Transparent Thin Film Heaters Research Articles

Directly patterned ITO nanoparticle-based transparent electrode using co-solvent-based aerosol jet printing for transparent thin film heaters

High-performance transparent thin film heaters (TFHs) employing cost-effective nanoparticle-based transparent electrodes have attracted significant attention for applications in multifunctional smart windows. This study demonstrates the fabrication of high-performance TFHs featuring directly patterned Sn-doped In2O3 (ITO) nanoparticle electrodes using a methanol and ethylene glycol co-solvent-based aerosol jet printing (AJP) process. Our method utilizes a co-solvent ITO nanoparticle solution, blending methanol and ethylene glycol, to improve the surface uniformity and roughness of the AJP-processed ITO nanoparticle electrodes. The optimization of these electrodes was assessed using the figure of merit (FoM=T10/Rsh) value, derived from the transparency and sheet resistance of the electrodes. As a result of the AJP process and post-annealing optimization, the directly patterned ITO nanoparticle electrodes exhibited a high FoM value of 16.1 Ω−1, attributed to a high optical transmittance of 90.6 % and low sheet resistance of 23.20 Ω/sq. These optimized ITO nanoparticle electrodes were subsequently applied to TFHs through precise patterning of ITO electrodes via AJP. The ITO nanoparticle-based TFHs, benefiting from superior conductivity and precise AJP patterning, demonstrated a saturation temperature of 118.2 °C when subjected to a voltage of 9 V. Thermal stability and uniformity tests showed a temperature deviation of less than 5 %, validating the reliability of the TFHs. The performance of the AJP-processed ITO nanoparticle-based TFHs highlights the feasibility of cost-effective transparent TFHs for multifunctional smart windows, showing potential applications in electric vehicles and next-generation smart buildings.

Highly flexible and transparent electrodes for high-performance thin-film heaters with nanostructured micromesh Cu–Ag ultrathin films

Herein, we present a facile method to fabricate transparent, flexible, and high-performance thin-film heaters (TTFHs) based on nanostructured micromesh (NSMM) Cu–Ag/indium tin oxide bilayer transparent thin-film electrodes (TTFEs) with both nano- and microstructures. Our approach combines Ar-assisted thermal evaporation and pulsed laser ablation techniques to fabricate NSMM TTFEs with a low sheet resistance (∼2.1 Ω/sq) and high optical transmittance (∼80.1 %). By adjusting the spacing of microholes, we can achieve excellent figure of merit values (∼51.8 × 10−3 Ω−1) for the NSMM TTFEs. These electrodes also show excellent durability and flexibility, as evidenced by a resistance change of only 4 %–18 % in cyclic bending tests with up to 200,000 cycles. Finally, we demonstrate that the NSMM TTFEs can be used as high-performance TTFHs with short response times (3.97 s at a steady temperature of 87.5 °C under low-voltage operation (5 V)) and infrared shielding properties. Our results suggest that the Cu–Ag-alloy-based NSMM TTFEs are promising transparent electrodes for various applications in flexible electronics, smart windows, and wearable devices.

Conformal and Transparent Al2O3 Passivation Coating via Atomic Layer Deposition for High Aspect Ratio Ag Network Electrodes

We demonstrated conformal Al2O3 passivation via atomic layer deposition (ALD) of a flexible Ag network electrode possessing a high aspect ratio. The Ag network electrode passivated by the ALD-grown Al2O3 film demonstrated constant optical transmittance and mechanical flexibility relative to the bare Ag network electrode. Owing to the conformal deposition of the Al2O3 layer on the high aspect ratio Ag network electrode, the electrode exhibited more favorable stability than its bare Ag-network counterpart. To demonstrate the feasibility of Al2O3 passivation via ALD on a flexible Ag network, the performances of flexible and transparent thin-film heaters (TFHs) with both a bare Ag network and that passivated by ALD-grown Al2O3 were compared. The performance of Al2O3/Ag network-based TFHs was minimally altered even after harsh environmental tests at 85% relative humidity and a temperature of 85 °C, while the performance of bare electrode-based TFHs significantly deteriorated. The improved stability and reliability of the Al2O3/Ag network-based TFHs indicate that the ALD-grown Al2O3 film effectively prevents the introduction of moisture and impurities into the Ag network with a high aspect ratio. The improvement in the stability of the Ag network through Al2O3 passivation implies that the ALD-grown Al2O3 film represents a promising transparent and flexible thin film passivation material for high quality Ag network electrodes with high aspect ratios.

