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Silver Nanowire-Based Flexible and Stretchable Electronics with Enhanced Functionality and Reliability

  • Sungwoo Jun (전성우, 대학원 마이크로/나노시스템협동과정 마이크로/나노시스템협동과정전공)

초록/요약

Silver nanowires (AgNWs) have come to prominence in many research fields and applications due to their advantages, such as optically transparent characteristic despite high conductivity, deposition by the extremely facile fabrication, and availability combined with functional polymers. The properties of AgNW networks such as optical transparency, low sheet resistance, and stable mechanical reliability enhance their applicability to the next-generation electronic/optoelectronic devices and increase feasibility of the wearable products. And, going one step further, to achieve the stretchable devices, the AgNW-elastomer composite electrode should be optimized for good adhesion and mechanical elasticity. However, the low surface energy of most of elastic polymers prevents AgNWs from being coated and directly patterned on their surface, and the absence of the chemically/physically stable patterning limits their access to practical application. And, in addition to process improvements, it is also important to supplement the device structure or drive mechanism so as to reduce the mechanical stress that the AgNW-based electrodes receive. As well as, even if the AgNW-based electrode is deformable, unless other components except AgNW-based electrode exhibit that characteristic, the fabricated device does not have the deformable feature. Therefore, several methodologies were suggested here to address the presented issues and to develop mechanically reliable devices by appropriately combining various functional layers and processes. First, as a practical example of supplementing the mechanical instability of AgNWs from the perspective of the drive mechanism, a bending sensor was designed with a new operation concept replacing the conventional pressure and strain sensors. Because the proposed bending sensor is based on a bending curvature sensitive to the applied pressure without stretching and releasing, there is much less mechanical stress on the operating and it has superior performance in responding sensitively to the bending radius and induced pressure. Around 30% of the capacitance increased by pressing with 180 kPa, revealing that the sensitivity of the AgNW/PVB-based capacitive sensor was three times higher than that of the elastomer-based capacitive sensor. Moreover, the capacitance (about 4.95 pF) measured in the flat state increased with decreasing bend radius and reached 6.63 pF at 3 mm radius. Second, to realize the stretchable applications, industrially applicable electrode patterning procedure that does not damage the elastic polymer-based substrate is essential, and a facile, rapid, and chemical-free patterning method was employed as a solution. After patterning AgNWs deposited on the stretchable substrate, simultaneously embedded AgNWs enhanced the mechanical reliability of the stretchable touch sensor fabricated by the proposed patterning method. The precisely patterned AgNW onto the TPU film operated reliably as the touch sensor without degradation of the sensitivity even in 33 % stretched condition. Finally, for the deformability of the entire device as well as the AgNW-based electrodes, the AgNW-based stretchable electrodes and sandwiched ZnS:Cu-elastic polymer composite dielectric layer between the electrodes are employed and combined for a stretchable photodetector. Through the excellent stretchability of the entire device and the photo-responsive ZnS-based capacitive operation, the excellent performance and reliability could be improved and stabilized even in the stretched state. Light-irradiated capacitances increased by about 2.03, 2.53, 3.17, and 4.01 times compared with the non-irradiated capacitances in ZnS:Cu content ratios of 23 wt%, 33 wt%, 41 wt%, and 47 wt% constituting the ZnS:Cu-PUU composite, respectively. In addition, the photo-sensing functionality of the fabricated photodetector was not degraded even in the stretched state up to 50 % strain. The proposed methods of this dissertation are expected to contiribute to the development of future wearable devices in terms of operation mechanism, fabrication process, and device structure.

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Contents
Abstract i
List of Figures ⅶ
Chapter I. Introduction 1
I.1. Reference 8
Chapter II. Bending and Pressure-Derived Strain Sensor Based on Silver Nanowire/Polyvinyl Butyral Composite Electrode 13
II.1. Introduction 13
II.2. Experimental Section 17
II.2.1. Fabrition of capacitive sensor 17
II.2.2. Evaluation of sensor 21
II.3. Results and Discussion 22
II.4. Summary 49
II.5. Reference 50
Chapter III. Simple Fabrication of Stretchable Capacitive Sensor Based on Thermoplastic Polyurethane Using Photo-Induced Embedding of Silver Nanowires 57
III.1. Introduction 57
III.2. Experimental Section 61
III.2.1. Fabrition of touch sensor 61
III.3. Results and Discussion 64
III.4. Summary 82
III.5. Reference 84
Chapter IV. Highly Stretchable Photodetector Based on ZnS:Cu Particle-Polyurethane Urea Composite and Silver nanowires 89
IV.1. Introduction 89
IV.2. Experimental Section 93
IV.2.1. Evaluation of Photodetector 93
IV.2.2. Fabrication of Capacitive Photodetector 93
IV.3. Results and Discussion 96
IV.4. Summary 108
IV.5. Reference 110
Chapter V. Conclusion 115
List of Publications 117

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