**3.4 Spray coating**

Spray coating is a low-temperature coating technology that is advanced. A nozzle sprays the tiny solution droplets over the pre-heated substrate at a high speed in this process. Pneumatic spraying, which provides the solution droplet as a rapid gas flow, is the most commonly used spray coater. The spray coating process consists of four steps: creation of the droplet at the nozzle, transportation of the droplet to the substrate, coalescence of the droplet on the substrate, and drying [92]. The droplets are formed through the nozzle in the first step, which is known as atomization. The substrate is ready for the annealing procedure to obtain the ultimate film after the droplet coalescence on the wet film. **Figure 8** depicts a schematic diagram of the overall coating process.

To achieve thorough wetting of the substrate, the solvent utilised in this approach must have a low surface tension and contact angle. Another way to produce a

**Figure 8.** *Schematic diagram of spray coating process.*

*Thin Film Solution Processable Perovskite Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.106056*

completely wet substrate is to use a pre-heated substrate, which lowers the surface tension and reduces the contact angle between the solution and the substrate [93]. Furthermore, the type of nozzle utilised, the pressure of the gas jet propeller, the distance between the nozzle and the substrate, and the temperature of the pre-heated substrate all have a significance in obtaining a uniform deposited film. An ultrasonic spray coater was utilised to create a high-quality smooth perovskite coating with homogeneous droplets.

#### **3.5 Ink-jet printing**

Ink-jet printing is a common method for fabricating electrical devices, particularly optoelectronics. This approach has a cost advantage over others due to its maskless on demand printing and, more crucially, contactless high-resolution printing. The printing procedure operates in the same way as spray coating. A piezo electronic transducer and a detector are also used to control the droplet size and trajectory, enabling this system to deposit material with precise patterning [94]. This technique is divided into two groups based on how ink droplets are emitted. Continuous ink-jet printing (CIP) and drop-on-demand ink-jet printing is two of them (DOD).

As the name implies, in continuous ink-jet printing, the solution droplet flows continuously towards the substrate under the influence of gravity. When the droplets fall from the nozzle, they acquire up an electric charge. The charged droplets are subsequently sent through a deflection coil, which directs them. A tiny voltage is applied between the nozzle and the ground to achieve this. **Figure 9(a)** depicts a schematic of the complete printing process. In this method, a piezoelectric transducer (PZT) provides an appropriate frequency for regular separation of the droplets, and the separation force involved is surface tension. When no printing is necessary, this approach also recycles the depositing material by collecting it in a reservoir [95]. Aside from that, because it is a non-contact printing technique, it enables excellent film deposition on both rough and curved substrates.

Again, DOD printing is a modern, high-precision printing technology that can print with a single drop precession, allowing it to save a lot of material that would otherwise be wasted throughout the CIP process which is depicted in the **Figure 9(b)**. In this technology, a computer programme controls the movement of either the printing head or the substrate. Due to the contraction of the ink contain volume, the printing material is ejected out the nozzle at a definite pressure pulse. The pressure pulse production method is split into two parts: piezoelectric DOD and thermal DOD.

In piezoelectric DOD, an ink droplet is created by passing an impulse current over the transducer, causing PZT to deform mechanically. The majority of the printing industry employs this printing process because it allows for variable actuation pulses to adjust the velocity and size of ink droplets released from the nozzle. The thermal DOD, on the other hand, uses thermal evaporation to create the ink droplet. This is accomplished by passing electricity through the small resistive heater. When the temperature rises above the boiling point of the printing ink, the vapour entrapment causes bubbles to form. When the heater's power is turned off, the bubble begins to collapse because of heat transfer to the surrounding tank depending on the temperature difference [96]. Due to the difficulty in generating ink bubbles for high vapour pressure solution ink, this approach is not appropriate for large-area printing.

#### **3.6 Screen printing**

The screen-printing process is used to print a pattern that has previously been created on a thread or steel mesh. When the ink has a high viscosity, this approach provides the best printing pattern. The printing ink is spread over the patterned mask with a squeegee, which prints the pattern on the substrate. Due to the high viscosity of the printing fluid, this method produces a somewhat thick film [97]. **Figure 10** depicts a schematic diagram of the printing process. Depending on the printing technique, this printing technology is divided into two categories: flatbed and rotary screen printing.

The printing is done in a stepwise manner in the flatbed method, with the screen held extremely close to the top of the substrate and the paste transferred over the screen by the squeegee. The screen is then lifted or transferred to continue the printing process over the entire substrate after the printing is completed. For roll to roll and large-area printing, the recurrence of this procedure is not suitable. However, rotary screen printing is a low-cost, high-efficiency large-area printing process for generating rapid, precise patterns. The squeegee and paste are stored in a folded tube in this way. The stationary squeegee constantly spreads the paste across the substrate

Printed structures

#### **Figure 10.** *Schematic diagram of screen-printing process and printed patterns.*

*Thin Film Solution Processable Perovskite Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.106056*


#### **Table 4.**

*A comparison of the performance of several perovskite large area modules prepared utilising various large scale techniques.*

through the mesh as the tubular screen rotates with the substrate, allowing for complete printing [98].

**Table 4** shows some of the best results obtained by all of the large-scale deposition processes.

Not only these methods are utilised to fabricate active perovskite layers, but they also provide high-quality deposition of all transport layers, including metal electrodes. However, the type of solvent employed in the precursor solution, the solute concentration, printing speed, surface tension, and viscosity of the solution ink, among other factors, all play key role in achieving high-quality pin hole-free morphological film deposition.