Thermally stable and transparent F-doped SnO2 (FTO) /Ag/FTO films for transparent thin film heaters used in automobiles

We investigated the characteristics of F-doped SnO2 (FTO)/Ag/FTO films prepared using thermal evaporation at room temperature for the application of the as-formed films in transparent thin film heaters (TFHs) of automobiles. To optimize the electrical and optical properties of the FTO/Ag/FTO multi-layer, the figure of merit (FoM) values of the FTO/Ag/FTO multi-layers were compared as a function of the thickness of the Ag and FTO layers. The sheet resistance and optical transmittance of the FTO/Ag/FTO multi-layer were primarily affected by the Ag inter-layer and bottom/top FTO thicknesses, respectively. At optimized Ag (10 nm) and FTO (40 nm) thicknesses, we fabricated a FTO/Ag/FTO electrode with a sheet resistance of 8.00 Ohm/square, an optical transmittance of 83.04 % at a visible wavelength (400–800 nm) and a FoM value of 19.49 Ohm-1. The TFHs comprising the optimal FTO/Ag/FTO electrode exhibited a saturated temperature of 117 °C at a low operating direct current of 6 V, owing to the low sheet resistance. In addition, the FTO/Ag/FTO-based TFHs exhibited thermally stable performances owing to the stability of the bottom and top FTO electrodes. The performance of the FTO/Ag/FTO-based TFHs demonstrated that the thermally evaporated FTO/Ag/FTO multi-layer is a promising, stable, and transparent electrode material for application in the front window TFHs used in automobiles.

ITO/AgPd/ITO Laminated Films for High-performance Transparent Thin Film Heaters

ITO/AgPd/ITO Laminated Films for High-performance Transparent Thin Film Heaters

Highly Transparent, Flexible, and Hydrophobic Polytetrafluoroethylene Thin Film Passivation for ITO/AgPdCu/ITO Multilayer Electrodes

AbstractTransparent and flexible polytetrafluoroethylene (PTFE) thin film passivation for ITO/AgPdCu (APC)/ITO (IAI) electrodes is developed to prevent severe degradation and maintain the reliable performance of the flexible IAI electrodes. Also, oxygen ion beam treatment (IBT) on the surface of the IAI electrode improves the adhesion between the top ITO and PTFE passivation layer, as well as the performance of the passivation layer. The IAI electrode with PTFE passivation exhibits constant sheet resistance, transmittance, and flexibility, even after the 85 °C and 85% RH test. However, the bare IAI electrode shows a significant increase in sheet resistance and a decrease in transmittance and flexibility, owing to the incorporation of moisture and impurities into the IAI layer, and agglomeration of the APC layer. To demonstrate the feasibility of PTFE passivation, the performance of flexible and transparent thin film heaters (TFHs) with the bare IAI and PTEF/IAI samples are compared. The better stability and reliability of the PTFE/IAI‐based TFHs than the bare IAI‐based TFHs indicate that the PTFE film effectively prevents the introduction of moisture and impurities. The performance of the TFH with PTFE/IAI electrode implies that sputtered PTFE film is a promising transparent and flexible thin film passivation for high‐quality oxide‐metal‐oxide multilayer electrodes.

