**1. Introduction**

For several decades, there has been substantial development in solar cells. Solar cells can be roughly divided into three generations, including (1) single-crystalline and polycrystalline silicon solar cells, (2) thin film solar cells (CIGS, CdTe, and amorphous silicon), and (3) new generations of solar cells (organic solar cells, dye-sensitized solar cells, and perovskite solar cells). Among these solar cells, the third generation of solar cells, which adopt organic materials and nanotechnology, shows relatively low cost; the fabrication process is relatively uncomplicated. Therefore, the booming of research interest in third generation solar cells has taken placed. Particularly,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the perovskite solar cells (PSCs), bearing the advantages including feasible solution process, low material cost, and rapid progress of efficiency, has drawn much attention. As one of the third generation photovoltaic material, the development of the perovskite solar cells (PSCs) efficiency chart, which was recorded by the National Renewable Energy Laboratory (NREL), is illustrated in **Figure 1** [1]. In 2009, Japanese scientist T. Miyasaka's group first reported methylammonium

efficiency (PCE) of 3.8% [2]. After that, many research groups dedicated to constructing various perovskite solar cells in the past eight years. By the apparent increase in the number of publications per year (**Figure 2**), PSCs have attracted the scientific community's attention for several years. Owing to the scientists' contribution, the PCE of PSCs has rapidly improved, and the latest record is up to 22.7% by the S. I. Seok group. However, the presence of lead ions in perovskitestructured materials is so toxic to both environment and human community that it will be an obstacle for possible commercialization. During the fabricating process of PSCs, the commonly

development will focus on the reduction of lead content and the use of low-hazardous solvents.

In 1839, Gustav Rose, a German mineralogist, first discovered a kind of calcium titanium

This mineral was then named after Russian mineralogist Lev Perovski. However, the mate-

crystal structure to perovskite. The chemical formula of this material is commonly denoted as

, 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**

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,

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

> √ \_\_

*a*, is geometrically related to the ionic radii and can be described with Eq. (1):

\_\_

H7

Cl), are also highly toxic, which may even result in cancer. Therefore, future

) as a light absorber in dye-sensitized solar cells with a power conversion

NO), acetonitrile (ACN, C2

2(*RA* + R*X*) = 2(*RB* + R*X*) (1)

2(*RB* <sup>+</sup> <sup>R</sup>*X*) (2)

H3

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

), in the Ural Mountains of Russia [3].

but a material that has a similar

N), and chlo-

79

lead halide (CH3

robenzene (CB, C<sup>6</sup>

**ABX3**

NH3 PbI<sup>3</sup>

H5

cation [4]. The crystal structure of ABX3

*a* = √

TF <sup>=</sup> (*RA* <sup>+</sup> <sup>R</sup>*X*) \_\_\_\_\_\_\_

equation of TF is as follows.

**2. What is perovskite?**

used solvents, such as dimethylformamide (DMF, C3

oxide mineral, composed of calcium titanate (CaTiO3

rial currently used in perovskite solar cells is not CaTiO3

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

**Figure 2.** The number of articles related to the "perovskite solar cell" in the science citation index (SCI;Thomson Reuters) from 2012 to 2017.

the perovskite solar cells (PSCs), bearing the advantages including feasible solution process, low material cost, and rapid progress of efficiency, has drawn much attention. As one of the third generation photovoltaic material, the development of the perovskite solar cells (PSCs) efficiency chart, which was recorded by the National Renewable Energy Laboratory (NREL), is illustrated in **Figure 1** [1]. In 2009, Japanese scientist T. Miyasaka's group first reported methylammonium lead halide (CH3 NH3 PbI<sup>3</sup> ) as a light absorber in dye-sensitized solar cells with a power conversion efficiency (PCE) of 3.8% [2]. After that, many research groups dedicated to constructing various perovskite solar cells in the past eight years. By the apparent increase in the number of publications per year (**Figure 2**), PSCs have attracted the scientific community's attention for several years. Owing to the scientists' contribution, the PCE of PSCs has rapidly improved, and the latest record is up to 22.7% by the S. I. Seok group. However, the presence of lead ions in perovskitestructured materials is so toxic to both environment and human community that it will be an obstacle for possible commercialization. During the fabricating process of PSCs, the commonly used solvents, such as dimethylformamide (DMF, C3 H7 NO), acetonitrile (ACN, C2 H3 N), and chlorobenzene (CB, C<sup>6</sup> H5 Cl), are also highly toxic, which may even result in cancer. Therefore, future development will focus on the reduction of lead content and the use of low-hazardous solvents.
