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High-performance copper mesh transparent flexible conductors based on electroplating with vacuum-free processing
Journal Pre-proof High-performance copper mesh transparent flexible conductors based on electroplating with vacuum-free processing Le Zhao, Shihui Yu, Xiaopeng Li, Muying Wu, Lingxia Li PII:
S1566-1199(19)30538-5
DOI:
https://doi.org/10.1016/j.orgel.2019.105511
Reference:
ORGELE 105511
To appear in:
Organic Electronics
Received Date: 11 July 2019 Revised Date:
28 September 2019
Accepted Date: 16 October 2019
Please cite this article as: L. Zhao, S. Yu, X. Li, M. Wu, L. Li, High-performance copper mesh transparent flexible conductors based on electroplating with vacuum-free processing, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105511. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
High-performance copper mesh transparent flexible conductors based on electroplating with vacuum-free processing Le Zhao,1 Shihui Yu,1† Xiaopeng Li,1 Muying Wu,2 Lingxia Li1 1
School of Microelectronics and Tianjin Key Laboratory of Imaging and Sensing
Microelectronic Technology, Tianjin University, Tianjin, 300072, P. R. China 2
School of Electronic Engineering and Intelligentization, Dongguan University of
Technology, Guangdong Dongguan 523808, China † Corresponding author. Tel./fax: +86 2227402838 E-mail: [email protected] (S. Yu)
ABSTRACT: Highly reliable Cu micron network transparent film on flexible polyethylene terephthalate (PET) substrate, which has excellent conductivity, improved thermostability and enhanced adhesion is prepared via electroplating and lithography without high temperature and vacuum environment. The sheet resistance of the Cu micron network flexible films obtained by this work is ~2.0 Ω/sq with the transmittance of ~86%, and almost unchanged in the peeling strength of 4.0 MPa. The fabricated Cu micron network establishes strong adhesion to PET substrates, thus mechanical stability against taping and compressive bending is attainable. Moreover, the Cu micro network films show a good stability to resist long-term storage and high temperature (300 °C). The thermal performance of the Cu micro network films as visible transparent heaters via Joule heating is also demonstrated. In addition, good durability and flexibility are also found by being dressed in the human body while
maintaining the excellent performance. Keywords: Cu micro network, transparent electrodes, electroplating, lithography, durability
1. Introduction Modern optoelectronic devices such as organic solar cells, organic light-emitting diodes (LED) and touch screens, etc., require transparent conductive electrode (TCE) with high transmittance, excellent conductivity and good flexibility [1-5]. Currently, Indium tin oxide (ITO) thin film is the most conventional material for TCE which is brittle but highly transparent, with good electro-optic performance [6-8]. However, because of the high cost of indium and the mechanical brittleness, the conventional ITO films are unable to meet the needs of some optoelectronic applications [9-12]. Recently, there has been a large number of ITO replacement technologies arising to fabricate TCE materials with high transmittance, good conductivity and durability, such as carbon nanotubes [13], conducting polymers [14], graphene [15-17], metal oxides [18-20] and metal nanowires (NWs) [21-24]. Besides, the transparent conductive films made from materials among them such as carbon nanotubes (CNTs), graphene and metal nanowires have excellent conductivity. And the optoelectronic performance of TCEs fabricated by solution-processed nanowires [25-29] is better than the ITO and metal-oxide thin films. The TCEs based on metal nanowires have reached an excellent performance with the resistance of ~10 Ω/sq. and transmittance of ~90% [30]. However, the opto-electronic properties of some TCE materials just
like carbon nanotubes and graphene depends on the thickness of the films [31-33]. The conductivity increases with thickening the materials, which results in the decrease of the transmittance. In addition, the nanowire based TCEs have exhibit high sheet resistance because of the high contact resistance when the nanowires are stacked to form a conductive film. Besides, the high sheet resistance and the poor adhesion of the solution-processed nanowires may cause uneven heating and reduced efficiency, which limits its application in flexible optoelectronic devices [34-35]. Recently, researchers have also used the template-method to prepare transparent conductive films. Although the optical and electrical properties have been significantly improved, the vacuum process is still used in the preparation process, which makes the process very complicated, which results in a very high preparation cost [36]. In addition, the adhesion of the sample obtained by that method is not ideal. It’s still a challenge to manufacture low-cost TCE with excellent optical and electrical performance which can substitute ITO. Electroplating is a facile and fast way to deposit metal particles onto a target surface while precisely controlling the thickness and composition. Here, we developed a rational method using lithography and electroplating to obtain a patterned Cu micro network, which exhibits the best transparent electrode performance (2.0 Ω/sq. at transmittance of 86%), excellent adhesion to PET substrate and outstanding flexibility. The sheet resistance is almost unchanged in the peeling strength of 4.0 MPa. Moreover, the Cu micro network films show good stability to resist long-term storage and high temperature (300 °C). The thermal performance of the Cu micro
network films as visible transparent heaters via Joule heating is also demonstrated. And, good durability and flexibility are also found by being dressed in the human body while maintaining the excellent performance. It combines advantages of the lithography with electroplating. In addition, the ITO glass can be recycled for the preparation of conductive film in this method, which reduces the cost to the maximum extent. 2.Experimental section Preparation of the Patterned Substrate: ITO glass substrate (purchased from the market), ultrasonic cleaned with acetone, ethanol and distilled water in the following solvent for 15 minutes each time, and then dried with a stream of N2. Applying photoresist to ITO glass to form a uniform sacrificial layer with a two-stage process: 550 rpm for 10s and 6000 rpm for 30s. Before exposure, the sample needs to be placed in a 95 °C incubator for 3 min. After exposure to an ultraviolet light source for 35s, development was carried out for 20s and rinsed with deionized water, followed by placing in a 125 °C incubator for 3 min. Cu Electroplating Deposition: The solution for depositing Cu includes Cu2+ (10 g/L ), K2CO3 (40 g/L) and HEDP (100 g/L). The whole process is carried out at room temperature. The sample is dipped into the solution for a period of time in order to form the pattern. One can control the thickness of the metal micro network by varying the voltage (the plating time). After rinsing with deionized water, UV glue was spin coated onto the sample. Followed by a flexible PET substrate attached onto the UV glue and place it under ultraviolet light for 10min so that the UV glue solidified.
