**5.2 Spray deposition**

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

#### **Figure 3.**

*(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].*

**289**

**Figure 4.**

*permission from Ref. [123].*

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

its widespread adoption for production of large area soft TEs.

the electrospray setup, utilized for deposition of zinc oxide (ZnO) and aluminum doped zinc oxide (AZO) films. [121] Despite such capabilities, spray processed TEs has the scalability problem, much more prominent as compared with other vacuum-free coating techniques. This limitation of low throughput has hammered

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

*(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* 

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

**5.3 Inkjet printing**

the electrospray setup, utilized for deposition of zinc oxide (ZnO) and aluminum doped zinc oxide (AZO) films. [121] Despite such capabilities, spray processed TEs has the scalability problem, much more prominent as compared with other vacuum-free coating techniques. This limitation of low throughput has hammered its widespread adoption for production of large area soft TEs.
