Open access peer-reviewed chapter

Recent Development of Lead-Free Perovskite Solar Cells

Written By

Anshebo Getachew Alemu and Teketel Alemu

Submitted: 21 February 2022 Reviewed: 25 April 2022 Published: 14 September 2022

DOI: 10.5772/intechopen.105046

From the Edited Volume

Recent Advances in Multifunctional Perovskite Materials

Edited by Poorva Sharma and Ashwini Kumar

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Abstract

Recently, the world energy demand has been raised up dramatically. Numerous energy sources have been developed to satisfy the urgent energy desires and to overcome the world energy crisis. Among them, solar energy has been considered an efficient energy source for current energy requirements. Nowadays, the lead-based perovskite solar cells achieved excellent power conversion efficiency exceeding 29.1%. However, to address major problems such as toxicity and underprivileged stability, several hardworks were made toward the replacement of lead-free perovskite material in perspective of device’s performance and stability. In this book chapter, we summarize material, dimensions, stability, and the current achievement of lead-free solar cells. Finally, we review the remaining challenges and future perspective for development of lead-free perovskite solar cells.

Keywords

  • material
  • dimensions
  • stability
  • lead-free photovoltaics

1. Introduction

Nowadays, among renewable energy alternatives, solar energy is the most abundant and has minimum impact on the environment compared with nonrenewable sources such as natural gas, fossil fuels, and nuclear energy. The development of photovoltaic has made possible the change of sunlight into electrical energy with high power conversion efficiencies with low cost [1, 2]. The demand of energy from the photon energy is primary significance because it is clean, renewable, abundant, and natural [3, 4]. Among the innovative photovoltaic, perovskite solar cells have hastily enhanced to the frontline for electricity production [5]. Solar-cells-based tandem perovskite achieved world high efficiency of 29.15% in the photovoltaic research field [6] and low production cost [7, 8, 9]. However, the increased concern of lead toxicity for extensive use in addition to the distress of disposal, widespread research effort has been dedicated to the path of lead-free PSCs [10, 11]. Due to encounters such as instability in ambient conditions [12], lack of accuracy in thickness [13], and device-incompatible solution growth processes [14, 15], PSCs have not yet gained sufficient trust for commercial applications.

Therefore, novel groups of lead-free halide PSCs have been discovered for substituting lead (Pb) with other elements such as antimony (Sb) [16], bismuth (Bi) [17], germanium (Ge), [18, 19], indium (In) [20, 21], tin (Sn), [22, 23], and double halide perovskite (Figure 1) possession the inherent perovskite properties unchanged. Furthermore, these alternative Pb-free materials show significant advantages such as highlight absorption coefficients, higher charge carrier mobilities, and narrow optical band gap compared with lead-based perovskites as shown in Figure 2 [32].

Figure 1.

Structure of (a) Pb-perovskite (b) tin-halide perovskites (c) double-halide perovskites. X is a halide; M and M′ stand for monovalent and trivalent metals, respectively [24, 25].

Figure 2.

Solar cell absorbers materials .A-site cations (organic MA and FA or inorganic Cs and Rb), metals, and halides (I, Br, Cl) for perovskite structure. b/ band gaps of different materials solar cells should have band gaps from 1.1 to 2.0 eV [26, 27, 28, 29, 30, 31].

This book chapter contains of the following sections: (1) introduction of metal halide PSCs, (2) origin of lead-free perovskite solar cells, (3) lead-free Pb-free materials, (4) dimensions of Pb-free materials, (5) limitations of Pb-lead materials, and (6) future prospective have also been discussed.

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2. Origin of lead-free perovskite solar cells

Later a revolutionary report by Kojima et al. [33], and a successive breakthrough by Kim et al. and Lee et al. [34, 35] showed that lead-based perovskite solar cells have significantly transmuted the field of photovoltaics. Different generations exhibit perovskite solar cells’ extraordinarily high power conversion efficiency because of high absorption coefficient [36] and defect tolerance [37] and to the long exciton diffusion length [38] of perovskite materials.

This noteworthy improvement in PSC solar cells restricted by critical intrinsic device instability of PSCs [39, 40] and toxic to the environment restricted for breakthrough outdoor application [41, 42, 43, 44, 45, 46]. According to the World Health Organization (WHO), report Pb through metabolism, and children are at a particularly high risk of Pb poisoning the human body cannot purge [47, 48]. Furthermore, Pb can simply spread into the air, water, and soil [49, 50].