Finally, the Cu micro network film was transferred to the flexible PET after peeling off. Characterization: The surface morphologies of Cu micro network TCEs were investigated by field emission scanning electron microscope (SEM, JSM-6700F, Japan). The samples were identified by X-ray diffractometer (XRD, Rigaku D/Max 2500PC, Japan) to measure the atom species of samples. The electrical and optical properties were characterized by four-point probe (Keithley 2400) and UV–vis spectrometer (Hitachi U-3900), respectively. 3.Results and discussion The process for fabricating Cu-based transparent conductive micro network based on electroplating welding technique is illustrated in Figure 1, as described in the Experimental section. Firstly, a layer of photoresist is deposited on the ITO glass substrate by the spin-coating process (Step 1). In the second step, the photoresist is exposed to transfer the pattern of the mask to the surface of ITO. Then the patterned photoresist layer (photoresist template) is hardened baking after development (Step 3). In step four, using an electroplating process, the sample is placed in a copper ion plating bath for copper plating. In step five, the Cu micro-network is produced by the lift-off technique. In step six, the UV-curable glue was coated on the micro network, followed by PET substrate attached to ITO and then peeled off from the ITO when the UV-glue is cross-linked under UV radiation (Step seven). Finally, Cu micro-network is transferred and embedded in the polymer to complete the preparation (Step eight). The entire process of preparation is carried out in natural atmospheric condition
without any vacuum processing, and it can thus be readily suitable for large-scale production. Figure 1b, c, and d show the optical micrographs of the Cu micro-networks with different mesh density. It can be seen that metallic Cu meshes keep well their shapes and patterns. The large space between the Cu metallic meshes is a benefit to optical transmission, resulting in the favorable visual comfortableness. Copper is the metal of choice in this work because of its low cost and excellent conductivity. As the electroplating time (voltage) increases, the thickness of the Cu micro network increases, the conductivity is improved. The scanning electron microscope (SEM) images of the copper micro network on ITO glass substrates formed by different number of exposures are illustrated in Figure 2a-f. The morphology of the micro network can be controlled by changing the chemical plating time, exposing time and the number of exposures, etc. The image in Figure 2a demonstrates that the Cu micro network is flat. In this case, the average diameter of micro networks shows different is about ~7 µm (as shown in Figure 2b), which leads to a high transmittance of ~91%. Figure 2c-f shows the different electroplating conditions of different patterns with a transmittance of ~86% and ~83%. The denser the network, the easier the electron transfers. A photograph of our copper micro networks with different mesh density on the PET substrate is shown in Figure 2g, which exhibits the predominant transmittance and mechanical flexibility. The XRD spectra of Cu micro network was shown in Figure 2h. Based on the Scherrer Formula, the grain size of the Cu film was calculated through the full width of half maximum (FWHM) of XRD results. The crystal size calculated is 38.3 nm, which is comparable
with the crystallinity size ( ~ 35 nm) that obtained by vacuum deposition [37]. Figure 3a shows the transmittance of the Cu micro network films with different resistance. For sample 1, the sheet resistance is 6.8 Ω/sq., and the transmittance is 91%, because of the low density of Cu meshes (see Figure 2a). As the increase of mesh density, the sheet resistance decreases rapidly to 1.7 Ω/sq for sample 3. But meanwhile, the transmittance also decreases to 83.5% due to the increase of light scattering in the Cu meshes. For all the samples, four thicknesses(200nm, 400nm, 600nm, and 800nm) for each sample, both the transmittance and sheet resistance reduce with increasing the thickness of Cu meshes. Low sheet resistance of 1.7 Ω/sq can be seen for the sample 3 with 0.8 µm thick Cu, while the optical transmittance (at 550 nm) is still above 83%. Figure 3b shows the optical transmittance-sheet resistance plot of the copper micro network transparent electrodes prepared in this work, which exhibits the excellent performance. What’s more, a figure of merit (F), given by the ratio of electrical
conductance to optical conductance (
), can be used to evaluate the optimal
trade-off between the optical transmittance and electrical conduction of a material better. The F value can be expressed by Tinkham formula [38-39]:
where T is the optical transmittance at 550 nm, and Rsh is the sheet resistance of the film, σOP is the optical conductivity and σDC is the direct current (DC) conductivity of the thin film. The higher F values indicate lower sheet resistance for a given
transmittance or higher transmittance for a given sheet resistance. Our flexible Cu micro network films (sample 2; Cu thickness:800nm) reveal an F value of 1147, which corresponds to low sheet resistance (2 Ω/sq.) at the high optical transmittance (86%). The F value is higher than that of commercial ITO on PET (100–150), carbon nanotube (31~64), graphene (~48), PEDOT:PSS films (~39), silver nanowire (100~350) and copper nanowire (25~66) and hybrid transparent conductive thin films (10~200). These data clearly indicate that the flexible Cu micro network thin films boast superior overall performance compared with most other transparent conducting thin films. Mechanical flexibility and thermal stability are extremely necessary for flexible electronic applications. In the bending test, the samples are fixed between two clamps and then bent by adjusting the distance between the two clamps (shown in Figure 4a). The photograph of Cu micro network films (sample 2; Cu thickness: 800 nm) on PET substrates is shown in Figure 4a, which demonstrates its flexibility and transparency. Figure 4b illustrates the sheet resistance changes (∆R/R0) of a typical bending-recovery cycle of Cu micro network thin films and ITO films. The bending radius is calculated by the following formula [40]: d L 2l 2r = sin ( 2r ) d > π d 2l r = d < 2 π
