Chapter 1. Introduction
1.1 Flexible/stretchable transparent electrodes
Soft electronics, as an emerging and interesting research field, have attracted enormous attention in the past few years to integrate electronics with dynamic nonplanar surfaces by expanding the limited capabilities of conventional rigid electronics. The rapid evolution of wearable electronics with the growing importance of human-machine interfaces and internet of things (IoT) has inspired transition from current rigid systems to flexible and ultimately to stretchable electronics, enabling the conformal integration of electronic devices with skin/clothes of robots or human.1, 2
Figure 1.1. Diverse soft optoelectronic and electronic device applications based on flexible/stretchable transparent electrodes. (C. Larson et al. Science 2016, 351, 1071, M. S. Lim et al. Nano Lett. 2020, 20, 1526, X. Zhang et al. Energy Environ. Sci. 2018, 11, 354, J. Lee et al. Nanoscale 2012, 4, 6408, L.-Q. Tao et al. Nat. Commun. 2017, 8:14579, A. G. Nasibulin et al. ACS Nano 2011, 5, 3214, T. Kim et al. Adv. Funct. Mater. 2013, 23, 1250, A. Miyamoto et al. Nat. Nanotechnol. 2017, 12, 907).
Figure 1.1 shows the representative soft optoelectronic and electronic device applications using flexible or stretchable transparent electrodes in the last decade. With the increasing demand for
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soft optoelectronic/electronic device applications, electroluminescent (EL) devices,3 organic light- emitting-diodes (OLEDs),4 organic solar cells,5 touch screens,6 acoustic sensors,7 loudspeakers,8 transparent heaters,9 and electronic skins10 have been developed with flexible or even stretchable form.
With the intensive researches on soft electronics, flexible/stretchable optoelectronic and electronic devices show tremendous advances in device performance with high durability at strained states.
Compared to the rigid electronics, flexible/stretchable electronics can provide opportunity to be used in wearable and conformable devices when being integrated into body parts or arbitrary curvilinear objects.
For example, soft optoelectronic devices such as EL devices, OLEDs, solar cells, and touch screens can be conformably installed on the three-dimensionally curved non-planar surfaces or human skins, enabling the provision of visual information with higher user experience and the user-display interfaces.
Besides, soft electronic devices with diverse functionalities such as acoustic sensors, loudspeakers, microphones, transparent heaters, and electronic skins can be further explored in various human- machine interface applications. In particular, the development of high-performance flexible or stretchable transparent conductors is critically important for the realization of highly robust and reliable soft optoelectronic and electronic devices. Despite the notable progress in the flexible/stretchable electronics, the development of flexible or stretchable transparent conductors having both high levels of electrical conductivity and optical transparency as well as high mechanical durability under repeated deformation is still challenging.11
1.1.1. Nanostructured transparent conducting materials
Flexible or stretchable transparent electrodes play a pivotal role in a variety of future soft optoelectronic and electronic device applications. Next-generation transparent electrodes should possess three main physical properties: high electrical conductivity, optical transparency, and mechanical durability. Doped metal-oxide films, especially indium tin oxide (ITO) films, are the most widely used transparent conducting material because of its optical transparency and electrical conductivity. Although ITO has excellent sheet resistance and optical transparency, its brittleness causes failure under mechanical deformation, which is not compatible with the soft electronic applications.12 In addition, price of raw indium material and high-cost manufacturing process based on vapor phase sputtering process make the ITO undesirable for the flexible or stretchable electronic applications.13 Given the fact that ITO has several limitations in manufacturing process and non-compatibility with soft electronics, nanostructured transparent conducting materials such as conducting polymer,14 graphene,15 metal mesh,16 carbon nanotube (CNT),17 electrospun nanotrough,18 and metallic nanowires19 have been thoroughly explored and investigated due to their promising properties in the past few years (Figure 1.2). Even though typical transparent conducting nanomaterials can be ideal for
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designing transparent conductors, trade-off between electrical conductivity and optical transmittance should be considered for the optimal performances (Figure 1.2a).
