ion composite ink

Journal Pre-proof Low temperature enhanced flexible conductive film by Ag flake/ ion composite ink

Yun Mou, Hao Wang, Yang Peng, Jiaxin Liu, Hao Cheng, Qinglei Sun, Mingxiang Chen PII:

S0264-1275(19)30777-4

DOI:

https://doi.org/10.1016/j.matdes.2019.108339

Reference:

JMADE 108339

To appear in:

Materials & Design

Received date:

9 July 2019

Revised date:

7 November 2019

Accepted date:

7 November 2019

Please cite this article as: Y. Mou, H. Wang, Y. Peng, et al., Low temperature enhanced flexible conductive film by Ag flake/ion composite ink, Materials & Design(2019), https://doi.org/10.1016/j.matdes.2019.108339

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© 2019 Published by Elsevier.

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Low Temperature Enhanced Flexible Conductive Film by Ag Flake/Ion Composite Ink Yun Moua, Hao Wangb, Yang Penga*, Jiaxin Liua, Hao Chenga, Qinglei Suna, and Mingxiang Chena,c* a

School of Mechanical Science and Engineering, Huazhong University of Science

National Laboratory for Optoelectronics, Huazhong University of Science and

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b

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and Technology, Wuhan 430074, China.

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Technology, Wuhan 430074, China. c

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State Key Laboratory of Digital Manufacturing Equipment and Technology,

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Huazhong University of Science and Technology, Wuhan 430074, China. *

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Abstract

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Corresponding authors: [email protected]; [email protected]

Poor flexibility and high contact resistance of conductive films are primary roadblock for the incorporation of Ag flake ink into practical applications. In this work, a novel and inexpensive Ag flake/ion composite ink was prepared to directly achieve the thick conductive films with excellent performances at 100°C. The effect of Ag ion content on the resistivity and microstructures of the modified conductive film was investigated, and the flexibility and relative resistance change of the modified conductive film were measured by the cyclic bending test and compared with the traditional Ag flake ink. Consequently, the modified conductive film sintered from Ag flake/ion-0.5 ink exhibits low electrical resistivity of 4.83 × 10-5 Ω·cm, 1

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which is 42.9% lower than the Ag flake film sintered from the Ag flake ink, and the modified conductive film still displays smaller changes in resistivity and relative resistance after the cyclic bending tests. The enhanced conductivity and flexibility are attributed to the bridging effect of Ag nanoparticles (NPs) generated from the in–situ decomposition of Ag ion ink. These results demonstrate that the low-cost Ag flake/ion

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composite ink provides a tempting prospect for fabricating high-quality flexible

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printed electronics at low temperature.

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Keywords: Ag flake/ion composite ink; conductive film; flexibility; resistivity

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1. Introduction

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Flexible printed electronics have attracted considerable attention in flexible displays, thin film transistors, wearable electronics, microelectronic packaging, solar

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cell arrays, and smart tags because of their cost-effectiveness, simple and eco-friendly

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manufacturing process, and scalability, compared with the traditional electrodeless plating and vacuum deposition [1–8]. Printable conductive inks are critical to fabricate the conductive patterns for printed electronics. Currently, Ag nanoparticle (NP) inks have been intensively explored owing to their high sintering efficiency, undemanding sintering condition, chemical stability, and excellent electrical conductivity [9–12]. High annealing temperatures exceed 200°C are required to remove the insulating organics and efficient sintering NPs, which is not compatible with heat-sensitive flexible substrates with low glass transition temperature, and induces the thermal degradation of substrates [13–16]. Additionally, the high cost of

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Ag NP ink limits their widespread applications and the commercialization of flexible electronics, and it is difficult to completely prevent the agglomeration of Ag NPs due to their high surface energy and high diffusivity, especially high concentration Ag NP inks [17-19], which reduces low temperature sintering properties and limits the choice of flexible substrates.

