The 3D-perovskite halides have gained a considerable reputation versus their counterpart semiconductor materials since they achieved a remarkable high-power conversion efficiency of 25.2% within a decade. Perovskite solar cells also have some problems as lattice degradation and sensitivity against moisture, oxygen, and strong irradiation. The perovskite instability is the drawback in front of this emerging technology towards mass production and commercialization. 2D-perovskites, with the general formula A2Bn − 1MnX3n + 1, have been recently introduced to overcome some of the drawbacks of the stability of 3D-perovskites; however, this is at the expense of sacrificing a part of the power conversion efficiency. Mixed 2D/3D perovskites could solve this dilemma towards the way to high stability-efficiency perovskites. The research is expected to obtain highly stable and efficient mixed 2D/3D perovskite solar cells in the few coming years. This chapter reviews 2D-perovskites’ achieved progress, highlighting their properties, current trends, challenges, and future prospects.
- perovskite solar cells
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, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 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, 30, 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, 33, 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 . 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, 37, 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 . 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 . 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 . 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. . The perovskite absorber composition has been optimized using a 2D-perovskite, and stable performance over 12000 h without the HTM was observed .
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 .
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 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.
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.  applied CH3NH3PbI3 film in highly efficient inverted planar heterojunction perovskite solar cells obtaining an efficiency of 17.04%. In 2020, Shuai Gu et al.  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 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 . 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) .
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 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 .
Despite the high performance of the 3D-perovskite , 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 . Although the researchers’ focus was first on 3D, they turned to 2D to solve the stability problem that plagues 3D . 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.
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
2D-halide perovskite layers are conceptually obtained by cutting along the crystallographic planes <100>, <110> or < 111> of the 3D-perovskite structure  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-Popper (R.P.), and the second is Dion-Jacobson (D.J.) [60, 61].
In Ruddlesden-Popper (R.P.), the most used and studied type (owing to its superior ambient stability ) 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%  with a structure of glass/FTO/TiO2 as compact layer/(PEA)2(MA)m-1PbmI3m + 1/spiroOMeTAD/Au as shown in Figure 4.
Dion–Jacobson (D.J.) perovskites adopt a general formula ABn − 1MnI3n + 1. This type has layered structures where the stacking of inorganic layers is unique as they lay precisely on top of another, and this is quite the opposite of Ruddlesden-Popper (R.P.) . The difference between R.P. and D.J. is shown in Figure 5.
In 2018, Sajjad Ahmad and co-workers developed a series of Dion-Jacobson phase 2D-perovskites that record a cell power conversion efficiency of 13.3% with high stability. Unencapsulated devices retain over 95% efficiency upon exposure to ambient air (40–70% relative humidity [R.H.]) for 4,000 hours, damp heat (85°C and 85% R.H.) for 168 hours, and continuous light illumination for 3,000 hours. This device is more stable than the R.P. counterpart, as shown in Figure 6. The improved device stability over the R.P. counterpart is attributed to alternating hydrogen bonding interactions between diammonium cations and inorganic slabs, strengthening the 2D-layered perovskite structure .
3.2 Applications in regular or inverted structure perovskite solar cells
The 2D-perovskites have many applications; it is used in solar cells, light emitting diodes, etc. Here we will concentrate on their applications in the n-i-p normal structure and p-i-n inverted structure solar cells as follows: In 2015, Cao and co-workers fabricated a 2D-perovskite thin-film and recorded an efficiency of 4.02% with a regular structure n-i-p with a device structure FTO/TiO2/2D perovskite/spiro-OMeTAD/Au . In 2016, Hsinhan Tsai and co-workers reported a photovoltaic efficiency of 12.52% with no hysteresis for an inverted structure solar cell FTO/PEDOT:PSS/(BA)2(MA)3Pb4I13/PCBM/Al . In 2018, Xinqian Zhang and co-workers fabricated a vertically orientated highly crystalline 2D (PEA)2(MA)n–1PbnI3n + 1, n = 3, 4, 5) films with the assistance of an ammonium thiocyanate (NH4SCN) additive. Planar-structured PSC with the device structure of ITO/PEDOT:PSS/(PEA)2(MA)4Pb5I16 (
3.3 Optoelectronic properties
In this subsection, we will discuss some of the 2D-perovskite’s optoelectronic properties, like how reducing dimensionality can affect the bandgap, exciton’s influence on charge transport, and so on. In the 2D-perovskites, the large-sized organic cation interlayers can restrict or limit the charge carriers. These organic interlayers act as dielectric regulators, determining the electrostatic force on the electron–hole pairs. The alternating arrangement of inorganic sheets and bulky organic interlayers results in a multiple-quantum-well (MQW) electronic structure . In other words, the inorganic slabs serve as the potential “wells” while the organic layers function as the potential “barriers”  as shown in Figure 7. The high organic and inorganic dielectric contrast leads to a huge electron–hole binding energy (Eb) in 2D perovskites [69, 70]. The confinement effect of 2D-perovskites directly affects the bandgap. For an R.P. hybrid perovskite, the bandgap depends on the good width of layer thickness . The total bandgap energy is determined by the base 3D-structure and extra quantization energies of the electron–hole . The optical bandgap of 2D-perovskite generally decreases as the “n” value increases. For example, the bandgap value for the 2D (CH3(CH2)3NH3)2(CH3NH3)n–1PbnI3n + 1 perovskites decreases with increased layer thickness from 2.24 eV (at n = 1) to 1.52 eV (at n = ∞) due to the quantum-confinement effects associated with the dimensional increase . This flexibility of bandgap tuning can facilitate various optoelectronic applications with targeted optical bandgap materials like tandem solar cells. Those in tandem solar cells, the upper absorber layer needs to have a higher bandgap than the bottom one .
