**5.3 Inkjet printing**

*Nanofibers - Synthesis, Properties and Applications*

**5.2 Spray deposition**

alone. Therefore, this wastage of material by spin coating is not financially viable

It is a coating process that uses a spray of particles or droplets to deposit a material onto a substrate using a nozzle, as schematically illustrated in **Figure 3a**. The spray nozzle creates a spray that comprises small drops of TE material and leads the materials transportation to the substrate by the help of carrier gas or electric charge. [119] Compared with other vacuum-free deposition techniques, the main benefit of spray coating is its capability of uniform coating of materials on non-flat substrates. **Figure 3b** displays organic photodetector (fiber-based) using PEDOT:PSS TE, that was realized using spray coating. it difficult to coat smooth PEDOT:PSS film though spin coating on the curved optical fiber surface. [120] It is also useful for subsequent processing, for instance, to spray coat on uneven surfaces, for instance, metal NWs, metal mesh coated substrates, as spin-coating of solutions can create non-continuous surface coverage. [33] Besides condense and smooth TCO-free films, spray coating has also the capability to deposit TCO films. **Figure 3c** shows

*(a) Schematic illustration of spray coating process. (b) Schematic (left) and photo (right) of fiber-based organic photodetector produced by spray coating. Reproduced with permission from Ref. [120] (c) schematic* 

*illustration of electrospray system. Reproduced with permission from Ref. [121].*

for industrial mass-production, even though partially this may be reused.

**288**

**Figure 3.**

It is another highly used technique for making soft TEs. Inkjet printing is devised from dispenser printing where ink droplets exit the nozzles by a vibrant practice. By controlling the contraction expansion of the piezoelectric actuator, discrete ink drops are ejected from the nozzle making the anticipated design on top of the substrate, as schematically illustrated in **Figure 4a**. It is direct printing technique for high-resolution patterning, without the need of lithography other advantage key advantages that the printed design can be easily changed by modifying the digital pattern that controls the actuator. [122] Inkjet printing is an effective approach to producing large area soft TEs. **Figure 4b** displays a large-area organic solar cell (OSC) having silver current collecting mesh fabricated by inkjet-printing. The printed silver mesh consisted only small portion (~8%) of the total substrate area due to the mesh relatively small line width (∼160 μm). The thickness of the printed silver mesh lines was >2 μm, which caused large height variation for the subsequent processing i.e. spin-coating of PEDOT:PSS and other active materials of the solar cell. This problem was resolved by embedding the silver mesh into an extra barrier film. The large-area OSCs having flexible Ag/PEDOT:PSS mesh TEs shown excellent performance as compared to that of TCO-solar cell, due to the high conductivity (sheet resistance ~1 Ω/□) of the silver mesh. [87] Inkjet printing processes based on mechanisms other than piezoelectric actuation are also utilized

#### **Figure 4.**

*(a) Schematic illustration of inkjet printing process. Reproduced with permission from Ref. [34]. (b) Photographs of inkjet-printed silver current mesh for large-area OSCs. Reproduced with permission from Ref. [87]. (c) Schematic illustration of high-aspect ratio metal grid along with electrohydrodynamic inkjet printing and the SEM image of the printed gold metal mesh electrode. Reproduced with permission from Ref. [65] (d) schematic diagram, SEM image, and photographs of the inkjet printed CNTs based TEs by "coffee ring effect". Reproduced with permission from Ref. [123].*

for fabricating TEs. For instance, electrohydrodynamic inkjet process, as shown in **Figure 4c**, enabled the printing of high resolution gold meshes (feature size line 80 to 500 nm line widths) for realizing high performance TEs (8 Ω/□ at 94% optical transmittance), that can be custom-made for the application in different electronic devices. [65]

