*2.4.1 Additives*

The wettability of the substrate and consequently the film formation is affected by the viscosity and solvation ability of the solvent. The rate of solution evaporation is determined by the solvent's boiling point and vapour pressure. Therefore, a selection of extremely polar aprotic organic solvents is required for a weakly soluble inorganic lead salt. Lewis-base character of some highly polar aprotic organic solvents can cause a solvent-solute coordination, which can alter the perovskite crystallisation process. The colloidal skeleton of the perovskite precursor solution is made up of numerous coordination complexes, and it is viewed as a colloidal cluster with a soft colloidal skeleton [49, 50]. Additives (Cl<sup>−</sup> and Br− ) influence the size of the colloidal clusters. According to Liao et al., adding chlorine to perovskite films improves their optoelectronic capabilities and environmental resilience. The addition of 10% Cl− to a MAPbI3 precursor solution increases film uniformity and coverage greatly [51].

#### *2.4.2 Ageing time and solvent*

The size and shape of colloidal clusters, as well as the nucleation and growth process, can be affected by the ageing duration of the precursor solution. Mohite et al. found a substantial link between film crystallinity and ageing time. The crystallinity and grain size of perovskite films were greatly improved when the precursor solution was aged for more than 24 h, according to the findings. The precursor solution gradually develops big seeds (or crystals) as it matures. Grain development, phase purity, surface uniformity, and trap state density of the perovskite film have all been shown to be considerably affected by precursor ageing. Meanwhile, the phase development and crystalline characteristics of perovskite films are influenced by the solvents and content of the precursor. To deposit perovskite films, Wang et al. employed gammabutyrolactone (GBL) and DMF as co-solvents [36]. The grain size and photovoltaic characteristics of the device produced by the GBL solvent were found to be much lower than those produced by the DMF solvent. Iyer et al. also used DMSO as a Lewis base adduct to control perovskite crystallisation and grain development [37].

#### *2.4.3 Composition and other factors*

Processing parameters such as substrate temperature, rotation speed, and thermal annealing can be used to fine-tune the perovskite film morphology. Through heat energy and centrifugal force, these processing factors influence solute diffusion and perovskite crystallisation behaviour. The residual organic residue is evaporated during thermal annealing. To create a high-quality perovskite layer, Moon et al. employed MAI and lead acetate (PbAc2) as precursors. By using by-product gas (3MAI + PbAc2 MAPbI3 + 2MAAc) to remove the PbAc2 residue, the crystal development can be accelerated to generate a fully covered, pinhole-free, and highly crystalline perovskite film [52]. Janssen et al. used a mixture of PbAc2, PbI, and MAI to make a high-quality perovskite layer. In ambient condition, Huang et al. used methylammonium acetate (MAAc) as a general solvent to produce high-quality perovskite films. To facilitate solvent evaporation, a constant substrate temperature (100°C) was used, resulting in supersaturation and fast nucleation and crystal formation [53].

#### *2.4.4 Atmosphere*

The crystallinity and surface morphology of as-cast perovskite films are affected by various deposition circumstances. One benefit of hot-casting method is that the deposition is not affected by the processing environment. Therefore, the perovskite films may be produced in ambient air, and the device revealed great stability under high humidity [54]. Mori et al. combined a gas flow with hot-casting method to make MAPbI3 films in ambient circumstances (relative humidity = 42–48%) [55]. The flowing gas can greatly expedite mass transfer and eliminate thickness nonuniformity due to the difference in centrifugal force between the centre and edge of the substrate. Yang et al. also used a combination of hot-casting and methylamine (MA) gas treatment to create dense and homogeneous perovskite films at high relative humidity. With MA gas treatment, porous and rough MAPbI3 perovskite films made by hot casting can be turned into dense and high-quality films. In addition, Hao et al. used non-destructive ethanol/chlorobenzene to treat MAPbI3 perovskite films, which resulted in coarsening of the perovskite grains and lateral grain expansion of the MAPbI3 perovskite films. To overcome the challenges of temperature gradient and moisture intrusion during the deposition process, Cheng et al. developed a thermal radiation hot-casting method [56].

#### **2.5 Advantages of hot-casting technique**

### *2.5.1 Grain size, orientation, and film thickness*

To create reasonably thick and preferentially oriented large-grain perovskite films, hot-casting process has been frequently used. The grain boundaries of the perovskite films are reduced due to the higher grain size. At the same time, the absorption, charge transport, and crystallinity of perovskite films are all positively affected. Increases in grain size, on the other hand, may increase the density of unwanted pinholes, resulting in direct contact between the HTL and ETL and leakage current [57]. The device's performance will be severely harmed by the voids in the perovskite coating. A thick perovskite layer helps to gather enough light absorption across the visible light spectrum [58]. To tune the thickness of the perovskite layer, the concentration of the precursor solution and the rotation speed can be varied. The inorganic halide octahedrons are joined at a shared apex and stretched in a 2D direction to form 2D perovskites when a long-chain organic cation layer is placed into the inorganic framework to deviate the tolerance factor from a value of 1. The introduction of an organic chain will make charge extraction and collecting more difficult. Controlling the growth direction of a 2D perovskite film is critical for carrier transport. A hot-casting technique can produce a selectively oriented development of 2D perovskites [59, 60].

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

### *2.5.2 Defects and recombination*

The performance of the PSCs is harmed by nonradiative recombination in the following ways. Radiative recombination occurs when electrons return to the valence band. Minority recombination occurs at the interface as holes (electrons) are transported back to the perovskite layer. The porous perovskite layer will then come into direct contact with the functional layers, resulting in carrier recombination. A nonradiative recombination process is caused by a number of defect states in the device forming recombination centres to trap the carriers. The major avenue for a carrier loss is through some deep-level traps. Furthermore, the nonradiative recombination of carriers within the device has a direct impact on the PSCs' Voc. As a result, forming low-defective perovskite thin films is critical for preventing nonradiative recombination. Large grain sizes help limit the number of grain boundaries in hot-casted perovskite films, which reduces charge trapping [30].
