**5. Vacuum-free fabrication approaches for soft TEs**

Vacuum-free thin film fabrication techniques are favored by soft electronic industry because of low cost, low material waste and high output as compared with conventional vacuum fabrication processes. Yet, accomplishing equivalent quality solution-processed TEs is a challenging job due to several reasons, including the substrate/TE adhesion, the solvent volatility, surface wettability, and solution rheology need to be accustomed. Following are the most commonly reported vacuumfree printing and coating approaches for the fabrication of soft TEs.

#### **5.1 Spin coating**

It is a simple technique used to coat continuous thin films onto rigid flat surfaces. Typically a small amount of coating material is put on the substrate's center, that is ideally spinning at low speed. The substrate is then rotated at high speed (max ~10 k rpm) to uniformly spread the coat-material utilizing the centrifugal force, as schematically illustrated in **Figure 2a**. One main benefit of the spin coating process is its capacity of dense coating of uniform and thin films onto rigid flat surfaces. This ability is quite attuned along the requirement of excellent TEs, as the thickness of TEs needs to be optimized. It is an attractive method to fabricate transparent thin

**287**

**Figure 2.**

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

graphene films (few nanometer-thick), as the optical transparency of these films will decline considerably with increase in thickness. For instance, each graphene layer absorbs 2.3% of white light. [117] Therefore, graphene-based TEs needs to be ultra-thin to obtain appropriate optical transmittance. Thin (3.1 nm) graphene TEs are fabricated using spin-coated for realizing OSCs. [118] Occasionally, the smoothness of spin-coated TEs is not perfect because of the material properties itself. For instance, silver NWs have decent dispersion in isopropanol, water, and few other frequently employed solvents and therefore can be easily spin coated on several substrates for the fabrication of TEs. Conversely, the spin coated silver NWs typically creates a nano-mesh (with certain thickness) on the substrates, making roughness for the subsequent processing, and therefore limits the applications of bottom TEs. In addition to the roughness concern, the weak silver NW/substrates adhesion causes mechanical failure of the devices, particularly in soft electronics. [88] This issue is resolved by spin coating a TiOx buffer layer (~200 nm) over the silver NWs to get a comparatively uniform film, as displayed in **Figure 2b**. [32] Despite such potential, spin coating process have few limitations for the realizing of soft TEs. First, flatness of the spin-coated TEs is typically sensitive to spin speed, humidity, and substrate cleanness, which make the processing difficult to reproduce in ambient environment. Second, spin coating on large-area substrates is precisely difficult as it is challenging to clamp a hefty substrate and keep it stable at a high rotating speed. As a result, the spin coated films thickness is spatially different over a large substrate due to the variation of the localized centrifugation speed. Third, majority of the material is spun-off the substrate in spin coating, making this material-wasting approach. Bearing in mind, major portion of the total price of the raw materials of the soft electronic devices comprises of the material cost of TEs

*with the TiOx buffer layer (right). Reproduced with permission from Ref. [32].*

*(a) Schematic illustration of spin coating process. (b) AFM topographies of silver NW TEs without (left) and* 

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

*Vacuum-Free Fabrication of Transparent Electrodes for Soft Electronics DOI: http://dx.doi.org/10.5772/intechopen.96311*

**Figure 2.**

*Nanofibers - Synthesis, Properties and Applications*

**4.2 Transparent conducting polymers**

scale soft electronics. [113, 114]

soft electronics.

Numerous vacuum-free approaches, for example, spin coating [111] and transfer printing, [112] are developed to produce CNTs based soft TEs. While, CNTs based TEs have attractive characteristics, such as higher optical transmittance and superior mechanical deformation capability, these have typically poor electrical conductivity. This limitation makes CNTs less suitable for large-area commercial

*PEDOT:PSS:* Few transparent polymers, having intrinsically poor electrical conductivity, are transformed into conducting polymers via addition of conductive dopants into their iterating chains. Poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is one of the classic model of such conducting polymers. In PEDOT: PSS unit chain, PEDOT acts as the conducting polymer, while the PSS plays the role of a dopant, enhancing its electrical conductance via significantly increasing the charge carriers. Since, PEDOT:PSS has no visible absorptive resonances, therefore it is routinely used as TEs in small scale soft electronic devices. Yet, a number of concerns, for example, instable molecular structure and high water solubility have limited the use of PEDOT:PSS in large-