More recently, the scientific community has been pointed for Pb-material replacements that have been reported promising efficiency progress [51, 52, 53]. Figure 3a bar graph shows the recent the highest certified power conversion efficiency (PCE) of the different types of photovoltaics, perovskite solar cells (PSC, 29.1%), organic solar cells (OSC, 17.4%), dye-sanitized solar cells (DSC, 13.8%), CIGS (23.4%), CdTe (22.1%), polycrystalline siliconcells (PCSSC, 22.3%), and monocrystalline silicon solar cells (MCSSC, 26.7%). Figure 3b shows that reported PCEs of Pb-based, Bi-based, and Sn-based PSCs from the preliminary stage of development to date. Sn-based PSCs (Sn-PSCs) have thus far shown the greatest prospects though there are fewer reports on Sn-PSCs compared with those on Pb-PSCs, the PCE and stability of Sn-based PSCs have been enhanced quickly. For example, Sn-based perovskites such as cesium tin iodide (CsSnI3), formamidinium tin iodide (FASnI3), and methylammonium tin iodide (MASnI3), have direct band gaps of approximately 1.3 eV, 1.41, and 1.20 eV, respectively, which are narrower than Pb-based perovskites [31, 54].

Figure 3.

(a) Highest certified PCEs of the different types of photovoltaics. (b) Reported PCEs of Pb-based, Bi-based, and Sn-based PSCs.

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3. Materials

The existence of Pb is an urgent problem in contradiction of the final application of PSCs and the toxicity of lead disturbs the operational of the blood kidneys, liver, testes, brain, and nervous system [55]. In order to address toxicity and poor stability lead (Pb)-based perovskite, several hard works were made toward the replacement of lead-free perovskite material. In this section, we give comprehensive review of the Pb-substitutes, such as Ge-based perovskite, bismuth-based perovskite, Sn-based perovskite, alkaline-earth metals perovskite, transition-metal-based perosivikate, and heterovalent perovskite.

3.1 Ge-based perovskite

Ge, is finest substitute for lead in the same group as Pb. For example, AGeI3 perovskite group, CsGeI3 has the narrowest band gap of about 1.6 eV (Figure 2), but MAGeI3 and FAGeI3 have direct band gaps of 1.9–2. 2 eV. The band gap (Eg) of the MAGeX3 perovskites estimated, and the Eg of MAGeI3 was 1.61 eV, in contrast to 2.81 eV and 3.76 eV for the Br and Cl anions, correspondingly. And high-quality CsSn0.5Ge0.5I3 perovskite films with a band gap of 1.5 eV solar cells reported an incredible PCE of 7.11% [56]. The germanium-based-perovskite instability due to from the tendency of Ge2+ to oxidize into Ge4+ [57]. The new result was achieved CsSn0.5Ge0.5 I3, which provided a PCE of 7.11% with improved stability concerning the CsSnI3 [58]. Furthermore, germanium in 0.75 MA0.25Sn1 − xGexI3 was newly provided PCE of 7.9% [59] as shown in Table 1.

Germanium halide perovskitesDimensionalityBand gap (eV)PCEs (%)References
RbGeCl3x H2O3D3.84[60]
RbGeBr33D2.74[60]
(RbxCs1-x)GeBr33D2.4[61]
CsGeCl33D3.4–3.67[61]
CsGeBr33D2.32–2.4[61]
CsGe(BrxCl1-x)33D2.65[62]
CsGeI33D1.53–1.630.11[62]
CH3NH3GeCl33D3.74–3.76[61]
CH3NH3GeBr33D2.76–2.81[60]
CH3NH3GeI33D1.9–2.00.2[61]
CH(NH2)2GeI33D2.2–2.35[61]
MFOGeI33D2.5[60]
GUAGeI33D2.7[61]
TMAGeI33D2.8[60]
IPAGeI33D2.7[61]

Table 1.

Germanium halide perovskites PCEs in photovoltaic devices.

