*4.2.3 Heat*

*Perovskite and Piezoelectric Materials*

ity under ambient conditions.

to the following reaction (1): [63].

environment [33].

without encapsulation.

lize, forcing the forward reaction. Moreover, MA<sup>+</sup>

be correlated with the improved stability [65].

nature of the organic components of perovskite materials are spontaneously affected by moisture access and then degrade [62]. It has basically been supposed that moisture-induced degradation is the leading issue, imitating MNH3PbI3 stabil-

Prolonged exposure of perovskite material to water vapor activates an irreversible degradation which eventually leads to transformation of the perovskite back to the initial precursors (such as PbI2). In detail, perovskite forms hydrate complexes with water such as (CH3NH3)4PbI6·2H2O and leaves out PbI2, which tend to crystal-

water to form volatile methylamine (CH3NH3) and hydroiodic acid (HI), according

Some researchers have reported that the compositions, microstructures (such as grain size) also affected the moisture stability of perovskite devices and concluded that larger grains resulted in a smaller area density of grain boundaries, which can

In demand to progress the chemical stability of MAPbI3-based PSCs against

Smith et al. [66] discussed that Low-dimensional 2-D perovskites exhibited better moisture stability than 3D perovskites due to the hydrophobic nature of organic cations. Though, the insulating aspect of the organic cations with poorer charge transport resulted in lower PCE as compared to 3D perovskites. Therefore, various efforts have been made to form a quasi-2D (or 2D–3D mixture) and 2D on top of 3D (2D@3D) to use the benefits of both 2D and 3D perovskites. The use of 2D perovskite is mostly to improve the moisture stability, a thin 2D layer was deposited on top of 3D MAPbI3 perovskite to cover it fully and shield the 3D perovskite from moisture. The highest PCE for 2D@3D perovskite solar cell was observed to be of 18.0%, with an enhanced device stability under both inert (90% of initial PCE for 32 d) and ambient conditions (72% of initial PCE for 20 d)

Polymers, such as poly(4-vinylpyridine) (PVP), poly (methyl methacrylate) (PMMA) covering p-type and n-type semiconductors, or insulators, were also reported to improve stability. These long chain polymer acts as defect passivator and a moisture blocker by forming a network along perovskite grains and resulted

Light-induced perovskite solar cell degradation and environmental stability are the most frequently cited villains. Early on, stability of PSC was a big issue. But just as there were quick improvements in efficiency of PSCs, there has also been similar quick progresses in stability. Ultraviolet light (UV) can also cause the degradation of MAPbI3 perovskite. For e.g. the commonly used TiO2 electron transport layer

in improved device efficiency and stability [58, 60, 64].

tive components at the A position. For example, FAPbI3 has been presented to be further thermally stable than MAPbI3 because of its larger tolerance factor. Though, FAPbI3 suffers a phase transition from the a-FAPbI3 (black triangular) phase to the d-FAPbI3 (yellow hexagonal) phase due to the presence of moisture. Furthermore, degradation of FA0.9Cs0.1PbI is prevented by adding a small amount of cesium (Cs) into orbital lead-iodine to form FA0.9Cs0.1PbI in high humidity

moisture, scientists have proposed replacing the organic cation MA<sup>+</sup>

(CH NH PbI .2H O bI 4CH NH 4HI 2H O 33 6 2 2 33 )<sup>4</sup> → + ++ *P* <sup>2</sup> (2)

is slightly acidic and reacts with

with alterna-

**82**

*4.2.2 Light*

Heat is also another factor that influences stability due to the inherent matter with low formation energies, and perovskites thus have a great response to a small increase in external temperature [71]. Organic-inorganic perovskites tend to decompose due to the instability of organic A<sup>+</sup> cations under thermal atmosphere. Commercial solar cells should be able to work efficiently above 85°C, to have any influence in the market.

MAPbI3 is basically unstable upon thermal stress which produces a discharge of I2 and the presence of metallic Pb at 40°C in the dark [63]. This is produced by the decomposition reactions (2) and (3):

$$\text{CH}\_3\text{NH}\_3\text{PbI}\_3 \rightarrow \text{CH}\_3\text{NH}\_3\text{I} + \text{PbI}\_2 \tag{3}$$

$$\text{PbI}\_2 \rightarrow \text{PbO} + \text{I}\_2 \tag{4}$$

Although reaction (3) is reversible at just 80–85°C, methylammonium iodide decomposes into more volatile compounds as represented by reactions (4) and (5):

$$\text{CH}\_3\text{NH}\_3\text{I} \rightarrow \text{CH}\_3\text{I} + \text{NH}\_3 \tag{5}$$

$$\text{CH}\_3\text{NH}\_3\text{I} \rightarrow \text{CH}\_3\text{NH}\_2 + \text{HI} \tag{6}$$

It was found that HI(g) and CH3NH2(g) were dominant products during the decomposition of MAPbI3 and only trace amounts of CH3I and NH3 were found. Though, the ratio of CH3I and NH3 increased at higher temperature and lesser than HI(g) and CH3NH2(g). In short, HI(g) and CH3NH2(g) were the dominant decomposition products at ambient temperature under vacuum while CH3I and NH3 gases were obtained at high temperature. Both processes occurred simultaneously near ambient temperature in vacuum and the later was favored at high temperature.

