**2.2 Two-step dip-coating**

*Perovskite and Piezoelectric Materials*

3.thermal evaporation technique

However, deposition of organic-inorganic perovskite materials is often challenging due to different physical and chemical properties of the organic and inorganic parts of perovskite materials [15]. Despite of this, several significant methods are used for thin film deposition of organic-inorganic hybrid perovskites. Various methods used for perovskite deposition are solution-processed (one-step and two-step) deposition, evaporation method, and vapor assisted solution process (VASP) are the

2.two-step sequential deposition (including the vapor-assisted solution process)

One-step processing (spin-coating) is a suitable technique extensively applied for uniform thin film deposition and is based on the co-deposition of both the inorganic and organic components either through solution processing or thermal evaporation. In solution processing, a mixture of both MX2 (M ¼ Pb, Sn; X ¼ Cl-, Br-, I-) and AX (A ¼ methylammonium MA); formamidinium, FA) is dissolved in an organic solvent and deposited through the spin coating to form a film (**Figure 4**), followed by annealing to produce the perovskite layer [15]. The post deposition annealing of the films at low temperature (T < 250°C) is sometimes used to increase phase purity and crystallinity [6]. Spin-coating allows deposition of hybrid perovskites on various substrates, containing glass, quartz, plastic, and silicon. Selection of suitable parameters such as substrate, spin speed and the substrate temperature are essential for this technique and can be selected accordingly. The wetting properties of the solution on the chosen substrate can be improved by pre-treating the substrate with a suitable adhesion agent. The spin-coating technique does not involve cumbersome equipment and it gives high-quality films in quite short time at room temperature. It is considered as a distinct case of solution crystal growth, which results in the formation of highly oriented perovskites layer on a substrate. In order to obtain a layer with the desired thickness, optimization of various parameters such as concentration of perovskites solution, and spin-coating parameters (spin speed, acceleration and spin duration) can be carried out. Generally, 2D homogeneous perovskites films with a thickness ranging 10 nm to 100 nm can be obtained by carefully choosing the parameters. The selection of solvent is also important by considering the solubility for both the organic ammonium and the inorganic lead halide. The usually used solvents for spin coating technique are Dimethylformamide (DMF) or Dimethyl sulfoxide (DMSO) [39]. These spin-coated perovskites films are very reproducible, and this technique is suitable for all PSC structures (mesoporous vs. planar) [39].

typically adopted methods for film deposition [15, 37, 38].

1.one-step precursor solution deposition (Spin-coating technique)

**2.1 One-step precursor solution deposition (spin-coating technique)**

**76**

**Figure 4.**

*Schematic of the spin-coating process [4].*

Mitzi [40] first time reported the two-step dipping technique in 1998, and later by Burschka et al. [41] in 2013. In a two-step dip-coating deposition process, a metal halide PbI2 layer is first deposited by vacuum evaporation or spin-coated on a substrate. Then this coated film is altered into the perovskite by dipping into an organic MAI solution as it is shown in **Figure 4**. This method offered PCE of 15% and certified 14.14% [39]. Suitable selection of solvent is important for the dipping process. The solvent is selected such that can dissolve organic salt but cannot metal halide and the final organic-inorganic perovskite, toluene/2-propanol mixture is an appropriate solvent for the organic salt. The organic cations in solution intercalate into and react with metal halide on the substrate and form a crystalline film [6]. The dipping times are quite short: several seconds to minutes, depending on the system. This method is a suitable method for a variety of inorganic and organics, even if they have an incompatibility in solubility. This process effectively reduces the chemical reaction between the perovskite and the underlying ETL. The development of successive deposition methods has offered a variety of ETL options, though allowing for perovskite films to be prepared successfully at room temperature [15]. In addition, Chen et al. [42] developed a vapor assisted solution processing (VASP) method that used the reaction between MAI vapor and pre-deposited PbI2 to form the completed perovskite film. The resulting MAPbI3 exhibits excellent film quality.