One major obstacle in attaining uniform inkjet printed structures is the coffeering effect, that initiates because of the capillary flow in the solvent evaporation step. [124] Though, this effect is also occasionally useful for making TEs with particular ring shapes. [125] **Figure 4d** demonstrates a CNTs based TE having joined ring patterns, that was made through inkjet printing the CNT ink on top of a pre-heated PET film. The height and diameter of the rings were the functions of applied temperature. Post heat-curing further lowered the sheet resistance of the CNT coatings. [123]

### **5.4 Screen printing**

It is one of the attractive methods used to print soft TEs. In this technique, viscous inks are forced across stencils or patterned mesh (typically used as the template) using a scraper as shown in **Figure 5a**. The density of the used mesh and ink viscosity define the printing resolution and thickness of the pattern. [35] This handy and relatively simple technique is utilized mainly for graphene and PEDOT:PSS, however metals can also be printed. The resolution of conventional screen printing processes is not high, however, it can be improved to tens of micrometer using an improved screen-offset approach. [126] **Figure 5b** displays the screen printed graphite oxide (GO) arrays on PET film, which was afterwards reduced to rGO using hydriodic acid (HI) in modest environments. This technique developed an easy way to manufacture large-area graphene TEs (patterned), having thickness of few hundred nanometers. [127] Similar to other screen printable materials, mesh-patterned PEDOT:PSS TEs can be realized with various width/period ratios by adjusting the wire diameter, mesh size, and photoresist thickness. [128] Beside graphene and PEDOT:PSS, screen printing is also utilized for the patterning of metallic inks. **Figure 5c** shows the schematic illustration of the structure of OSC having printed silver mesh as TEs. This work relates the screen printed hybrid TEs having PEDOT:PSS on top of silver mesh with various other printing approaches for

#### **Figure 5.**

*(a) Schematic illustration of screen printing process. Reproduced with permission from Ref. [35]. (b) Photographs of GO (left) and rGO (right) films, fabricated by screen printing. Reproduced with permission from Ref. [127]. (c) Schematic illustration of the OSCs containing the layers of P3HT:PCBM, ZnO, PEDOT:PSS, and silver electrodes. The back silver electrode is printed by various processes including screen printing. Reproduced with permission from Ref. [129].*

**291**

**Figure 6.**

*with permission from Ref. [131].*

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

the vacuum-free and TCO free OSCs. It concludes that the uniformity of screenprinted silver meshes was superior as compared to inkjet printed and flexographic printed TEs, which were damaged by de-wetting in the subsequent PEDOT:PSS film processing. Consequently, the OSCs having screen-printed silver TEs showed better performance equated with inkjet printed and flexographic printed solar

Transfer printing is an emergent method for fabrication of soft TEs, that empowers the processing of various materials into the chosen useful shapes. This produces manufacturing prospects in the field of soft electronics with comparable performance to that of traditional wafer-based processes, however with capacity to be deformed. In this technique, first the materials structures are fabricated on the conventional donor substrate and then wisely transferred onto unconventional soft substrates, as described in **Figure 6a**. [36] For instance, graphene ultra-thin films are first coated on Ni or Cu foils using the standard chemical vapor deposition (CVD) technique. [130] In order to be used as TEs, the this graphene has to be transferred directly to top of the devices or transparent substrates. There are two different transfer approaches (wet transfer and dry transfer) to transfer CVD graphene onto various soft substrates, as shown in **Figure 6b**. In wet transfer,

*(a) Schematic illustration of the transfer printing technique. Reproduced with permission from Ref. [36]. (b) Schematics of wet (right) and dry (lift) transfer printing for graphene soft TEs fabrication. Reproduced* 

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

cells. [129]

**5.5 Transfer printing**

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

the vacuum-free and TCO free OSCs. It concludes that the uniformity of screenprinted silver meshes was superior as compared to inkjet printed and flexographic printed TEs, which were damaged by de-wetting in the subsequent PEDOT:PSS film processing. Consequently, the OSCs having screen-printed silver TEs showed better performance equated with inkjet printed and flexographic printed solar cells. [129]
