**3. 2D-perovskites**

*Solar Cells - Theory, Materials and Recent Advances*

solution contains all the chemicals deposited on the substrate. With a two-stage spin coating, organic and inorganic chemicals are deposited separately on the substrate. For example, CH3NH3PbI3 perovskite material can be deposited onto a substrate by one-step or two-step spin-coating methods. In the one-step process, CH3NH3I and PbI2 are mixed in solution with the solvent (dimethyl formamide (DMF)), and the solution is spin-coated onto the substrate. In the two-step method, PbI2 is dissolved in the solvent (DMF) and spin-coated onto the substrate. Then, CH3NH3I is dissolved in the solvent (isopropanol (IPA)) and spin-coated onto the PbI2-coated

*(a) Method of one-step spin coating. (b) Method of two-step spin coating (c) a diagram of the CVD technique.* 

(ii) Vacuum treatment is a technique in which a CVD (Chemical Vapor Deposition) machine is used to achieve high temperatures in a glass housing [54]. Gases can flow through the pipe ends through the glass holder. This property is commonly used to achieve desired pressures or add reactive gas to the system. CVD has a temperature gradient along the tube so that the positions near the center are warmer than the positions near the ends of the tube. This temperature gradient is a critical aspect of the CVD manufacturing technique. The technology begins with the selection of the materials for the solar cell. The substrate is placed near the end of the tube and the materials for the solar cell are placed in a solid phase towards the center. When the CVD machine reaches the appropriate temperature, the solids in the center of the tube evaporate. An inert carrier gas such as argon flows through the tube and pushes the vaporized solids away from the substrate. The substrate has a lower temperature and causes condensation of the evaporated materials when they meet the substrate. This causes the materials to deposit on the substrate and form a thin layer. A diagram of the CVD technique is shown in **Figure 2(c)** [55]. The CVD technique can be used in several steps to deposit each material layer separately if desired. CVD technology has two distinct advantages over spin coating and other processes. First, the film layer produced is exceptionally clean since no solvent was used in the process, eliminating the possibility of impurities being

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substrate [7, 53].

**Figure 2.**

*Reprinted with permission from Ref. [53, 55]*

Despite the high performance of the 3D-perovskite [57], which qualified it to be a strong competitor to the various other types of solar cells, the stability or the ability of 3D to resist various factors of humidity, heat, and so on represent a critical issue in the direction of the possibility of becoming commercial [48]. Although the researchers' focus was first on 3D, they turned to 2D to solve the stability problem that plagues 3D [58]. In the next section, we are going to talk about the structure of the 2D-perovskites, their optoelectronic properties, preparation methods, layers orientation, and applications in regular or inverted structure PSCs.
