**2. Structure of perovskite materials**

Although solar cells based on the photovoltaic effect have attracted great attention due to the advantage of decentralization and sustainability, yet they suffer low cost effectiveness. Another emerging class of thin‐film energy devices based on amorphous silicon also tried to capture the market, making headway by processing of costs per unit area [3–5]. The manu‐ facturing of inorganic thin‐films solar cells needs high‐temperature and high vacuum‐based techniques [6]. In addition, these techniques are limited and due to the inclusion of toxic ele‐

In 1991, a new breakthrough emerged in the form of dye‐sensitized solar cells (DSSCs) that have attracted considerable attention due to their potential application in low‐cost solar energy conversion [8–16]. A high efficiency exceeding 12% was obtained by using 10 μm mesoporous

[23]. So attempts were made to design various types of cells to increase the efficiency of solar

This efficiency criterion was increased by the introduction of the perovskite sensitizer ABX3

has opened a new era in the field of DSSCs due to the excellent light‐harvesting capabilities [24–37]. These materials are composed of earth abundant materials, inexpensive, processable at low temperatures (printing techniques), generate charges freely (after absorption) in bulk materials, which qualify them as low energy‐loss charge generators and collectors [38–40].

) have an optical bandgap between 2.3 and 1.6 eV depending on halide content, while

PbX3

eV. The minimum bandgap is closer to optimum for a single‐junction cell than methylam‐ monium lead trihalide, which enhance to higher efficiencies [41]. The power conversion effi‐ ciency (PCE) of perovskite cells was improved from 7.2 to 15.9%, which is associated with the comparable optical absorption length and charge‐carrier diffusion lengths, making this device the most outperforming relative to the other third‐generation thin‐film solar cell tech‐

ciency exceeding 16%, have been reported [26, 42], provided few issues related to the stability

Here, it is necessary to mention that the lack of hysteresis that was an obstacle for stable operation in perovskite was observed recently using thin films of organometallic perovskites with millimeter‐scale crystalline grains with efficiencies approximately equal to 18% [44].

The three recent reports have given high hopes in the field of solar cells as EPFL scientists have developed a new hole‐transporting material FDT that can reduce the cost and achieve

NH3 PbX3

NCHNH2

nologies. Although two different configurations using CH3

and hysteresis are to be solved effectively [43].

solid‐state DSSC and in a thin‐film planar configuration with CH3

 film sensitized with a cobalt redox electrolyte and an organic dye [17]. Furthermore, solid‐state DSSCs were also investigated where the liquid electrolyte was replaced by a solid hole‐transporting material (HTM) [e.g., poly(3‐hexylth‐iophene)(P3HT),2,2′,7,7′‐tetra‐ kis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′spirobifluorene (spiro‐MeOTAD)], polyaniline, and polypyrrole [8] to increase the open circuit voltage and stability of solar cells [18–22]. However, these ss‐DSSCs also suffer from faster electron recombination dynamics between

) and holes (hole transporter), which results in the low efficiency of ss‐DSSCs

, B = Pb, Sn, and X = Cl, Br, I), introduced by Prof. Grätzel and team, which

, where X is a halogen ion such as I−

NH3 PbI3

) also have a bandgap between 2.2 and 1.5

NH3

PbI3−*<sup>x</sup>* Cl*<sup>x</sup>* , Br−

, having effi‐

perovskite in a classical

, and

ments, they are limited to large‐scale production and wide applications [7].

TiO2

electrons (TiO2

246 Nanostructured Solar Cells

cells [24].

(A = CH3

Cl−

NH3

Methylammonium lead trihalide (CH3

formamidinum lead trihalide (H2

The basic structure of perovskite consists of a 3D network corner‐sharing BX6 octahedra, where A (e.g., A = Cs, CH3 NH3 , NH2 CHNH2 ) cations are located in the larger 12‐fold coordi‐ nated holes between the octahedra [44]. It is composed of a metal cation (M = Sn, Pb, Ge, Cu) and its ligantanions (X = O2−, Cl− , Br− , I<sup>−</sup> , or S2−). In the case of inorganic perovskite compounds, the structures can be distorted as a result of the cation displacements, which give rise to some useful properties of ferroelectricity and antiferroelectricity due to the stereochemically active pairs of A cations [48]. The simple cubic structure of CH3 NH3 PbI3 is given in **Figure 1**.

These inorganic‐organic hybrid compounds have the advantages of inorganic components that include structural order and thermal stability with interesting characteristics of organic materials such as low cost, mechanical flexibility, and functional versatility [49–53]. Numerous compounds have been reported by the covalent bonding between the inorganic and organic bonds [54]. Although the degree of interactions in organic‐inorganic systems with the van der Waals interacting system is relatively small, the reason for the small van der Waals inter‐ action is the choice of organic cations, which is limited as the restricted dimension of the cuboctahedral hole formed by the 12 nearest‐neighbor X atoms. The synthesis of compounds CH3 NH3 MX3 with M = Sn, Pb and X= Cl, Br, and I has been successfully carried out by some groups [55–57]. These organic cations show orientational disorder at high temperature, while at lower temperature the cubic phase results in a structural phase transition as the tolerance factor is smaller than unity. Upon cooling, the structure distorts to lower its symmetry as there are many restrictions to the motion of methylammonium cations [57].

