*2.6.2 Stability*

H2O and O2 have a direct impact on PSC device performance and stability throughout film deposition, device testing, characterisation, and storage. In the case of MAPbI3, water vapour can dissolve the perovskite material, and MAI is dissolved to generate a combination of MA and HI; however, HI will either react with O2 to form H2O and I2, or self-decompose. After being exposed to moisture, MAPbI3 continues to degrade. The inability hinders the commercialization of PSC devices. Improving long-term stability requires adjusting the ABX3 perovskite content and boosting crystalline quality [42, 63]. Long-chain organic cations have been routinely used as a site cation substitution in this regard. The organic and inorganic layers alternatively form a layered structure in these 2D perovskites, which have great long-term stability. As long-chain organic cations are hydrophobic, the bigger organic cations in the 2D perovskite crystal structure improve humidity stability. Pure 2D PSCs are stable, but have a low PCE. Combining 3D perovskite and 2D perovskite yields exceptional optoelectronic characteristics and stability. The intrinsic performance of the 2D perovskite deposited using a hot-casting approach can be maintained for a long time, showing greater stability in humid and other environmental conditions [64, 65].

#### **2.7 Two-step coating**

Mitzi et al. first introduces the two-step coating method in order to improve the morphology and quality of the active perovskite layer [66]. Coating is done in the first phase by using conventional spin coating process, and the second step uses other coating methods such as immersion and spin coating depending on the materials' requirements.

#### *2.7.1 Immersion method*

The organic and inorganic components of the perovskite material are treated separately in the immersion method. The perovskite material's inorganic salt (PbI2) is first spin coated on the substrate at a particular RPM. The spin-coated PbI2 film substrate is then immersed in an organic salt (MAI) precursor solution for a period of time. After taking the substrates from the precursor solution, rinse them using the same solvent and concentration that was used to prepare the organic precursor solution. This step is used to remove any surplus organic material from the substrate's surface. Finally, the ultimate perovskite film is obtained by annealing the substrate for a few minutes at a specific temperature. The procedure is depicted schematically in **Figure 3**. Grätzel et al. used this strategy for the first time in 2013 to obtain MAPbI3 film in order to optimise the morphology of active perovskite material for photovoltaic devices [67].

The concentration of inorganic compound in the spin coated film, the concentration of organic salt in the precursor solution, and the immersion duration all seem to have a significant impact on the growth of the perovskite film, which defines the morphology and quality of the ultimate perovskite film.

After immersion in the precursor solution, two distinct reaction pathways convert the spin coated film to the ultimate perovskite film. The first is the solid-liquid interface conversion reaction, which happens when the precursor solution concentration is low. Due to the low concentration of the MAI precursor solution, MAI tends to diffuse into the PbI2 structure during immersion of the PbI2 coated film. Finally, the reaction of MAI with the PbI2 film at the interface produces the ultimate MAPbI3 perovskite film. The conversion reaction at the solid-liquid interface is show by Eq. (1).

$$\text{PbI}\_2\text{(s)} + \text{CH}\_3\text{NH}\_3^+\text{(sol)} + \text{I}^\cdot\text{(sol)} \rightarrow \text{CH}\_3\text{NH}\_3\text{PbI}\_3\text{(s)}\tag{1}$$

**Figure 3.** *Schematic diagram of immersion coating process of perovskite (MAPbI3) film.*

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

In the above reaction initially, MAI diffuses into the PbI2 structure as well as the reaction start occurs at the interface of PbI2. Once the perovskite crystal is formed at the surface of PbI2 the diffusion of the organic cation MAI into the PbI2 structure is at a standstill and the conversion reaction is completed [68].

When the concentration of organic cation is high enough (>10 mg/mL), another reaction called dissolution-recrystallization evolution mechanism occurs. As the concentration of organic cation in this process is high, it causes rapid crystallisation of perovskite on the surface of PbI2. As a result, organic cation diffusion into the PbI2 structure is totally stopped after a few times, as stated by Eq. (2). According to Yang et al., a high concentration of the organic cation MAI leads to production of the lead iodide complex PbI4 2−, as shown in the Eq. (3). The excess iodine in the system began to dissolve previously created MAPbI3 crystals and PbI2 components that had not been covered during the crystallisation process, and the reaction continued until the system could not reach thermal equilibrium [69].

$$\mathrm{CH\_3NH\_3Pbl\_3(s)} \star \mathrm{I^\cdot(sol)} \rightarrow \mathrm{CH\_3NH\_3^+(sol)} \star \mathrm{PbI\_4^{2-}(sol)}\tag{2}$$

$$\text{PbI}\_2\text{(s)} + 2\text{I}^\cdot\text{(sol)} \rightarrow \text{PbI}\_4^{2-}\text{(sol)}\tag{3}$$

