**1. Introduction**

Perovskites are among the essential material science topics in the last decades due to their low-cost, solution-processed devices and exceptional optoelectronic properties [1–28]. The most studied compositions are represented by the formula ABX3 (organic cation A is larger than the metal cation B, and X is a halide anion). For example, methylammonium lead tri-iodide (CH3NH3PbI3 or MAPbI3), the other halide variants such as CH3NH3PbBr3 and mixed halides, CH3NH3PbI3-xClx [29–31]. The most advantages of 3D-perovskite (CH3NH3PbI3) are combining direct bandgap with high molar extinction coefficient (≈104 –105 M−1 cm−1), low trap densities, low exciton binding energies (≈10–50 meV), which cause long-range free-carrier diffusion lengths (≈100 nm). The perovskite is unique in such a way that its efficiency boosted up from 3.8% to 23.7% in just few years as compared to all other types of traditional solar cells. However, the lack of durability of these materials (hydrophilic cations) due to thermal instability and degradation upon exposure to humidity, U.V. radiation, and the electric field is still a significant barrier to

commercialization [32–34]. 3D-perovskite solar cells' performance decreases due to ion migration and segregation, the ionic nature of the materials, and their low formation energies, making them vulnerable to water-induced hydrolysis [35]. The instability issues associated with perovskite materials have been overcome by using additives, introducing intermediate phases, encapsulating the layers (to avoid spreading Pb toxicity into the external environment), etc. [36–38]. Several studies have also shown that mixed cations and halides tend to enhance perovskites' stability and efficiency.

A 2D Ruddlesden–Popper perovskite has a general formula of A2Bn − 1MnX3n + 1, where "A" represents a large-sized organic cation [39]. Incorporating a bulky organic cation (2D) into the 3D-perovskite layer's crystal lattice passivates the vulnerable 3D-perovskite against oxygen and moisture intrusion, resulting in enhanced stability while maintaining the efficiency of 3D-Perovskite solar cell [40]. 2D-perovskites are very stable but have larger bandgaps and higher exciton binding energies than 3D-perovskites. On the other hand, the exceptional stability under heat and light soaking conditions of low-dimensional perovskites makes them essential to protect the highly efficient 3D [41]. Phenyl ethyl ammonium iodide (PEAI) as a bulky cation in 2D-perovskite was investigated and fabricated as a layered 2D/3D structure demonstrating an impressive PCE 20.1% with 85% PCE retention after 800 h in ambient conditions by Cho et al. [40]. The perovskite absorber composition has been optimized using a 2D-perovskite, and stable performance over 12000 h without the HTM was observed [42].

The 2D-perovskite solar cells (PSCs) have shown superiority over 3D ones, such as improved stability towards humidity and light, improved processability, long-term durability, and higher chemical versatility. All this makes 2D-perovskites a promising alternative and one that has attracted substantial attention over the past few years. The ability of 2D-perovskites to incorporate bigger, less volatile, and generally hydrophobic organic cations; which makes the materials with improved thermal and chemical stability. Furthermore, the ability to use an enormous variety of organic cations and various metals, halides, and combinations of all of the above make this family of materials can be employed in different applications such as solar cells and others [43, 44]. Though there are several approaches to stabilize the 3D-perovskite, the most common one is cation tuning. The bigger cations beyond the Goldsmith tolerance limit produce low dimensional perovskites at least 2.5 eV, restricting photons conversion to less than 500 nm. 2D-perovskites are very stable, but unfortunately they have larger bandgaps and higher exciton binding energies (≈300 meV), penalizing output photovoltage and, therefore, the power conversion efficiencies [45, 46]. Improved instability in metal halide PSCs is one of the most interesting issues to open the door for them towards commercialization. The reduced-dimensional perovskites (RDPs) have shown increase in ambient and operating lifetimes of PSCs. In other words 2D/3D heterostructure PSCs consisting of a thin layer of RDPs a top and a 3D active layer, improved both stability and efficiency compared to pure 3D counterparts [31, 47, 48], as shown in **Figure 1(a)**. Indeed, 2D/3D engineering aimed to combine both advantages, namely the outstanding optoelectronic properties of the 3D-perovskite and the 2D Ruddlesden–Popper phase's high robustness. Recently, based on the 2D/3D heterostructured lead halide perovskites, high-efficiency stable PSCs with an average PCE of 17.5 ± 1.3% was demonstrated. The "post-burn-in" efficiency could be retained over 80% after the air's operation for 1000 h and encapsulated process for 4000 h. Therefore, the performances of PSCs are brought closer to meeting the commercial requirements [49].

In this chapter, the 2D, 3D, and 2D/3D hybrid systems for perovskites will be discussed, focusing on the crystal structure, optoelectronic properties, synthesis methods, and layer orientation. Finally, application in regular or inverted structure

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solar cells.

