Flexible Transparent Heaters Research Articles

Flexible transparent conductive ZnSnO/Cu/ZnSnO multilayer films for flexible transparent heaters

The characteristics of low-cost ZnSnO/Cu/ZnSnO multilayer films as potential flexible transparent electrodes and heaters are investigated. The multilayer films show a low sheet resistance of 9.92 Ω/sq. at optical transmittance of 81.2%. Calculation of figure of merit shows a large value of 173.2, which is superior to that obtained from the flexible ITO and other Cu based hybrid films. The resistance remains nearly constant after 1000 bending cycles and 100 tape tests, indicating good flexibility and adhesivity. Furthermore, the multilayers also exhibit excellent stability under hot and humid environment. Finally, the successful implementation of the multilayer films into a flexible transparent film heater is demonstrated, verifying the applicability.

Degradable, ultra-flexible, transparent and conductive film made of assembling CuNWs on chitosan

The rapid development of wearable and disposable electronic devices and the rising awareness of environmental sustainability impose growing new demands on the nature degradability of current electronic systems. It is a challenge to fabricate degradable, ultra-flexible, transparent and conductive films (TCFs), which are essential component of wearable devices. Here, a new type of degradable, ultra-flexible TCFs made from green and earth abundant materials are reported. The TCFs are achieved by embedding copper nanowires (CuNWs) into chitosan strategy via simple sol-gel transition. The prepared TCFs exhibit favorable optical and electrical properties (83.7% at the sheet resistance of 14.1 Ω/sq.). Calculation of figure of merit displays a high value of 143.7, which is superior to that obtained from the flexible ITO, carbon nanotubes, graphene, and other CuNWs based TCFs. In particular, excellent insensitivity conductivity are achieved with a bending radius of 0.1 mm and the sheet resistance remains nearly to be constant after 1000 bending cycles with a bending radius of 1.0 mm. Moreover, the TCFs show good stability to resist tape test and ultrasonication, and which can be completely dissolved in city running water contains 2% acetic acid after ten days under the natural condition, demonstrating an excellent nature degradability. Finally, the successful implementations of the degradable flexible transparent heater and electroluminescent device demonstrate the practical application value of the TCFs.

Rapid nanowelding of silver nanowires by focused-light-scanning for high-performance flexible transparent electrodes

Silver nanowires (AgNWs) have been considered as one of the most promising flexible transparent electrodes (FTEs) material for next-generation optoelectronic devices. However, the large contact resistance between AgNWs could deteriorate the conductivity of FTEs. In the present work, high-performance AgNWs FTEs were obtained by means of focused-light-scanning (FLS), which could lead to the large-area, rapid and high-quality welding between AgNWs within a short time, forming the reliable and stable AgNWs network. The results of the optoelectronic tests show that after FLS, the sheet resistance of the AgNWs FTEs sharply decreased from 5113 Ω/sq to 7.7 Ω/sq, with maintaining a high transmittance (∼94%). Finally, a high-performance flexible transparent heater was fabricated by using FLS, showing reach a relatively high temperature in a short response time and rapid response at low input voltage. The findings offer an effective pathway to greatly improve the conductivity of AgNWs FTEs.

Folding-insensitive, flexible transparent conductive electrodes based on copper nanowires

