*3.6.2 Gold halide perovskite*

It is similar to thallium-based via the mixed-valence approach. Subsequently, gold has to be existing in grouping of mono valent Au<sup>+</sup> (5d10, t2g 6 eg 4 ) and trivalent Au3 + (5d8 , t2g 6 eg 2 ) to form ABX3-type perovskite structures, like in the case of Cs2AuIAuIIIX6(X = Cl, Br, I) compounds. Moreover, hybrid gold halide perovskites


#### **Table 6.**

*Optical data of gold and thallium halide perovskites.*

have been investigated such as [NH3(CH2)7NH3]2[(AuII2)(AuIIII4) (I3)2], and [NH3(CH2)8NH3]2[(AuII2) (AuIIII4) (I3)2] as **Table 6** [69, 112].

#### *3.6.3 Antimony halide perovskite*

Antimony halide perovskite is potential alternative to lead perovskite for photovoltaic applications to address problems of chemical stability and toxicity. The valance three Sb3+metal cation, isoelectronic to Sn2+(4d10 5s<sup>2</sup> ) and has a comparable s<sup>2</sup> valence electronic arrangement as Pb2+(5s2 lone pair), and equivalent electronegativity (Sb:2.05, Sn:1.96, Pb:2.33) but considerable lesser ionic radius (76 pm) compare to the valance two Sn2+(110 pm) and Pb2+ (119 pm) metal cations [69, 113].

#### **3.7 Lanthanide and actinide halide perovskites**

It is another substituent for Pb2+ giving rise toward lanthanide and actinide halide perovskites. According to Liang and Mitzi report, europium halide perovskites: CH3NH3EuI3 is a 3D ABX3-type perovskite with a tetragonal distorted structure of BX6 corner-connected octahedral, which can be synthesized via a diffusion based solid-state synthesis route from CH3NH3I and EuI2.The applicability of materials, in optoelectronic devices, limited by the environment sensitivity. However, another group such as lanthanide perovskite exhibits remarkable optical characteristics; it is possible candidates as new light absorbing materials for solar application. In addition, lanthanides such as (Ce3+, Dy3+, Er3+, Eu3+, Gd3+, La3+, Lu3+, Pr3+, Nd3+, Sm3+, Tm3+), and actinides (Am3+, Bk3+, Pu3+) have been functioning in halide double perovskites, but until now no reported study on their solar application [69, 111, 113, 114].

#### **3.8 Ferroelectronic perovskite**

It is mainly as magnetic material but investigation of ferroelectronic perovskites have the potential to be employed as light absorbers. Moreover, they can be treated as a type of Pb-free perovskites. According to Nechache et al. (2009), Jiang et al. (2020) report, Bi2CrFeO6 as solar material, attaining the PCE of 8% [115].

### **4. Dimension**

The structural chemistry and dimensionality of perovskites are significantly influenced by the performance of solar devices. Depending on the dimensionality, the crystal structures of perovskites can be divided into three categories [116, 117]. As the nonstop novelty of preparation methods has developed, (PCE) of solar cells with three-dimensional (3D) lead (Pb) perovskites has rapidly rushed from 3.9% to over 29.1% within nearly one decade, which incomparable to monocrystalline silicon solar cells [6]. In this section, we summarize applicable dimensions such as 0D, 1D, 2D, and 3D.

#### **4.1 Zero-dimensional Pb-free PSC**

It is emerging class of material condensed dark current, and improved environmental stability compared with different dimension perovskites. To create a stable 0D compound is based on the assumption that a quantum-well structure would result in

*Recent Development of Lead-Free Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.105046*

**Figure 4.**

*Chemical structure of (a) Bi1, (b) Bi2, (c) [(Bi/Sb) Br6]3– and (d) Bi/Sb ((Bi/Sb azure, Br orange, N blue, C gray [120].*

stronger quantum confinement. The low-dimensional perovskites are proven to inhibit ion migration, sensitivity for humidity and chemical stability. It is required to develop, Pb-free, quality samples for optoelectronic function [118, 119]. For example, Cs3BiBr6crystal has orthorhombic space group *Pbcm* to form a 0D perovskite structure (**Figure 4**). An otherair-stable, mixed antimonybismuth perovskite,(C8NH12) 4Bi0.57Sb0.43Br7H2O, was synthesized, which reported 0D structure with isolated [(Bi/Sb) Br6]3–, Bi/Sb metal-halide octahedrons(**Figure 4**) [120, 121].

#### **4.2 One-dimensional Pb-free PSCs**

Recently 2D and 3D perovskites in various dimensions have investigated in different perspective. But current research attention been diverted to the lower dimensions (1D and 2D). According to Zhou et al. report, bulk assemblies of 1D and 0D coreshell quantum confined materials, reproducible low-dimensional tin bromide PSCs.

