**2.1 Structural properties of 2D perovskite**

The stability of perovskite structure can be described by tolerance factor-t, whose calculation formula is: ( ) t 2 <sup>+</sup> <sup>=</sup> <sup>+</sup> *Ra Rx Rb Rx* , where Ra, Rb and Rx are the radius

of the atom located at A, B, X site respectively [20]. The perovskite structure with tolerance factor in the range of 0.8–1.01 is stable and can form an ideal 3D cubic structure [21]. In the stable 3D structure, the octahedrons formed by lead ions and halogen ions are infinitely connected, and organic cations are located between the spaces formed by the octahedrons [22]. When the size of the organic cation at the A site increases, the tolerance factor will exceed the above range finally. Then the infinitely connected octahedral structure is broken, and a conductive inorganic layer and an insulating organic layer are alternately connected to each other [23]. The 2D layered perovskite film can be regarded as the infinitely connected octahedron structure separating by the large-size organic cations, and the thickness of the octahedron contained in each layer is n. The value of n is closely related to the ratio of large organic spacer cations, and represents the periodicity in the crystal structure. The perovskite crystal structure with n value ranging from n = 1 to n = ∞ is shown in **Figure 1**. The L-value there denotes the thickness of the inorganic layer in each compound [25].

All organic cations capable of forming a 2D perovskite structure have groups that can interact with the inorganic layer at their ends and can stably exist in the crystal structure. According to the structural characteristics of large-size organic cations, the obtained 2D perovskites can be divided into different types. And the corresponding PSCs have different performances accordingly. The 2D perovskite formed by organic cation similar to BA+ and PEA+ , which have only one amino group at the end, is called the RP phase, which was first applied to 2D PSCs in 2014 [19]. According to the characteristics of the crystal structure and the chemical ratio of each ion, the chemical formula of the 2D RP phase perovskite is AAn − 1BnX3n + 1. [26]

**255**

**Figure 2.**

forces. BA+

*Chemical Society.*

**Figure 1.**

and PEA+

*2D Organic-Inorganic Hybrid Perovskite Light-Absorbing Layer in Solar Cells*

As shown in **Figure 2(a)**, the RP phase crystal is coupled by weak van der Waals

*Crystal structures of the 2D lead iodide perovskites, (BA)2(MA)n − 1PbnI3n + 1, ranging from n = 1 to n =* ∞*. The L-value denotes the thickness of the inorganic layer in each compound [24]. Copyright 2016, American* 

extensively studied [27–29]. The development of new organic spacers cations is an important way to improve the performance of 2D perovskite solar cells. Recently, 2D RP PSCs with a record PCE more than 19% was prepared by using an organicsalt-assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the losses caused by charge recombination [30]. The 2D perovskite formed by organic cation similar to EDA2+, which have two amino groups that can interact with inorganic layers at both ends is called the DJ phase. Its chemical formula and crystal structure are shown in **Figure 2(b)** [31]. The DJ phase has better stability than RP perovskite (van der Waals interaction) because the spacer cations with two amino groups at both ends can form hydrogen bonds with inorganic plates without any gaps [32]. Ke et al. used 3-(amino methyl) piperidine (3-AMP2+) as organic spacers. Compared with the single A-site cation, mixing cation (3AMP)(MA0.75FA0.25)3Pb4I13 perovskite has a narrower band gap, less inorganic skeleton distortion and the larger Pb-I-Pb angle [33]. Cohen used BDA2+ as an organic spacer cation and achieved 15.6% PCE

*Schematic representation of (a) RP phase (b) DJ phase (c) ACI phase [26, 31, 35]. Copyright 2000, springer.* 

*Copyright 2018, American Chemical Society. Copyright 2017, American Chemical Society.*

PSCs. In subsequent studies, organic cations such as AVA+

are the most widely studied organic spacer cations in 2D RP

, PEI+

, PPA+

have been

*DOI: http://dx.doi.org/10.5772/intechopen.93725*

*2D Organic-Inorganic Hybrid Perovskite Light-Absorbing Layer in Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93725*