Quasi-two-dimensional perovskites have been shown to have strong excitonic effects, and their structure generally shows a large exciton binding energy (Eb) of several hundreds of meV. This improves the interaction between electrons and holes compared to the exchange in 3D-perovskites . The sizeable binding energy Eb in low “n” 2D-perovskites may be detrimental for charge separation in solar cells. So, the considerable binding energy of excitons is one of the main reasons for declining performance standards.
3.4 Device fabrication of 2D-perovskite films
The finite preparation methods of 2D-perovskite films are different from the multiple preparation methods of 3D-perovskite films.
3.5 Layer orientation
The device performance has been enhanced using the vertical alignment of the inorganic sheets of the 2D-perovskites concerning the substrate. The vertically oriented inorganic slabs provided a direct pathway for charge transport between layers, whereas the bulky organic separators act as electrical insulators hindering out-of-plane conduction of charges . 2D single-layered (n = 1) halide perovskites have shown to align horizontally to the substrate on which they were grown. When “n” was greater than 1, competition arose between horizontal and vertical to the perovskite layers alignment (caused by BA and MA cations). When n = 1–4, it was found that the devices with n ≥ 3 had a better performance than lower “n”, as shown in previous studies [84, 85]. It showed that for BA2MA3Pb4I13, the nucleation process and film growth orientation occurred at the liquid-air interface rather than at the liquid–liquid or liquid–substrate interfaces (since surface tension made nucleation and growth at the liquid-air interface more favorable) and that the initial nuclei are oriented in vertical configuration . With increasing “n”, the probability of obtaining an n-homogeneous-film decreased, giving rise to a mixture of a 3D-like dominant phase with some 2D-perovskite phases. Finally, films with n = 5–10 tend to align perpendicular to the substrate (Figure 9) . This crystal orientation was examined using a scanning electron microscope (SEM), X-ray diffraction (XRD), and grazing incidence wide-angle X-ray scattering (GIWAXS) tests. XRD spectra of (BA)2(MA)n − 1PbnI3n + 1 (n = 1, 2, 3, 4, ∞), with (n = 1) showed (00 k) peaks implying the preferential growth along (110) direction, in (n = 2), (0 k0) reflections occurred in addition to (111) and (222) in the obtained XRD spectra corresponding to in-plane and out-of-plane orientations. In contrast, (111) and (222) reflections were noticed for n = 3 and n = 4 quasi 2D-perovskite films (110) and (220) analogously for n = ∞, which implies the vertical growth orientation (n = 3, 4), increasing towards 3D-perovskite; The preferential layer alignment vanished because few BA cations doped in 3D-perovskite and no influence on orientations (Figure 9) . From (SEM, GIWAXS) it was concluded that the hot-cast films grew along certain orientations, confirmed by the most remarkable reflections of (111) and (202) planes. The inorganic crystal plates <(MA)n − 1PbnI3n + 1 > 2− are perpendicular to the substrate, forming continuous charge transfer channels favorable to charge transport for optoelectronic applications .
4. Mixed 2D/3D perovskites
Downsides of 2D-perovskites can be reduced by mixing 2D with the ordinary 3D-perovskites to form 2D/3D perovskite. 2D/3D designing intends to combine the advantages of 3D-perovskite (high optoelectronic properties) and 2D-perovskite (high stability)  to produce an efficient and stable perovskite material that could contribute to the advancement of PSCs towards commercial industrialization. So, owing to these reasons, the 2D/3D perovskite attracted the researcher’s attention during the last few years .
4.1 Optoelectronic properties
The 2D/3D, in comparison to 2D-perovskite, has higher charge mobility, less non-radiative charge recombination, smaller bandgap as illustrated in Figure 10, higher power conversion efficiency, long-term stability, and in some cases, do not even need encapsulation.