3.2 Bismuth-based perovskite

Bi is a nontoxic and has the comparable properties to Pb and has satisfactory tolerance factor rule and enhanced the stability. It has unique properties such as 0D dimensionality, indirect band gaps, and mobilities (Figure 3a) [60, 61]. According to Park report [63], Bi-based perovskite A3Bi2I9 (A to be Cs and MA) as a photovoltaic absorber has estimated band of A3Bi2I9 to be ca. 2.1 eV for MA and 2.2 eV for Cs, and the exciton binding energy as 70 meV (while Pb perovskite is at 2550 meV). The reported photovoltaic parameters as (for CS3Bi2I9 PCE =1.09%, FF = 0.6, Voc = 0.85 V, Jsc = 2.15mAcm−2, 0.12%, for MA3Bi2I9 FF = 0.33, Voc = 0.68 V, Jsc = 0.52mAcm−2). This low efficiency because of the number of reasons, such as excess reactant residue, extra band-gap states, poor morphology, and interface contact. Therefore, the Bi-based perovskite is ideal as solar cell absorbers due to low mobilities and good stability [62, 63] as shown in Table 2.

Bismuth halide perovskitesDimensionalityBand gap (eV)PCEs (%)References
(CH3NH3)3Bi2I90D1.94–2.110.42[64]
(CH3NH3)3Bi2I9-xClx2.40.003[65]
Cs3Bi2I90D1.8–2.21.09[66]
HDABiI51D2.050.027[67]
CsBi3I102D1.770.40[68]
LiBiI4 5 H2O1D1.7–1.76[69]
MgBi2I8 H2O1D1.7–1.76[69]
MnBi2I8 H2O1D1.7–1.76[69]
KBiI4H2O1D1.7–1.76[69]
Cs2AgBiCl63D2.2–2.77[70]
Cs2AgBiBr63D1.95–2.19[70]
Cs2AgBiI63D1.6[70]
Cs2AuBiX63D1.6[70]
K3Bi2I92D2.9[70]
Rb3Bi2I92D1.89–2.1[70]
Cs3Bi2Br92D2.50[70]

Table 2.

Bismuth halide perovskites and the highest obtained PCEs.

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, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92].

PerovskiteDimensionalityBand gap (eV)PCE (%)References
CH3NH3SnBr33D2.15–2.24.27[77]
CH3NH3SnIBr23D1.755.73[78]
CH3NH3SnI2Br3D1.565.48[79]
CH3NH3SnI33D1.27–1.355.23[80]
CH(NH2)2SnI2Br3D1.681.72[81]
CH(NH2)2SnI33D1.4–1.416.22[82]
CsSnBr33D1.75–1.82.1[83]
CsSnIBr23D1.63–1.653.2[84]
CsSnI2Br3D1.37–1.411.67[85]
CsSnI33D1.27–1.313.31[86]
CsSnI2.95F0.053D1.38.51[87]
Cs2SnCl63D3.90.07[69]
Cs2SnBr63D2.70.04[69]
Cs2SnI63D1.26–1.620.86,[69]
Cs2SnI3Br33D1.433.63[69]
MAPb0.85Sn0.15I3163DN/A10.10[70]
MA0.5FA0.5Pb0.75Sn0.25I33D1.3314.35[70]
MAPb0.5Sn0.5I33D1.1813.60[70]
MAPb0.5Sn0.5I33D1.1210.0[70]
(MAPbI3)0.4(FASnI3)0.63D1.2015.08[70]

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 alkaline-earth 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 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].

PerovskiteDimensionalityBand gap (eV)PCE (%)Crystal system
(space group)
References
CH3NH3MgI31.5Tetragonal[70, 96]
CH(NH2)2MgI30.9Trigonal(P3m1)[70, 96]
CsMgI31.7Orthorhombic
CH3NH3CaI32.95,Tetragonal[70, 96]
CH3NH3CaI3-xClx[70, 97]
CH3NH3SrI33.6Tetragonal[70, 97]
CH3NH3BaI33.3Tetragonal[70, 97]

Table 4.

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

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 Cu2+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+ 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).

PerovskiteDimensionalityBand gap (eV)PCE (%)References
(p-F-C6H5C2H4NH3)2CuBr42D1.740.51[99, 101]
(CH3(CH2)3NH3)2CuBr42D1.760.63[99]
(CH3NH3)2CuCl42D2.48[102, 103]
(CH3NH3)2CuCl2Br22D2.120.017[102]
(CH3NH3)2CuClBr32D1.90[102]
(CH3NH3)2CuCl0.5Br3.52D1.800.0017[104, 105]
(CH3NH3)2FeCl42D[104]
(C2H5NH3)2FeCl42D[104]
(C3H7NH3)2FeCl42D[104]
(C6H5CH2NH3)2FeCl42D[104]
(CH3NH3)2FeCl2Br22D[104]
(CH3NH3)2FeCl3Br2D[104]
(CH3NH3)2PdCl42D[104]
(C8H17NH3)2PdCl42D[104]

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+ [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, 110, 111].