Flexible multilayered transparent electrodes with less than 50 nm thickness using nitrogen-doped silver layers for flexible heaters

In this study, Al-doped ZnO/Ag/Al-doped ZnO multilayered electrodes are fabricated on a colorless polyimide substrate through an optimized nitrogen insertion technique, and subsequently applied to transparent flexible thin film heaters. The resulting electrodes have remarkable performance and durability. A completely continuous and smooth Ag thin film with extremely small thickness is fabricated by optimizing the N2 gas flow rates at room temperature. The multilayered films exhibit a high optical transmittance of 96.0% at a wavelength of 550 nm and low sheet resistance of 8.5 Ω sq.−1. The bending test results show that there is no significant change in the sheet resistance even after 5,000 bending cycles, highlighting their mechanical flexibility. For its application in transparent flexible heaters, the performance of the multilayered films exceeds that of conventional ITO-based films, as confirmed by their higher saturation temperatures and flexibility, indicating their outstanding thermal properties and high conductivity with small thickness.

Silver alloy-based metal matrix composites: a potential material for reliable transparent thin film heaters

A Zn–SnOx/optimized Ag-0.07 at%-0.67 at%Cr/Zn–SnOxmultilayer thin film with reliable and reversible heating performance as a flexible transparent thin film heater was fabricated.

Roll to roll coating of carbon nanotube films for electro thermal heating

Carbon nanotube (CNT) films are gaining traction in applications such as transparent conductive films, electro-magnetic shields and thin film heaters. However, to date, few cost-effective large-area CNT coating methods have been reported. Here, we present a roll-to-roll (R2R) slot-die coating process for thin film CNT heaters. In this process, a CNT suspension is continuously coated on a PET film substrate and subsequently dried and packaged. This process allows for continuous square-meter-size CNT coating. The electrical resistance and thermal map of these samples are measured by high definition infrared (IR) thermography. Anti-/de-icing demonstrations of R2R CNT coated samples are performed inside a cold room and outdoor atmospheric icing conditions. The successful R2R coating of CNTs and anti-/de-icing demonstrations show promise for application of CNTs in large area applications, such as the de-icing of ships, for which strict regulations are put in place for vessels operating in polar waters.

Rapid Defrost Transparent Thin-Film Heater with Flexibility and Chemical Stability

Zn-doped SnOx/Ag/Zn-doped SnOx(ZTO/Ag/ZTO) multilayer thin films fabricated on a polyethylene terephthalate (PET) substrate using an optimized N2-to-(Ar + O2) gas ratio are used for transparent thin-film heaters with high performance and chemical stability. The ZTO/Ag/ZTO-based multilayer thin film exhibits enhanced durability at high temperatures and humid environments by incorporating nitrogen. The bending test results-there was no significant change in the sheet resistance even after 10,000 bending cycles-highlight the mechanical flexibility of the ZTO/Ag/ZTO multilayer thin film. The ZTO/Ag/ZTO-based thin-film heater on PET, fabricated under optimized deposition gas conditions, exhibits a fast thermal response time of 30 s and a low driving voltage of 6 V to attain 100 °C. It also exhibits uniform heat distribution at saturated temperature and chemical stability after 100 heating-cooling cycles. Hence, the proposed ZTO/Ag/ZTO-based thin-film heater is applicable for use in front and rear window automobile and building applications.

Deposition Rate Effect on Optical and Electrical Properties of Thermally Evaporated WO3\u2212x/Ag/WO3\u2212x Multilayer Electrode for Transparent and Flexible Thin Film Heaters

We investigated the deposition rate effect on the optical, electrical, and morphological characteristics of thermally evaporated WO3−x/Ag/WO3−x (WAW) multilayer electrodes. By controlling the deposition rate of the WO3−x and Ag layers, we can control the interface structure between WO3−x and Ag and improve both the optical and electrical properties of the thermally evaporated WAW multilayer electrodes. At the optimized deposition rate of WO3−x (2.5 Å/sec) and Ag (10 Å/sec), the symmetric WAW multilayer exhibited a high optical transmittance of 92.16% at a 550 nm wavelength and low sheet resistance of 3.78 Ω/square. During repeated bending, rolling, and twisting, there was no resistance change indicating the superior flexibility of WAW multilayer electrodes. As a promising application of the WAW multilayer electrodes, we suggested the transparent and flexible thin film heaters (TFHs) to substitute the high cost indium tin oxide-based TFHs. In comparison to the ITO-based TFHs, the WAW based TFHs showed higher convective heat transfer property and higher saturation temperatures are achieved at lower input voltages due to lower sheet resistance. This indicates that the WAW multilayer is suitable as the electrode for high performance transparent and flexible TFHs.