#### **Figure 1.**

*Solar Cells - Theory, Materials and Recent Advances*

of perovskite [19, 20].

is low, and molecular dissociation and ion migration are prone to occur within the

In recent years, the two-dimensional (2D) perovskite structure formed by introducing large-size organic cations is proved to be more stable than its threedimensional (3D) counterpart and it has become a potential light-absorber in the PSCs. There are many reasons for the 2D perovskite to exhibit higher stability. The 2D perovskite has higher formation energy and it is more difficult to be oxidized than the 3D structure [16]. Compared with 3D perovskite crystals, the bonding forces between organic ions and [PbI6] octahedral units such as van der Waals forces and hydrogen bonds are stronger [17]. Due to the presence of large size organic cations, ion migration is blocked [18]. Meanwhile, the 2D perovskite layer can work as passivation layer and blocking layer of moisture and oxygen to enhance the stability

Although 2D perovskite materials show great potential in terms of stability, the relatively lower PCE needs to be improved. In this chapter, based on the structural and photophysical properties of 2D perovskite, the latest progress made in 2D PSCs in recent years and strategies to improve the performance of 2D PSCs are summarized, which is of great significance for the further development of PSCs based on 2D perovskite materials. Finally, a brief conclusion and outlook is promoted.

The stability of perovskite structure can be described by tolerance factor-t,

( )

of the atom located at A, B, X site respectively [20]. The perovskite structure with tolerance factor in the range of 0.8–1.01 is stable and can form an ideal 3D cubic structure [21]. In the stable 3D structure, the octahedrons formed by lead ions and halogen ions are infinitely connected, and organic cations are located between the spaces formed by the octahedrons [22]. When the size of the organic cation at the A site increases, the tolerance factor will exceed the above range finally. Then the infinitely connected octahedral structure is broken, and a conductive inorganic layer and an insulating organic layer are alternately connected to each other [23]. The 2D layered perovskite film can be regarded as the infinitely connected octahedron structure separating by the large-size organic cations, and the thickness of the octahedron contained in each layer is n. The value of n is closely related to the ratio of large organic spacer cations, and represents the periodicity in the crystal structure. The perovskite crystal structure with n value ranging from n = 1 to n = ∞ is shown in **Figure 1**. The L-value there denotes the thickness of the inorganic layer in

All organic cations capable of forming a 2D perovskite structure have groups that can interact with the inorganic layer at their ends and can stably exist in the crystal structure. According to the structural characteristics of large-size organic cations, the obtained 2D perovskites can be divided into different types. And the corresponding PSCs have different performances accordingly. The 2D perovskite

at the end, is called the RP phase, which was first applied to 2D PSCs in 2014 [19]. According to the characteristics of the crystal structure and the chemical ratio of each ion, the chemical formula of the 2D RP phase perovskite is AAn − 1BnX3n + 1. [26]

and PEA+

*Rb Rx* , where Ra, Rb and Rx are the radius

, which have only one amino group

structure, which limits the structural stability of these materials [15].

**2. Material properties of 2D perovskite materials**

t

2 <sup>+</sup> <sup>=</sup> <sup>+</sup> *Ra Rx*

**2.1 Structural properties of 2D perovskite**

whose calculation formula is:

each compound [25].

formed by organic cation similar to BA+

**254**

*Crystal structures of the 2D lead iodide perovskites, (BA)2(MA)n − 1PbnI3n + 1, ranging from n = 1 to n =* ∞*. The L-value denotes the thickness of the inorganic layer in each compound [24]. Copyright 2016, American Chemical Society.*

