**2. Sn-based perovskites**

Tin (Sn) element has been widely used as an alternative to Pb, since both occur in group IVA of the periodic table with similar ionic radii (Pb: 1.49 Å and Sn: 1.35 Å). Therefore, Pb substitution by Sn would cause no obvious lattice distortion in perovskites [27]. The intrinsic instability that results in the decomposition of unstable products such as SnI2 and HI (acidifier) and toxicologically inactive oxygenated Sn precipitates are still remained the toxicity issues in the Sn-based perovskites [16]. However, benefiting from the stability of PSCs based on Sn perovskites and easy cleaning of Sn from the human body compared to Pb, Sn-based perovskites could be a better choice than perovskite with Pb cation. Density functional theory (DFT) and GW approximation are typically used for the structural and electrical properties of Sn-based perovskites [28, 29]. For instance, the theoretical–experimental calculations about the bandgap of MASnI3 showed

**3**

**Figure 2.**

*with permission from [30, 31].*

*Quest for Lead-Free Perovskite-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83360*

in a photocurrent of 21 mA cm<sup>−</sup><sup>2</sup>

cation by Cs+

a range of 1–1.3 eV, charge mobility of 1.6 cm2

However, plasticity of the tin-halide-tin angle and Cs<sup>+</sup>

and 8.1%, respectively (**Figure 3C** and **D**) [42, 43].

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

in ASnX3 yields higher charge mobility, lower exciton binding energy,

. In order to obtain low doping level in MASnI3-

30 nm (**Figure 2A**) [29, 30, 32, 33]. Additionally, SnF2-doped MASnI3 showed tenfold larger carrier lifetime with diffusion length of 500 nm (**Figure 2B**) [31]. Replacing MASnI3 by FASnI3 results in a desirable bandgap of 1.41 eV and minimized oxidation of Sn2+ to Sn4+ [34, 35]. It is also reported that replacement of "A"

and large optical absorption coefficient compared to conventional MAPbI3 [36, 37].

Since Sn-based perovskites possess low crystallization barrier and high solubility in solvents, their thin films can be fabricated at low temperature in the PSCs. For example, a high crystalline MASnI3-perovskite thin film has been prepared from a transitional SnI2∙3DMSO intermediate phase (**Figure 3A**) [40]. This high-quality perovskite film formation in a hole-selective layer-free PSCs resulted

It is reported that the oxidation of Sn-based perovskites (self-doping from Sn2+ to Sn4+) leads to carrier recombination and poor device performance. In this context, specific amount of SnF2 can be used as an inhibitor for Sn4+. The large quantity of SnF2 may generate phase separation such as plate-like aggregates on the perovskite film surface. Here, the strong binding affinity in SnF2-pyrazine complex was helpful to

*(A) Transient terahertz photoconductivity and simulated crystal structure of MASnI3. (B) Time-resolved photoluminescence for 20 mol (%) SnF2-doped MASnI3 thin film and schematic device illustration. Reprinted* 

CsSnX3 a phase transitional perovskite with respect to temperature [39].

perovskite thin film, a low-temperature vapor-assisted solution process was employed (**Figure 3B**), where the excess of Sn2+ compounds due to Sn(OH)2 and SnO resulted in low hole-doping level [41, 44]. Furthermore, anti-solvent dripping process was used to control fast crystallization and fabricate pinhole-free thin films of Sn-based perovskites. The diethyl ether dripping on FASnI3 and chlorobenzene on (FA)0.75(MA)0.25SnI3 enabled the as-prepared PSCs to obtain efficiency of 6.2

