**3. Metal based soft TEs**

Due to the high density of free electrons, metals demonstrate the uppermost electrical conductance among all the conductive materials. Yet, metallic materials in bulk are unable to work as TEs directly as it has high light reflection at visible wavelength. [11] Thus, shape structuring is essential for metallic materials to attain the required optoelectronic characteristics. Following are the classes of metal-based TEs frequently reported in recent years. These typically include metal nanoparticle/nanowire/nanofiber networks, regular metal meshes, and ultra-thin metal films.

#### **3.1 Metal nanoparticle/nanowire/nanofiber networks**

One of the major classes of soft TEs is prepared from the metal NPs/NWs networks [21, 22], that have exhibited enormous performance in optical transparency, electrical conductivity, and mechanical deformation. The metal NPs or NWs must be gathered to form transparent metal meshes using several vacuum-free fabrication methods to realize soft TEs. In reality, the porous arrangement of these class of TEs permit the light to go across the free spaces in the grids. Therefore, the electrical and optical conductivity of these electrodes are greatly reliant on the grid arrangement. Simply, the electrical conductance depends on the density of metallic materials, while the optical transmittance is determined by the area fraction of metal coverage. Among these, TEs prepared from the metal NWs got much attention because of their shape and that they can easily be dispersed in various solvents. Therefore, these can be processed by multiple vacuum-free techniques to create TEs having decent optoelectronic performance for soft electronic applications. **Table 1** reviews the electrical and optical performance, and applications of metal NWs based soft TEs published in recent literature. Similar to other classes, metal NWs soft TEs also suffer from quite a few difficulties such as problem in achieving smooth NWs distribution across the large-area substrates, and the NWs delamination from the substrate during deformation. [9] In addition, the dispersed NWs network cannot be employed directly as further processing steps are normally required to eliminate the polymer capping around the NWs to decrease the junction resistance. This is achieved either using selective welding, bulk heating, or chemical processes. In addition to metal NWs, nanofiber based TEs have also got great interest due to their wide range of unique capabilities. Nanofibers are fabricated by employing various approaches, however, electrospinning technique is considered to be facile and low-cost to realize nanofibers with decent reproducibility, wellcontrolled shape, high aspect ratio, and saleable size. Moreover, the production of nanofibers can be enhanced by means of electrospinning system with multi-nozzles. [98] Despite this potential, TEs based on nanofibers [24, 67, 84, 99] have the randomly distributed patterns and because of this, the reproducibility of placing the nanofibers in precise locations and alignments remains a foremost challenge in these TEs. [15, 100]

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*Vacuum-Free Fabrication of Transparent Electrodes for Soft Electronics*

required to efficiently mass-produce these thin metal films.

Compared with metal NPs/NWs, metal-mesh based soft TEs look extra proficient as their electrical and optical conductivity can easily be adjusted in a broad assortment via changing the line width, mesh opening, and thickness. [26] Besides, numerous metals can be employed as metal-mesh based TEs to attain the desired chemical characteristics and work functions for the targeted soft electronic applications. [24] **Table 2** summarizes the electrical and optical performance, and applications of metal mesh based soft TEs published in recent literature. The presented data shows that the FoM values of metal-mesh based TEs are comparatively higher than that of metal NPs/NWs based TEs. This is mainly due to the low junction resistances, offered by the regular metal meshes. Regardless of the superior performances, rough surface topography and poor adhesion between the meshes and substrates constrained the extensive use of metal-mesh based TEs in soft electronic

Mostly, bulk metallic films having tens to hundreds of nanometers thicknesses are utilized as back-electrodes (opaque-cathodes). But, ultra-thin metal films with only few nanometers thicknesses can also be utilized as front-electrodes (transparent-anodes). Since, these metal layers are thinner in comparison with the light visible wavelength, and thus are optically transparent to human-eye. The thickness and uniformity of the metal films determine the optoelectronic performance of these TEs for the desired soft electronic applications. Several metals having different work-functions, including silver, nickel, gold, and platinum are effectively employed as transparent electrodes in soft electronic devices. [100] However, the vacuum-free fabrication of these ultra-thin transparent metallic films over large area is difficult, and thus substantial advancements in the fabrication methods are

*Graphene:* Graphene efficiently conducts electricity and heat, is stronger than steel (~200 times), and is nearly transparent. [101] Due to these unique characteristics, it has been suggested as a substitute soft TE material. Over the years, various vacuum-free approaches are established to produce thin films of graphene on soft substrate materials. [102–104] Recently, significant advancement has been made to enhance the optoelectronic properties of the graphene based TEs. Large-area graphene film was made-up on copper catalyst (~30 inches diagonal size), which was then accurately transferred to the target soft substrate using transfer printing technique. [105] In an ideal world, graphene has massive capability and is currently offering the assurance of being the vital transparent material for soft TEs. However as a matter of fact, uniform ultra-thin films of graphene are exceedingly challenging and are costly to produce. In addition, optoelectronic performances of graphene based TEs reduce quickly, due to the wrinkles/folds and crystallographic defects formed in these ultra-thin films during mechanical deformation. [105, 106] *Carbon Nanotubes:* Similar to graphene, CNT is one of the hardest materials recognized. Due to its decent electronic and mechanical characteristics, CNTs are productively employed as TE material in soft electronic devices. [107–110]

*DOI: http://dx.doi.org/10.5772/intechopen.96311*

**3.2 Regular metal meshes**

**3.3 Transparent thin metal films**

industry.

**4. Other soft TEs**

**4.1 Carbon materials**