As shown in **Figure 2(a)**, the RP phase crystal is coupled by weak van der Waals forces. BA+ and PEA+ are the most widely studied organic spacer cations in 2D RP PSCs. In subsequent studies, organic cations such as AVA+ , PEI+ , PPA+ have been extensively studied [27–29]. The development of new organic spacers cations is an important way to improve the performance of 2D perovskite solar cells. Recently, 2D RP PSCs with a record PCE more than 19% was prepared by using an organicsalt-assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the losses caused by charge recombination [30]. The 2D perovskite formed by organic cation similar to EDA2+, which have two amino groups that can interact with inorganic layers at both ends is called the DJ phase. Its chemical formula and crystal structure are shown in **Figure 2(b)** [31]. The DJ phase has better stability than RP perovskite (van der Waals interaction) because the spacer cations with two amino groups at both ends can form hydrogen bonds with inorganic plates without any gaps [32]. Ke et al. used 3-(amino methyl) piperidine (3-AMP2+) as organic spacers. Compared with the single A-site cation, mixing cation (3AMP)(MA0.75FA0.25)3Pb4I13 perovskite has a narrower band gap, less inorganic skeleton distortion and the larger Pb-I-Pb angle [33]. Cohen used BDA2+ as an organic spacer cation and achieved 15.6% PCE

**Figure 2.**

*Schematic representation of (a) RP phase (b) DJ phase (c) ACI phase [26, 31, 35]. Copyright 2000, springer. Copyright 2018, American Chemical Society. Copyright 2017, American Chemical Society.*

without additives and any additional treatment [34]. The 2D perovskite formed by organic cation similar to GA+ , which can alternate interact with MA+ in the organic layers, is called the ACI phase. Its chemical formula and crystal structure are shown in **Figure 2(c)** [35]. Due to its relatively small difference in ion size from MA+ and FA+ , it has smaller exciton binding energy and weaker quantum confined effect. So, it is expected to achieve higher efficiency. The perovskite solar cells with BEA2+ ligand achieved high efficiency of 14.48 and 17.39% when doped with and without Cs+ respectively [36]. Zhao's team achieved a high PCE of 18.48% by adding methyl ammonium chloride as an additive to effectively control the film quality of ACI 2D perovskite (GA)(MA)nPbnI3n + 1 (n = 3), showing great potential of ACI perovskite with high stability and PCE [37].

#### **2.2 Photophysical properties of 2D perovskite materials**

Compared with 3D perovskites, 2D perovskite has a greater chemical and structural flexibility. The optical, electrical, and charge transfer properties can be regulated by controlling the width and composition of the potential well and barrier. In the 2D perovskite structure, a quantum well structure is formed between the insulating organic layer and the conductive inorganic layer, resulting in a quantum confinement effect [38]. The dielectric confinement effect is caused by the different dielectric constants of the potential well and the barrier, coupled with the quantum space limitation. The optical gap of 2D perovskites has a higher value than its 3D counterparts [39]. Zhang's work explored the inherent properties of 2D layered perovskites (PEA)2PbI4(N) and Cs2PbI4(N), and demonstrated that their structure and properties vary with N. The results reveal that both (PEA)2PbI4(N) and Cs2PbI4(N) are direct bandgap semiconductors. When N ≥ 3, their band/optical gap and exciton binding energy vary linearly by 1/N. This work shows that ultrathin 2D materials can become potential candidates for nano-optoelectronic devices, and nanoplates with N ≥ 3 can have similar properties to bulk materials in terms of carrier migration and exciton separation, so they can be effectively applied for photovoltaic devices [40].