Flexible transparent conductive electrodes (FTCEs) have received a great deal of attention for various applications due to their versatile ability. To demonstrate high-performance of FTCEs, numerous efforts have been focused on high conductivity, transparency and mechanical durability against bending. However, it is still a challenge for FTCEs to apply in foldable applications due to the degradation of electrical conductivity during folding. In this paper, high-performance FTCEs with folding-insensitivity are achieved by embedding copper nanowires (Cu NWs) into polymethyl methacrylate strategy via simple sol-gel transition. The prepared film demonstrates a very high transmittance of ~94 % with a low sheet resistance of 56.3 Ω/sq. In particular, excellent insensitivity conductivity are achieved with a bending radius of 2.5 μm and the resistance remains nearly to be constant after 1000 folding cycles with a bending radius of 50 μm. Moreover, the film shows a good stability to resist long-term storage and ultrasonication. Finally, flexible transparent heater is fabricated based on the FTCEs, the immediate response make it have a great promise to be used as defrost, antifog and (light emitting diode) LED devices. • Folding-insensitive FTCEs are prepared by sol-gel technology. • The conductivity enable to achieve excellent insensitivity to the folding. • The FTCE shows a good stability to resist long-term storage and ultrasonication. • The FTCE has very high transmittance of ~94 % with sheet resistance of 56.3 Ω/sq.

Fast Welding of Silver Nanowires for Flexible Transparent Conductive Film by Spatial Light Modulated Femtosecond Laser

Silver nanowires (AgNWs) conductors with fine flexibility and high conductivity have been considered to have a promising perspective in the area of flexible transparent electrode. However, many challenges still exist in improving the conductivity of AgNWs films, such as high temperature and time‐consuming. Herein, an efficient and rapid method to weld AgNWs by spatial light modulated femtosecond (fs) laser is proposed. The Gaussian laser beam is modulated into a line‐shaped laser beam with the length of 773 μm. Compared with traditional spherical lens focus irradiation welding, the welding efficiency can be improved up to tens of times. The mechanism of fs laser welding is mainly to use high‐energy laser beam irradiation to generate local field enhancement which causes local high temperature at the nanowire junction. After laser welding, the sheet resistance of AgNWs films can be reduced to 14.7 Ω sq−1 at the transmittance of 89.5% without substrate damage. The welded AgNWs films exhibit excellent electrical conductivity even after 10 000 bending cycles. Moreover, the welded AgNWs films are used to make a flexible transparent heater, showing superior sheet resistance uniformity.

Quick Thermal Response‐Transparent‐Wearable Heater Based on Copper Mesh/Poly(Vinyl Alcohol) Film

High‐performance, transparent, wearable heaters with fast thermal response are developed. To accomplish this, highly conductive and transparent copper (Cu) mesh films are transfer printed onto flexible poly(vinyl alcohol) (PVA) substrates of different thicknesses. The thermal response characteristics of the heaters, with different PVA substrate thicknesses, are analyzed and results are recorded. The Cu mesh/PVA film heater shows a favorable figure of merit value of 228.4 and outstanding durability in response to mechanical stress tests for bending, crumpling, and detaching. Through the test results, it is concluded that the thermal response time of the heater is directly related to the thickness of the PVA substrate. The optimized Cu mesh/PVA film heater, with a substrate thickness of ≈60 μm and input voltage of 3 V (DC), exhibits a fast Joule heating effect with a response time of ≈12 s and a ramping rate of ≈7.5 °C s−1. These are excellent results, compared with the previously reported studies with other metal‐based flexible transparent heaters. A flexible transparent heater with low applied voltage and fast response time is expected to have innovative electrothermal applications, such as transparent soft actuators, wearable heaters, and medical thermotherapy pads.

Transparent planar layer copper heaters for wearable electronics

To keep up with the progress in wearable electronics, it is imperative to create flexible transparent heaters with low sheet resistance and high transmittance. This allows for low-voltage operation. In this study, we fabricated highly flexible ultrathin transparent heaters composed of Cu layers with a thickness in the range of 4–40 nm sandwiched between ZnO layers. The sheet resistance (0.8–165 Ω/sq) and optical transparency (average visible light transmittance of 29.3–90.0%) can be considerably modulated by varying the Cu layer thickness. The fabricated transparent heaters exhibited superior heating capability over reported network-type transparent heaters, i.e., extremely fast thermal response (90% of target temperature reached within 5 s), excellent thermal uniformity (less than 2.5% thermal variation at a heater temperature of 100 °C), and outstanding heater durability (no degradation with a continuous heating test at 80 °C for 10,000 min). Based on the area-dependent thermoelectrical parameters, it was shown that large heaters could reach 50 °C with a portable rechargeable lithium-ion battery with 12 V DC output. The excellent flexibility in the bent and unbent state, when subjected to various bending tests, confirmed the high suitability of the ultrathin transparent Cu heaters for wearable electronics, such as biomedical devices.

