**2. What is perovskite?**

**Figure 2.** The number of articles related to the "perovskite solar cell" in the science citation index (SCI;Thomson Reuters)

from 2012 to 2017.

**Figure 1.** Efficiency chart of perovskite-structured solar cells.

78 Solar Panels and Photovoltaic Materials

In 1839, Gustav Rose, a German mineralogist, first discovered a kind of calcium titanium oxide mineral, composed of calcium titanate (CaTiO3 ), in the Ural Mountains of Russia [3]. This mineral was then named after Russian mineralogist Lev Perovski. However, the material currently used in perovskite solar cells is not CaTiO3 but a material that has a similar crystal structure to perovskite. The chemical formula of this material is commonly denoted as **ABX3** , where **A** is a monovalent cation, **B** is a bivalent metal cation, and **X** is a halogen anion. Furthermore, the **A** cation with larger ionic radii occupying a cuboctahedral site is shared with 12 **X** anions. The 6 **X** anions surround the **B** cation with the smaller ionic radii occupying octahedral coordination and form a stable structure. The octahedron will connect with each other by corning sharing arrangement, and the center of each octahedron is the location of **A** cation [4]. The crystal structure of ABX3 is shown in **Figure 3**.

The proposed perovskite structure and its stability can be determined by Goldschmidt's tolerance factor (TF) and octahedral factor (μ) [5]. For an ideal cubic perovskite, the unit cell axis, *a*, is geometrically related to the ionic radii and can be described with Eq. (1):

$$a = \sqrt{2}(\mathbf{R\_A} + \mathbf{R\_\chi}) = \mathcal{Z}(\mathbf{R\_g} + \mathbf{R\_\chi}) \tag{1}$$

where *R*A, *R*B, and *R*X are the ionic radii of **A** cation, **B** cation, and **X** anion, respectively. Goldschmidt's tolerance factor is the ratio of the two expressions of the unit cell axis. The equation of TF is as follows.

$$\text{TF} = \frac{\text{(R}\_{\text{A}} + \text{R}\_{\text{X}}\text{)}}{\sqrt{2}(\text{R}\_{\text{y}} + \text{R}\_{\text{x}}\text{)}} \tag{2}$$

For the stable perovskite structure, the coordination number is 6, and the corresponding μ will be in the range of 0.414 < μ < 0.732. Calculating the tolerance factor and the octahedral factor could be an effective way to construct a new structural map leading to new principles

To further complement the suitable metal cation candidates to replace toxic lead in perovskite while not to destroy its structure, **Figure 4** summarizes the comparison of tolerance and octahedral factors based on Goldschmidt's rule for non-toxic alkaline-earth metal cations. The homovalent substitution of Pb to alkaline-earth metal cations shows that it can meet the requirement of not only the charge balance but also the tolerance factor of Goldschmidt's rule in perovskite. As a result, alkaline-earth metal cations are widely considered as suitable candidates for replacing toxic lead.

Perovskite-structured photovoltaic materials can be divided into four groups according to the demand of the research objectives. The first group, organolead halide perovskite, is used to improve the power conversion efficiency up to its theoretical limit ~31%. The second group is lead-free perovskite. The purpose of the lead-free perovskite-structured material is to find an alternative to replace the toxic lead. The third group is lead-reduced perovskite-structured material. Owing to the unstable performance of lead-free perovskite, partial substitution of lead can not only reduce the lead content but also retain the power conversion efficiency. The last group, two-dimensional (2D) perovskite-structured material, with the presence of hydrophobic alkyl ammonium, exhibits good moisture resistance. In this section, we provide a report of the four types of perovskite-structured photovoltaic materials and their relevant applications.

Up to now, the organolead halide perovskite is still the highest performance material for PSCs. The high performance perovskite-structured solar cells are commonly based on

of charge carrier kinetics and recombination, and thus influences the photovoltaic performance. It is believed that less non-radiative pathways in grain boundaries can lead to higher photovoltaic performance. However, formation of grain boundaries is unavoidable while the

group reported a grain boundary healing process and achieved a PCE of 20.4% [6]. The grain boundary healing process involves adding a slight excess of MAI to the precursor coating

excess MAI, the excess MAI will not influence the crystal structure but form MAI on the sur-

architecture of PSCs, deposition process, and compositional manipulation have been seen as

is forming on the substrate during the solution process. Therefore, the N. G. Park

. After spin coating the non-stoichiometric precursor solution with slightly

grain. This MAI film prevents carrier recombination at the grain boundar-

. To obtain high efficiency, one of the effective approach is

thin film. The grain boundary is a critical factor

Perovskite-Structured Photovoltaic Materials http://dx.doi.org/10.5772/intechopen.74997 81

PbI3–γBrγ. The

**3. Development of perovskite-structured photovoltaic materials**

of the formability of perovskite compounds.

**3.1. Organolead halide perovskite-structured materials**

NH3

ies and also maximizes the extraction ability of the electron and hole.

Another notable organolead halide perovskite solar cell is based on FA1–xMAx

, where MA is CH3

to improve the grain boundaries of MAPbI3

MAPbI3

MAPbI3

solution of MAPbI3

face of the MAPbI3

**Figure 3.** Crystal structure of an organic–inorganic metal halide perovskite.

**Figure 4.** Comparison of calculated octahedral and tolerance factors for metal cations as a candidate for replacing toxic lead.

The perovskite structure is stable when the TF is in the range of 0.82 < TF < 1.00. The octahedral factor calculated by Pauling's rule can determine the coordination numbers of the metal cation and the halogen anion. The equation of the octahedral factor is described in Eq. (3).

$$
\mu = \frac{R\_y}{R\_\chi} \tag{3}
$$

For the stable perovskite structure, the coordination number is 6, and the corresponding μ will be in the range of 0.414 < μ < 0.732. Calculating the tolerance factor and the octahedral factor could be an effective way to construct a new structural map leading to new principles of the formability of perovskite compounds.

To further complement the suitable metal cation candidates to replace toxic lead in perovskite while not to destroy its structure, **Figure 4** summarizes the comparison of tolerance and octahedral factors based on Goldschmidt's rule for non-toxic alkaline-earth metal cations. The homovalent substitution of Pb to alkaline-earth metal cations shows that it can meet the requirement of not only the charge balance but also the tolerance factor of Goldschmidt's rule in perovskite. As a result, alkaline-earth metal cations are widely considered as suitable candidates for replacing toxic lead.