Robust ultrathin and transparent AZO/Ag-SnOx/AZO on polyimide substrate for flexible thin film heater with temperature over 400 °C

We developed flexible and transparent thin film heaters (TFHs) with a structure of AZO/Ag-SnOx/AZO/polyimide (PI) which exhibited superior stability under operational conditions. Ultrathin and robust doped silver (Ag) films were produced by introducing a small amount of SnOx during the sputtering process. The AZO/Ag-SnOx/AZO stacks showed the best figure of merit value of 139.9, which was higher than that of ITO counterpart (25.5) with the same thickness. In addition, it exhibited excellent durability, which showed unaffected optical and electrical properties under the heat treatment of 500 °C for 30 min in air, highly-accelerated temperature and humidity stress test (HAST) at 121 °C and 97%RH for 36 h, 10,000 times of scratching and 1500 times of inner and outer bending test. Furthermore, the TFHs based on AZO/Ag-SnOx/AZO/PI achieved a high temperature of 438.8 °C with response time in several seconds, which outperformed most previous studies. The robust high-temperature TFHs in this research hold promising commercial applications in flexible and high temperature occasions.

Core/shell copper nanowire networks for transparent thin film heaters

Copper nanowires (Cu NWs) appear as the strongest alternative to silver nanowires (Ag NWs) in transparent conductors. Cu NWs; however, are more prone to oxidation compared to Ag NWs even at room temperature. This problem becomes more severe when Cu NWs are used as transparent thin film heaters (TTFHs). In this work, we have utilized ALD deposited zinc oxide (ZnO) shell layers, and provide a comparison with typically used aluminum oxide (Al2O3) shell layers to improve the TTFH performance. While Cu NW network TTFHs barely withstood temperatures around 100 °C, critical thickness of ALD deposited Al2O3 and ZnO layers were determined to find out TTFH limits. Maximum stable and reproducible temperatures of 273 °C and 204 °C were obtained for Al2O3 and ZnO deposited Cu NW network TTFHs, respectively. An extensive parametric study on the NW density and oxide type in conjunction with the electrical conductivity and optical transmittance was conducted. A remarkably high heating rate of 14 °C s−1 was obtained from the fabricated core/shell networks with improved oxidation stability under ambient and high humidity conditions. Finally, these high performance core/shell Cu NW network TTFHs were utilized as efficient defrosters.

Transparent heater based on Al,Ga co-doped ZnO thin films

Al,Ga co-doped ZnO thin films were deposited on to glass substrates by aerosol assisted chemical transport (AACT) and were studied for application as transparent thin film heaters. The film thickness was around 400 nm after 60 min of AACT deposition; this gave a sheet resistance of 142.5 Ω/sq, which corresponded to a resistivity of 5.7 × 10−3 Ω·cm. The thin films exhibited a maximum transmittance of 90% in the visible region. A mean temperature of 132.3 °C was reached after applying a voltage of 18 V for 10 min, giving a power consumption of 2.11 W. These results could provide a possibility to use Al,Ga co-doped ZnO thin films as transparent heaters to replace the more expensive indium tin oxide.

