Novel Flexible Transparent Conductive Films with Enhanced

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Novel Flexible Transparent Conductive Films with Enhanced Chemical and Electro-Mechanical Sustainability: TiO2 Nanosheet-Ag Nanowire Hybrid Hiesang Sohn, Se Yun Kim, Weon Ho Shin, Jong Min Lee, Hyangsook Lee, DongJin Yun, Kyoung-Seok Moon, Intaek Han, Chan Kwak, and Seong-Ju Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13224 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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ACS Applied Materials & Interfaces

Novel Flexible Transparent Conductive Films with Enhanced

Chemical

and

Electro-Mechanical

Sustainability: TiO2 Nanosheet-Ag Nanowire Hybrid Hiesang Sohn, 1,2‡* Seyun Kim, 2‡ Weonho Shin, 2 Jong Min Lee, 2 Hyangsook Lee,3 Dong-Jin Yun, 3

Kyoung-Seok Moon,2 In Taek Han,2 Chan Kwak2 and Seong-Ju Hwang4

1

Department of Chemical Engineering, Kwangwoon University, 20 Kwangwoon-Ro, Nowon-

Gu, Seoul 01897, Korea 2

Inorganic Materials Laboratory, Materials Center, Samsung Advanced Institute of Technology,

Samsung Electronics, 130 Samsung-ro, Yeongtong-gu, Suwon, 443-803, Korea 3

Analytical Engineering Group, Platform Technology Laboratory, Samsung Advanced Institute

of Technology, Samsung Electronics, 130 Samsung-ro, Yeongtong-gu, Suwon, 443-803, Korea 4

Department of Chemistry and Nanoscience, Ewha Womans University, Seoul, 03760, Korea

E-mail: Hiesang Sohn ([email protected])

KEYWORDS. TiO2 nanosheet-Ag nanowire hybrid; Flexible transparent conductive film; Ag sulfidation; Chemical/aging stability; Electro-mechanical stability.

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ABSTRACT

Flexible transparent conductive films (TCFs) of TiO2 nanosheet (TiO2 NS) and silver nanowire (Ag NW) network hybrid were prepared through a simple and scalable solution-based process. As-formed TiO2 NS-Ag NW hybrid TCF shows a high optical transmittance (TT: 97% (90.2% including plastic substrate)) and low sheet resistance (Rs: 40 ohm/sq.). In addition, TiO2 NS-Ag NW hybrid TCF exhibits a long-time chemical/aging and electro-mechanical stability. As for the chemical/aging stability, the hybrid TCF of Ag NW and TiO2 NS reveals a retained initial conductivity (∆Rs/Rs 4000%) or RuO2 NS-Ag NW hybrid (∆Rs/Rs >200%). As corroborated by the density functional theory (DFT) simulation, the superb chemical stability of TiO2 NS-Ag NW hybrid is attributable to the unique role of TiO2 NS as a barrier which prevents Ag NW’s chemical corrosion via the attenuated adsorption of sulfidation molecules (H2S) on TiO2 NS. With respect to the electro-mechanical stability, in contrast to Ag NWs (∆R/Ro ~152.9%), our hybrid TCF shows a limited increment of fractional resistivity (∆R/Ro ~14.4%) after 200,000 cycles of the 1R bending test (strain: 6.7%) owing to mechanically welded Ag NW networks by TiO2 NS. Overall, our unique hybrid of TiO2 NS and Ag NW exhibits the excellent electrical/optical properties and reliable chemical/electro-mechanical stabilities.

1. Introduction

Transparent conductive electrode (TCE) has become a focus of considerable research activities for a long time owing to its essential role in various optoelectronic devices such as flat panel


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displays, organic light-emitting diode (OLED), and touch screen panels.1-7 Indium tin oxide (ITO) has been widely used as TCE material due to its low electrical resistance and high optical transmittance. However, recent interests for flexible and stretchable optoelectronics (e.g., bendable displays and wearable devices) have driven to replace ITO films with novel TCE materials since ITO-based devices suffer from 1) inadequate process condition (high temperature) for plastic substrate, 2) limited flexibility (strain 90% on PET substrate), 3) cost-effective processability, and 4) good flexibility.11,12 Nevertheless, there remain challenges of insufficient chemical/mechanical sustainability of Ag NWs, restricting their practical applications.11,12 For instance, Ag NW can be easily oxidized/sulfidized under exposure to air and water for a long time, exhibiting the debilitated electrical, optical and mechanical properties of Ag NW films.1-5,14,15 Such sulfidations/corrosions of Ag NW can be initiated by very small amount of sulfur compounds (e.g., H2S, OCS, S2, SO2, and CS2) under exposure to atmosphere, making Ag NWs less conductive by contact ripening and fretting.2-5,14-21 In addition, Ag NW networks reveal