where, d is the distance between two clamps and L is the length of the electrode. The value of ∆R/R0 increases by slightly less than a factor of 0.2, as the Cu micro network thin films bend up to a 6 mm bending radius. The reasonable electromechanical
stability is attributed to the Cu micro network embedded in the UV glue, resulting in strong adhesion between the Cu micro network and the PET substrates, which helps preserve the electrical properties under considerable bending stress. In comparison, the ITO thin films started to crack at a bending radius of 6 mm, showing 300 times increase of sheet resistance. The ITO thin film is stretched to some extent or even broken after bending, resulting in a sharp drop in electrical conductivity. Another folding-recovery cycle test was conducted to indicate the excellent performance of anti-folding which is displayed in Figure 4c. The sheet resistance of the Cu micro networks as a function of folding times is shown in Figure 4d. The sheet resistance of the Cu micro network just only increases less than a factor of 0.15 after 10000 times folding-recovery test, indicating that the film made of the Cu micro network has good durability and tenacity. The softening temperature of PET substrate is 160 °C, thus the Cu micro networks are prepared on glass substrates to evaluate the thermal stability at high temperature. Figure 5a shows the sheet resistance of the Cu micro networks as a function of temperature. The sheet resistance of the Cu micro networks changes slightly with temperature increasing. The copper micro network film can remain unchanged under 200 °C, and the sheet resistance increases only from 5 Ω/sq. to 5.5 Ω/sq. at 300 °C, which illustrates the excellent thermal stability when heated. In addition, the long-term stability of the samples is scrutinized and the samples are stored at ambient environmental conditions (at room temperature and relative humidity of about 45 %) without encapsulation. Figure 5b illustrates that the sheet
resistance of the Cu micro network films only increases by 4% within 7 days, indicating that Cu micro network has perfect stability. To further explore the enhanced adhesion of the Cu micro network film obtained by this method in our previous work, the samples are subjected to an adhesion test using a 3 M Scotch® Magic™ tape. Figure 5c displays the image of the Cu micro network film before and after peeling off the tape (press:0.6 MPa ). It is obvious that nothing is remaining on the tape and the film keeps the original appearance due to its strong adhesion to the PET substrates. The sheet resistances as a function of the number of peeling tests are also shown in Figure 5c. The Cu micro network films have changed little in electrical conductivity retaining its initial sheet resistance after 50 anti-sticking test cycles, the sheet resistance increases only from 5 Ω/sq. to 5.4 Ω/sq.. In addition, supernumerary pressure is applied on the 3M tape by a pressure applicator to further explore the enhanced adhesion of the film. As shown in Figure 5d, the samples are under the pressure applicator and the size of the pressure can be seen on the display disk. The pressure can be changed by turning the upper turntable. The sheet resistances as a function of the size of pressure are also displayed in Figure 5d. The sheet resistance kept constant until the applied pressure exceeds 4.0 MPa, whose adhesion is better than most films prepared by magnetron sputtering. In Figure 6a, an LED was connected on a Cu micro network film at 2.0 V, and the film operated well during peeling the tape off, demonstrating that the Cu micro network film electrode has excellent durability, as shown in Figure 6b-d. Combined with the results of flexible, stable, optical and electrical properties, which suggests that our Cu micro
network films with low sheet resistance, high optical transmittance, strong adhesion, outstanding thermal stability, and good flexibility was successfully prepared on PET substrates via the proposed vacuum-free preparation process, which is important for the application in high-performance flexible opto-electronic devices. Figure 7 shows an example of applications of the Cu micro network film. First is the Thermographic IR camera images demonstrating the temperature uniformity of the heater (shown in Figure 7a). The transparent heaters exhibit uniform temperature distribution due to the uniform micro network distribution obtained by lithography and electroplating, whose average surface temperature is 160 ± 10 °C. As shown in Figure 7b, constant DC bias voltage is applied between two side ends of the target electrode, while the voltage is increased by 0.5 V every 30s. Figure 7b shows the time-dependent temperature curves of the Cu micron network film prepared on PET substrate when applied different voltages with insets of the corresponding thermal images. The increase of temperature is very fast and nearly steady state temperatures are reached in below 15s. The heater has a resistance of 2.0 Ω/sq., and it is subjected to ten voltage from 0.5 V to 5.0 V. According to the Joule law, the power dissipated (P) of the resistive conductor is defined as P = V²/R, where V is the bias voltage. That is to say, higher heat dissipation can be achieved at lower resistance at a fixed voltage. The lower the resistance, the more heat production, while consuming fewer. It is obvious that the average temperature of Cu micro network films can reach a plateau (160 °C) at 5.0 V in a short time and the steady state can maintain for several minutes. After the voltage (5.0 V) is switched off, the average temperature drops suddenly to
below 30 °C in several seconds. It is worth mentioning that the copper network can reach a higher temperature and keep stable, for the PET substrate can't withstand temperature above 170 °C in the test so that the film in this study only reached 160 °C. The Joule effect is utilized to demonstrate the use of the TCEs as a visible transparent heater for thermochromic displays [41]. Photographs of the heater with letters painted with Coloured Chameleon® Reversible thermochromic paint (ABC) whose transition temperature is about 37 °C are shown in Figure 7c. With the temperature increasing, the pattern of the letters gradually fades away, illustrating the principle of the thermochromic signage. In addition, the images also illustrate that the transparent heater can maintain excellent performance. Lower temperature can be seen at the edge of the heater, and the maximum temperature of the heater is about 162 °C. The cycling tests of these displays are carried out, any sign of deterioration isn’t observed even after tens of test cycles. In addition, flexible electronic devices play an important role in electronics. This application can be applied to circuits with good electrical performance in the bent state, as shown in Figure 7d, the LED light can still be kept lighted when the bending radius change to 5 mm. What’s more, while wearing the film in which the Cu micro network electrode was attached to a finger, the film still worked well during finger bending/unbending cycles (Figure 7e-f), providing the basis for the production of sensors in the future. Both examples demonstrate the practicality and feasibility of thin films in electronic devices. 4.Conclusions