The number of inorganic layers (n) affects the performance parameters of 2D/3D perovskite. The higher the (n) number, the better the performance parameters (as the 2D/3D perovskite gets closer to 3D). Moreover, 2D in 2D/3D can work as a capping layer to protect the 3D layer from air, moisture, heat; It serves as a hydrophobic encapsulation layer. In 2014, Karunadasa et al. introduced the mixed dimensional perovskite in the PSCs for the first time . In this work , a mixture of phenylethylammonium (PEA) and MA cations was prepared to obtain a Ruddlesden–Popper structure of (PEA)2(MA)2[Pb3I10] at (n = 3) with an achieved efficiency of 4.73% . The bandgap of the corresponding 2D and 3D-perovskite was 2.10, and 1.63 eV, respectively. After that, Sargent et al. investigated the efficiency and stability of (PEA)2(MA)
4.2 Device fabrication methods
The 2D/3D multidimensional perovskite can be synthesized via many ways: one-step deposition, two-step deposition, anti-solvent method, the self-assembly method, vapor-assisted solution deposition approach , etc. In the one-step deposition process, the 2D and 3D precursors are mixed, and the layers are grown simultaneously. In a two-step process, the 3D layer is first deposited then the 2D-perovskite is grown on top of it in a consecutive step. In 2019, Zhang and his co-workers reported a 2D/3D perovskite by post-treated n-butylamine iodide (BAI) and the residual PbI2 on a one-step deposited MAPbI3 film. They added a thin 2D-perovskite layer on the top of the 3D-perovskite and grain boundaries. The formed 2D/3D perovskite has the stability of 2D (after three months the perovskite still support 80% of its initial efficiency) and the high performance of 3D (VOC = 1.09 V, JSC = 22.55 mA/cm2 a, FF =0.74, PCE = 18.3%) . The anti-solvent method has been used as an ordinary recipe for getting a 3D-perovskite film . The anti-solvent (e.g., C.B., toluene, etc.) is employed as a second solvent to cause instantaneous nucleation for the 3D film’s fast crystallization. When the anti-solvent is used as a solvent for the 2D organic salt, the in-situ 2D growth method can be practically combined with the usual anti-solvent method, as illustrated in Figure 11. The anti-solvent containing the organic salt for 2D is directly dripped on the substrate, which is wet with a precursor solution of the 3D-perovskite. The dripping process of the anti-solvent is followed by a one-step process. Toluene containing phenylethylammonium iodide (PEAI) was used as the anti-solvent for a 3D/2D perovskite structure .
In the low-pressure vapor assisted solution process, the larger organohalide (PEA) doped with a metal halide (PbI2) is first spin-coated on the substrate and then reacts with the smaller organohalide (MAI) vapor in a low-pressure oven .
4.3 Applications in regular or inverted structure perovskite solar cells
Planer structures are always employed, n-i-p standard structure and p-i-n inverted structure. The researchers used 2D/3D perovskite in these two structures as follows: In 2019, Abbas and his co-workers  developed a way to add a 2D-layered perovskite with MAPbI3 bulk 3D-perovskite by a vertically oriented 2D-layered mixed with a bulk 3D-perovskite they select the anilinium ion as the sizeable organic cation. In this research, they used a normal n-i-p structure and recorded an efficiency up to 16%. The planar solar cell heterojunction with ITO/PEDOT-PSS/perovskite/PCBM/BCP/Ag was fabricated . In 2020, Jia Zhang and Bin Hu recorded a 15.93% efficiency with 2D/3D Pb-Sn alloyed perovskite. They applied the inverted structure p-i-n on their work with the solar cell structure (ITO/PEDOT:PSS/PEAxMA1xPb0.5Sn0.5I3/PC61BM/PEI/Ag) .
Multi-dimensional 2D/3D hybrid perovskites have proven to be one of the most promising approaches to improving PCE and stability than pure 3D PSCs. Many novel 2D-perovskites can be synthesized in terms of materials engineering and consistent unknown properties studied. Besides, the recently proposed computational theoretical study would speed up selecting and applying 2D materials. In particular, long-chain alkyl cations seem to be suitable for stabilizing perovskite materials based on 2D-Pb-Sn or Sn. The performance can be improved by technical composition and structure. Besides, it remains essential to study the mechanism of formation of 2D-perovskite layers, which allows us to produce 2D-perovskites with high phase purity and out-of-plane orientation and to cultivate 2D/3D perovskite materials in a more controllable approach. Also, a thorough understanding of the chemical or physical interaction between 2D and 3D perovskite materials, 2D/3D perovskite materials, and neighboring charge transport layers will help achieve efficient and stable PSCs. As the bandgap increases, 2D-perovskite could be ideal for tandem solar cells to drive energy generation. In summary, the development strategy for multi-dimensional 2D/3D perovskites offers an opportunity for efficient and stable PSCs that enable ingenious modification of PSCs and inform their future marketing.
The authors acknowledge Science and Technology Development Fund (STDF) and (CMRDI) for their support to this study through the project grant no. 25250 in Egypt.
Conflict of interest
The authors declare no conflict of interest.