Following these two reactions, when the concentration of excess PbI4 2− approaches the supersaturation state, the gradual re-crystallisation of MAPbI3 via the reaction mechanism shown in Eq. (4) begins anew.

$$\text{PbI}\_4^{2-} \text{(sol)} \text{+CH}\_3\text{NH}\_3^+ \text{(sol)} \rightarrow \text{CH}\_3\text{NH}\_3\text{PbI}\_3 \text{(s)} + \text{I'} \text{(sol)}\tag{4}$$

#### *2.7.2 Sequential spin coating method*

The two-step spin coating method, also known as sequential spin coating, is a well-studied laboratory technique for perovskite thin film deposition. Im et al. presented this technology in 2014 to fabricate high efficiency (>16%) perovskite solar cells by growing cuboid perovskite grains of controlled size [70]. The organic precursor is spin coated above the inorganic layer at a certain spin-rpm in this technique, which starts with the inorganic part of the perovskite material being spin coated onto the substrate. The final perovskite thin film is obtained by periodically annealing the substrate, which exhibits the perovskite film's growth by changing colour during the

process. As long as the concentration of organic precursor is high enough, the dissolution crystallisation mechanism produces the resulting perovskite film. **Figure 4** illustrates the perovskite film fabrication process using this technique.

Huang et al. devised a two-step spin coating process to produce a perovskite film with fewer pinholes at low temperatures. The inorganic precursor is spin coated onto the substrate and dried on the hot plate in the first stage. After that, the inorganic film substrate is spin coated with the organic precursor solution. The solvent used to dissolve the organic component of the perovskite material should have a low solubility of the inorganic part in this approach, such that no inorganic part washed out during the organic precursor coating. Due to the inter-diffusion of spin coated organic and inorganic assembly layers, the substrate ultimate compact, pin-hole free perovskite film is achieved after thermal annealing [71].

Panzer et al. revealed the reaction mechanism for obtaining the ultimate perovskite layer using a two-step spin coating method. According to their findings, the reaction mechanism can be broken down into five steps: perovskite capping layer formation, change in organic layer concentration during solvent evaporation, initial caping layer dissolution, rapid dissolution recrystallization, and complete conversion of residual PbI2 to perovskite crystal. The perovskite capping layer begins to form immediately in the second step of the spin coating process, i.e., on the surface of PbI2 after the introduction of MAI, which initiates the formation of the MAPbI3 crystal. The size of perovskite grains and the time it takes to produce them both decrease as the MAI concentration rises [72]. Furthermore, the reaction temperature has a major impact on the production of perovskite crystals and their grain sizes. Solvent evaporation occurs in the organic precursor during thermal annealing. During this method, a dense perovskite crystal layer is first generated on top of the PbI2 layer to prevent MAI from penetrating the PbI2 structure and preventing further perovskite crystal formation in the system [72]. The concentration of MAI solution continues to rise due to the suppression of MAI solution penetration onto PbI2 and the volatile nature of its precursor solvent. As the concentration of MAI rises, so does the concentration of iodine ions, which react with the perovskite layer on the surface of PbI2 to generate complex lead iodine PbI4 2− as a by-product. The degradation of the previously produced perovskite layer occurs primarily at grain boundaries and smaller grains in this process [72]. Finally, when the concentration of volatile PbI4 2− approaches the supersaturation point it start recrystallisation of perovskite grain with all the uncovered inorganic component PbI2 converted into MAPbI3 crystal.

Two-step coating, also known as sequential deposition, has a number of advantages, including increased processing window flexibility. Solvent engineering, concentration variation, annealing time, annealing temperature adjustment, spin rpm variation etc. can be used to optimise each deposition stage separately, resulting in a superior morphological perovskite film. Furthermore, depending on the solubility of the additive, different additives can be doped directly into the precursor solution to form a defect-free perovskite layer. These approaches also produce a high-quality perovskite film with great reproducibility. When comparing the performance of solar cell devices, sequential deposition delivers a comparable efficiency to the single-step deposition procedure.

Aside from these benefits, the two-step deposition process has a few disadvantages. The formation of a perovskite layer in this approach relies heavily on molecular exchange. The organic component is frequently overreacted in the second step of the deposition technique, making it difficult to precisely control the chemical composition of the film [73]. Another issue with this approach is incomplete inorganic

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


**Table 3.**

*Photovoltaic parameters of different perovskite material using two step deposition method.*

compound conversion to perovskite crystal, which affects overall molecular exchange with the organic component. **Table 3** shows some of the best results obtained using these two-step coating methods.

The methods described above are only suitable for making solar cells on a laboratory scale because a large amount of precursor solution is wasted during the spinning process, and the deposited film does not provide uniformity across the entire area of the film, resulting in device performance degradation.