Figure 1.2. Various next-generation conducting nanomaterials. (a) Sheet resistance versus optical transmittance for various transparent electrodes (W. Xiong et al. Adv. Mater. 2016, 28, 7167). (b) Conducting polymer (N. Kim et al. Adv. Mater. 2014, 26, 2268). (c) Graphene (J. C. Meyer et al. Nature 2007, 446, 60).15 (d) Metal mesh (A. Khan et al. Small 2016, 12, 3021). (e) Carbon nanotube (F. Mirri et al. ACS Nano 2012, 6, 9737). (f) Eletrospun nanotrough (B. W. An et al. Nano Lett. 2016, 16, 471).
(g) Metallic nanowire (Y. Sun et al. Nat. Electron. 2019, 2, 513).
Conducting polymers are one of the most promising alternative material to ITO due to their solution-processability and high mechanical flexibility (Figure 1.2b).20 A complex of poly(3,4- ethylenedioxythiophene) (PEDOT) and poly(4-styrenesulfonate) (PSS), in which PSS acts as counter- ion is the most widely used conducting polymer for flexible transparent electrodes that are often used in OLED and organic solar cells as hole transport layer.14 Despite high optical transparency (>85%) and mechanical flexibility of PEDOT:PSS, low electrical conductivity (>100 Ω sq-1) limits its applications requiring high current carrying capacity.
From the discovery of single atomic layer of carbon, graphene has attracted enormous attention for replacing ITO films due to outstanding electrical, mechanical, and chemical properties in the past decade (Figure 1.2c).21 The atomically thin single-crystal graphene sheets have an excellent transparency in visible region. Although graphene sheets can be synthesized in large-scale using roll- to-roll process, the fabrication process is based on complicated wet-chemical doping and chemical vapor deposition (CVD) with high vacuum condition.22 Moreover, the electrical conductivity of graphene is still not comparable with metallic nanomaterials.
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Recently metal mesh structures have been reported to replace ITO films for flexible transparent electrodes because of their superior optical transmittance and sheet resistance compared to ITO films (Figure 1.2d).16, 23 Due to the inherent light scattering and low haze factor of metal mesh structure, it can be readily applied to the flexible optoelectronic applications including solar cells and touch screens by enhancing optical path length of photons in solar cells and improving image quality of touch screen display.24 However, fabrication process of metal mesh is mostly based on photolithography and laser- etching process for high precision patterning that involves complicated procedures.
Transparent conducting networks of CNTs have received increasing attention to replace ITO due to the flexibility, durability, and solution-processability (Figure 1.2e).17, 25 Especially, single-walled CNTs have been intensively utilized in flexible optoelectronic devices because of its high optical transparency compared to the multi-walled CNTs. Even though CNTs are easily coated on a large-area substrate by solution-based roll-to-roll coating processes, the electrical conductivity of CNT networks is not comparable to ITO films owing to the large junction resistance between CNT bundles.
More recently, electropun metal nanotrough networks having superior optoelectronic performances and mechanical durability have been presented for future flexible transparent electrodes (Figure 1.2f).18, 26 Thereafter, Cui research group further enhanced the optoelectronic performance (0.36 Ω sq-1 at 92% transmittance) using hybrid of mesoscale and nanoscale metal nanowires synthesized by electrospinning methods.27 Although the proposed metal nanotrough networks show excellent optical transparency and electrical conductivity, electrospinning process is not adequate to mass production with low production efficiency.
Among various alternatives to ITO film, metallic nanowires are currently the most promising material due to the solution-processability and comparable optoelectronic performance to ITO (<30 Ω sq-1 at >90% transmittance) (Figure 1.2g).13 The metallic nanowires including copper nanowires (CuNWs)28-30 and silver nanowires (AgNWs)19, 31-33 have been intensively researched in the past decade for the fabrication of flexible and even stretchable transparent electrodes.
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