Roll-to-roll sputtered, indium-free ZnSnO/AgPdCu/ZnSnO multi-stacked electrodes for high performance flexible thin-film heaters and heat-shielding films

As a cost-effective, indium-free, flexible, and highly transparent electrode for flexible thin-film heaters and heat-shielding films, ZnSnO(ZTO)/AgPdCu(APC)/ZTO multi-stacked films were fabricated by using 1500-mm–width, roll-to-roll (RTR) sputtering at room temperature. The optimized RTR-sputtered ZTO/APC/ZTO film showed a very low sheet resistance (3.43 ohm/square), high optical transmittance (80.99%), and small inner/outer critical bending radii (1 and 2 mm, respectively), even when it was processed at room temperature. In particular, the better flexibility of the ZTO/APC/ZTO films, relative to typical ITO films, was confirmed by lab-scale bending, rolling, twisting, and folding tests. Due to the very low sheet resistance, superior flexibility, and low transmittance in the near-infrared wavelength region, flexible and transparent thin-film heaters (TFHs) and flexible heat-shielding films (HSFs) made with the ZTO/APC/ZTO films showed better performance than typical ITO-based TFHs and HSFs. This suggests that the RTR–sputtered, indium-free ZTO/APC/ZTO films can be applied as transparent and flexible electrode materials in cost-effective and multi-functional smart windows for buildings and automobiles.

Highly Transparent, Deformable, and Multifunctional AgPdCu/ITO/PTFE Hybrid Films for Self‐Cleaning, Flexible, and Energy‐Saving Smart Windows

AbstractRollable, bendable, twistable, and foldable polymer–oxide–metal multistacked hybrid films are demonstrated, which simultaneously act as multifunctional electrodes in flexible polymer‐dispersed liquid crystal (PDLC) windows and flexible and transparent thin‐film heaters (TFHs) for self‐cleanable and flexible smart windows. By continuous sputtering of a thin polytetrafluoroethylene (PTFE)–indium tin oxide (ITO)–AgPdCu (APC) multilayer on a colorless polyimide substrate, transparent and multifunctional films are fabricated with a low sheet resistance of 7 Ω □−1, high optical transmittance of 83% at a wavelength of 550 nm, low transmittance in the infrared region, and a high contact angle of 109°. Based on inner/outer bending, rolling, twisting, and folding tests, the outstanding flexibility of these PTFE/ITO/APC multistacked films is demonstrated. The flexible PDLC and flexible TFHs with PTFE/ITO/APC multistacked electrodes show a controllable transmittance range between the on and off states that is nearly 40%, and a high saturation temperature of 111 °C is observed at a low applied voltage of 4 V. The multifunctional properties of the flexible PDLC and TFHs fabricated on PTFE/ITO/APC hybrid films indicate that the polymer–oxide–metal hybrid film is a versatile transparent electrode that can be used for flexible, smart, and energy‐saving windows.

Effective passivation of Ag nanowire network by transparent tetrahedral amorphous carbon film for flexible and transparent thin film heaters

We developed effective passivation method of flexible Ag nanowire (NW) network electrodes using transparent tetrahedral amorphous carbon (ta-C) film prepared by filtered cathode vacuum arc (FCVA) coating. Even at room temperature process of FCVA, the ta-C passivation layer effectively protect Ag NW network electrode and improved the ambient stability of Ag NW network without change of sheet resistance of Ag NW network. In addition, ta-C coated Ag NW electrode showed identical critical inner and outer bending radius to bare Ag NW due to the thin thickness of ta-C passivation layer. The time-temperature profiles demonstrate that the performance of the transparent and flexible thin film heater (TFH) with the ta-C/Ag NW network is better than that of a TFH with Ag NW electrodes due to thermal stability of FCVA grown ta-C layer. In addition, the transparent and flexible TFHs with ta-C/Ag NW showed robustness against external force due to its high hardness and wear resistance. This indicates that the FCVA coated ta-C is promising passivation and protective layer for chemically weak Ag NW network electrodes against sulfur in ambient.

Hydrophobic and stretchable Ag nanowire network electrode passivated by a sputtered PTFE layer for self-cleaning transparent thin film heaters.