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enervated electrical/mechanical properties after long-term cyclic bending/unbendings owing to their cracking, sliding, and delaminations under repeated mechanical deformations.12 To address above issues, it is required to suppress the oxidation/sulfidation of Ag NWs and enhance their electro-mechanical stabilities. Existing approaches to address this problem mainly focus on the formation of hybrid structure by providing Ag NW with secondary protection layers or conductive paths.2-5,11,12,14-18,22,23 For instance, reduced/non-reduced graphene oxide-Ag NW2,14, graphene-Ag NW6,7,15, carbon nanotube-Ag NW22,23, nanoparticle-Ag NW24 and Ag NW-conducting polymer25 hybrid-based transparent electrodes endow Ag NW with partial moisture protection and conductivity enhancement. Recently, some metal oxide (e.g., ITO, Al2O3, TiO2) nanostructures hybridized with Ag NW network17,18,24,26 prevent the oxidation/sulfidation of Ag NW by protecting them from atmospheric molecules through the sol-gel coating or atomic layer deposition (ALD) owing to their excellent chemical stability (inertness)17,18,24,26 and good barrier property.17,18,27 However, the conventional hybrid of Ag NW network and secondary nanostructure is not sufficient to improve both chemical and electro-mechanical stabilities without compromise of electrical/optical properties.17,18,22-26 What is more, above hybrids based flexible TCE materials are hard to be commercialized due to 1) limited commercial viability (difficulty in scalability, high cost, and complicated process condition), 2) poor optical properties (TT 95%), mechanical flexibility and good solution processability, we selected TiO2 NS as an appropriate counter layer of Ag NW in the hybrid conductor.17,18,27,28 To assess the characteristics of our hybrid material, we investigated the electrical/optical properties and chemical/aging stability of TiO2 NS-Ag NW hybrid by comparing it with those of bare Ag NW and other metal oxide (RuO2) NS-Ag NW hybrids. Then, we analyzed and discussed the chemical stability of TiO2 NS-Ag NW hybrid through the comparative elemental/surface analyses and density functional theory (DFT) simulation. We also evaluated the electro-mechanical stability of TiO2 NS-Ag NW hybrid TCF by performing the bending fatigue test at 1R (1 mm of bending radius: 6.7% of bending strain) for 200,000 (200K) times. Based on the analyses, we found that an as-formed TiO2 NS-Ag NW hybrid exhibits 1) a low sheet resistance (Rs: 40 ohm/sq.), 2) high optical transparency (TT >90% including substrate), 3) long-term chemical/aging stabilities (∆R/Rs 90%).2-5,14,36 As shown in 3D reconstructed AFM image (Figure 1i), TiO2 NS-Ag NW hybrid displays its unique structure of Ag NW networks conformally covered by TiO2 NS. Such a homogeneous distribution of Ag NW networks in the hybrid structure suggests a strong physical adhesion of TiO2 NS on hydrophilic PET surfaces as well as on Ag NW networks owing to strong hydrogen-bonding interactions between hydroxyl groups from TBAOH on TiO2 NS and polyvinylpyrrolidone (PVP) on Ag NWs.11,12 As shown in Figure 1h, the thicknesses of Ag NW and TiO2 NS are 21 and 4.1 nm, respectively, consistent with the TEM images (Figures 1a and c).


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Figure 1. Electronic images for TCFs: TEM images for (a) Ag NW, (b) TiO2 NS, (c) TiO2 NSAg NW (cross-section view); SEM images for (d) Ag NW, (e) TiO2 NS, (f) TiO2 NS-Ag NW (plane view); AFM images for (g) TiO2 NS, (h) 2D plane view, and (i) 3D reconstructed image of (h) for TiO2 NS-Ag NW.


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3.2 Electrical and optical characteristics Figure 2 shows the electrical and optical properties of conductors (Ag NW and TiO2 NS-Ag NW hybrid) prepared at various coating conditions. Figures 2a and b compare sheet resistance and optical transmittance of Ag NW and TiO2 NS-Ag NW hybrid by altering the number of deposition layers of TiO2 NS at fixed Ag NW networks layer. Figures 2c and d present the plot of sheet resistance and optical transmittance for Ag NW and its hybrid by varying deposition layer thickness of Ag NW at fixed layers of TiO2 NS. As for electrical properties of conductors (Figure 2a and 2c), the conductivity of hybrid is enhanced ~up to 13.5% (∆Rs/Rs) by applying thicker TiO2 NS at fixed Ag NW network (Figure 2a) or ~up to 63% (∆Rs/Rs) by applying thinner (or lower density) Ag NW network (thickness: 20~30 nm) at fixed TiO2 NS layer (Figure 2c) without additional treatments (e.g., thermal annealing, plasmonic welding, and current ripening). However, as shown in Figure 2c, there is no significant increment of conductivity of hybrid at thicker (or higher density) Ag NW network (thickness >30 nm). As for optical properties (Figure 2b and 2d), TiO2 NS hybrid shows a slight (∆TT/TT

Novel Flexible Transparent Conductive Films with Enhanced

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