**4. Toxicity and stability issue**

#### **4.1 Lead: the toxicity problem**

Regardless of the excellent properties and high efficiencies, the poor stability of organic–inorganic perovskite materials are yet a serious challenge, inhibiting PSCs from being commercialized. To be marketable for commercial purposes, PSCs need to be capable of work constantly for over 20 years under outdoor conditions. Thus, large consideration has recently been centered to overcome barriers associated with stability and environmental compatibility of perovskite materials [10].

Presently there is a debate on the use of lead (main component) in PSCs, which causes toxicity problems during device manufacture, placement, and disposal. Hence, the toxicity of lead-based perovskites is an obvious problem due to leaching of lead into the environment [9]. Lead toxicity has been pointed out as one of the most challenging barriers towards the commercialization of solar cells, as compared to stability issues and cost-effective production ways. The environmental impact benefits of lead-free (or lead-reduced) solar cells have been analyzed by Life Cycle Assessment (LCA) [15].

#### *4.1.1 Lead-free perovskite solar cells*

Up to now, several research groups have ambiguously proven their solution to this challenge. Thus, it is critical to test alternatives to attain similar optical and photovoltaic performances for the commercialization of PSCs. Several research groups have tried to replace lead with other elements (Sn, Ge) and organic cations with inorganic cations to form new appropriate non-toxic and stable perovskite materials, which may be a long journey before the final commercialization of PSCs [9].

It is worth studying alternatives using lead-free PSCs, but Lead-free PSCs reached a PCE of only 6% at a time when lead-based PSCs produced efficiency of 17%. Moreover, the Sn-based solar cells display poorer stability than Pb-based solar cells. [14]. Bivalent Sn is the most favorable choice for replacing Pb as they both are in the same group and possess analogous lone-pair s orbitals [10]. Both Sn- and Pb-based materials have a tetragonal structure under ambient conditions; however, Sn-based perovskite have a higher symmetrized α phase as compared to the Pb-based materials lower symmetrized ß phase [47]. Chung et al. [54] first demonstrated CsSnI3 as a solid electrolyte in DSSCs. Afterward Chen et al. [58] fabricated a photovoltaic device, ITO/CsSnI3/Au/Ti, attaining very low PCE of 0.88%. However, Sn2+ based perovskite undergoes oxidation from Sn2+ to Sn4+, which is destructive

**81**

*4.2.1 Moisture*

**4.2 Stability**

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

this material as a solid hole transport material in DSSCs.

moisture, oxygen, light, and heat [63, 64].

been considered as a major contributor to environmental impact [61].

of perovskite solar cells is the most significant issue in this field [15].

Perovskite solar cells (PSCs) have been established with promising PCEs. Regardless of the great potential as PV material in terms PCE, the instability of the PSCs is one of the core barriers for larger scale applications [8, 9, 39]. At present, PSCs can only perform for several months under active conditions, whereas traditional silicon cells can operate for more than 25 years. Therefore, stability issues must be reasonably dealt with before its actual use and commercialization [62]. Poor stability of PSCs is due to several affiliated factors resulting from exposure to

Nevertheless, the importance of stability has been highlighted and recognized as the foremost problem, in the past five years to solve for the perovskite solar cells (PSCs) to be able to challenge in the market arena. So, how to increase the stability

In this section, the effect of environmental factors will be discussed on PSCs along with approaches developed to improve stability of perovskite solar cells.

The first environmental factor observed to degrade perovskites was Moisture/ water. The instability of perovskite at high humidity is the serious issue that needs to be focused. Solar cells when exposed to moisture (water), due to the hygroscopic

for the charge transport properties, and PCE. Recently, Lv 2019 [59] reported the replacement of spiro-OMeTAD by a Zn-derivative porphyrin in a lead-free solar cell

There is another approach of mixed Pb/Sn perovskite Solar cell have also been reported. Lead and tin were revealed to be arbitrarily spread in the [MX6] octahedra in the perovskite and percentage of tin could be altered from 0 to 1 [15]. These devices presented the best photocurrent at a 50% mixing ratio. SnO used as ETL has also resulted with good PCE of (13%) and stability (>700 h storage) [15, 60]. LCA showed the replacement of lead did not decrease the environmental impacts, meanwhile the loss of PCE and stability generates an environmental burden. However, those studies are also interesting because they draw consideration to other toxicity problems occurring from the solvent use during processing of charge transport layers (ETLs) [15]. For lead-free inorganic perovskites Tetravalent cations have also been thought to replace Pb. A new chemical formula of A2BX6 structure is designed by eliminating half of the B-site ions in the ABX3 perovskite for adjusting the heterovalent cation substitution as shown in **Figure 1**. Due to the lack of connectivity in the [BX6] octahedral structure, the A2BX6 can be considered as a 0D non-perovskite which results in different optical and optoelectronic properties of the A2BX6 from those of the ABX3. [3, 9, 10]. Amongst the A2BX6 perovskites, Cs2SnI6, Cs2TiBr6 and Cs2PdBr6 have been employed in photovoltaic devices [9, 10]. Chung et al. [54] first utilized