In this work, we have developed a fast and low-cost process to prepare transparent conductive film with excellent performance which requires no sputtering or evaporation to replace ITO. It is worth mentioning that many preparation methods involve high temperature and vacuum, which results in energy consumption and increased processing difficulty. This method in the paper can be processed in the air. The sheet resistance of the flexible Cu micro network films shows perfect performance with a sheet resistance of ~2.0 Ω/sq. at transmittance of ~86%, and almost unchanged in the peeling strength of 4.0 MPa. The fabricated Cu micron network establishes strong adhesion to PET substrates, thus mechanical stability against taping and compressive bending is attainable. Moreover, the Cu micro network films show a good stability to resist long-term storage and high temperature (300 °C). The thermal performance of the Cu micro network films as visible transparent heaters via Joule heating is also assessed. Good thermal resistance and durability characteristics are found using measured thermal imaging data and human body-worn test. The proposed process of Cu micro network films provides a low-cost, fast and scalable method to prepare practical ITO replacement for many electronic applications. Acknowledgements This work was supported by the Natural Science Foundation of Tianjin City (Grant no. 18JCQNJC01300), National Natural Science Foundation of China (Grant No. 61701338), Foundation (B) for Peiyang Scholar-Young Core Faculty of Tianjin University (Grant no. 2018XRG-0020), Scientific Research Foundation for
High-Level Talents (Innovative Team) of Dongguan University of Technology (Grant no. KCYCXPT2017003).
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Figure captions Figure 1. (a) Process for fabricating Cu micro network based transparent conductive network based on lithography and electroplating technique. (b,c,d)optical micrographs of the Cu micro-networks with different mesh density
Figure 2. FESEM morphologies and XRD spectra of the Cu micro network. (a) and (b) SEM images of once-exposure Cu network films on the ITO glass. (c) and (d) SEM images of double-exposure Cu network films on the ITO glass. (e) and (f) SEM images of triple-exposure Cu network films on the ITO glass. (g) Photograph of the Cu network on a PET substrate, demonstrating its flexibility and transparency. (h) XRD spectrum of Cu micro networks.
Figure 3. (a) Transmittances of the Cu micro network transparent films with different sheet resistances. (b) Optical transmittance−sheet resistance plot of the Cu micro network films with different times of exposures.
Figure 4. Flexibility of the Cu micro network thin films. (a) Bending test setup. The Cu micro network transparent films, with varying the bending radius, was conducted. (b) Sheet resistance changes (∆R/R0) of a typical bending status of Cu micro network thin films. (c) Folding cycle test setup. Laboratory self-assembled motor for film bending test (d) Sheet resistance changes (∆R/R0) of a typical folding-recovery cycle
of Cu micro network thin films. Figure 5. (a) Adhesion test images of the Cu micro network transparent films. The blue rectangles indicate the area where the peeling tests were applied. And sheet resistances as a function of the number of peeling tests for the Cu micro network transparent films. (b) Schematic diagram of the device for applying pressure and sheet resistances as a function of the size of the pressure for the Cu micro network transparent films. (c)Thermal performances of Cu micro network transparent conductive electrodes. (d)Sheet resistance of the Cu micro network transparent films changes with time at room temperature.
Figure 6. (a) Copper micro network film is used in the circuit with one LED lighted (b-d) The brightness of the LED light remains unchanged during the 3M tape peeled off.
Figure 7. (a) Thermographic IR camera image of the heater (heated area size: 10 × 20 mm). (b) Plots of temperature versus time for a transparent heater, for ten square voltage pulses. (c) A sequence of photographs of our sample on PET with letters ‘ABC’ using the blue thermochromic paint, and subjected to a single voltage pulse (2 V), the painted letters are shown fading away after 2 s, completely disappearing after 4 s, and appearing again after 6s when turn off the power. (d) Flexible film is applied in the circuit with one LED. (e) Flexible film is worn on the human body with the Thermographic IR camera image. (f) The Thermographic IR camera image of the film
when bending the finger.
Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Highlights (for review): 1. In this work, the Cu micron network transparent film was fabricated on a flexible polyethylene terephthalate (PET) substrate, via electroplating and lithography without high temperature and vacuum environment. It’s a simple, low cost and fast preparation method. 2. The Cu micron network transparent film has excellent optical electrical properties and a high F value (1147). The sheet resistance of the Cu micron network flexible films is ~2.0 Ω/sq at transmittance of ~86%. 3. The adhesion of the Cu micron network transparent film is excellent. The sheet resistance almost unchanged in the peeling strength of 4 MPa and even scratched by sharp objects due to the UV glue. Moreover, the Cu micro network films show a good stability to resist long-term storage and high temperature (200 °C). In addition, good durability and flexibility are also found by being dressed in the human body while maintaining the excellent performance.