Flexible high-performance SnO2/AgNWs bilayer transparent conductors for flexible transparent heater applications

Because of high transparency, conductivity and flexibility, silver nanowire (AgNW) networks have been widely explored as a promising alternative to indium tin oxide (ITO). However, their poor adhesiveness, corrosion and thermal stability still remain limiting factors for widespread adoption in a range of devices. Here, we introduce SnO2 to improve the adhesion and stability performances of AgNWs by controlling the preparation process, enhance the conductivity by providing the electric channels at contacts of AgNWs. The unique structural feature of composite film allows for a novel transparent conductor with low sheet resistance of 7.9 Ω/sq. and high transmittance of 87.6% (at 550 nm), as well as good flexibility. Unlike bare AgNW networks, the composite film retains its original conductivity under immersion in HCl and Na2S solutions, or long-term storage at up to 85% relative humidity at 85 °C. This chemical durability can be attributed to the absence of silver corrosion products for the AgNW encapsulated by SnO2. Furthermore the resistance is nearly unchanged after tape and ultrasonication tests, indicating a good adhesion to the substrate. Finally, a flexible transparent heater employing a hybrid film is realized.

Templateless, Plating-Free Fabrication of Flexible Transparent Electrodes with Embedded Silver Mesh by Electric-Field-Driven Microscale 3D Printing and Hybrid Hot Embossing.

Flexible transparent electrodes (FTEs) with an embedded metal mesh are considered a promising alternative to traditional indium tin oxide (ITO) due to their excellent photoelectric performance, surface roughness, and mechanical and environmental stability. However, great challenges remain for achieving simple, cost-effective, and environmentally friendly manufacturing of high-performance FTEs with embedded metal mesh. Herein, a maskless, templateless, and plating-free fabrication technique is proposed for FTEs with embedded silver mesh by combining an electric-field-driven (EFD) microscale 3D printing technique and a newly developed hybrid hot-embossing process. The final fabricated FTE exhibits superior optoelectronic properties with a transmittance of 85.79%, a sheet resistance of 0.75 Ω sq-1 , a smooth surface of silver mesh (Ra ≈ 18.8nm) without any polishing treatment, and remarkable mechanical stability and environmental adaptability with a negligible increase in sheet resistance under diverse cyclic tests and harsh working conditions (1000 bending cycles, 80 adhesion tests, 120 scratch tests, 100min ultrasonic test, and 72 h chemical attack). The practical viability of this FTE is successfully demonstrated with a flexible transparent heater applied to deicing. The technique proposed offers a promising fabrication strategy with a cost-effective and environmentally friendly process for high-performance FTE.

All-Solution-Processed Molybdenum Oxide-Encapsulated Silver Nanowire Flexible Transparent Conductors with Improved Conductivity and Adhesion.

A novel transparent conductive conductor composed of a silver nanowire (AgNW) network and MoOx on a flexible polyethylene terephthalate (PET) substrate, with contemporaneously improved adhesion and reduced resistivity, is prepared using the full-solution process without high-temperature annealing. Under the optimized conditions, a MoOx/AgNW/MoOx multilayer is achieved, which shows much superior optoelectronic performance to that obtained from ITO with a high optical transmittance of 89.2% and a low sheet resistance of ∼12.5 Ω/sq. Unlike pure AgNW films, the sheet resistance is little changed after the tape and ultrasonication tests, illustrating a very strong adhesion to the PET substrate after the encapsulation of MoOx. Moreover, the multilayer film exhibits excellent stability to resist mechanical bending and acid damage. In addition, the successful implementation of the flexible transparent heater demonstrates the practical application value of the electrode.