Furthermore, a special concern for toxicity must be upraised during experimental work in the laboratory, since hazards arise primarily by the absorption of the toxic lead when used in solution, which is significantly higher, particularly through the dermal and respiratory routes; some of the lead derivatives are soluble both in water and fat, posing a high risk. Solvents such as dimethylformamide (DMF) and dimethylsulfoxide (DMSO) are not only toxic, but also raise the risk of bio incorporation as they are miscible in all ratios with water. Thus, these solvents have also

has resulted in stability up to 60 h for water and 100 h for thermal stability.

*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*

for the charge transport properties, and PCE. Recently, Lv 2019 [59] reported the replacement of spiro-OMeTAD by a Zn-derivative porphyrin in a lead-free solar cell has resulted in stability up to 60 h for water and 100 h for thermal stability.

There is another approach of mixed Pb/Sn perovskite Solar cell have also been reported. Lead and tin were revealed to be arbitrarily spread in the [MX6] octahedra in the perovskite and percentage of tin could be altered from 0 to 1 [15]. These devices presented the best photocurrent at a 50% mixing ratio. SnO used as ETL has also resulted with good PCE of (13%) and stability (>700 h storage) [15, 60]. LCA showed the replacement of lead did not decrease the environmental impacts, meanwhile the loss of PCE and stability generates an environmental burden. However, those studies are also interesting because they draw consideration to other toxicity problems occurring from the solvent use during processing of charge transport layers (ETLs) [15].

For lead-free inorganic perovskites Tetravalent cations have also been thought to replace Pb. A new chemical formula of A2BX6 structure is designed by eliminating half of the B-site ions in the ABX3 perovskite for adjusting the heterovalent cation substitution as shown in **Figure 1**. Due to the lack of connectivity in the [BX6] octahedral structure, the A2BX6 can be considered as a 0D non-perovskite which results in different optical and optoelectronic properties of the A2BX6 from those of the ABX3. [3, 9, 10]. Amongst the A2BX6 perovskites, Cs2SnI6, Cs2TiBr6 and Cs2PdBr6 have been employed in photovoltaic devices [9, 10]. Chung et al. [54] first utilized this material as a solid hole transport material in DSSCs.

Furthermore, a special concern for toxicity must be upraised during experimental work in the laboratory, since hazards arise primarily by the absorption of the toxic lead when used in solution, which is significantly higher, particularly through the dermal and respiratory routes; some of the lead derivatives are soluble both in water and fat, posing a high risk. Solvents such as dimethylformamide (DMF) and dimethylsulfoxide (DMSO) are not only toxic, but also raise the risk of bio incorporation as they are miscible in all ratios with water. Thus, these solvents have also been considered as a major contributor to environmental impact [61].

### **4.2 Stability**

*Perovskite and Piezoelectric Materials*

resulted in a reduction in the bandgap. A tunable bandgap can be obtained (between 1.48 and 2.23 eV) by replacing the methylammonium with a slightly larger formamidinium cation. Significantly, the reduced bandgap led to a PCE of up

CH3NH3SnI3 is demanded to be a low-carrier-density p-type metal. Theoretical calculations on perovskite recommended that their electronic properties intensely depend on the structure of the inorganic cage and formation of the perovskite octahedral network. By changing the inorganic and organic components and their stoichiometric ratio, it is probable to control the system dimensionality and electronic and optical properties. Furthermore, the presence of weak bonds in the perovskite structures ensures malleability and flexibility that could permit the

Regardless of the excellent properties and high efficiencies, the poor stability of organic–inorganic perovskite materials are yet a serious challenge, inhibiting PSCs from being commercialized. To be marketable for commercial purposes, PSCs need to be capable of work constantly for over 20 years under outdoor conditions. Thus, large consideration has recently been centered to overcome barriers associated with