### **3. Techniques for large area device fabrication**

The performance of PSCs in large area modules must be maintained for practical implementation in the industrial environment. The methods described above are exclusively used to create small-scale laboratory devices. The performance of a perovskite solar cell declines as the active area grows. With the scaling up of device area, the PCE value reduces by 0.8% in most large-scale devices [83]. The PCE is lost due to an increase in series resistance caused by the huge transparent substrate's resistance. Furthermore, when the active area of the device grows, it becomes more difficult to maintain homogeneity across all layers, which has a significant impact on its PCE. The scaling up of perovskite solar cells with morphology manipulation for high quality pin hole free perovskite layer is the most significant and challenging of all the deposition layers.

Several attempts have been made to scale up PSCs towards commercialization. For the fabrication of large area devices, numerous solution-based scalable deposition processes have been developed that can retain overall good device performance. Blade coating, slot die coating, bar coating, spray coating, and inkjet printing, screen printing are examples of these processes.

#### **3.1 Blade coating**

For the printing of large area devices, the blade coating process is a relatively simple and inexpensive technique. The blade is utilised to spread the precursor solution throughout the substrate. A micrometre screw is used in the system to adjust the distance between the blade and the substrate by rotating it, allowing the thickness, homogeneity, and crystallinity of the deposited layer to be determined. This approach can be utilised to deposit not just the active perovskite layer, but also other charge transport layers [84]. **Figure 5(a)** illustrates a schematic illustration of the deposition procedure. This approach allows for a slower drying of the wet film, allowing for a broader coverage and higher-quality perovskite film. The creation of a bigger grain perovskite coating as a result of the slow solvent drying process enhances the carrier diffusion length and thus the device's PCE [85]. In the film creation process of these technologies, two significant regimes exist: the evaporation regime and the Landau-Levich regime, which are depicted in **Figure 5(b)** and **(c)**, respectively.

The thickness of the film tends to decrease in the evaporation regime as the substrate's movement speed increases. This occurs as a result of the solute's shorter residence duration, which results in a lower accumulation amount. In the Landau-Levich regime, the thickness of the deposited layer grows as the coating speed increases, owing to the viscous force that pulls the liquid film out and then dries [86]. The Landau-Levich regime is advantageous in terms of practical use. However, if the solvent present in the precursor solution has a high surface-tension, the perovskite material can generate islands with a high roughness surface [87].

#### **3.2 Slot die coating**

The slot die coating process is similar to blade coating in that it uses a different coater to apply a thin and homogeneous film. The coater is made up of a head that includes a downstream and upstream die. The precursor solution is initially pushed to the die head using a syringe pump in this procedure. The solution forms a liquid bridge between the head and the substrate during the procedure. As the substrate begins to move, a moist layer of solution forms. The flow rate of the solution, coating

#### **Figure 5.**

*(a) Schematic diagram of blade coating process, (b) schematic diagram of evaporation regime, (c) schematic diagram of Landau-Levich regime.*

#### **Figure 6.**

*(a) Schematic diagram of slot-die coating process, (b) schematic diagram of gas quenching annealing of slot-die coated film.*

speed, spacing between the head and the substrate, viscosity of the solution, and other factors all influence the quality and shape of the deposited film in this process [88]. **Figure 6(a)** depicts a schematic diagram of the coating process. This method can be used to deposit the inorganic part of the perovskite material in the first step of sequential deposition. Again, annealing can be done by N2 gas quenching in this process for solvent evaporation to fabricate a pin hole free perovskite layer which is shown by **Figure 6(b)**. It is the only approach that has shown perovskite solar cell role-to-role manufacturing.

Apart from the many advantages of using the slot die coating method to fabricate high-quality large module perovskite films, it also has some failure mechanisms that prevent the development of good morphological films. The failure mechanism includes a low flow limit of the solution, which causes a breakup of the downstream meniscus, resulting in a discontinuity in the wet film [89]. The creation of an air entrainment defect in the wet film is caused by the breaking of upstream meniscus bubbles inside the film. Flooding or dripping occurs when the ink flow to the head is greater than the coating speed, leading in a progressive build-up of ink at the coating head, preventing the desired film thickness from being attained.

#### **3.3 Bar coating**

A well-known method for producing high-efficiency PSCs is bar coating, often known as D-bar coating. The procedure is similar to blade coating, but it spreads the solution across the substrate with a cylindrical bar. In this case, the precursor solution is put onto a bar with a small cylindrical tube. As the substrate moves below the cylindrical bar, a wet film is formed due to the solution passing through the cylinder's wire gap [90]. **Figure 7** depicts a schematic diagram of the coating procedure. After annealing, the deposited solution is converted into the ultimate film. The deposited film thickness and quality is directly proportional to the amount of solution passing through the wire gap of the cylinder and the type of solvent used to prepare the

**Figure 7.** *Schematic diagram of Bar coating process.*

solution in this technique, providing for high repeatability and minimal precursor solution loss [91].