S1566-1199(19)30538-5
DOI:
https://doi.org/10.1016/j.orgel.2019.105511
Reference:
ORGELE 105511
To appear in:
Organic Electronics
Received Date: 11 July 2019 Revised Date:
28 September 2019
Accepted Date: 16 October 2019
Please cite this article as: L. Zhao, S. Yu, X. Li, M. Wu, L. Li, High-performance copper mesh transparent flexible conductors based on electroplating with vacuum-free processing, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105511. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
High-performance copper mesh transparent flexible conductors based on electroplating with vacuum-free processing Le Zhao,1 Shihui Yu,1† Xiaopeng Li,1 Muying Wu,2 Lingxia Li1 1
School of Microelectronics and Tianjin Key Laboratory of Imaging and Sensing
Microelectronic Technology, Tianjin University, Tianjin, 300072, P. R. China 2
School of Electronic Engineering and Intelligentization, Dongguan University of
Technology, Guangdong Dongguan 523808, China † Corresponding author. Tel./fax: +86 2227402838 E-mail: [email protected] (S. Yu)
ABSTRACT: Highly reliable Cu micron network transparent film on flexible polyethylene terephthalate (PET) substrate, which has excellent conductivity, improved thermostability and enhanced adhesion is prepared via electroplating and lithography without high temperature and vacuum environment. The sheet resistance of the Cu micron network flexible films obtained by this work is ~2.0 Ω/sq with the transmittance of ~86%, and almost unchanged in the peeling strength of 4.0 MPa. The fabricated Cu micron network establishes strong adhesion to PET substrates, thus mechanical stability against taping and compressive bending is attainable. Moreover, the Cu micro network films show a good stability to resist long-term storage and high temperature (300 °C). The thermal performance of the Cu micro network films as visible transparent heaters via Joule heating is also demonstrated. In addition, good durability and flexibility are also found by being dressed in the human body while
maintaining the excellent performance. Keywords: Cu micro network, transparent electrodes, electroplating, lithography, durability
1. Introduction Modern optoelectronic devices such as organic solar cells, organic light-emitting diodes (LED) and touch screens, etc., require transparent conductive electrode (TCE) with high transmittance, excellent conductivity and good flexibility [1-5]. Currently, Indium tin oxide (ITO) thin film is the most conventional material for TCE which is brittle but highly transparent, with good electro-optic performance [6-8]. However, because of the high cost of indium and the mechanical brittleness, the conventional ITO films are unable to meet the needs of some optoelectronic applications [9-12]. Recently, there has been a large number of ITO replacement technologies arising to fabricate TCE materials with high transmittance, good conductivity and durability, such as carbon nanotubes [13], conducting polymers [14], graphene [15-17], metal oxides [18-20] and metal nanowires (NWs) [21-24]. Besides, the transparent conductive films made from materials among them such as carbon nanotubes (CNTs), graphene and metal nanowires have excellent conductivity. And the optoelectronic performance of TCEs fabricated by solution-processed nanowires [25-29] is better than the ITO and metal-oxide thin films. The TCEs based on metal nanowires have reached an excellent performance with the resistance of ~10 Ω/sq. and transmittance of ~90% [30]. However, the opto-electronic properties of some TCE materials just
like carbon nanotubes and graphene depends on the thickness of the films [31-33]. The conductivity increases with thickening the materials, which results in the decrease of the transmittance. In addition, the nanowire based TCEs have exhibit high sheet resistance because of the high contact resistance when the nanowires are stacked to form a conductive film. Besides, the high sheet resistance and the poor adhesion of the solution-processed nanowires may cause uneven heating and reduced efficiency, which limits its application in flexible optoelectronic devices [34-35]. Recently, researchers have also used the template-method to prepare transparent conductive films. Although the optical and electrical properties have been significantly improved, the vacuum process is still used in the preparation process, which makes the process very complicated, which results in a very high preparation cost [36]. In addition, the adhesion of the sample obtained by that method is not ideal. It’s still a challenge to manufacture low-cost TCE with excellent optical and electrical performance which can substitute ITO. Electroplating is a facile and fast way to deposit metal particles onto a target surface while precisely controlling the thickness and composition. Here, we developed a rational method using lithography and electroplating to obtain a patterned Cu micro network, which exhibits the best transparent electrode performance (2.0 Ω/sq. at transmittance of 86%), excellent adhesion to PET substrate and outstanding flexibility. The sheet resistance is almost unchanged in the peeling strength of 4.0 MPa. Moreover, the Cu micro network films show good stability to resist long-term storage and high temperature (300 °C). The thermal performance of the Cu micro
network films as visible transparent heaters via Joule heating is also demonstrated. And, good durability and flexibility are also found by being dressed in the human body while maintaining the excellent performance. It combines advantages of the lithography with electroplating. In addition, the ITO glass can be recycled for the preparation of conductive film in this method, which reduces the cost to the maximum extent. 2.Experimental section Preparation of the Patterned Substrate: ITO glass substrate (purchased from the market), ultrasonic cleaned with acetone, ethanol and distilled water in the following solvent for 15 minutes each time, and then dried with a stream of N2. Applying photoresist to ITO glass to form a uniform sacrificial layer with a two-stage process: 550 rpm for 10s and 6000 rpm for 30s. Before exposure, the sample needs to be placed in a 95 °C incubator for 3 min. After exposure to an ultraviolet light source for 35s, development was carried out for 20s and rinsed with deionized water, followed by placing in a 125 °C incubator for 3 min. Cu Electroplating Deposition: The solution for depositing Cu includes Cu2+ (10 g/L ), K2CO3 (40 g/L) and HEDP (100 g/L). The whole process is carried out at room temperature. The sample is dipped into the solution for a period of time in order to form the pattern. One can control the thickness of the metal micro network by varying the voltage (the plating time). After rinsing with deionized water, UV glue was spin coated onto the sample. Followed by a flexible PET substrate attached onto the UV glue and place it under ultraviolet light for 10min so that the UV glue solidified.