**Figure 1.** The crystal structure of perovskites, ABX3 , a large cation (A) at center together with metal cation (B) bonded to the surrounded halides (X). Color code: A (CH3 NH3 ),blue; B (Pb), green; and X (I), pink.

**Figure 2.** Graphical representation of phase transitions of MA(Pb, Sn)X3 perovskite materials (a) α‐phase, (b) β‐phase, (c) γ‐phase. Precision images are taken at the [006] view. (d) The structural transformation of Br included in MAPbI3 . Adapted with permission from reference [37].

MA, FA, Pb, and Sn perovskite combinations to identify three distinct phase transitions that occur are classified as a high temperature **α** phase, an intermediate **β** temperature phase, and a low temperature **γ** phase [54]. These different phases are represented in **Figure 2**.

The perovskites were first investigated by Goldschmidt in the 1920s [58] in work related to tolerance factors. The tolerance factor, *t*, with respect to the ionic radius of the actual ions is given in Eq. (1), where *r*A, *r*B, and *r*C are the ionic radius of the A, B, and C ions, respectively.

$$t = \frac{r\_{\text{A}} + r\_{\text{C}}}{\sqrt{2} \left(r\_{\text{B}} + r\_{\text{C}}\right)}\tag{1}$$

The tolerance factor of (0.9–1) is for an ideal cubic structure, for a cubic structure with the tolerance factor (0.7–0.9), the A ion is too small or the B ion is too large. This can be resulted in orthorhombic, rhombohedral, or tetragonal structure. For a large A cation, *t* becomes larger than one, which results in layered perovskite structures [59, 60]. The compiled results are given in **Table 1** and the different forms of perovskite material CH3 NH3 PbI3 are given in **Figure 3**. The expected structure is also related to Pauling's rules (PRs) [61], given the expected coordi‐ nation around a two‐component radii (cation/anion) system which is summarized in **Table 2**.

The smaller tolerance factor is related to lower symmetry tetragonal or orthorhombic struc‐ tures, whereas larger *t* (*t* > 1) could destabilize the three‐dimensional (3D) B‐X network.


**Table 1.** Tolerance factors for the perovskite structures

MA, FA, Pb, and Sn perovskite combinations to identify three distinct phase transitions that occur are classified as a high temperature **α** phase, an intermediate **β** temperature phase, and

(c) γ‐phase. Precision images are taken at the [006] view. (d) The structural transformation of Br included in MAPbI3

The perovskites were first investigated by Goldschmidt in the 1920s [58] in work related to tolerance factors. The tolerance factor, *t*, with respect to the ionic radius of the actual ions is given in Eq. (1), where *r*A, *r*B, and *r*C are the ionic radius of the A, B, and C ions, respectively.

> ( ) A C B C 2 *r r*

(1)

.

are given in **Figure 3**.

perovskite materials (a) α‐phase, (b) β‐phase,

*r r* <sup>+</sup> <sup>=</sup> <sup>+</sup>

The tolerance factor of (0.9–1) is for an ideal cubic structure, for a cubic structure with the tolerance factor (0.7–0.9), the A ion is too small or the B ion is too large. This can be resulted in orthorhombic, rhombohedral, or tetragonal structure. For a large A cation, *t* becomes larger than one, which results in layered perovskite structures [59, 60]. The compiled results are given

The expected structure is also related to Pauling's rules (PRs) [61], given the expected coordi‐ nation around a two‐component radii (cation/anion) system which is summarized in **Table 2**. The smaller tolerance factor is related to lower symmetry tetragonal or orthorhombic struc‐ tures, whereas larger *t* (*t* > 1) could destabilize the three‐dimensional (3D) B‐X network.

NH3 PbI3

a low temperature **γ** phase [54]. These different phases are represented in **Figure 2**.

*t*

in **Table 1** and the different forms of perovskite material CH3

**Figure 2.** Graphical representation of phase transitions of MA(Pb, Sn)X3

Adapted with permission from reference [37].

248 Nanostructured Solar Cells

**Figure 3.** The crystal structure of perovskites (CH3 NH3 PbI3 ) in different forms: (a) cubic, (b) tetragonal, (c) rhombohedral, and (d) orthorhombic. Color code: CH3 NH3 , pink; Pb, green and I, blue.


**Table 2.** Coordination and ideal *rc ra* (Pauling's rules). *rc* and *ra* represent the cationic and anionic radii

The other important parameter is an octahedral factor that plays an important role in these materials, and is given by,

$$
\mu = \mathbb{R}\_{\mathbb{B}} \;/\, \mathbb{R}\_{A} \tag{2}
$$

where *R*B is the ionic radii of the B cation and *R*A is the ionic radii of A anion. If *μ* > 0.442, the formation of halide perovskite achieves, whereas below this value BX6 octahedron will become unstable and a perovskite structure will not form, although these factors provide a guidelines for the formation of halide perovskite, yet they are not sufficient to predict the structural formations within the perovskite family [62].