, and diffusion length up to

cation migration [38] showed

## *Quest for Lead-Free Perovskite-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83360*

*A Guide to Small-Scale Energy Harvesting Techniques*

way, Cu2+, Ge2+, Bi3+, Sb3+, and Sn2+ with *ns*<sup>2</sup>

**2. Sn-based perovskites**

**Figure 1.**

where *r*A, *r*B, and *r*X denote the ionic radii of A, B, and X, respectively. If the value of "*t*" results in the range between 0.813 and 1.107, then it is considered as a high-symmetry cubic three-dimensional (3D) perovskite, while two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) perovskites are formed when "t" gives other values than the abovementioned range [21, 22]. The structural dimensionality approach is considered one of the essential factors because different dimensions of perovskite influence the kinetics of charge carriers. Nonetheless, this evaluation is not enough to be applied to all perovskite semiconductors. Therefore the probe for electronic dimensionality is equally important [23]. For example, perovskite materials with low electronic dimensionality but high structural dimensionality have less promises as light absorbers because of the barrier to isotropic current flow, large effective masses of holes/electrons, and deeper defect states. The excellent performance of Pb-based perovskites is mainly due to high structural symmetry and strong antibonding coupling between Pb and I [24]. In a similar

*Basic crystal structure of perovskite semiconductor, where A, B, and X represent (CH3NH3*

*(Cu2+, Pb2+, Sn2+, Ge2+, Bi3+, Sb3+), and (I<sup>−</sup>, Cl<sup>−</sup>, Br<sup>−</sup>), respectively.*

to obtain octahedral structure; therefore, they are investigated as alternatives to the toxic Pb element [15, 25, 26]. Herein, we will introduce the Pb-free perovskites from

Tin (Sn) element has been widely used as an alternative to Pb, since both occur in group IVA of the periodic table with similar ionic radii (Pb: 1.49 Å and Sn: 1.35 Å). Therefore, Pb substitution by Sn would cause no obvious lattice distortion in perovskites [27]. The intrinsic instability that results in the decomposition of unstable products such as SnI2 and HI (acidifier) and toxicologically inactive oxygenated Sn precipitates are still remained the toxicity issues in the Sn-based perovskites [16]. However, benefiting from the stability of PSCs based on Sn perovskites and easy cleaning of Sn from the human body compared to Pb, Sn-based perovskites could be a better choice than perovskite with Pb cation. Density functional theory (DFT) and GW approximation are typically used for the structural and electrical properties of Sn-based perovskites [28, 29]. For instance, the theoretical–experimental calculations about the bandgap of MASnI3 showed

previously reported theoretical calculations and experimental studies.

lone pairs could be used with halides

*+*

*, CH(NH2)2*

*+ , Cs+ ),* 

**2**

a range of 1–1.3 eV, charge mobility of 1.6 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> , and diffusion length up to 30 nm (**Figure 2A**) [29, 30, 32, 33]. Additionally, SnF2-doped MASnI3 showed tenfold larger carrier lifetime with diffusion length of 500 nm (**Figure 2B**) [31]. Replacing MASnI3 by FASnI3 results in a desirable bandgap of 1.41 eV and minimized oxidation of Sn2+ to Sn4+ [34, 35]. It is also reported that replacement of "A" cation by Cs+ in ASnX3 yields higher charge mobility, lower exciton binding energy, and large optical absorption coefficient compared to conventional MAPbI3 [36, 37]. However, plasticity of the tin-halide-tin angle and Cs<sup>+</sup> cation migration [38] showed CsSnX3 a phase transitional perovskite with respect to temperature [39].

Since Sn-based perovskites possess low crystallization barrier and high solubility in solvents, their thin films can be fabricated at low temperature in the PSCs. For example, a high crystalline MASnI3-perovskite thin film has been prepared from a transitional SnI2∙3DMSO intermediate phase (**Figure 3A**) [40]. This high-quality perovskite film formation in a hole-selective layer-free PSCs resulted in a photocurrent of 21 mA cm<sup>−</sup><sup>2</sup> . In order to obtain low doping level in MASnI3 perovskite thin film, a low-temperature vapor-assisted solution process was employed (**Figure 3B**), where the excess of Sn2+ compounds due to Sn(OH)2 and SnO resulted in low hole-doping level [41, 44]. Furthermore, anti-solvent dripping process was used to control fast crystallization and fabricate pinhole-free thin films of Sn-based perovskites. The diethyl ether dripping on FASnI3 and chlorobenzene on (FA)0.75(MA)0.25SnI3 enabled the as-prepared PSCs to obtain efficiency of 6.2 and 8.1%, respectively (**Figure 3C** and **D**) [42, 43].

