**3.1 Structures**

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

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

#### **Figure 3.**

*(a) Crystal structures of a 3D-perovskite and the 2D-hybrid perovskite with monovalent and divalent ammonium cations. Reprinted with permission from Ref. [43]. (b) Cuts along <100>, <110> and < 111> directions and the 2D-perovskites that result from such cuts. Reprinted with permission from Ref. [43].*

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.) [63]. 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

#### **Figure 4.**

*A schematic device architecture of planar (PEA)2(MA)m-1PbmI3m + 1 based perovskite solar cell, consisting of glass/FTO/TiO2CL/(PEA)2(MA)m-1PbmI3m + 1/spiroOMeTAD/Au. Reprinted with permission from Ref. [60].*

**169**

**Figure 5.**

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

improved device stability over the R.P. counterpart is attributed to alternating hydrogen bonding interactions between diammonium cations and inorganic slabs,

*Illustration of R.P. and D.J. phase 2D-layered perovskites. Reprinted with permission from Ref. [64].*

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 [65]. 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 [66]. 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 (*n* = 5)/PC61BM/BCP/Ag was fabricated. They got an efficiency up to 11.01% with the optimized NH4SCN addition at *n* = 5 [67]. In the same year, Chunqing Ma and co-workers fabricated a 2D-PSC with the device configuration ITO/PEDOT:PSS/PDAMA3Pb4I13/C60/BCP/Ag and recorded an efficiency of 13% with PDAMA3Pb4I13 (PDM: propane-1,3-diammonium) as the

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

**3.2 Applications in regular or inverted structure perovskite solar cells**

strengthening the 2D-layered perovskite structure [64].

2D-perovskite layer using p-i-n inverted structure [67].

**3.3 Optoelectronic properties**

*Solar Cells - Theory, Materials and Recent Advances*

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

*(a) Crystal structures of a 3D-perovskite and the 2D-hybrid perovskite with monovalent and divalent ammonium cations. Reprinted with permission from Ref. [43]. (b) Cuts along <100>, <110> and < 111> directions and the 2D-perovskites that result from such cuts. Reprinted with permission from Ref. [43].*

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

*A schematic device architecture of planar (PEA)2(MA)m-1PbmI3m + 1 based perovskite solar cell, consisting of glass/FTO/TiO2CL/(PEA)2(MA)m-1PbmI3m + 1/spiroOMeTAD/Au. Reprinted with permission from Ref. [60].*

(R.P.) [63]. The difference between R.P. and D.J. is shown in **Figure 5**.

**168**

**Figure 4.**

**Figure 3.**

**Figure 5.** *Illustration of R.P. and D.J. phase 2D-layered perovskites. Reprinted with permission from Ref. [64].*

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 [64].