For example, novel set of one-dimensional (TMHD)BiBr5 (TMHD=N,N,N,Ntetramethyl-1,6-hexanediammonium), comprising infinite 1D chains of BiBr6 octahedra and organic TMHD cations as shown **Figure 5** [118]. Another 1D structured Rb2CuBr3 was reported with the orthorhombic space group P*nma* where the Cu atom was shown to be harmonized by four Br atoms as shown in **Figure 5c** and **d** [122, 123].

#### **Figure 5.**

*Structural (a) packing. (b) unit of inorganic BiBr6 octahedra [118, 122]. (c) Crystal structure of Rb2CuBr3, (d) Rb2CuBr3 structure as viewed down the a-axis (red, blue, and brown indicate Rb, Cu, and Br atoms, respectively). (e, f) Isosurface plots of the wave function |Ψ|2 of CBM and VBM [123].*

### **4.3 Two-dimensional Pb-free PSCs**

Counter 2D layered structure in the monoclinic framework with the P21 space group is that of (BA) 2CsAgBiBr7 which behaves like a quantum-constrained structure where the perovskite layers look like to the quantum wells and the massive cations as the expected obstructions (**Figure 6a** and **b**) [124]. The Crystal show high symmetry in the orthorhombic space group Pnma required properties. Another Zhang et al. explained bidoped two-layered tin-based halide perovskite series PEA2Sn1 xBixBr4 + x. For undoped PEA2SnBr4, the [SnBr6]4 octahedra show up in the form of sheets lying between the huge natural moieties of PEA+, exhibits as brand of 2D layered morphology as shown **Figure 6c** and **d** [125].

#### **4.4 Three-dimensional Pb-free PSCs**

Up to this point; most research work has been conducted to PSCs with a 3D structure and ABX3 stoichiometry. Nonetheless, since rearrangement into various phases is energetically good in the ABX3 has been a blast in the synthesis of a several perovskite crystals shape in various designs and morphology as displayed in **Figure 7**. There has been a boom in the synthesis of several that crystallize in different structures and morphology into lower dimensions for attaining stability [126, 127].

### **5. Stability**

The most common attention grabbed limits of Pb-based perovskites poor stability for photovoltaic application. The stability challenge of perovskites is being addressed through the use of low-dimensional perovskites as well as improved device

#### **Figure 6.**

*2D perovskite (a) BA cations are bound to two kinds of octahedra through N HBr hydrogen bonds, as shown by the dashed lines. (b) (BA)2CsAgBiBr7 describing the 2D perovskite quantum-confined motif (c) crystalstructural diagram of PEA2SnBr4 (d) distorted [SnBr6]4 octahedron [124, 125].*

*Recent Development of Lead-Free Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.105046*

#### **Figure 7.**

*3D crystal structure perovskites Cs2SnCl6* � *xBrx, (a) MA2TeI6; and (b) MA2TeBr6; and (c) discrete [Sn(Cl, Br)6]2 [126, 127].*

engineering. The development of low-toxicity ideal Pb-free materials should have low toxicity, narrow direct band gaps, high optical absorption coefficients, high mobilities, low exciton-binding energies, long charge-carrier lifetimes, and better stability. The intrinsic tolerance of perovskite to humidity, light, and temperature has made an embarrassment of device applications that rendering for commercialization [128].

#### **5.1 Moisture stability**

Upon exposure to moisture, lead-based perovskite damaged by forming coordinate bonds with the H2O molecule, thus subsequent in the chemical decomposition and the structure of perovskite structure. The perovskite layer with insufficient time for device operation with the moisture, oxygen, air, and high energy photon. The organic halide would remain the hydrolysis and release HI. The HI would be constantly disbursed with the oxygen and the photon. The decomposition reaction described in Eqs. (1)–(4) as bellow with acceptable amount of moisture.

$$\mathrm{CH\_3NH\_3PbI\_3(s)} \stackrel{\mathrm{H\_{20}}}{\Leftrightarrow} \mathrm{PbI\_2(s)} + \mathrm{CH\_3NH\_3I(aq)}\tag{1}$$

$$\text{CH}\_3\text{NH}\_3\text{I}(\mathbf{aq}) \Leftrightarrow \text{CH}\_3\text{NH}\_2(\mathbf{aq}) + \text{HI}(\mathbf{aq}.) \tag{2}$$

$$\text{4HI(aq)} + \text{O}\_2 \Leftrightarrow \text{2I}\_2(\text{s}) + \text{2H2O(l)}\tag{3}$$

$$2\text{HI}(\text{aq})\Leftrightarrow\text{H}\_2(\text{g}) + I\_2(\text{s})\tag{4}$$

There is new lead-free Cs2PdBr6 was reported to moisture stable after no indication of chemical decomposition was detected even after immersion in water for 10 min [129]. Another type leadless perovskiteCs2NaBiI6 was found to hold all its properties after contact to humid air for 5 months, and no deprivation peak was observed [130]. Alternatively, FA4GeIISbIIICl12 showed no change when visible to 60% humidity for up to 3 months. This natural stability occurring in Pb-free SCs thus demonstrates to be one of the foremost reasons for them to be pitched as excellent candidates dignified to bring about the next big wave in Pb-free perovskite optoelectronics application.

