategy is further applied to fabricate high‐accuracy arbitrary AgNW circuits to realize flexible transparent heaters with adjustable localized heat sources. This is a universal and customizable method for producing functional nanomaterials with hardly any scale or shape limitations in a streamlined fashion and provides great freedom for prototyping and manufacturing high‐performance soft optoelectronics.

Farming‐Inspired Continuous Fabrication of Grating Flexible Transparent Film with Anisotropic Conductivity

AbstractFlexible transparent conductive films (FTCFs) are critically important for the emerging flexible electronic devices. To this end, FTCFs with conductive gratings have drawn more interests in the past decades. However, there are great challenges to fabricate such FTCFs containing conductive gratings in a facile way. Borrowing the idea from traditional farming, a homemade roll‐to‐roll plowing technique combined with blade coating is proposed to fabricate silver grating FTCFs with anisotropic conductivity in a continuous, cost‐effective, and environmental friendly way. The as‐prepared FTCFs exhibit satisfied optical and electrical properties (i.e., transmittance is 78.2% and sheet resistance is 3.1 Ω □–1). After coated with thin polyvinyl butyral layer through hot‐pressing, it shows a higher transmittance of 84.2% and remarkable mechanical flexibility. Based on the FTCFs with anisotropic conductivity, flexible transparent heater and capacitive keyboard are successfully fabricated. Furthermore, based on such flexible capacitive keyboard, smart wireless controlled human–machine interface system is developed to wirelessly control tablet personal computer working and quadrotor flying. This study provides a promising strategy to fabricate high‐performance FTCFs with anisotropic conductivity via a continuous, cost‐effective, and environmental friendly way.

Flexible Transparent Heater Fabricated from Spray-Coated In:ZnO/Ag-NWs/In:ZnO Multilayers on Polyimide Foil.

A flexible transparent heater is presented, based on an all-sprayed composite architecture of indium-doped zinc oxide (IZO) layers that sandwich a network of silver nanowires, on a polyimide-foil substrate. This architecture could be materialized through the development of a low-temperature (240 °C) spray-pyrolysis process for the IZO layers, which is compatible with the thermal stability of the transparent polyimide substrate and allows for the formation of compact and transparent layers, without precipitates. The IZO layers entirely embed the silver nanowires, offering protection against environmental degradation and decreasing the junction resistance of the nanowire network. The resulting transparent heaters have a high mean transmittance of 0.76 (including the substrate) and sheet resistance of 7.5 Ω/sq. A steady-state temperature of ~130 °C is achieved at an applied bias of 3.5 V, with fast heater response times, with a time constant of ~4 s The heater is mechanically stable, reaching or surpassing 100 °C (at 3.5 V), under tensile, respectively, compressive-bending stress. This work shows that high-performance transparent heaters can be fabricated using all-sprayed oxide/silver-nanowire composite coatings, that are compatible with large-scale and low-cost production.

Simple, Fast, and Scalable Reverse-Offset Printing of Micropatterned Copper Nanowire Electrodes with Sub-10 μm Resolution.

Copper nanowires (CuNWs) possess key characteristics for realizing flexible transparent electronics. High-quality CuNW micropatterns with high resolution and uniform thickness are required to realize integrated transparent electronic devices. However, patterning high-aspect-ratio CuNWs is challenging because of their long length, exceeding the target pattern dimension. This work reports a novel reverse-offset printing technology that enables the sub-10 μm high-resolution micropatterning of CuNW transparent conducting electrodes (TCEs). The CuNW ink for reverse-offset printing was formulated to control viscoelasticity, cohesive force, and adhesion by adjusting the ligands, solvents, surface energy modifiers, and leveling additives. An inexpensive commercial adhesive handroller achieved a simple, fast, and scalable micropatterning of CuNW TCEs. Easy production of high-quality CuNW micropatterns with various curvatures and shapes was possible, regardless of the printing direction. The reverse-offset-printed CuNW micropatterns exhibited a minimum of 7 μm line width and excellent pattern qualities such as fine line spacing, sharp edge definition, and outstanding pattern uniformity. In addition, they exhibited excellent sheet resistance, high optical transparency, outstanding mechanical durability, and long-term stability. Flexible light-emitting diode (LED) circuits, transparent heaters, and organic LEDs (OLEDs) can be fabricated using high-resolution reverse-offset-printed CuNW micropatterns for applications in flexible transparent electronic devices.