**3.3 Sn-based perovskite**

Tin (Sn) is group 14 element less-toxic metal having comparable properties [71]. As the primary report on Sn-based perovskite solar cells, power conversion efficiency (PCE) of 6% [72, 73]. Later the tin-based perovskites have been dominant and providing the highest efficiency of 13.24% [74]. The representative cesium tin iodide (CsSnI3), formamidinium tin iodide (FASnI3), and methylammonium tin iodide (MASnI3) have direct band gaps of 1.3, 1.2, and 1.41 eV [75, 76], respectively. Recently, Sn-based perovskite highest (MAPbI3) 0.4 (FASnI3) 0.6 15.08% [70] efficiency reported (**Table 3**) [69, 77–92].


#### **Table 2.**

*Bismuth halide perovskites and the highest obtained PCEs.*


#### **Table 3.**

*Tin halide perovskites and the highest obtained PCEs.*

For example, more recently in 2016, Li et al. fabricated an inverted structured device with MAPb0.5Sn0.5I3, achieved PCE of 13.6% [93]. And Liao et al. employed (MAPbI3)0.4(FASnI3)0.6 in inverted device and achieved a PCE of 15.08% [94]. Finally, Sn-based perovskite has become a hopeful alternative material for replacement the Pb-based perovskite. Though, instability, low PCE and certain degree of toxicity problems hasty more research to find other substitute materials, which can be more stable with less toxicity.

#### **3.4 Alkaline-earth metals perovskite**

Another friendly unleaded perovskite such as magnesium (Mg), calcium (Ca), strontium (Sr), and barium Ba) are also interesting candidate for Pb. The alkalineearth metals and their compounds are usually low cost and have advantage to industrial applications [68, 95].

### *3.4.1 Magnesium halide perovskite*

Magnesium halide perovskite is low effective masse, reasonable absorption coefficients, and direct band gaps. AMgI3 perovskites, the band gap was predicted to be


**Table 4.**

*Alkaline-earth metal halide perovskites: NB. Dimensionality and PCE values have not been reported.*

tunable using different A-site cations with band gaps of 0.9 eV (CH(NH2)2MgI3), 1.5 eV (CH3NH3MgI3), and 1.7 eV(CsMgI3) (**Table 4**). Until now magnesium halide perovskites have not been applied as materials in solar cells, which might be because of the sensitivity toward moisture [69, 95].

## *3.4.2 Calcium halide perovskite*

Calcium halide perovskite is low-cost, nontoxic, abundant in the Earth's crust. The divalent Ca2+ ion has suitable ionic radius (100 pm) similar to Pb2+ (119 pm) to exchange lead in the perovskite structure. It is the high band gap, the low mobility, and the instability. This material is not appropriate for photovoltaic applications due to environmental instability but might be probable candidates for charge-selective contacts [69, 97].

## *3.4.3 Strontium halide perovskite*

Strontium halide perovskite is an impartially less toxic, inexpensive, highly abundant alkaline-earth metal with an ionic radius (Sr2+:118 pm) very similar to lead (Pb2+:119 pm), which makes strontium an appropriate candidate for homovalent substitution of lead in the perovskite without affecting the crystal structure. It exhibits an underprivileged stability under ambient conditions because of its hygroscopic nature. It recommended a potential application as charge-selective contact material [70, 97].

### *3.4.4 Barium halide perovskite*

Barium halide perovskite is the stable Ba2+ metal cation shows a slightly larger ionic radius (135 pm) compared with Pb2+ (119 pm). It is expected to have a similar crystal structure as CH3NH3PbI3. According DFT calculations predicted CH3NH3BaI3 to form stable perovskite materials with an estimated band gap of 3.3 eV. It is sensitivity to moisture; it hampers the synthesis characterization and applicability in photovoltaics [97].

#### **3.5 Transition-metal-based perovskites**

There is significant interest in the field of transition metal halide perovskites rises from the rich chemistry and high abundance of metals [95]. Divalent transition metals Cu2+(73 pm), Fe2+ (78 pm) Zn2+, and Pd2+(86 pm)) have been as the replacement of Pb perovskites photovoltaic devices [77]. Their small ionic radii and good tolerance factor of 1, 3D structures. This material has potential for photovoltaic applications in bulky crystal [98].

#### *3.5.1 Copper halide perovskite*

Copper halide perovskite is less-toxic, low-cost earth abundant. The divalent Cu<sup>2</sup> + gets particular attention for replacement for Pb2+due to ambient stability and the high absorption coefficient in visible region. According to Cortecchia et al. report, a noticeable photoluminescence with higher bromine contents resulting from the in-situ formation of Cu<sup>+</sup> ions and the consistent charge carrier recombination at the charge traps [99].

### *3.5.2 Iron halide perovskite*

Iron halide perovskite is smaller ionic radius of the Fe2+ (78 pm) compared to Pb2+ (119 pm) hampers the development of 3Dstructures [95]. The limitations of iron halide perovskites are the multiple oxidation states of iron that hinder the constancy reaction, i.e., oxidation of Fe2+to Fe3+ comparable to tin and germanium perovskite [99, 100]. Therefore, iron halide perovskite has not been suitable for solar applications (**Table 5**).


#### **Table 5.**

*Optical data of transition metal halide perovskites.*

#### *3.5.3 Palladium halide perovskites*

Palladium halide perovskites is only a few studies on palladium-based perovskite have been reported so far [106]. In additional the investigation of palladium halide perovskite confirm that the general formula A2PdX4, Where A is an organic aliphatic cation (RNH3 + ) such as CH3NH3 <sup>+</sup> [107] and n-octyl ammonium [106] and X is a halide. It characterized 2D layered structures contain an alternating organic and inorganic layers [107]. Thus, palladium halide perovskites solar cell has not been reported. (**Table 5**) [108].

#### **3.6 Heterovalent substitution**

Among the ideal substitute of lead heterovalent substitution is a second viable approach toward lead-free perovskite. It is standby of the divalent lead cation with a cation in a diverse valence such as mono-, tri-, or tetravalent cation. Then, two different procedures such as the mixed-valence approach and heterovalent substitution accompanied with a significant change in the structure from ABX3-type to A3B2X9-type to maintain charge neutrality [109–111].

#### *3.6.1 Thallium halide perovskites*

It is a p-block metal with a Tl+ cation isoelectronic to Pb2+ (6s<sup>2</sup> 6p0 electronic configuration). The monovalent Tl<sup>+</sup> cation, though, cannot substitute the divalent Pb<sup>2</sup> <sup>+</sup> metal cation directly due to the violation of the charge neutrality. According to Giorgi et al. report, thallium halide perovskites (CH3NH3Tl0.5Bi0.5I3) is projected to be a potential alternative solar cell material (**Table 6**). The thallium-based compounds are presumably no substitute to lead-based perovskites in terms of photovoltaic applications due to the toxicity of thallium [110, 111].