Fabrication of extremely conductive high-aspect silver traces buried in hot-embossed polycarbonate films via the direct gravure doctoring method

Depositing inks on a planar substrate by printing is a facile way to fabricate conductive traces and other complicated functional devices. However, irrespective of the printing methods used, the thickness of the inks has an upper limit due to their fluidic property and subsequent wetting on the substrates. Herein, we present a method for creating high-aspect ratio conductive traces by combining hot-embossing and gravure doctoring techniques. Binary-sized colloidal pastes containing Ag nanoparticles and micrometer-sized spherical Ag powder additives were filled using a doctor blade in the grooves of polycarbonate (PC) films inscribed via hot-embossing to create buried traces. Under optimal mixing conditions in which the minimum resistivity was achieved, voids between the microparticles provided a pathway for the volatile solvents to smoothly escape from the filled ink and minimized thickness reduction during the thermal sintering process. A fabricated trace buried in the PC film with an aspect ratio of around 3:1 and a linewidth of 55 μm showed extremely low resistance of less than 10 Ω/m. A flexible transparent heater was developed using the reported binary colloidal paste.

Facile Post Treatment of Ag Nanowire/Polymer Composites for Flexible Transparent Electrodes and Thin Film Heaters

Typical polyol-based synthesis of silver nanowire employs insulating polymer as a surfactant for the silver nanowire growth, which limits direct contact between each nanowire and thus its optoelectronic properties. We herein demonstrate that a simple solvent treatment effectively removes the insulating polymer around Ag NWs, leading to significantly decreased sheet resistance (~12 Ω/sq) with an increased transmittance (81% @ T550), as compared to other post-treatments. We successfully demonstrate the transparent film heaters using the solvent-treated Ag NWs network, which rapidly exhibited 150 °C under a bias of 5 V. Flexible film heaters on plastic substrate is also demonstrated, suggesting a great potential of the solvent treatment process of Ag NWs for flexible transparent electrode and film heater applications.

Flexible Transparent Polypyrrole-Decorated MXene-Based Film with Excellent Photothermal Energy Conversion Performance.

Flexible transparent heaters based on photothermal energy conversion are highly desired for next-generation electronic devices. However, how to balance the photothermal conversion efficiency and transparency is still a huge challenge. In this work, we demonstrate a flexible polycarbonate (PC) film with balanced photothermal energy conversion performance and transparency obtained from the spraying polypyrrole (PPy)-modified Ti3C2Tx MXene (MXene@PPy) layer. Due to the synergistic light-to-heat effects of MXene and the attached PPy nanoparticles, the resulted transparent film heater (MP-PC) can obtain a satisfying photothermal conversion performance (47.5 °C at 100 mW/cm2) at a low spraying density of MXene and thus show an effective transmittance of 51.61%, simultaneously. Moreover, the photothermal conversion performance reveals an outstanding stability without significant deterioration after exposing to an outdoor environment for seven months. Besides, arising from the excellent surface electrical resistance (413 Ω/sq), the MP-PC film also exhibits an effective Joule heating capacity with a high heating temperature of 108 °C at 24 V input voltage. As one of the promising applications, the MP-PC film exhibits the effectiveness and feasibility as a light-triggered thermal therapy film for human skin in cold environments.