Finally, the Cu micro network film was transferred to the flexible PET after peeling off. Characterization: The surface morphologies of Cu micro network TCEs were investigated by field emission scanning electron microscope (SEM, JSM-6700F, Japan). The samples were identified by X-ray diffractometer (XRD, Rigaku D/Max 2500PC, Japan) to measure the atom species of samples. The electrical and optical properties were characterized by four-point probe (Keithley 2400) and UV–vis spectrometer (Hitachi U-3900), respectively. 3.Results and discussion The process for fabricating Cu-based transparent conductive micro network based on electroplating welding technique is illustrated in Figure 1, as described in the Experimental section. Firstly, a layer of photoresist is deposited on the ITO glass substrate by the spin-coating process (Step 1). In the second step, the photoresist is exposed to transfer the pattern of the mask to the surface of ITO. Then the patterned photoresist layer (photoresist template) is hardened baking after development (Step 3). In step four, using an electroplating process, the sample is placed in a copper ion plating bath for copper plating. In step five, the Cu micro-network is produced by the lift-off technique. In step six, the UV-curable glue was coated on the micro network, followed by PET substrate attached to ITO and then peeled off from the ITO when the UV-glue is cross-linked under UV radiation (Step seven). Finally, Cu micro-network is transferred and embedded in the polymer to complete the preparation (Step eight). The entire process of preparation is carried out in natural atmospheric condition
without any vacuum processing, and it can thus be readily suitable for large-scale production. Figure 1b, c, and d show the optical micrographs of the Cu micro-networks with different mesh density. It can be seen that metallic Cu meshes keep well their shapes and patterns. The large space between the Cu metallic meshes is a benefit to optical transmission, resulting in the favorable visual comfortableness. Copper is the metal of choice in this work because of its low cost and excellent conductivity. As the electroplating time (voltage) increases, the thickness of the Cu micro network increases, the conductivity is improved. The scanning electron microscope (SEM) images of the copper micro network on ITO glass substrates formed by different number of exposures are illustrated in Figure 2a-f. The morphology of the micro network can be controlled by changing the chemical plating time, exposing time and the number of exposures, etc. The image in Figure 2a demonstrates that the Cu micro network is flat. In this case, the average diameter of micro networks shows different is about ~7 µm (as shown in Figure 2b), which leads to a high transmittance of ~91%. Figure 2c-f shows the different electroplating conditions of different patterns with a transmittance of ~86% and ~83%. The denser the network, the easier the electron transfers. A photograph of our copper micro networks with different mesh density on the PET substrate is shown in Figure 2g, which exhibits the predominant transmittance and mechanical flexibility. The XRD spectra of Cu micro network was shown in Figure 2h. Based on the Scherrer Formula, the grain size of the Cu film was calculated through the full width of half maximum (FWHM) of XRD results. The crystal size calculated is 38.3 nm, which is comparable
with the crystallinity size ( ~ 35 nm) that obtained by vacuum deposition [37]. Figure 3a shows the transmittance of the Cu micro network films with different resistance. For sample 1, the sheet resistance is 6.8 Ω/sq., and the transmittance is 91%, because of the low density of Cu meshes (see Figure 2a). As the increase of mesh density, the sheet resistance decreases rapidly to 1.7 Ω/sq for sample 3. But meanwhile, the transmittance also decreases to 83.5% due to the increase of light scattering in the Cu meshes. For all the samples, four thicknesses(200nm, 400nm, 600nm, and 800nm) for each sample, both the transmittance and sheet resistance reduce with increasing the thickness of Cu meshes. Low sheet resistance of 1.7 Ω/sq can be seen for the sample 3 with 0.8 µm thick Cu, while the optical transmittance (at 550 nm) is still above 83%. Figure 3b shows the optical transmittance-sheet resistance plot of the copper micro network transparent electrodes prepared in this work, which exhibits the excellent performance. What’s more, a figure of merit (F), given by the ratio of electrical
conductance to optical conductance (
), can be used to evaluate the optimal
trade-off between the optical transmittance and electrical conduction of a material better. The F value can be expressed by Tinkham formula [38-39]:
where T is the optical transmittance at 550 nm, and Rsh is the sheet resistance of the film, σOP is the optical conductivity and σDC is the direct current (DC) conductivity of the thin film. The higher F values indicate lower sheet resistance for a given
transmittance or higher transmittance for a given sheet resistance. Our flexible Cu micro network films (sample 2; Cu thickness:800nm) reveal an F value of 1147, which corresponds to low sheet resistance (2 Ω/sq.) at the high optical transmittance (86%). The F value is higher than that of commercial ITO on PET (100–150), carbon nanotube (31~64), graphene (~48), PEDOT:PSS films (~39), silver nanowire (100~350) and copper nanowire (25~66) and hybrid transparent conductive thin films (10~200). These data clearly indicate that the flexible Cu micro network thin films boast superior overall performance compared with most other transparent conducting thin films. Mechanical flexibility and thermal stability are extremely necessary for flexible electronic applications. In the bending test, the samples are fixed between two clamps and then bent by adjusting the distance between the two clamps (shown in Figure 4a). The photograph of Cu micro network films (sample 2; Cu thickness: 800 nm) on PET substrates is shown in Figure 4a, which demonstrates its flexibility and transparency. Figure 4b illustrates the sheet resistance changes (∆R/R0) of a typical bending-recovery cycle of Cu micro network thin films and ITO films. The bending radius is calculated by the following formula [40]: d L 2l 2r = sin ( 2r ) d > π d 2l r = d < 2 π