To find out the decomposition temperature of perovskites, Thermogravimetric analysis (TGA) was used. From the mass loss of TGA curve for MAPbI3, the decomposition onset temperature was found to be 234°C [62]. This indicates that as the practical application temperature usually is less than 100°C, so this high decomposition temperature made the stability of MAPbI3 not a big issue. The as prepared film did not show any changes in XRD patterns when stayed inside the vacuum for up to three days. This might be owing to the purer perovskite films without any exposure to the ambient atmosphere. Though, the commonly degradation of the perovskite solar cell was apparent even with encapsulation. This could be inadequate to estimate the long-term stability of a photovoltaic material, which is essential to work for a long time at temperatures lower than the decomposition temperature [72]. The fact that inert condition and encapsulation cannot completely avoid MAPbI3 perovskite degradation. At low temperature, the degradation of MAPbBr3 was found by only releasing HBr and CH3NH2 gases [69]. The encapsulation of devices is essential not only to prevent exposure to oxygen and moisture, but also to avoid leakage of volatile decomposition products. Photostability can also be increased by replacing MA cation with more stable Cs/FA combination.

Substituting organic cations with inorganic Cs<sup>+</sup> or Rb+ cations is also valuable to stabilize perovskite solar cells [73, 74]. Grancini et al. [74] stated an ultra-stable 2D/3D (HOOC(CH2)4NH3)2PbI4/CH3NH3PbI3 perovskite, presenting a PCE of 12.9% with carbon electrodes and 14.6% with the normal mesoporous structure and stability of one-year.

By introducing n-butylammonium iodide (BAI) to MAPbI3 perovskite, a mixed 2D (BA)2PbI4 structure is formed, which probably provide an improved protection for the 3D perovskite against heat stress [75]. Octylammonium (OA) cation has also been reported to enhance the thermal stability of perovskites and keep 80% of their initial efficiency for 760 h aged at 85°C in ambient atmosphere without encapsulation [76]. Other additives, such as π-conjugated polymer, nonvolatile ionic liquids, bifunctional hydroxylamine hydrochloride guanidinium isothiocyanate, have also been reported to improve the thermal stability of various perovskites [77–79].

#### **5. Conclusion and perspective**

The discovery and development of organic inorganic perovskite materials have become a hot research topic in the field of photovoltaics. This chapter deals with a comprehensive discussion on the properties and applications of organic inorganic perovskites materials in PSCs. The extraordinarily outstanding performances of organic inorganic perovskites result of their excellent properties. Solar cells based

**85**

**Author details**

Peshawar, Pakistan

Madeeha Aslam, Tahira Mahmood\* and Abdul Naeem

provided the original work is properly cited.

\*Address all correspondence to: tahiramahmood@uop.edu.pk

National Centre of Excellence in Physical Chemistry, University of Peshawar,

© 2020 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,

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material*

on organic inorganic perovskite materials have achieved much advancement, both in PCE and stability, in a short very time. To PSCs, though great progress has been attained, there are still various obstacles in terms of stability and toxicity until its practical usage in the PV market. However, for large-scale performance, are required to be overcome. So far, great research has been made to overcome these issues by changing the composition of organic inorganic perovskite material either by replacing Pb with Sn or Ge or organic methyl with other organic or inorganic cations. However, commercialization of an organic inorganic perovskite solar cell needs further development in both efficiency and long-term stability, with low-cost photovoltaic materials and ease of printability. To increase stability, various methods such as the use of buffer layers, varying the composition of organic inorganic perovskite materials, and better techniques of encapsulation. In inference, the research which has been enduring for the past five years has attained significant results. Future research needs to endeavor for longer stability with high efficiency.

*DOI: http://dx.doi.org/10.5772/intechopen.94104*

### *Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material DOI: http://dx.doi.org/10.5772/intechopen.94104*

on organic inorganic perovskite materials have achieved much advancement, both in PCE and stability, in a short very time. To PSCs, though great progress has been attained, there are still various obstacles in terms of stability and toxicity until its practical usage in the PV market. However, for large-scale performance, are required to be overcome. So far, great research has been made to overcome these issues by changing the composition of organic inorganic perovskite material either by replacing Pb with Sn or Ge or organic methyl with other organic or inorganic cations. However, commercialization of an organic inorganic perovskite solar cell needs further development in both efficiency and long-term stability, with low-cost photovoltaic materials and ease of printability. To increase stability, various methods such as the use of buffer layers, varying the composition of organic inorganic perovskite materials, and better techniques of encapsulation. In inference, the research which has been enduring for the past five years has attained significant results. Future research needs to endeavor for longer stability with high efficiency.