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Recently, Ag ion inks have been considered as promising printing materials to trade off sintering temperature and conductivity simultaneously [20–27]. Compared

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with Ag NP inks, they not only evade the high sintering temperature of ink and the

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thermal degradation of substrates, but also eliminate the complicated synthesis and

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purification processes of NPs. However, the low Ag content is an intractable problem

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to plague the application of Ag ion inks, which usually requires multi-time printing to

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obtain the high conductive film with desired thickness for enhancing the current capacity and broadening the application fields [26–31]. Furthermore, the sub-micron

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Ag flake inks with high content are developed to directly fabricate the conductive film with desired thickness and reduce the number of contact resistances between adjacent particles [32–34], but it is difficult to obtain the high conductive film at low temperature due to the insufficient sintering of Ag flakes. Aiming to decreasing the electrical resistivity of the conductive film, Ag flake/NP composite inks have been developed to enhance the sintering of Ag flakes [35–38]. However, they inevitably introduce the problems faced by NPs. Therefore, it is still huge challenge to develop a conductive ink that can efficiently transform into the high-quality conductive film at lower sintering temperature. 3

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Herein, Ag flake/ion composite ink was designed and prepared to directly fabricate the thick conductive film with high conductivity on flexible substrate at 100°C. This approach evades the inherent defects of Ag NPs and enhances the conductivity and flexibility of the conductive film at low temperature, contributing to expanding the choice of flexible substrates and practical application prospects. The

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effect of Ag ion content on the resistivity and microstructures of the modified conductive film was investigated. Furthermore, the flexibility and relative resistance

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change of the modified conductive film were measured by the cyclic bending test and

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compared with the traditional Ag flake ink, and the sintering enhancement mechanism

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of Ag flake/ion composite ink was further discussed.

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2. Experimental section

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2.1. Materials

Silver acetate, ethylene glycol butyl ether, formic acid (88%), and anhydrous were

provided

from

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alcohol

Sinopharm

Chemical

Reagent

Co.,

Ltd.

Polyvinylpyrrolidone (PVP, K30) and 1-amino-2-propanol were purchased from Aladdin reagent Co. Ag flakes (2 µm) were supplied by Ningbo Jinlei Nano Materials Technology Co., Ltd. Polyimide (PI) substrates with 100 µm thickness were obtained from Toray Industries, Inc. 2.2. Preparation of Ag flake/ion composite ink Firstly, ethylene glycol butyl ether (2 g), 1-amino-2-propanol (4 g), and formic acid (0.1 g) were mixed in a flask at 5°C. Subsequently, silver acetate powder (0.5 g) was introduced to the mixture under magnetic stirring for 30 min. After that, PVP 4

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(adhesive agent) was dissolved in the resulting solution by magnetic stirring to obtain Ag ion ink, and the weight percent of PVP was designed at 10 wt%. Finally, the Ag flake/ion composite ink was prepared by dispersing 50 wt% Ag flakes into 50 wt% Ag ion ink through vacuum mixer at 1500 rpm for 5 min. The prepared Ag ion ink and composite ink were hermetically stored in a refrigerator to maintain their stability. To

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adjust the Ag ion content of composite ink, the weight of silver acetate powder was designed from 0.25 to1.0 g, and the weight ratio of silver acetate to formic acid was

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controlled at 5:1. For ease of description, they were denoted as Ag flake, Ag

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flake/ion-0.25, Ag flake/ion-0.5, Ag flake/ion-0.75, and Ag flake/ion-1 inks, in which

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figures represented the weight of silver acetate.

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2.3.Low temperature fabrication of flexible conductive film

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Before printing, PI substrates were ultrasonically cleaned by anhydrous alcohol and deionized water to remove the surface contaminants, and dried with a nitrogen

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gun. The Ag flake ink and composite inks were uniformly printed on PI substrates to form the rectangular patterns (1 cm × 5 cm) through mask-printing method, and sintered at 100°C for 30 min on a hot-plate in the air. 2.4. Characterization and measurement Ultraviolet−visible (UV−vis) absorption spectra of Ag ion inks were recorded on an UV−vis spectrophotometer (Hitachi U-4100) with the wavelength range of 290−800 nm. The crystalline structures of the conductive films were measured by using a X-ray diffractometer (XRD, PANalytical PW3040/60) with Cu Kα radiation in the range of 20–90°(2θ). The surface compositions of the conductive films were recorded on an 5

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X-ray

photoelectron

spectrometer

(XPS,

AXIS-ULTRA

DLD-600W).