#### **5.2 Photostability**

In PSCs mesoscopic device structure was used to photo-generated electrons transportation. Though, it is sensitive to ultraviolet (UV) light. According to Snaith *et al.* report, photoinduced instability of PSCs degraded quicker unshaded device under sun light described as Eqs. (5)–(8).

$$\text{CH}\_3\text{NH}\_3\text{PbI}\_3(\text{s}) \stackrel{\text{hv}}{\Leftrightarrow} \text{PbI}\_2(\text{s}) + \text{CH}\_3\text{NH}\_2\uparrow + \text{HI}\uparrow\tag{5}$$

$$\text{2I}^- \Leftrightarrow \text{I}\_2 + \text{2e}^- \tag{6}$$

$$\text{\textbulletCH}\_3\text{NH}\_3^+ \overset{hw}{\Leftrightarrow} \text{\textbulletCH}\_3\text{NH}\_2\uparrow + \text{\textbulletH}^+ \tag{7}$$

$$\rm I^- + I\_2 + \bf 3H^+ + 2e^- \Leftrightarrow \bf 3HI \uparrow \tag{8}$$

In recent times, innovative group of lead-free material, for example, Cs2AgBiBr6 indicators do not give the impression to harmed even subsequently nonstop experience to X-ray radiation in neighboring conditions [131]. The essential driver for this is a consolidation of higher actuation energy (a few times that of organic–inorganic half breeds) and high dispersion hindrances for constituent particles suggesting lesser opportunity of underlying unwinding and thus, greater dependability [132]. (BA)2CsAgBiBr7 additionally shows an extraordinarily steady reaction even on nonstop openness to X-beams [133].

#### **5.3 Thermal stability**

In normal state, direct lighting of PSCs will increase operation temperature of panel. The temperature as high as 85°C that the ecological temperature is 40°C. According to Conings et al. reports, the intrinsic thermal stability of MAPbI3 � xClx was found that the degradation happened at 85°C condition. This means that PSCs may not be widely used in actual daytime if the device temperature exceeds 85°C. The perovskite phase would transit from lower symmetry to higher symmetry (orthorhombic-tetragonal-cubic). Recently Weber et al. also reported MAPbBr3 and MAPbCl3 could maintain better symmetry than MAPbI3 from �40–85°C.Recently new group of lead-free perovskites, for instance, (MA)2AgBiBr6 [134] is steady up to �550 K which is not exactly that of Cs2AgBiBr6 (stable up until�700 K) [135]. The short fall of Pb appear as attractive variable also since the lead-containing counterpart (MAPbBr3) is just steady until �490 K. On practically equivalent to lines, telluriumbased A2TeX6 SCs were additionally observed to be entirely steady up to �270°C alongside being phase tolerant to moisture and air [127].

### **6. Recent advances**

The stability and harmfulness challenge in organic� inorganic hybrid lead halide perovskites cells being addressed through low-poisonousness without lead materials. Similar as the Pb-containing perovskites, the helpful properties for such lead-free solar devices would be appropriate direct band gaps, high assimilation coefficients, high mobilities, low exciton restricting energies, long charge transporter lifetimes.

*Recent Development of Lead-Free Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.105046*


#### **Table 7.**

*Lead-free perovskite solar cell parameters.*

The Sn-based perovskite have got much interest along with all pb-free solar cells, because of their fundamentally the same as properties to lead-based ones, and the most encouraging exhibition accomplished by gadgets utilizing this class of materials. In **Table 7**, recorded a solar cell parameters based on pb-free perovskites. The controlled crystallization of lead-free perovskite material shows improved performance in solar oriented cells, which is roused toward the manufacture of without pb perovskite film-based sun-powered cells [101, 103, 105].

## **7. Conclusions and future prospective**

It is known until now Pb-based solar cells could not be open commercial market because of the poor stability and Pb toxic nature. In this concern the device showed PCE of 8.12%–15.08% as shown in **Table 7**. These obtained results showed the excellent optical properties of the lead-free perovskite materials and suggested their potential as light absorbers in the construction of PSCs.Finally, we have concluded the recent development of lead-free perovskite materials in perspective of solar cell application. Lead-free perovskites empower to circumvent the problems of instability and toxicity to improve commercial production. Simultaneously, this study overview understanding of the fundamental challenges behind the efficiency, stability, and environmental of lead-free PSCs. Here, we are looking forward the materials, dimension, and the further development of lead-free perovskite materials and PSCs. Finally, we summarize the latest highest performance of lead-free perovskites. In spite of faster development of lead-free perovskites, we believe that the efficiency based on lead-free perovskite materials can breakthrough over 15% after further

comprehensive study, and we should keep forward research until the achievement of commercial leadless perovskite solar cell.