*Other Conducting Polymers:* Besides PEDOT:PSS, other conducting polymers comprising poly(p-phenylene-vinylene) (PPV), polyaniline (PANI), polyfuran (PF), polypyrrole (PPy), are utilized as TE materials for several soft electronic devices, due to their decent electrical and optical conductivity. [115, 116]

As discussed above, each class of soft TEs offers unique set of favorable properties, and also has some disadvantages. Researchers have combined different classes of TEs into a single electrode structure to fabricate hybrid soft TEs. The objectives of developing this new class TEs are: (1) take advantage of the benefits offered by individual electrode. (2) overcome those challenges associated with the electrode once employed individually. **Table 3** summarizes, the optical and electrical perfor-

mance, and applications of hybrid soft TEs published in recent literature.

Vacuum-free thin film fabrication techniques are favored by soft electronic industry because of low cost, low material waste and high output as compared with conventional vacuum fabrication processes. Yet, accomplishing equivalent quality solution-processed TEs is a challenging job due to several reasons, including the substrate/TE adhesion, the solvent volatility, surface wettability, and solution rheology need to be accustomed. Following are the most commonly reported vacuum-

It is a simple technique used to coat continuous thin films onto rigid flat surfaces. Typically a small amount of coating material is put on the substrate's center, that is ideally spinning at low speed. The substrate is then rotated at high speed (max ~10 k rpm) to uniformly spread the coat-material utilizing the centrifugal force, as schematically illustrated in **Figure 2a**. One main benefit of the spin coating process is its capacity of dense coating of uniform and thin films onto rigid flat surfaces. This ability is quite attuned along the requirement of excellent TEs, as the thickness of TEs needs to be optimized. It is an attractive method to fabricate transparent thin

**5. Vacuum-free fabrication approaches for soft TEs**

free printing and coating approaches for the fabrication of soft TEs.

**286**

**5.1 Spin coating**

*(a) Schematic illustration of spin coating process. (b) AFM topographies of silver NW TEs without (left) and with the TiOx buffer layer (right). Reproduced with permission from Ref. [32].*

graphene films (few nanometer-thick), as the optical transparency of these films will decline considerably with increase in thickness. For instance, each graphene layer absorbs 2.3% of white light. [117] Therefore, graphene-based TEs needs to be ultra-thin to obtain appropriate optical transmittance. Thin (3.1 nm) graphene TEs are fabricated using spin-coated for realizing OSCs. [118] Occasionally, the smoothness of spin-coated TEs is not perfect because of the material properties itself. For instance, silver NWs have decent dispersion in isopropanol, water, and few other frequently employed solvents and therefore can be easily spin coated on several substrates for the fabrication of TEs. Conversely, the spin coated silver NWs typically creates a nano-mesh (with certain thickness) on the substrates, making roughness for the subsequent processing, and therefore limits the applications of bottom TEs. In addition to the roughness concern, the weak silver NW/substrates adhesion causes mechanical failure of the devices, particularly in soft electronics. [88] This issue is resolved by spin coating a TiOx buffer layer (~200 nm) over the silver NWs to get a comparatively uniform film, as displayed in **Figure 2b**. [32] Despite such potential, spin coating process have few limitations for the realizing of soft TEs. First, flatness of the spin-coated TEs is typically sensitive to spin speed, humidity, and substrate cleanness, which make the processing difficult to reproduce in ambient environment. Second, spin coating on large-area substrates is precisely difficult as it is challenging to clamp a hefty substrate and keep it stable at a high rotating speed. As a result, the spin coated films thickness is spatially different over a large substrate due to the variation of the localized centrifugation speed. Third, majority of the material is spun-off the substrate in spin coating, making this material-wasting approach. Bearing in mind, major portion of the total price of the raw materials of the soft electronic devices comprises of the material cost of TEs

alone. Therefore, this wastage of material by spin coating is not financially viable for industrial mass-production, even though partially this may be reused.