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microstructures of the conductive films were observed by field-emission scanning electron microscopy (SEM, FEI Nova Nano SEM 450) and transmission electron microscopy (TEM, FEI Talos F200X). The porosity of the conductive film was calculated by Matlab software based on the SEM images at the same magnification

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[39], and the corresponding results were shown in Fig. S1 (Supporting information). Firstly, the gray SEM image was converted into a binary image by setting a

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reasonable threshold, and then the porosity is obtained by calculating the area of the

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binary image. A reasonable threshold was very important for calculating porosity,

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which needed to truly reflect the porosity of the conductive film. The sheet resistance

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and relative resistance change of the conductive films were acquired by a four-point

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probe tester (Probes Tech RTS-8), and the thickness of the conductive films was measured the step profiler (ET4000 Series).

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3. Results and discussion

The UV−vis absorption spectra of Ag ion inks were characterized to analyze their chemical composition forms, as shown in Fig. 1. The absorption bands are detected in the range from 320 to 380 nm, which are associated with Ag ion complex [20, 26–29], and the intensity of the absorption band is more remarkable with the increase of Ag ion content. The significant absorption bands of Ag ion-0.25 and Ag ion-0.5 inks are not discovered in the visible light range (400-800 nm), which indicates that silver acetate is absolutely dissolved and converted into the Ag ion complex. With the further increase of Ag ion content, the weak absorption bands are observed in the 6

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range from 400 to 500 nm, and the color of Ag ion ink changes from colorless transparency to yellowish or grayish tints, suggesting that a small amount of Ag ion inks has been decomposed into Ag particles. To determine whether the composite ink is decomposed into the Ag conductive film during sintering process, the various inks were printed and sintered at 100°C for

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30 min, and their crystallographic characteristics were displayed in Fig. 2. The diffraction peaks are not detected in the Ag ion ink, which indicates that the Ag ion

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ink is amorphous and particle-free. As expected, the intensities and angles (2θ) of

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diffraction peaks from the sintered layers are consistent with the face centered cubic

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Ag (JCPDS 04-0783), and no obvious diffraction peaks of oxides or other impurity

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thermal decomposition.

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phases are observed. These results indicate that the Ag ion ink is converted into Ag by

The ultralow content and amorphous substances are undetectable in XRD

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measurement, thus XPS was performed to in-depth elaborate the surface chemical compositions of the sintered Ag ion ink and Ag flake/ ion-0.5 ink , as presented in Fig. 3(a-d). The symmetric peaks of Ag 3d spectra of sintered Ag ion ink and Ag flake/ ion-0.5 ink are identified as the zero valent Ag, and the peaks of monovalent Ag ion are absent. In addition, the C 1s peaks positioned at 284.5, 285.8, and 288 eV are ascribed to the C–C, C-N, and C=O groups, respectively, in agreement with the molecular groups of PVP. Moreover, the O 1s peaks at 531.2 and 532.5eV are determined as the adsorbed oxygen and C=O groups, and the significant peaks of Ag oxides are not resolved from the O 1s spectrum. The results demonstrate that the 7

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composite ink can be absolutely transformed into pure Ag conductive film without the oxides at 100°C, which is beneficial to fabricate the high conductive film. Low electrical resistivity and high density are essential requirements for the practical application of flexible conductive film, thus the resistivity and porosity of the conductive films sintered from the different inks were measured, as described in

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Fig. 4. The high conductive films with low resistivity (10-5 Ω·cm) have been achieved from Ag flake ink and composite inks at low sintering temperature of 100°C, and the

