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

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).

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

*RX*

(3)

*<sup>μ</sup>* <sup>=</sup> *<sup>R</sup>*\_\_\_*<sup>B</sup>*

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

80 Solar Panels and Photovoltaic Materials

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 MAPbI3 , where MA is CH3 NH3 . To obtain high efficiency, one of the effective approach is to improve the grain boundaries of MAPbI3 thin film. The grain boundary is a critical factor 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 MAPbI3 is forming on the substrate during the solution process. Therefore, the N. G. Park 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 solution of MAPbI3 . After spin coating the non-stoichiometric precursor solution with slightly excess MAI, the excess MAI will not influence the crystal structure but form MAI on the surface of the MAPbI3 grain. This MAI film prevents carrier recombination at the grain boundaries and also maximizes the extraction ability of the electron and hole.

Another notable organolead halide perovskite solar cell is based on FA1–xMAx PbI3–γBrγ. The architecture of PSCs, deposition process, and compositional manipulation have been seen as important factors for high-efficiency PSCs. The S. I. Seok group devoted themselves to overcoming these obstacles. They proposed a sandwich-type architecture consisting of CH3 NH3 PbI<sup>3</sup> perovskite as a light harvester on mesoporous TiO2 and achieved a PCE of 12% [7]. Then, they demonstrated a solution-based process to deposit a uniform and dense perovskite layer. The adoption of γ-butyrolactone and dimethyl sulfoxide (DMSO) mixed solvent followed by toluene drop-casting will form an intermediate phase, CH3 NH3 I–PbI2 –DMSO, which leads to the formation of a uniform layer and significantly enhances the PCE to 16.2% with no hysteresis [8]. They also developed a two-step process based on an intramolecular exchange between organic cations and DMSO molecules to fabricate FAPbI3 -based PSCs with 20.2% of PCE [9]. To date, the S. I. Seok group have combined the contributions mentioned earlier and modified the content of iodide in FAPbI3 for efficient PSCs as shown in **Figure 5**. The addition of iodide ion by a two-step process can decrease the deep-level defects that are seen at the nonradiative recombination centers and improve the PCE to 22.1% [10].

FAx

MA1-xSnI<sup>3</sup>

to the fast crystallization of MASnI3

PCE of 8.12% as shown in **Figure 6**.

oxidation of Sn2+ to Sn4+, lead-free CH3

critical step because it will react with SnI<sup>2</sup>

free perovskite-structured material, FA0.75MA0.25SnI<sup>3</sup>

**3.3. Lead-reduced perovskite-structured materials**

served as the light harvesting layer with SnF2

[13]. Mixing cations such as MA and FA is a conventional method in the composition engineering to improve the PSCs performance. The addition of additives to perovskite precursor solution can reduce the doped-hole density and enhance the stability of Sn-base PSCs. Due

DMSO is further introduced to the perovskite precursor solution. Adding DMSO is seen as a

suppresses the fast crystallization and thus obtains a homogeneous film. Up to date, the lead-

Although lead-free perovskite-structured photovoltaic materials solve the toxicity issues, efficiency is sacrificed for the replacement of lead. Partial substitution of lead in perovskitestructured materials is the alternative solution which can not only reduce the toxicity but also maintain the power conversion efficiency. Many literature indicates that owing to the facile

its reduced efficiency and lack of reproducibility [14]. Therefore, many scientists attempted to prepare perovskite film with partial replacement of lead. The M. G. Kanatzidis group fab-

found that doping Sn into the perovskite active layer can efficiently regulate the band gap of the perovskite material from 1.55 to 1.17 eV [15]. With the tunable band gap, it is also observed

superior film coverage and better film morphology, which ensures connectivity between

NH3 SnI<sup>3</sup>

ricated a perovskite material with 50% of Sn doping concentration (CH3

that light absorption extends to the near-infrared region. In addition, CH3

grains and overcomes short-circuiting and charge leaking issues.

**Figure 6.** Schematic diagram of a perovskite device and crystal structure based on FAx

employed as an additive in DMSO

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

as an additive can achieve a

·3DMSO, that

83

, which makes it difficult to control the film morphology,

perovskite-structured material usually exhib-

NH3

NH3

MA1-xSnX3

as an active layer.

Sn0.5Pb0.5I<sup>3</sup>

Sn0.5Pb0.5I<sup>3</sup>

), and

shows

and form an intermediate phase, SnI<sup>2</sup>

, with SnF<sup>2</sup>