Fabrication of metal mesh flexible transparent electrodes and heaters by a cost-effective method based on ultrafast laser direct writing

A new ultrafast laser direct writing process to fabricate completely embedded silver mesh flexible transparent electrodes (TEs) using sputtered silver thin films and their applications for flexible transparent heaters (FTHs) are reported. The optimal laser etching parameters were determined, and the effects of mesh pitch and height on photoelectric properties of the silver mesh flexible TEs were systematically studied, with an aim to obtain a TE with excellent comprehensive performance. The resulting TE had a lower sheet resistance (0.86 Ω/sq) and a higher optical transmittance (91.26%) than traditional TEs. Good environmental stability of the TE, i.e. maintaining good electrical conductivity in air or deionized water without significant changes for 40 days, was also demonstrated. The fabricated representative FTH also presented stable heating performance, reaching a high temperature in 60 s under a DC voltage of 1.0 V and finally stabilizing at 98 ℃. This heating performance was confirmed to be stable and consistent after 20 cycles or under the conditions of bending and twisting. In addition, a deicing experiment was performed to illustrate the possible application of the FTH. The results indicated the availability of the laser direct writing method for fabricating high-performance metal mesh-based flexible TEs.

Facile Synthesis of Highly Oxidation Stable Nanosilver-Coated Copper Nanowires for Transparent Flexible Electrodes

This study reports an efficient electroless plating approach using an amino-alcohol precursor to synthesize the oxidation stable copper@silver (Cu@Ag) core–shell nanowires. The synthesized nanowires were characterized by different techniques. The as-synthesized nanowires were used to fabricate transparent flexible electrodes (TFEs) and transparent stretchable heaters (TSHs). The nanowire network on the substrates was sintered using the 5% aqueous solution of H2O2, which welded the nanowires at their junction and strongly attached to the substrate. Therefore, the optical-electrical performance (∼13 Ω sq–1 @ 89% transmittance), oxidation stability, and mechanical flexibility of as-prepared TFEs and TSHs were improved to a large extent. In addition, the performance of the TSHs in terms of high-saturation temperature and stability against repeated stretching cycles upon 20% mechanical strain temperature are also enhanced. The overall processing strategy was suitable to synthesize the Cu@Ag core–shell nanowires and to fabricate the stable TFEs and TSHs for modern electronic and thermoelectric devices.

Flexible biodegradable transparent heaters based on fractal-like leaf skeletons

We present a facile method to prepare flexible, transparent, biodegradable, and fast resistive heaters by applying silver (Ag) nanowires onto fractal-like leaf skeletons. The fractal-like structure of the leaf skeleton maximizes its surface area, improving the transfer of heat to its surroundings and thus making the heater fast, without compromising transparency. Ag ion layer on the leaf skeleton helps to conformally cover the surface with Ag nanowires. The sheet resistance of the heater can be controlled by the loading of Ag nanowires, without sacrificing the optical transmittance (~80% at 8 Ω sq−1). The heating is uniform and the surface temperature of a 60 mm × 60 mm heater (8 Ω sq−1) can quickly (5–10 s) raise to 125 °C with a low voltage (6 V). The heater displays excellent mechanical flexibility, showing no significant change in resistance and heating temperature when bent up to curvature of 800 m−1. Finally, we demonstrate the potential of the bioinspired heater as a thermotherapy patch by encapsulating it in a biodegradable tape and mounting it on the human wrist and elbow. This study shows that fractal-like structures from nature can be repurposed as fractal designs for flexible electronics.

Metal mesh embedded in colorless shape memory polyimide for flexible transparent electric-heater and actuators

The current electroactive shape memory polymer composites (SMPCs) show low electric heating efficiency, and the transparency of the matrix is seriously damaged by the conductive fillers. In this work, transparent metal mesh is used to replace the conductive fillers to construct a conductive network for electrical actuation for the first time. The self-cracking aluminum mesh is embedded in the transparent shape memory polyimide film (Alm@TSMPI) by solution casting. As a new type of flexible and transparent heater, the Alm@TSMPI has advantages of fast response and high steady-state temperature. As an electric actuator, the Alm@TSMPI is active and deformable depending on its variable stiffness characteristics, and reverts to its original shape within 13 s under electric stimulation. This is the first report on an electroactive transparent shape memory polymer while maintaining light transmission. Moreover, the transition temperature of 230 °C is much higher than that of all other reported electroactive SMPCs. Furthermore, the Alm@TSMPI is attached to transparent shape memory polystyrene as a flexible transparent heater, and the deformation is triggered by electric field. According to this strategy, the Alm@TSMPI can be applied for the electrical actuation of other SMPs, extending the applications in the electric actuators.