**Figure 3** shows the absorption and emission spectra of a series of ultra-thin (BA)2(MA)n-1PbnI3n + 1 (n = 1–5) crystal layers mechanically peeled off from the pure phase (fixed n) single crystal by Blancon et al. In the exfoliated crystal, as n decreases from 5 to 1 (quantum well thickness varies from 3.139 to 0.641 nm), the band edge absorption and emission peaks monotonically increase from 1.85 to 2.42 eV. Due to the quantum and dielectric confinement effect, exciton binding

**Figure 3.**

*(Left) absorption and (right) PL Spectrum of the exfoliated (BA)2 (MA)n-1PbnI3n+1 crystals (from n = 1 to 5) [41]. Copyright 2017, American Association for the Advancement of Science (AAAS).*

**257**

with MA<sup>+</sup>

*2D Organic-Inorganic Hybrid Perovskite Light-Absorbing Layer in Solar Cells*

energies are approximately one order of magnitude higher than the values found in 3D perovskites. Their results also show that in the film, the optical band gap is consistent with that of the exfoliated crystal at n = 1 and n = 2, but the red shifts

Consistent with the situation in the 3D PSCs, it is of vital importance to control the film quality of absorption layer for a high-performance 2D PSC. For the synthesis of the light-absorption layer, the most-commonly used spin-coating method is applied. However, things become more difficult when it comes to the crystallization process of the 2D perovskite films. To improve the film quality of the 2D peroskite, many strategies are used to optimize the film quality, mainly focusing on adjusting

The precursor solution of 2D perovskite is prepared by mixing and dissolving ammonium salt of the organic cation spacers, ammonium salt of the A-site organic cations and metal halide according to a certain proportion (depending on n value). During the crystallization process, competition was confirmed to occur between the large organic cations and A-site organic cations [18]. To be more specific, the large organic spacers tend to form a low-dimensional perovskite while the A-site cations tend to form a 3D structure. The low-n phases are prone to have a horizontal orientation instead of vertical orientation, which is unfavorable to the charge transfer. Since the presence of insulating large-size organic cations will hinder the charge transport out-of-plane, Tsai, H. et al. used a hot-casting technology in 2016 to prepare high-quality films, which means that the substrate is preheated before spin-coating the perovskite precursor solution. With this method, a more beneficial crystal growth along (111) and (202) planes is observed instead of random orientation [42]. The application of the hot-casting method has promoted the efficiency of 2D PSCs and confirmed the importance of the crystal orientation perpendicular to the substrate for the performance improvement. At present, hot-casting method is widely used for better performance of the 2D PSCs. However, it is hard to keep temperature accurate and uniform when transferring the substrate from the hotplate to the spin-coater. To solve this problem, Li et.al partially replaced the BA+

 in the BAMA quasi-2D perovskite, reducing the dependence on the preheating of the substrate. After being replaced, the quantum confinement effect of the perovskite film is weakened, the crystallization barrier is reduced, and higher quality perovskite film crystals and fewer defect states are obtained [43]. Beyond ion replacement, the morphology of the 2D DJ perovskite film with rigid piperidine ring was adjusted by MASCN additive at room temperature. By optimizing the amount of added MASCN, the perovskite film deposited on the substrate has good crystallinity, preferred orientation, reduced defects and better energy level alignment with the transport layer. The device had an inverted planar structure with a maximum PCE of 16.25%, which is the highest PCE of 2D DJ PVSCs without hot casting. After being exposed to air for 35 days, the unencapsulated device maintains about 80% of its initial efficiency (Hr 45 ± 5%). It provides a possiblely practical

way for the development of high-performance 2D DJ PSCs [44].

Many other fabrication methods besides hot-casting have also been applied to obtain a vertical crystal orientation. Ke et al. used a two-step method of spincoating a stoichiometric precursor containing PEAI and PbI2, then performing FAI,

the crystal orientation and the phase distribution, as summarized below.

*DOI: http://dx.doi.org/10.5772/intechopen.93725*

about 200–300 meV at n = 3–5 [41].

**3.1 Vertical orientation**

**3. Film quality control of 2D perovskite**

#### *2D Organic-Inorganic Hybrid Perovskite Light-Absorbing Layer in Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93725*

energies are approximately one order of magnitude higher than the values found in 3D perovskites. Their results also show that in the film, the optical band gap is consistent with that of the exfoliated crystal at n = 1 and n = 2, but the red shifts about 200–300 meV at n = 3–5 [41].