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composite inks achieve lower resistivity and porosity than the Ag flake ink. The

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modified conductive film originated from the Ag flake/ion-0.5 ink has a resistivity of

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4.83 × 10-5 Ω·cm and a porosity of 3.13%, which represents 42.9% and 69.4%

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reduction as compared with the Ag flake ink, respectively. Although the resistivity and

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porosity of the modified conductive film slightly increases with the further increase of Ag ion content, they are still significantly lower than that of the Ag flake film

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originated from the Ag flake ink. The high content Ag ion ink is in-situ decomposed to produce a large number of gases and Ag NPs during sintering process. The rapid evaporation of massive gases may damage the integrity and density, and the excessive Ag NPs undoubtedly increase the contact resistance, thus displaying the increase of resistivity and porosity. In addition, the appropriate printing thickness of the inks is very important to obtain the high conductive film with low porosity, because it is related to the volatilization of solvents and the continuity of films. The microstructures of the conductive films were investigated to further clarify the change trend of their resistivity, as displayed in Fig. 5. Many micron-scale voids 8

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are clearly observed in the conductive film formed from Ag flake ink, and its density is significantly lower than that formed from the composite inks at the same magnification. It is because that the Ag NPs decomposed from Ag ion inks precipitate on Ag flakes fill the voids between Ag flakes. Compared with the Ag flake film where Ag flakes are interconnected through their own few contact points, the contact points

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between Ag flakes are enlarged by the bridging effect afforded by in situ formed Ag NPs in the modified conductive film, thus effectively enhancing the conductivity and

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reducing the porosity [38]. In addition, the bridge-like structures can be further

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observed from TEM images, and the dislocations and lattice fringes with different

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orientations are observed at the junction interface, as shown in Fig. 5(d-e). Therefore,

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it can be inferred that the bridge-like structures are metallurgical interconnections

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rather than simple physical contacts, which is conducive to increase the contact sites and reduce the contact resistance between the Ag flakes.

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The flexibility performance is another indispensable consideration for the flexible conductive films, thus the changes of resistivity and relative resistance (R/R0) of the flexible conductive films sintered from Ag flake ink and Ag flake/ion-0.5 ink were measured during the cyclic bending tests, as depicted in Fig. 6. The resistivity and relative resistance change of the conductive films increase with the increase of cyclic bending times. After the 200 times cyclic bending tests, the resistivity of the modified conductive film is 7.82 × 10-5 Ω·cm, which is significantly lower than that of the Ag flake film with a resistivity of 17.85 × 10-5 Ω·cm. Moreover, the relative resistance change (R/R0) of the modified conductive film is 1.62, which is still lower than the Ag 9

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flake film of 2.11. These results indicate that the modified conductive film sintered from Ag flake/ion-0.5 ink represents better bending durability and flexibility. Based on the microstructural observations and experimental results, a possible mechanism is proposed to explain the enhanced conductivity and flexibility by the Ag flake/ion composite ink, as illustrated in Fig. 7. The adjacent Ag flakes contact each

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other to form the conductive paths by low temperature sintering Ag flake ink. However, these contacted Ag flakes are difficult to realize the metallurgical

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interconnections at such a low temperature, thus representing high contact resistance

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and poor flexibility. The sintering situations of Ag flake/ion composite ink are

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different from that the Ag flake ink. The Ag ion ink is in situ decomposed into Ag

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nucleus during the sintering process, and the Ag flakes are preset to provide the

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nucleation sites for Ag nucleus. On this account, the Ag nuclei trends to be adsorbed and grown on the surfaces of Ag flakes, which is described as heterogeneous

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nucleation. Heterogeneous nucleation is a surface assisted nucleation process, which effectively reduces the nucleation barrier and facilitate the fast growth of Ag nuclei [39–42]. Furthermore, the Ag nucleus further grow on the surface of Ag flakes and form into Ag NPs to achieve the sintering and bridging of Ag flakes, which contributes to constructing more continuous conductive networks and reducing the contact resistance. Therefore, the Ag flake/ion ink can directly fabricate the thick conductive films with excellent conductivity and flexibility at low sintering temperature. 4. Conclusions 10

