Micro-networked metal coating using self-cracked WO₃ inorganic thin film as sacrificial layer: Application to transparent flexible electrodes

Highlights

• Micro-networked metal electrode using a self-cracked WO3 film as a sacrificial layer.

• Electrical conductivity of approximately ∼100 Ω/sq was measured.

• An average transmittance was 95.6% in the visible light region.

• The electrical resistance was maintained stable with repeated bending tests (>10000).

Abstract

A method of fabricating a micro-networked metal electrode using a self-cracking WO₃ oxide thin film as a sacrificial layer is presented. The width of the formed crack can be adjusted to a size of 10 to 40 µm by adjusting the wet-etching time in dilute NaOH solution, and accordingly, the electrical conductivity and transmittance of the electrode can be adjusted. An electrical conductivity of approximately 100 Ω/sq was measured at an average transmittance of 95.6% in the visible light region, and when applied to a flexible substrate, the electrical resistance was maintained stable even with repeated bending tests. The method proposed in this study can be applied to various transparent flexible electrode devices.

Introduction

The market for flexible-photoelectric devices (displays, mobile devices, sensors, and energy conversion devices) has expanded rapidly of late, giving rise to considerable interest in flexible materials with good transmittance and electrical conductivity both in industry and in research [1], [2], [3]. Conventional transparent conductive oxides such as indium tin oxide (ITO) and aluminum zinc oxide have excellent light transmittance (> 90%) and high electrical conductivity (∼ 10⁴ S/cm), but they are prone to brittleness when subjected to bending conditions exceeding a certain curvature. This behavior results in fracture, making it difficult to apply these materials to flexible devices [4], [5], [6]. Alternative materials such as sp² bond-based carbon materials (carbon nanotubes and graphene) [7], [8], [9] and conductive polymers, particularly poly(3,4-ethylenedioxythiophene) and its complex with poly(styrene sulfonate) [10,11], have been explored, but they have limitations with regard to conductivity (∼10³ S/cm), large-scale processability, and chemical and thermal stability [7], [8], [9], [10], [11], [12].

Metals have high free electron densities and are very flexible when fabricated into thin components, making them ideally suited as flexible conductors. However, the light incident on these materials is reflected and absorbed as indicated by only imaginary numbers in the refractive indices, and hence, these materials have low transmittance. However, it is possible to use such metals as transparent flexible electrodes (TFEs) by fabricating them into fine grids, as evidenced by recent studies [13], [14], [15], [16], [17], [18]. Such metal grids are generally fabricated either by utilizing their self-connection by randomly providing a thin coating of nanorods [14,15], or by depositing metal through a lithography process and subsequent micropatterning [16], [17], [18]. The process using nanorods can achieve high transmittance but is not suitable for extending over a large area as it is difficult to ensure uniformity and good thermal stability [15,19,20]. Conversely, the patterning method has the disadvantage of a higher process cost [21].

In this study, we propose a method to fabricate a low-cost metal microgrid by forming a sacrificial layer on a flexible substrate. The sacrificial layer material forms micro-sized cracks without a patterning process, coating the metal electrode, and the sacrificial layer is then subjected to wet etching. If two materials with a large difference in the coefficient of thermal expansion are used, a self-cracking phenomenon can be induced by the difference in stress due to temperature change [22], [23], [24], [25]. This phenomenon has been verified by the micro-crack formation of several metallic thin films for application in surface-enhanced Raman scattering. In this study, an oxide with a high thermal expansion coefficient (TEC) (WO₃, 8 × 10⁻⁶ ∼ 15 × 10⁻⁶ K⁻¹ [26]) was deposited on a flexible substrate in an amorphous state to induce as much thermal expansion and contraction as possible. After depositing the metal layer, a microgrid was formed by wet etching using a solution (NaOH, WO₃ (s) + NaOH (l) → Na₂WO₄ + H₂O [27]) that will not damage the metal layer. Thus, we developed a process for manufacturing a TFE that can easily be adapted for processing large areas. We can create a cracked template very simply by reducing the process steps and variables compared to the common method (spin coating, process using mechanical force) [28], [29], [30], [31], [32], [33], [34]. In addition, the micro-networked metal structure was fabricated using the template produced herein, demonstrating the possibility that it could be applied to the industrial process.