**Figure 1.**

**2. 3D-perovskites**

**2.3 Device fabrication methods**

**2.1 Structures**

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

PSCs which remain to be addressed are herein highlighted while giving the outline on the perspectives of 3D and 2D/3D perovskites for high efficiency stable

*(a) Dimensional mixed 2D/3D increases the soaking time and the stability of perovskite solar cells. Reprinted with permission from Ref. [31]. (b) The perovskite unit cell consists of an a cation (red) at the center, B cations in the corners (blue), and X anions (green) on the edges. The B cation forms an octahedral with the surrounding X anions (all eight octahedral are shown). Reprinted with permission from Ref. [50].*

Perovskite materials generally contain a cubic unit cell with the general formula ABX3. Cation A, which is larger than cation B, is in the center of the unit cell. The B cations are in every corner of the unit cell; Cation B also serves as the center of an octahedron with an X anion surrounding cation B, corner-sharing between each cation B. As shown in **Figure 1(b)**, the full picture of cation A is surrounded by eight octahedra, each of which contains a cationic center B and anions X. In this orientation, the cubic structure of the perovskite has 6-fold the coordination number for cation A and 12-fold the coordination number for cation B. It should be noted that the ionic radii are quite crucial in maintaining a stable cubic unit cell.

**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%.

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

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

**Figure 1.**

*Solar Cells - Theory, Materials and Recent Advances*

ity and efficiency.

commercialization [32–34]. 3D-perovskite solar cells' performance decreases due to ion migration and segregation, the ionic nature of the materials, and their low formation energies, making them vulnerable to water-induced hydrolysis [35]. The instability issues associated with perovskite materials have been overcome by using additives, introducing intermediate phases, encapsulating the layers (to avoid spreading Pb toxicity into the external environment), etc. [36–38]. Several studies have also shown that mixed cations and halides tend to enhance perovskites' stabil-

A 2D Ruddlesden–Popper perovskite has a general formula of A2Bn − 1MnX3n + 1,

where "A" represents a large-sized organic cation [39]. Incorporating a bulky organic cation (2D) into the 3D-perovskite layer's crystal lattice passivates the vulnerable 3D-perovskite against oxygen and moisture intrusion, resulting in enhanced stability while maintaining the efficiency of 3D-Perovskite solar cell [40]. 2D-perovskites are very stable but have larger bandgaps and higher exciton binding energies than 3D-perovskites. On the other hand, the exceptional stability under heat and light soaking conditions of low-dimensional perovskites makes them essential to protect the highly efficient 3D [41]. Phenyl ethyl ammonium iodide (PEAI) as a bulky cation in 2D-perovskite was investigated and fabricated as a layered 2D/3D structure demonstrating an impressive PCE 20.1% with 85% PCE retention after 800 h in ambient conditions by Cho et al. [40]. The perovskite absorber composition has been optimized using a 2D-perovskite, and stable

performance over 12000 h without the HTM was observed [42].

The 2D-perovskite solar cells (PSCs) have shown superiority over 3D ones, such as improved stability towards humidity and light, improved processability, long-term durability, and higher chemical versatility. All this makes 2D-perovskites a promising alternative and one that has attracted substantial attention over the past few years. The ability of 2D-perovskites to incorporate bigger, less volatile, and generally hydrophobic organic cations; which makes the materials with improved thermal and chemical stability. Furthermore, the ability to use an enormous variety of organic cations and various metals, halides, and combinations of all of the above make this family of materials can be employed in different applications such as solar cells and others [43, 44]. Though there are several approaches to stabilize the 3D-perovskite, the most common one is cation tuning. The bigger cations beyond the Goldsmith tolerance limit produce low dimensional perovskites at least 2.5 eV, restricting photons conversion to less than 500 nm. 2D-perovskites are very stable, but unfortunately they have larger bandgaps and higher exciton binding energies (≈300 meV), penalizing output photovoltage and, therefore, the power conversion efficiencies [45, 46]. Improved instability in metal halide PSCs is one of the most interesting issues to open the door for them towards commercialization. The reduced-dimensional perovskites (RDPs) have shown increase in ambient and operating lifetimes of PSCs. In other words 2D/3D heterostructure PSCs consisting of a thin layer of RDPs a top and a 3D active layer, improved both stability and efficiency compared to pure 3D counterparts [31, 47, 48], as shown in **Figure 1(a)**. Indeed, 2D/3D engineering aimed to combine both advantages, namely the outstanding optoelectronic properties of the 3D-perovskite and the 2D Ruddlesden–Popper phase's high robustness. Recently, based on the 2D/3D heterostructured lead halide perovskites, high-efficiency stable PSCs with an average PCE of 17.5 ± 1.3% was demonstrated. The "post-burn-in" efficiency could be retained over 80% after the air's operation for 1000 h and encapsulated process for 4000 h. Therefore, the performances of PSCs are brought closer to meeting the commercial requirements [49]. In this chapter, the 2D, 3D, and 2D/3D hybrid systems for perovskites will be discussed, focusing on the crystal structure, optoelectronic properties, synthesis methods, and layer orientation. Finally, application in regular or inverted structure

**164**

*(a) Dimensional mixed 2D/3D increases the soaking time and the stability of perovskite solar cells. Reprinted with permission from Ref. [31]. (b) The perovskite unit cell consists of an a cation (red) at the center, B cations in the corners (blue), and X anions (green) on the edges. The B cation forms an octahedral with the surrounding X anions (all eight octahedral are shown). Reprinted with permission from Ref. [50].*

PSCs which remain to be addressed are herein highlighted while giving the outline on the perspectives of 3D and 2D/3D perovskites for high efficiency stable solar cells.