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In summary, Ag flake/ion composite ink was prepared to directly fabricate the thick conductive film with excellent conductively and flexibility on PI substrate at 100°C. The more reliable metallurgical contacts between Ag flakes are realized by the bridging effect of Ag NPs generated from the in–situ decomposition of Ag ion ink, which contributes to decrease the contact resistance and form more continuous

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conductive networks. Ag flake/ion composite ink evades the inherent defects of Ag NPs and enhances the conductivity and flexibility of the conductive film at low

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temperature. The modified conductive film sintered from Ag flake/ion-0.5 ink exhibits

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low electrical resistivity of 4.83 × 10-5 Ω·cm and a porosity of 3.13%, which are 42.9%

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and 69.4% lower than the Ag flake film sintered from the Ag flake ink. Moreover,

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after the 200 times cyclic bending tests, the resistivity and relative resistance change

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(R/R0) of the modified conductive film are 7.82 × 10-5 Ω·cm and 1.62, respectively, which are still significantly lower than the Ag flake film. Therefore, the Ag flake/ion

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composite ink is a promising and low cost printing material for low temperature fabricating high-quality printed electronics. Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (NSFC, 51775219 and 51805196), and the Graduates' Innovation Fund, Huazhong University of Science and Technology (2019ygscxcy044). Thanks to the facility support of Analytical and Testing Center of Huazhong University of Science and Technology for XPS, SEM, and TEM measurements. References 11

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Captions of figures Fig. 1. UV−vis absorption spectra of Ag ion inks. The insets are the corresponding photographs of Ag ion inks. Fig. 2. (a) XRD patterns of various inks sintered at 100°C for 30 min and unsintered

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Ag ion ink. Fig. 3. (a) XPS Ag 3d spectrum of the sintered Ag ion ink at 100°C for 30 min; (b) Ag

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3d, (c) C 1s, and (d) O 1s spectra of the sintered Ag flake/ion-0.5 ink at 100°C for 30

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min.

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Fig. 4. Electrical resistivity and porosity of the conductive films sintered from Ag

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flake ink and composite inks at 100°C for 30 min.

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Fig. 5. SEM images of the conductive film sintered from (a) Ag flake ink, (b) Ag flake/ ion-0.5 ink, and (c) Ag flake/ ion-1 ink. (d-e) TEM images of the modified

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conductive film sintered from Ag flake/ ion-0.5 ink. 2θ (°) Fig. 6. Change of resistivity and relative resistance (R/R0) of the conductive films sintered from Ag flake ink and Ag flake/ion-0.5 ink during the bending test at the bending radius of 5 mm. Fig. 7. Schematic diagrams of the sintering mechanism of (a) Ag flake ink and (b) Ag flake/ion composite ink at low temperature.

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Graphical abstract

Credit Author Statement

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Yun Mou: Data curation, Experimental design, Investigation, Methodology, Writing original draft, and Writing - review & editing.

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Hao Wang: Experimental design and Investigation. Yang Peng: Conceptualization, Data curation, Formal analysis, and Writing - review & editing.

Jiaxin Liu: Data curation and Formal analysis. Hao Cheng: Data curation and Formal analysis. Qinglei Sun: Formal analysis and Investigation. Mingxiang Chen: Formal analysis, Funding acquisition and Writing - review & editing.

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Highlights

The inexpensive Ag flake/ion composite ink was prepared

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The modified conductive film with low electrical resistivity of 4.83 × 10-5 Ω·cm

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The conductivity of the modified conductive film were enhanced by 42.9%

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was achieved at 100°C

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the modified conductive film represents better bending durability and flexibility compared with the traditional Ag flake film

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

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compared with the traditional Ag flake film

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