Mechanical properties of Metal Based Flexible Transparent Conductive Electrode: From Fracture Mechanics perspective

Abstract

With increasing interests for transparent conductive electrode for flexible electronic devices, many researchers have developed flexible electrodes from curved devices to wearable and foldable devices. But they are faced with new challenges because these devices not only require high optical transmittance and low sheet resistance value, but also must be able to change its shape to flexible, bendable, and even stretchable form. Studies in the past focused heavily on the process and material development to improve functionality and flexibility without a thorough mechanical investigation and a deep understanding failure mechanisms. Dielectric/Metal/Dielectric (DMD) layer consists of few tens of nanometer thick structure, and despite its outstanding optoelectronic performance, research on the mechanical and electrical relationship has not yet been investigated thoroughly. One of the reasons is that studies on nanoscale mechanics and flexible electronics have been conducted independently by researchers in different fields. Moreover, fracture mechanics confront new challenges at the nanoscale. It is known that the fracture mechanics of nanoscale materials are significantly different from those on the macroscopic scale. As the structural dimensions of materials are scaled down to nanoscale, only an extremely limited number of atoms exist in the vicinity of the crack tip, which challenges the conventional fracture mechanics theory. It brings up fundamental questions about what scale fracture mechanics effectively govern and what the basic principles and theories are in a nanometer scale. Due to experimental difficulties at the nanometer scale, very few attempts have tried to solve this important issue. xv In this study, we investigate the fracture characteristics of multilayer DMD structure and its unique cracking behavior. Abnormal crack propagation and toughening of multilayer DMD structures are analyzed and its underlying mechanism are explained. Various experiments and theoretical analyses are carried out to uncover the details of multilayered hierarchical structures and their underlying crack deflections. We analyze the fracture behavior of thin film structures during bending process and proposed a theoretical framework to identify the underlying principles of robustness for DMD multilayered structures. Bio-inspired ductile brittle combination strategy and experiments on crack deflection behavior of DMD layers are carried out to find the fracture toughness in thin materials. We also investigate fatigue failure for various conductive materials through cyclic experiments. Various bending speed experiments are performed to study the effect of strain rates, and we present that they control mobility of atoms, which changes its mechanical and electrical property. Most notably, we introduce a unique crack deflection propagating phenomenon where a crack can deflect along with the metallic layer and create a step-like fracture. Lastly, we generate multiple neutral plane and insert stretchable layer in the middle to minimize the strain exerted on the electrode during bending and also to achieve better flexibility in DMD samples. We provide a concise and accurate analytical model for this multilayer structure with dissimilar elastic properties and make it conclusive with numerical simulation and experimental results.