where, d is the distance between two clamps and L is the length of the electrode. The value of ∆R/R0 increases by slightly less than a factor of 0.2, as the Cu micro network thin films bend up to a 6 mm bending radius. The reasonable electromechanical
stability is attributed to the Cu micro network embedded in the UV glue, resulting in strong adhesion between the Cu micro network and the PET substrates, which helps preserve the electrical properties under considerable bending stress. In comparison, the ITO thin films started to crack at a bending radius of 6 mm, showing 300 times increase of sheet resistance. The ITO thin film is stretched to some extent or even broken after bending, resulting in a sharp drop in electrical conductivity. Another folding-recovery cycle test was conducted to indicate the excellent performance of anti-folding which is displayed in Figure 4c. The sheet resistance of the Cu micro networks as a function of folding times is shown in Figure 4d. The sheet resistance of the Cu micro network just only increases less than a factor of 0.15 after 10000 times folding-recovery test, indicating that the film made of the Cu micro network has good durability and tenacity. The softening temperature of PET substrate is 160 °C, thus the Cu micro networks are prepared on glass substrates to evaluate the thermal stability at high temperature. Figure 5a shows the sheet resistance of the Cu micro networks as a function of temperature. The sheet resistance of the Cu micro networks changes slightly with temperature increasing. The copper micro network film can remain unchanged under 200 °C, and the sheet resistance increases only from 5 Ω/sq. to 5.5 Ω/sq. at 300 °C, which illustrates the excellent thermal stability when heated. In addition, the long-term stability of the samples is scrutinized and the samples are stored at ambient environmental conditions (at room temperature and relative humidity of about 45 %) without encapsulation. Figure 5b illustrates that the sheet
resistance of the Cu micro network films only increases by 4% within 7 days, indicating that Cu micro network has perfect stability. To further explore the enhanced adhesion of the Cu micro network film obtained by this method in our previous work, the samples are subjected to an adhesion test using a 3 M Scotch® Magic™ tape. Figure 5c displays the image of the Cu micro network film before and after peeling off the tape (press:0.6 MPa ). It is obvious that nothing is remaining on the tape and the film keeps the original appearance due to its strong adhesion to the PET substrates. The sheet resistances as a function of the number of peeling tests are also shown in Figure 5c. The Cu micro network films have changed little in electrical conductivity retaining its initial sheet resistance after 50 anti-sticking test cycles, the sheet resistance increases only from 5 Ω/sq. to 5.4 Ω/sq.. In addition, supernumerary pressure is applied on the 3M tape by a pressure applicator to further explore the enhanced adhesion of the film. As shown in Figure 5d, the samples are under the pressure applicator and the size of the pressure can be seen on the display disk. The pressure can be changed by turning the upper turntable. The sheet resistances as a function of the size of pressure are also displayed in Figure 5d. The sheet resistance kept constant until the applied pressure exceeds 4.0 MPa, whose adhesion is better than most films prepared by magnetron sputtering. In Figure 6a, an LED was connected on a Cu micro network film at 2.0 V, and the film operated well during peeling the tape off, demonstrating that the Cu micro network film electrode has excellent durability, as shown in Figure 6b-d. Combined with the results of flexible, stable, optical and electrical properties, which suggests that our Cu micro
network films with low sheet resistance, high optical transmittance, strong adhesion, outstanding thermal stability, and good flexibility was successfully prepared on PET substrates via the proposed vacuum-free preparation process, which is important for the application in high-performance flexible opto-electronic devices. Figure 7 shows an example of applications of the Cu micro network film. First is the Thermographic IR camera images demonstrating the temperature uniformity of the heater (shown in Figure 7a). The transparent heaters exhibit uniform temperature distribution due to the uniform micro network distribution obtained by lithography and electroplating, whose average surface temperature is 160 ± 10 °C. As shown in Figure 7b, constant DC bias voltage is applied between two side ends of the target electrode, while the voltage is increased by 0.5 V every 30s. Figure 7b shows the time-dependent temperature curves of the Cu micron network film prepared on PET substrate when applied different voltages with insets of the corresponding thermal images. The increase of temperature is very fast and nearly steady state temperatures are reached in below 15s. The heater has a resistance of 2.0 Ω/sq., and it is subjected to ten voltage from 0.5 V to 5.0 V. According to the Joule law, the power dissipated (P) of the resistive conductor is defined as P = V²/R, where V is the bias voltage. That is to say, higher heat dissipation can be achieved at lower resistance at a fixed voltage. The lower the resistance, the more heat production, while consuming fewer. It is obvious that the average temperature of Cu micro network films can reach a plateau (160 °C) at 5.0 V in a short time and the steady state can maintain for several minutes. After the voltage (5.0 V) is switched off, the average temperature drops suddenly to
below 30 °C in several seconds. It is worth mentioning that the copper network can reach a higher temperature and keep stable, for the PET substrate can't withstand temperature above 170 °C in the test so that the film in this study only reached 160 °C. The Joule effect is utilized to demonstrate the use of the TCEs as a visible transparent heater for thermochromic displays [41]. Photographs of the heater with letters painted with Coloured Chameleon® Reversible thermochromic paint (ABC) whose transition temperature is about 37 °C are shown in Figure 7c. With the temperature increasing, the pattern of the letters gradually fades away, illustrating the principle of the thermochromic signage. In addition, the images also illustrate that the transparent heater can maintain excellent performance. Lower temperature can be seen at the edge of the heater, and the maximum temperature of the heater is about 162 °C. The cycling tests of these displays are carried out, any sign of deterioration isn’t observed even after tens of test cycles. In addition, flexible electronic devices play an important role in electronics. This application can be applied to circuits with good electrical performance in the bent state, as shown in Figure 7d, the LED light can still be kept lighted when the bending radius change to 5 mm. What’s more, while wearing the film in which the Cu micro network electrode was attached to a finger, the film still worked well during finger bending/unbending cycles (Figure 7e-f), providing the basis for the production of sensors in the future. Both examples demonstrate the practicality and feasibility of thin films in electronic devices. 4.Conclusions