Hybrid PEDOT:PSS to obtain high-performance Ag NW-based flexible transparent electrodes for transparent heaters

Silver nanowire (Ag NW) networks are remarkable components that may be able to replace indium tin oxide (ITO) as transparent electrodes. In this paper, the Ag NW based flexible transparent electrodes are prepared via PEDOT:PSS sol–gel transition without vacuum process, provides very high figure of merit by minimizing junction resistance, and also exhibit excellent mechanical robustness against bending. The sheet resistance of the flexible Ag NW-based films reduces from 1000 to 9.4 Ω/sq. at transmittance of ~ 90%. The conduction mechanism involves Ag NWs to PEDOT:PSS carrier injection prior to the formation of a continuous metal conduction pathway. Calculations of figure of merit display a large value of 340, which is superior to that obtained from the flexible ITO, carbon nanotube, graphene, copper nanowire, and other silver nanowire and hybrid thin films. Meanwhile, the sheet resistance does not show great change after tape test due to the effect of PEDOT:PSS. Finally, the successful implementation of the prepared Ag NWs-based transparent electrodes into a flexible transparent heater is demonstrated, verifying the applicability of the electrodes. It is believed that our study is the key step toward realizing the practical use of NW flexible transparent electrodes in various flexible electronic devices.

Effect of AlOx protection layer on AgNWs for flexible transparent heater

We indicated high performance and stability transparent heaters based on AlOx covered Ag nanowires. We obtained an AlOx covered Ag nanowire thin film which has a 47 ohm/sq of sheet resistance and 88.1% (substrate included) of transmittance at 600 nm on a flexible substrate. We demonstrate that the thin AlOx layer leads to increased contact area at the junction of Ag nanowires, which contributes to lower sheet resistance and improved adhesion of Ag nanowires. Furthermore, high stability and flexibility of Ag nanowire have been achieved by the AlOx layer. Finally, we fabricated a flexible transparent heater with AlOx covered Ag nanowire, and obtained a temperature of 81 °C within 40 sec at the driven voltage of 7 V with fast response and uniform temperature distribution. Therefore, the AlOx covered Ag nanowire film is a promising candidate for the application of the flexible transparent heaters.

Intense pulsed light-induced structure-transformed ultrathin Ni shell for improving the chemical, thermal, and electrical reliability of metal nanowire electrodes without transmittance loss

Metal nanowires (NWs) boost the development of flexible transparent conductive electrodes (TCEs), but they are hardly applied in industrial productions due to poor reliability. Although various coating technologies have been used to resolve this issue, most of them deteriorate the transmittance of TCEs due to the thick coating shell. Herein, we propose a facile and highly compatible structural transformation process for fabricating core-shell NW TCEs with highly improved reliability. Instead of increasing the thickness of the coating shell to guarantee sufficient coverage around NWs, an instantaneous and surface heating technology of intense pulsed light is creatively applied to realize the structural transformation of scattered Ni crumbs into an ultrathin and uniform Ni coating shell. Results show that without the decline of the transmittance, the reliability of structure-transformed Ag@Ni (STAN) core-shell NW TCEs has been increased to 10 times higher than that of pristine Ag@Ni NW TCEs, which are much better than most of the state-of-the-art publications on core-shell NW TCEs. Flexible transparent heaters fabricated by the STAN NW TCEs exhibit both rapid response and long-term electrical stability. It is believed that the proposed structure-transformed coating method provides a novel and highly versatile way to generate ultrathin protective shells around NWs, which breaks the contradiction between achieving the high transmittance and obtaining the long-term reliability during the fabrication of NW TCEs.