3.6.1 Thallium halide perovskites

It is a p-block metal with a Tl+ cation isoelectronic to Pb2+ (6s26p0 electronic configuration). The monovalent Tl+ cation, though, cannot substitute the divalent Pb2+ 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].

PerovskiteDimensionalityBand gap (eV)PCE (%)References
Cs2AuIAuIIICl63D2.04[69]
Cs2AuIAuIIIBr63D1.60[69, 104]
Cs2AuIAuIIII63D1.31[112]
[NH3(CH2)7NH3]2[(AuII2)(AuIIII4)(I3)2]2D0.95[112]
[NH3(CH2)8NH3]2[(AuII2)(AuIIII4)(I3)2]2D1.14[69]
CsTlF33D0.0017[69]
(CsTl +0:5 Tl3+0:5 F3) CsTlCl33D2.5[69]
(CsTl+0.5Tl3+0.5 F3Cl3) CH3NH3Tl0.5 I33D1.6[69]

Table 6.

Optical data of gold and thallium halide perovskites.

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+(5d10, t2g6eg4) and trivalent Au3+(5d8, t2g6eg2) to form ABX3-type perovskite structures, like in the case of Cs2AuIAuIIIX6(X = Cl, Br, I) compounds. Moreover, hybrid gold 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 5s2) and has a comparable s2 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].

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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 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 antimony−bismuth perovskite,(C8NH12)4Bi0.57Sb0.43Br7·H2O, was synthesized, which reported 0D structure with isolated [(Bi/Sb) Br6]3–, Bi/Sb metal-halide octahedrons(Figure 4) [120, 121].

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].

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 core−shell quantum confined materials, reproducible low-dimensional tin bromide PSCs.

For example, novel set of one-dimensional (TMHD)BiBr5 (TMHD=N,N,N,N-tetramethyl-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 Pnma 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].

Figure 6.

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

Figure 7.

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

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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 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.

CH3NH3PbI3sH2OPbI2s+CH3NH3IaqE1
CH3NH3IaqCH3NH2aq+HIaq.E2
4HIaq+O22I2s+2H2OlE3
2HIaqH2g+I2sE4

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).

CH3NH3PbI3shvPbI2s+CH3NH2+HIE5
2II2+2eE6
3CH3NH3+hv3CH3NH2+3H+E7
I+I2+3H++2e3HIE8

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, tellurium-based A2TeX6 SCs were additionally observed to be entirely steady up to ∼270°C alongside being phase tolerant to moisture and air [127].

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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.

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].

lead-free perovskiteVoc(V)Jsc (mA cm2)FF (%)η (%)References
FASnI30.6421.950.7310.16[116, 117, 136]
CsSnI30.8623.26512.96[137]
PEAxFA1 − xSnI3 + NH4SCN0.9417.47512.4[115]
(FA)0.75(MA)0.25 SnI3 + 10% SnF20.6121.262.78.12[138]
PEA2SnI40.6122.070.19.41[139]
(BA0.5 PEA0.5)2FA3Sn4I130.6021.8266.738.82[140]
AVA2FAn − 1SnnI3n + 10.6121.068.88.71[141]
CsSnBr30.8521.235810.46[137]
FASnI3 + 1% EDAI20.5821.30.728.9[136]
FASnI3 + 5% PHCl0.7623.56411.4[120]
0.92FASnI3 + 0.08PEAI +10% SnF20.5324.1719.0[142]
MAPb0.5Sn0.5I30.7536.307513.60[143]
MA0.5FA0.5 Pb0.75 Sn0.25I30.7822.448214.35[144]
(MAPbI3)0.4(FASnI3)0.60.70626.8679.515.08[145]
CsSnCl30.8719.82569.66[146]

Table 7.

Lead-free perovskite solar cell parameters.

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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.

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Acknowledgments

The authors acknowledge Intech open access Dr. Sara Tikel for invitation and constructive comments.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Anshebo Getachew Alemu and Teketel Alemu

Submitted: 21 February 2022 Reviewed: 25 April 2022 Published: 14 September 2022