We demonstrated hydrophobic, flexible/stretchable, and transparent electrodes made up of Ag nanowire (NW) networks passivated by a sputtered polytetrafluoroethylene (PTFE) layer to produce self-cleaning transparent thin film heaters (TFHs). Using carbon nanotubes and a PTFE mixed conducting target, we successfully sputtered a transparent PTFE layer on the Ag NW network using mid-frequency magnetron sputtering. The hydrophobic surface of the PTFE/Ag NW electrodes led to water-repelling and self-cleaning transparent Ag NW electrodes, which are beneficial for transparent TFH-based smart windows. Furthermore, hydrophobic PTFE/Ag NW electrodes coated on polyethylene terephthalate (PET) and polyurethane (PU) substrates showed outstanding flexibility and stretchability, respectively, due to the capping effect of the PTFE layer. Based on outer/inner bending and stretching test results, we demonstrated the superior mechanical properties of the PTFE/Ag NW electrode compared to a bare Ag NW electrode. Finally, we investigated the feasibility of the PTFE/Ag NW film coated on a PU substrate as a transparent and stretchable electrode for stretchable and self-cleaning transparent TFHs. The effective heat generation of the stretchable PTFE/Ag NW electrode indicates the potential for energy-efficient multi-functional PTFE/Ag NW-based TFHs attached to automobile windows.

Thermally evaporated indium-free, transparent, flexible SnO2/AgPdCu/SnO2 electrodes for flexible and transparent thin film heaters

We investigated the characteristics of themally evaporated SnO2/Ag-Pd-Cu (APC)/SnO2 multilayer films for applications as damage-free, indium-free, flexible, and transparent electrodes for high performance flexible and transparent thin film heaters (TFHs). The top and bottom SnO2 layers and APC interlayer were prepared by a multi-source evaporation process, and the effect of the thickness of each layer on the resistivity, optical transmittance, and mechanical flexibility of the SnO2/APC/SnO2 electrodes was investigated in detail. Based on a figure of merit value, we obtained a SnO2/APC/SnO2 electrode with a low sheet resistance of 9.42 Ohm/square and a high optical transmittance of 91.14%. In addition, we examined the mechanical properties of the SnO2/APC/SnO2 electrode using various bending tests such as inner bending, outer bending, dynamic fatigue, and a twisting test. By comparing the crack shape of the SnO2/APC/SnO2 electrode bent beyond the critical bending radius (2~3 mm), we suggest a possible crack formation mechanism for the SnO2/APC/SnO2 electrodes. Furthermore, we evaluated the feasibility of the SnO2/APC/SnO2 electrodes for flexible and transparent TFHs. By correlating the sheet resistance of the SnO2/APC/SnO2 electrode and the performance of TFHs, we show the importance of transparent electrodes for high performance flexible and transparent TFHs.

Transparent Electrodes Based on Silver Nanowire Networks: From Physical Considerations towards Device Integration

The past few years have seen a considerable amount of research devoted to nanostructured transparent conducting materials (TCM), which play a pivotal role in many modern devices such as solar cells, flexible light-emitting devices, touch screens, electromagnetic devices, and flexible transparent thin film heaters. Currently, the most commonly used TCM for such applications (ITO: Indium Tin oxide) suffers from two major drawbacks: brittleness and indium scarcity. Among emerging transparent electrodes, silver nanowire (AgNW) networks appear to be a promising substitute to ITO since such electrically percolating networks exhibit excellent properties with sheet resistance lower than 10 Ω/sq and optical transparency of 90%, fulfilling the requirements of most applications. In addition, AgNW networks also exhibit very good mechanical flexibility. The fabrication of these electrodes involves low-temperature processing steps and scalable methods, thus making them appropriate for future use as low-cost transparent electrodes in flexible electronic devices. This contribution aims to briefly present the main properties of AgNW based transparent electrodes as well as some considerations relating to their efficient integration in devices. The influence of network density, nanowire sizes, and post treatments on the properties of AgNW networks will also be evaluated. In addition to a general overview of AgNW networks, we focus on two important aspects: (i) network instabilities as well as an efficient Atomic Layer Deposition (ALD) coating which clearly enhances AgNW network stability and (ii) modelling to better understand the physical properties of these networks.