In Situ Electro‐Mechanical Experiments and Mechanics Modeling of Fracture in Indium Tin Oxide‐Based Multilayer Electrodes

Abstract

Indium Tin Oxide (ITO) is one of the most widely used transparent conducting oxides in applications such as electronic displays and solar cells. The low resistivity and high transmittance of ITO have led to the recent explorations of polymer-supported ITO films as the transparent electrodes in flexible electronics. Flexible devices are often subject to repeated large deformation. While compliant polymer substrates can sustain large strain, brittle ITO films often fracture at a small strain. The cracking of ITO electrodes leads to loss of electrical conductance, posing crucial challenge to the reliability of flexible devices. As an effort to address this challenge, here we report a multilayer structural design of ITO-based electrodes with enhanced electro-mechanical durability. In particular, our in situ electro-mechanical experiments and coherent mechanics modeling reveal that, a top protective polymeric coating above and an intermediate polymeric layer below the ITO electrode can effectively reduce the driving force for cracking of the ITO electrode. The findings of the present paper suggest a feasible solution to durable transparent electrodes for flexible electronics.The multilayer structural design of ITO-based electrodes in the present study is inspired by the recent progress in designing high performance permeation barriers for flexible electronics. While flexible electronics is being developed toward an array of promising applications (e.g., paper like displays and sensitive electronic skins, etc.), [1][2][3][4][5][6] the service life of flexible devices is limited due to the vulnerability of functional organic layers in flexible devices to the attack of environmental water vapor and oxygen. High performance COMMUNICATION Indium Tin Oxide (ITO) films are widely used as transparent electrodes in electronic displays and solar cells. However, the small fracture strain of brittle ITO films poses significant challenge to their applications in flexible electronics devices that often undergo large deformation. Inspired by recent development of inorganic/organic hybrid permeation barriers for flexible electronics, we design and fabricate ITO-based multilayer electrodes with enhanced electro-mechanical durability. In situ electro-mechanical experiments of five structural designs of ITO-based multilayer electrodes are performed to investigate the evolution of crack density and the corresponding variance of electrical resistance of such electrodes. A coherent mechanics model is established to determine the driving force for crack propagation in the ITO layer in these electrodes. The mechanics model suggests that a top protective polymeric coating above and an intermediate polymeric layer below the ITO layer can effectively enhance the mechanical durability of the ITO electrodes by reducing the crack driving force up to 10-folds. The modeling results offer mechanistic understanding of the in situ experimental measurements of the critical fracture strains of the five types of ITO-based multilayer electrodes. The findin...

Indium Tin Oxide (ITO) serpentine ribbons on soft substrates stretched beyond 100%

“…A representative resistance vs. strain curve for straight serpentine ribbon is given in Fig.3B, with a blown up view included as the inset. The stretchability of a straight ITO ribbon is found to be between 0.77% and 1.5%, with an average of 1.12%, which is in good agreement with previous stretchability tests on blanket ITO films [34,35] or straight ITO ribbons [36], indicating that stretchability obtained by our electromechanical measurement is reliable. For the serpentine specimen shown in Fig.…”

“…3B. It has been well proved that the explosion of the curve is closely correlated with the mechanical failure of the ITO film [34][35][36]. The quantitative definition of stretchability is somewhat arbitrary and in this paper, we define stretchability, i.e., the critical strain-to-rupture, , as the tensile strain applied to the substrate at which R/R 0 = 6.…”

“…Thin ITO films are widely used as the electrode material in flat panel displays [31] and solar cells [32] attributing to their combined high electrical conductivity and optical transparency. However, cracks are observed at applied tensile strains around 1% in polymer-supported blanket thin ITO films [33][34][35]. Even ITO serpentine ribbons are found no more stretchable than their linear counterparts when supported by a stiff polymer such as Kapton [36].…”

“…Since ITO is mechanically brittle and electrically conductive, resistance vs. applied strain curves have been widely adopted to indicate the overall mechanical integrity of thin ITO films as a function of strain [34][35][36]. The resistance vs. applied strain curve can be tested by an in situ electromechanical setup as shown in Fig.…”

Direct Optical Visualization of Graphene and Its Nanoscale Defects on Transparent Substrates

“…Our IRM results reveal that for graphene, strain-induced nanocracks are largely, but not strictly (as found in metal oxide films 34,35 ), perpendicular to the stretching direction. Frequent changes in crack orientation and position are observed, and wrinkles parallel to the stretching direction often block crack propagation and lead to discontinued cracks (Figure 4d,h).…”