It is reported that the oxidation of Sn-based perovskites (self-doping from Sn2+ to Sn4+) leads to carrier recombination and poor device performance. In this context, specific amount of SnF2 can be used as an inhibitor for Sn4+. The large quantity of SnF2 may generate phase separation such as plate-like aggregates on the perovskite film surface. Here, the strong binding affinity in SnF2-pyrazine complex was helpful to

#### **Figure 2.**

*(A) Transient terahertz photoconductivity and simulated crystal structure of MASnI3. (B) Time-resolved photoluminescence for 20 mol (%) SnF2-doped MASnI3 thin film and schematic device illustration. Reprinted with permission from [30, 31].*

#### **Figure 3.**

*(A) The schematic illustrations of the SnI(DMSO)3+ ions linked with lone I<sup>−</sup> ions, unit cell of SnI2***∙***3DMSO, and the film formation of the MASnI3 perovskite starting from SnI2 through SnI2***∙***3DMSO intermediate. (B) Scanning electron microscopy images of MASnI3 thin films obtained through low-temperature vaporassisted solution process and vapor-assisted solution method. (C) Current–voltage characteristic curves of the cell based on FASnI3 with 10% SnF2 additives. (D) Current-voltage curves of the best device based on (FA)0.75(MA)0.25SnI3 perovskite. Reprinted with permission from [40–43].*

inhibit the phase separation caused by excess SnF2 (**Figure 4A**) [45]. Furthermore, the built-in potential can also be optimized through SnF2 and thus high open-circuit voltage of the device by energetic landscape alignments [46] as illustrated in (**Figure 4B**). Besides SnF2, other additives such as SnBr2, SnI2, and SnCl2 have also been explored where SnCl2 exhibits the highest stability by inhibiting the decomposition/oxidation [34, 47]. Moreover, the employment of hypophosphorous acid in CsSnIBr2 showed seed-like perovskite where Sn2+ oxidation was significantly reduced [48]. As a result, the charge recombination rate was decreased by fourfold compared to the control devices. The as-prepared cells displayed excellent oxygen-moisture stability at ambient conditions and thermal stability in vacuum environment.

The inclusion of large ammonium cations for tuning the dimensionality or generating massive Schottky defects has also been tested in Sn-based perovskites to optimize the device performance. For example, a different ratio of phenylethylammonium (PEA) as a cation can yield perovskites with two-dimensional, threedimensional, and three-dimensional-two-dimensional-mixed dimensionalities. The high-quality thin film of the mixed PEA-FA perovskite (20% of PEA) delivered a stable device efficiency of 5.9% [49]. The composition of ethylenediammonium (en) and formamidinium (FA) in FASnI3/MASnI3/CsSnI3 crystal indicates no effect on the dimensionality of the perovskite because the size of en is too large for the unit cell cage and it can remove a certain {SnI}<sup>+</sup> species. Here, the point defects are recognized as the Schottky defects. However, the optimized devices based on {en} MASnI3 and {en}FASnI3 showed efficiency of 6.6 and 7.1%, respectively [50, 51].

**5**

**Figure 4.**

*Quest for Lead-Free Perovskite-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83360*