Presently there is a debate on the use of lead (main component) in PSCs, which causes toxicity problems during device manufacture, placement, and disposal. Hence, the toxicity of lead-based perovskites is an obvious problem due to leaching of lead into the environment [9]. Lead toxicity has been pointed out as one of the most challenging barriers towards the commercialization of solar cells, as compared to stability issues and cost-effective production ways. The environmental impact benefits of lead-free (or lead-reduced) solar cells have been analyzed by Life Cycle

Up to now, several research groups have ambiguously proven their solution to this challenge. Thus, it is critical to test alternatives to attain similar optical and photovoltaic performances for the commercialization of PSCs. Several research groups have tried to replace lead with other elements (Sn, Ge) and organic cations with inorganic cations to form new appropriate non-toxic and stable perovskite materials, which may be a long journey before the final commercialization of PSCs [9]. It is worth studying alternatives using lead-free PSCs, but Lead-free PSCs reached a PCE of only 6% at a time when lead-based PSCs produced efficiency of 17%. Moreover, the Sn-based solar cells display poorer stability than Pb-based solar cells. [14]. Bivalent Sn is the most favorable choice for replacing Pb as they both are in the same group and possess analogous lone-pair s orbitals [10]. Both Sn- and Pb-based materials have a tetragonal structure under ambient conditions; however, Sn-based perovskite have a higher symmetrized α phase as compared to the Pb-based materials lower symmetrized ß phase [47]. Chung et al. [54] first demonstrated CsSnI3 as a solid electrolyte in DSSCs. Afterward Chen et al. [58] fabricated a photovoltaic device, ITO/CsSnI3/Au/Ti, attaining very low PCE of 0.88%. However, Sn2+ based perovskite undergoes oxidation from Sn2+ to Sn4+, which is destructive

stability and environmental compatibility of perovskite materials [10].

to 14.2% and high short circuit currents (>23 mA cm−2) [31].

deposition of thin films on flexible substrates [26].

**4. Toxicity and stability issue**

**4.1 Lead: the toxicity problem**

Assessment (LCA) [15].

*4.1.1 Lead-free perovskite solar cells*

**80**

Perovskite solar cells (PSCs) have been established with promising PCEs. Regardless of the great potential as PV material in terms PCE, the instability of the PSCs is one of the core barriers for larger scale applications [8, 9, 39]. At present, PSCs can only perform for several months under active conditions, whereas traditional silicon cells can operate for more than 25 years. Therefore, stability issues must be reasonably dealt with before its actual use and commercialization [62]. Poor stability of PSCs is due to several affiliated factors resulting from exposure to moisture, oxygen, light, and heat [63, 64].

Nevertheless, the importance of stability has been highlighted and recognized as the foremost problem, in the past five years to solve for the perovskite solar cells (PSCs) to be able to challenge in the market arena. So, how to increase the stability of perovskite solar cells is the most significant issue in this field [15].

In this section, the effect of environmental factors will be discussed on PSCs along with approaches developed to improve stability of perovskite solar cells.

#### *4.2.1 Moisture*

The first environmental factor observed to degrade perovskites was Moisture/ water. The instability of perovskite at high humidity is the serious issue that needs to be focused. Solar cells when exposed to moisture (water), due to the hygroscopic 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 stability under ambient conditions.

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 crystallize, forcing the forward reaction. Moreover, MA<sup>+</sup> is slightly acidic and reacts with water to form volatile methylamine (CH3NH3) and hydroiodic acid (HI), according to the following reaction (1): [63].

$$\left(\mathrm{CH}\_{3}\mathrm{NH}\_{3}\right)\_{4}\mathrm{PbI}\_{6}.2\mathrm{H}\_{2}\mathrm{O}\rightarrow\mathrm{PbI}\_{2}+4\mathrm{CH}\_{3}\mathrm{NH}\_{3}+4\mathrm{HI}+2\mathrm{H}\_{2}\mathrm{O}\tag{2}$$

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 be correlated with the improved stability [65].

In demand to progress the chemical stability of MAPbI3-based PSCs against moisture, scientists have proposed replacing the organic cation MA<sup>+</sup> with alternative 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 environment [33].

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) without encapsulation.