In this work, we have developed a fast and low-cost process to prepare transparent conductive film with excellent performance which requires no sputtering or evaporation to replace ITO. It is worth mentioning that many preparation methods involve high temperature and vacuum, which results in energy consumption and increased processing difficulty. This method in the paper can be processed in the air. The sheet resistance of the flexible Cu micro network films shows perfect performance with a sheet resistance of ~2.0 Ω/sq. at transmittance of ~86%, and almost unchanged in the peeling strength of 4.0 MPa. The fabricated Cu micron network establishes strong adhesion to PET substrates, thus mechanical stability against taping and compressive bending is attainable. Moreover, the Cu micro network films show a good stability to resist long-term storage and high temperature (300 °C). The thermal performance of the Cu micro network films as visible transparent heaters via Joule heating is also assessed. Good thermal resistance and durability characteristics are found using measured thermal imaging data and human body-worn test. The proposed process of Cu micro network films provides a low-cost, fast and scalable method to prepare practical ITO replacement for many electronic applications. Acknowledgements This work was supported by the Natural Science Foundation of Tianjin City (Grant no. 18JCQNJC01300), National Natural Science Foundation of China (Grant No. 61701338), Foundation (B) for Peiyang Scholar-Young Core Faculty of Tianjin University (Grant no. 2018XRG-0020), Scientific Research Foundation for
High-Level Talents (Innovative Team) of Dongguan University of Technology (Grant no. KCYCXPT2017003).
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Figure captions Figure 1. (a) Process for fabricating Cu micro network based transparent conductive network based on lithography and electroplating technique. (b,c,d)optical micrographs of the Cu micro-networks with different mesh density
Figure 2. FESEM morphologies and XRD spectra of the Cu micro network. (a) and (b) SEM images of once-exposure Cu network films on the ITO glass. (c) and (d) SEM images of double-exposure Cu network films on the ITO glass. (e) and (f) SEM images of triple-exposure Cu network films on the ITO glass. (g) Photograph of the Cu network on a PET substrate, demonstrating its flexibility and transparency. (h) XRD spectrum of Cu micro networks.
Figure 3. (a) Transmittances of the Cu micro network transparent films with different sheet resistances. (b) Optical transmittance−sheet resistance plot of the Cu micro network films with different times of exposures.
Figure 4. Flexibility of the Cu micro network thin films. (a) Bending test setup. The Cu micro network transparent films, with varying the bending radius, was conducted. (b) Sheet resistance changes (∆R/R0) of a typical bending status of Cu micro network thin films. (c) Folding cycle test setup. Laboratory self-assembled motor for film bending test (d) Sheet resistance changes (∆R/R0) of a typical folding-recovery cycle
of Cu micro network thin films. Figure 5. (a) Adhesion test images of the Cu micro network transparent films. The blue rectangles indicate the area where the peeling tests were applied. And sheet resistances as a function of the number of peeling tests for the Cu micro network transparent films. (b) Schematic diagram of the device for applying pressure and sheet resistances as a function of the size of the pressure for the Cu micro network transparent films. (c)Thermal performances of Cu micro network transparent conductive electrodes. (d)Sheet resistance of the Cu micro network transparent films changes with time at room temperature.
Figure 6. (a) Copper micro network film is used in the circuit with one LED lighted (b-d) The brightness of the LED light remains unchanged during the 3M tape peeled off.
Figure 7. (a) Thermographic IR camera image of the heater (heated area size: 10 × 20 mm). (b) Plots of temperature versus time for a transparent heater, for ten square voltage pulses. (c) A sequence of photographs of our sample on PET with letters ‘ABC’ using the blue thermochromic paint, and subjected to a single voltage pulse (2 V), the painted letters are shown fading away after 2 s, completely disappearing after 4 s, and appearing again after 6s when turn off the power. (d) Flexible film is applied in the circuit with one LED. (e) Flexible film is worn on the human body with the Thermographic IR camera image. (f) The Thermographic IR camera image of the film
when bending the finger.
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Highlights (for review): 1. In this work, the Cu micron network transparent film was fabricated on a flexible polyethylene terephthalate (PET) substrate, via electroplating and lithography without high temperature and vacuum environment. It’s a simple, low cost and fast preparation method. 2. The Cu micron network transparent film has excellent optical electrical properties and a high F value (1147). The sheet resistance of the Cu micron network flexible films is ~2.0 Ω/sq at transmittance of ~86%. 3. The adhesion of the Cu micron network transparent film is excellent. The sheet resistance almost unchanged in the peeling strength of 4 MPa and even scratched by sharp objects due to the UV glue. Moreover, the Cu micro network films show a good stability to resist long-term storage and high temperature (200 °C). In addition, good durability and flexibility are also found by being dressed in the human body while maintaining the excellent performance.