**3. Ge-, Bi-, Sb-, and Cu-based perovskites**

Although Sn-based PSC has achieved PCE over 8%, the oxidation of Sn and degradation of the perovskite still need to be addressed. In this context, other metal halide perovskites such as Ge, Bi, Sb, and Cu are considered as potential candidates to replace Pb element in the perovskite crystal. As Ge 4 s has higher orbital energy compared to Pb 6 s and Sn 5 s, thus Ge-based perovskites should exhibit smaller bandgaps. However, in practice the Ge-based perovskites such as CsGeI3, MAGeI3, and FAGeI3 displayed larger bandgaps (i.e., 1.63, 2.0, and 2.35 eV) than that of CH3NH3SnI3 (1.30 eV) and CH3NH3PbI3 (1.55 eV) [52]. The difference between experimental data and what we expected from high orbital energy is mainly due to the structural distortion of [GeI6] octahedral as the small ionic radius (0.73 Å) of Ge2+ substituting the bigger ionic radius of Pb2+ (1.19 Å) or Sn2+ (1.02 Å) [53]. Additionally, the Ge-based perovskites crystallize in polar space groups [54]. The Ge2+ cation cannot sustain at the center of octahedron and forms three long Ge▬I bonds (2.73–2.77 Å) and three short Ge-I bonds (3.26–3.58 Å) [53]. This means that the Ge cation cannot maintain

*(A) Schematic crystal structure of FASnI3 with SnF2 and SnF2-pyrazine complex with corresponding scanning electron microscopy images. (B) Representation of energetic landscapes of the TiO2, pure CsSnBr3, and 20 mol (%) of SnF2 in CsSnBr3 and spiro-OMeTAD with respect to vacuum level. Reprinted with permission from [45, 46].*

*Quest for Lead-Free Perovskite-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83360*

*A Guide to Small-Scale Energy Harvesting Techniques*

inhibit the phase separation caused by excess SnF2 (**Figure 4A**) [45]. Furthermore, the built-in potential can also be optimized through SnF2 and thus high open-circuit voltage of the device by energetic landscape alignments [46] as illustrated in (**Figure 4B**). Besides SnF2, other additives such as SnBr2, SnI2, and SnCl2 have also been explored where SnCl2 exhibits the highest stability by inhibiting the decomposition/oxidation [34, 47]. Moreover, the employment of hypophosphorous acid in CsSnIBr2 showed seed-like perovskite where Sn2+ oxidation was significantly reduced [48]. As a result, the charge recombination rate was decreased by fourfold compared to the control devices. The as-prepared cells displayed excellent oxygen-moisture stability at ambient

*(A) The schematic illustrations of the SnI(DMSO)3+ ions linked with lone I<sup>−</sup> ions, unit cell of SnI2***∙***3DMSO, and the film formation of the MASnI3 perovskite starting from SnI2 through SnI2***∙***3DMSO intermediate. (B) Scanning electron microscopy images of MASnI3 thin films obtained through low-temperature vaporassisted solution process and vapor-assisted solution method. (C) Current–voltage characteristic curves of the cell based on FASnI3 with 10% SnF2 additives. (D) Current-voltage curves of the best device based on* 

The inclusion of large ammonium cations for tuning the dimensionality or generating massive Schottky defects has also been tested in Sn-based perovskites to optimize the device performance. For example, a different ratio of phenylethylammonium (PEA) as a cation can yield perovskites with two-dimensional, threedimensional, and three-dimensional-two-dimensional-mixed dimensionalities. The high-quality thin film of the mixed PEA-FA perovskite (20% of PEA) delivered a stable device efficiency of 5.9% [49]. The composition of ethylenediammonium (en) and formamidinium (FA) in FASnI3/MASnI3/CsSnI3 crystal indicates no effect on the dimensionality of the perovskite because the size of en is too large for the

recognized as the Schottky defects. However, the optimized devices based on {en} MASnI3 and {en}FASnI3 showed efficiency of 6.6 and 7.1%, respectively [50, 51].

species. Here, the point defects are

conditions and thermal stability in vacuum environment.

*(FA)0.75(MA)0.25SnI3 perovskite. Reprinted with permission from [40–43].*

unit cell cage and it can remove a certain {SnI}<sup>+</sup>

**4**

**Figure 3.**

**Figure 4.**

*(A) Schematic crystal structure of FASnI3 with SnF2 and SnF2-pyrazine complex with corresponding scanning electron microscopy images. (B) Representation of energetic landscapes of the TiO2, pure CsSnBr3, and 20 mol (%) of SnF2 in CsSnBr3 and spiro-OMeTAD with respect to vacuum level. Reprinted with permission from [45, 46].*