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

#### *4.2.2 Light*

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

**83**

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

by a photogenerated reactive oxygen species (superoxide, O2

react with oxygen, hence decreasing the amount of superoxide O2

(ETL) for these PSCs is responsible for UV-induced degradation. According to the international standards for climate chamber tests (IEC 61646), solar cells need to

Bryant et al. [67] demonstrated that contact of MeNH3PbI3 films to both light and molecular oxygen can initiate quick degradation. Particularly, this reaction is started by the deprotonation of the methylammonium cation of the perovskite

MeNH3PbI3 based devices was checked under different operating (e.g. light and dark) and environmental conditions and infer that oxygen induced degradation, is relatively dominant than moisture induced degradation and limits the working stability of MeNH3PbI3 containing devices under ambient conditions. Moreover, they pointed out that this fall in device performance can be reduced by the addition of electron acceptor layers within device architecture. Such layers are exposed to augment electron extraction from the absorber (perovskite material) before they

It was noticed that by replacing MA with Cs and FA resulted in improved photostability of the PSCs. By systematically monitoring the development of PL intensity of perovskites, light-induced formation and annihilation of defects were reported to induce photo-instability [68]. Photostability can be improved through defect control by passivating which acted as a defect reservoir on the surface and grain boundaries. To stabilize surface defects, polyethylene oxide was applied and thus improved photostability was achieved. By substituting MA with FA, the degradation became slow with small pores forming on the surface after exposure to light. Moreover, Addition of Cs into the MAFA (forming CsMAFA) further lessen the degradation. XPS, XRD, Fourier transform infrared (FT-IR) spectrometry, and ultraviolet-visible absorption spectrometry were used to investigate the variation of MAPbI3 films under illumination. The result showed that light induced degradation is the main cause of degradation. Using polymer such as PTAA (Poly(triarylamine)) as the HTM, it was observed that pure MAPbI3 devices retained nearly 100% of their initial efficiency after 1000 h aging under constant illumination at room temperature. PTAA which act as a protection layer, inhibited the discharge of gaseous degradation products enhanced stability. However, for devices using Spiro as the HTM, their stability under illumination was lesser than

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

Commercial solar cells should be able to work efficiently above 85°C, to have any

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

CH NH PbI CH NH I PbI 3 33 3 3 2 → + (3)

PbI Pb0 I 2 2 → + (4)

−

). The stability of

−

cations under thermal atmosphere.

and increasing

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

tolerate long-term stability at 85°C.

the device stability.

that using PTAA [69, 70].

influence in the market.

decompose due to the instability of organic A<sup>+</sup>

decomposition reactions (2) and (3):

*4.2.3 Heat*

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

(ETL) for these PSCs is responsible for UV-induced degradation. According to the international standards for climate chamber tests (IEC 61646), solar cells need to tolerate long-term stability at 85°C.

Bryant et al. [67] demonstrated that contact of MeNH3PbI3 films to both light and molecular oxygen can initiate quick degradation. Particularly, this reaction is started by the deprotonation of the methylammonium cation of the perovskite by a photogenerated reactive oxygen species (superoxide, O2 − ). The stability of MeNH3PbI3 based devices was checked under different operating (e.g. light and dark) and environmental conditions and infer that oxygen induced degradation, is relatively dominant than moisture induced degradation and limits the working stability of MeNH3PbI3 containing devices under ambient conditions. Moreover, they pointed out that this fall in device performance can be reduced by the addition of electron acceptor layers within device architecture. Such layers are exposed to augment electron extraction from the absorber (perovskite material) before they react with oxygen, hence decreasing the amount of superoxide O2 − and increasing the device stability.

It was noticed that by replacing MA with Cs and FA resulted in improved photostability of the PSCs. By systematically monitoring the development of PL intensity of perovskites, light-induced formation and annihilation of defects were reported to induce photo-instability [68]. Photostability can be improved through defect control by passivating which acted as a defect reservoir on the surface and grain boundaries. To stabilize surface defects, polyethylene oxide was applied and thus improved photostability was achieved. By substituting MA with FA, the degradation became slow with small pores forming on the surface after exposure to light. Moreover, Addition of Cs into the MAFA (forming CsMAFA) further lessen the degradation. XPS, XRD, Fourier transform infrared (FT-IR) spectrometry, and ultraviolet-visible absorption spectrometry were used to investigate the variation of MAPbI3 films under illumination. The result showed that light induced degradation is the main cause of degradation. Using polymer such as PTAA (Poly(triarylamine)) as the HTM, it was observed that pure MAPbI3 devices retained nearly 100% of their initial efficiency after 1000 h aging under constant illumination at room temperature. PTAA which act as a protection layer, inhibited the discharge of gaseous degradation products enhanced stability. However, for devices using Spiro as the HTM, their stability under illumination was lesser than that using PTAA [69, 70].
