**5.5 Transfer printing**

*Nanofibers - Synthesis, Properties and Applications*

devices. [65]

CNT coatings. [123]

**5.4 Screen printing**

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

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

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

*(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].*

**290**

**Figure 5.**

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,

#### **Figure 6.**

*(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 with permission from Ref. [131].*

the graphene was initially covered by a PMMA thin film. Next, the underneath Cu film was removed by an etching step in FeCl3. The graphene film covered by PMMA was then lifted-off either using a PDMS stamp for transfer, or directly picked up using the target substrate itself. [131] To enhance the throughput and production speed, transfer printing has been integrated with R2R process for the fabrication of large-area graphene (30-in) soft TEs. [105] Despite such potential, wet transfer has a limitation for the fabrication of top graphene-based TEs for the soft thin-film devices as the functional materials used in these devices are sensitive to moisture. To overcome this a dry transfer approach is developed, where a the film is directly coated on the PDMS stamps before transfer. [132] Besides graphene, other major transfer printable material for soft TEs is the metal nanowire/ mesh films. These films typically have weak adhesion with the transfer substrates. This poor adhesion between the transfer substrates and metal films makes it easier to lift these films up with the PDMS, or another sticky polymeric stamp/target substrate. [60, 69] The high optical transmittance and superior conductivity of fabricated soft TEs using transfer printing ensure the high performance of soft electronic devices. [15, 60, 69, 70]
