**2.2 Applications in regular or inverted structured perovskite solar cells**

As mentioned, before we have n-i-p typical structure and p-i-n inverted structure. In 2018, Hua Dong et al. [51] applied CH3NH3PbI3 film in highly efficient inverted planar heterojunction perovskite solar cells obtaining an efficiency of 17.04%. In 2020, Shuai Gu et al. [52] applied tin and mixed lead in tin halide perovskite tandem solar cells with a power conversion efficiency (PCE) over 25%.

## **2.3 Device fabrication methods**

There are two ways to fabricate a PSC, solution and vacuum processing: (i) Spin coating is a solution deposition technique that uses high rotation speeds, as shown in **Figure 2(a** and **b)** [18, 26, 28]. A device rotates the substrate while a drop of the precursor solution is placed on the substrate. The high speed distributes the solution evenly on the substrate. After the material is deposited, the substrate is heated to evaporate and remove the solvent. This step is called the annealing step and the perovskite film is formed after removing the solvent. There are two methods of spin coating: a one-step and a two-step spin coating. In single-stage spin-coating, the

**Figure 2.**

*(a) Method of one-step spin coating. (b) Method of two-step spin coating (c) a diagram of the CVD technique. Reprinted with permission from Ref. [53, 55]*

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 substrate [7, 53].

(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

**167**

MeTAD/Au as shown in **Figure 4**.

*Mixed 2D-3D Halide Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.97684*

be used together to achieve the desired results [56].

**3. 2D-perovskites**

**3.1 Structures**

added to the solvent. Second, the process is relatively easy to scale up for large-scale manufacture. In addition, perovskite films produced with this technique have good uniformity, non-porous films, large grain sizes, and a long carrier life. Two parameters are used to optimize the vacuum deposition technique, deposition time and temperature. Deposition time determines the amount of material deposited on the substrate, so film thickness is the main effect of deposition time. The climate is essential for vacuum deposition technology. The temperature should be high enough to vaporize the materials, but most importantly, the temperature should not be high enough to melt the substrate. Adjusting the position of the substrate and solid materials in the tube will help heat the materials sufficiently and prevent the substrate from melting. However, the temperature and location of placement must

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

The general chemical formula of the 2D-perovskite is A2Bn − 1MnX3n + 1, where A can be a monovalent or divalent organic cation that intercalates between the inorganic A*n* − 1BnX3*n* + 1 2D sheets works as a spacer between the inorganic cation as shown in **Figure 3(a)**. *n* is the thickness or the number of the inorganic layers and

2D-halide perovskite layers are conceptually obtained by cutting along the crystallographic planes <100>, <110> or < 111> of the 3D-perovskite structure [59] as shown in **Figure 3(b)**, so we can classify the 2D perovskite depending on cutting the shape of the 3D-perovskite into <100>, <110>, and < 111> − oriented perovskites. Cutting layers along <110> direction (can be derived from the face diagonal) and along <111> direction (can be derived from the body diagonal) are less common in 2D-halide perovskites. Unlike these two types, <100> perovskites are the most common type of 2D-halide perovskites and are commonly used in solar cells. The general formula of <100> − oriented 2D-perovskites is A2Bn − 1MnX3n + 1, and their inorganic sheets are obtained by taking n-layers along the 100 direction of the 3D-perovskites. The <100> − oriented 2D-perovskites can be divided into two commonly used types. The first is Ruddlesden-

In Ruddlesden-Popper (R.P.), the most used and studied type (owing to its superior ambient stability [62]) has the chemical formula A2Bn-1MnX3n + 1. Each inorganic layer is confined between bilayers of bulky ammonium cations. The relatively weak van der Waals forces between the alkyl chains separating the layers generate a 2D structure. In 2017, Xiaoyan Gan and co-workers fabricated a 2D-perovskite (PEA)2(MA)n-1PbnI3n + 1 (phenylethylammonium = PEA, n = 1, 2, 3) with incorporation of TiO2 nanorod arrays into a solar cell harvesting efficiency of 3.72% [60] with a structure of glass/FTO/TiO2 as compact layer/(PEA)2(MA)m-1PbmI3m + 1/spiroO-

orientation, and applications in regular or inverted structure PSCs.

(n = 1 at the divalent A, and n = 2 at the monovalent A) [43].

Popper (R.P.), and the second is Dion-Jacobson (D.J.) [60, 61].

## *Mixed 2D-3D Halide Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.97684*

added to the solvent. Second, the process is relatively easy to scale up for large-scale manufacture. In addition, perovskite films produced with this technique have good uniformity, non-porous films, large grain sizes, and a long carrier life. Two parameters are used to optimize the vacuum deposition technique, deposition time and temperature. Deposition time determines the amount of material deposited on the substrate, so film thickness is the main effect of deposition time. The climate is essential for vacuum deposition technology. The temperature should be high enough to vaporize the materials, but most importantly, the temperature should not be high enough to melt the substrate. Adjusting the position of the substrate and solid materials in the tube will help heat the materials sufficiently and prevent the substrate from melting. However, the temperature and location of placement must be used together to achieve the desired results [56].
