Metal halide perovskites have recently emerged as one of the most promising classes of semiconductors for various applications, especially in the field of optoelectronics. Lead-based halide perovskite materials, virtually unexploited for decades, have become prominent candidates due to their unique and intrinsic physicochemical and optical properties. Current challenges faced by the scientific community to capitalize on the properties of Pb-based perovskites are mainly associated with environmental concerns due to the toxicity of Pb and their poor stability. Under this context, over recent years, a number of new Pb-free halide perovskite (and perovskite-like) semiconductor classes have been introduced. This Perspective reviews recent developments in Pb-free halide perovskites, which specifically target their application in solar cells, light-emitting devices, and photocatalysts. Each type of Pb-free material is paired with a specific optoelectronic application, and the latest record performances are reported. Although these materials do not yet exhibit as attractive intrinsic optoelectronic properties as the Pb-based halide perovskites, their potential as alternatives for well-suited applications is discussed.

For the past decade, halide perovskite semiconductors have received tremendous attention due to their tunable bandgaps, impressive optoelectronic properties, ease of process, and low fabrication cost, all of which lead to an ever-expanding range of potential device applications.1 This applies especially to their exploitation as solar cells, light-emitting diodes, photocatalysts, as well as various photodetectors.1–4 3D hybrid halide inorganic perovskites are ionic semiconductors typically composed of lead (Pb2+), halogen anions (X = I, Br, Cl), and small organic cations or cesium (Cs+). In 2009, MAPbI3 (MA = CH3NH3+, methylammonium) and MAPbBr3 were the first perovskites successfully applied as photovoltaic materials in a solar cell.5 MAPbI3 exhibits remarkable properties for solar cell applications, such as a sharp optical absorption with a large absorption coefficient of 104 cm−1 and a bandgap of 1.5–1.6 eV, as evaluated by UV photoelectron or optical spectroscopy.6 Moreover, MAPbI3 single crystals present ultra-low trap densities, namely, 109–1010 traps per cm3, and sizable charge carrier diffusion lengths ranging within 3–10 µm.7,8

The structural, thermodynamic, and electronic properties of Pb-based halide perovskites can be tuned by substituting MA with other cations, such as Cs+, K+, and FA [FA = HC(NH2)2+, formamidinium], and/or their mixtures. For example, by replacing MA with a suitable mixture of FA and Cs+, the materials’ stability increases and the bandgap decreases by 0.07 eV.9 Mixing halogen atoms to form mixed-halide perovskites is another efficient way of tuning the bandgap and spanning the absorption and emission energies over the UV–visible to near IR regions. Therefore, 3D halide perovskites with finely tuned Pb-based alloys of both cations and halogens are ideal for high performance solar cells, including tandems on silicon. To date, the highest certified Power Conversion Efficiency (PCE) of a Pb-based perovskite solar cell is 25.8%. This recent record was achieved, thanks to the insertion of a Cl-based coherent interlayer between the FAPbI3 and the SnO2 electrode.10 

Despite the remarkable performance of Pb-based halide perovskites, the materials’ stability under ambient conditions and the environmental concerns raised over the presence of Pb remain active challenges toward their large-scale application and commercialization.11–15 Various strategies are developed to overcome these hindrances. By device or module encapsulation, the halide perovskite materials can be indeed nicely protected from exposure to moisture, UV light, heating, or air, thus mitigating the fast degradation for their long-lasting purpose.16,17 On the other hand, Pb-free perovskites as well as materials derived from the perovskite structure (also called perovskite-like structures or perovskitoids), as introduced in Sec. II, are explored with the aim to obtain similar or even better properties than standard Pb-based materials.18 In this Perspective, we discuss how Pb-free perovskites or perovskite-like semiconductors are used for specific optoelectronic devices, namely, solar cells, light-emitting diodes, and photocatalysts. Other explored applications include memristors, transistors, lasers, photodetectors, and various sensors.19 Section II presents a classification of Pb-free materials, then we list and compare, for the best suited applications, the record performances reported so far for each class.

The cubic ABX3 perovskite structure within the Pm3̄m space group is the reference perovskite structure. It consists of a network of corner-sharing BX6 octahedra, with the A-site atom sitting in the cuboctahedral cavity formed in between the octahedra, as shown in Fig. 1. For typical halide perovskites, a monovalent organic or inorganic cation occupies the A-site, a divalent (2+) metal, most notably Pb, is at the B-site, and the halogen anion is at the X-site. Within the lattice, A-site atoms typically only have indirect effects on the electronic structure of the materials, as the electronic states of the typical A-site cations (i.e., Cs, MA, and FA) are far from the band edges. Yet, these atoms can induce structural modifications, which in turn can lead to modifications of their electronic structure and the exhibited bandgaps. Akkerman and Manna20 have recently categorized different structure types of materials based on the three- and two-dimensional (3D and 2D) ordered structures, as derived from the reference ABX3 lattice. Here, we summarize materials that are substitutionally engineered at the B-site so that all Pb atoms are replaced, only including those that have been applied in optoelectronic devices to date.

FIG. 1.

Structure of the conventional types of Pb-free metal halide perovskites. AB(2+)X3 perovskites (with B = Sn/Ge), A2B(3+)B′(1+)X6 double perovskites (with B = Bi/Sb/In and B′ = Ag/Tl), A2B(4+)X6 vacancy ordered double perovskites (with B = Sn/Ti/Te/Zr/Pd/Pt), and A3B2(3+)X9 2D layered perovskite (with B = Bi/Sb). A-site atoms can be Cs, MA, FA, and their combinations, while at the X-site, I, Br, Cl, and their combinations. Atoms A and X are in cyan and red, respectively, and atoms B or B′ in four different classes are shown as octahedra.

FIG. 1.

Structure of the conventional types of Pb-free metal halide perovskites. AB(2+)X3 perovskites (with B = Sn/Ge), A2B(3+)B′(1+)X6 double perovskites (with B = Bi/Sb/In and B′ = Ag/Tl), A2B(4+)X6 vacancy ordered double perovskites (with B = Sn/Ti/Te/Zr/Pd/Pt), and A3B2(3+)X9 2D layered perovskite (with B = Bi/Sb). A-site atoms can be Cs, MA, FA, and their combinations, while at the X-site, I, Br, Cl, and their combinations. Atoms A and X are in cyan and red, respectively, and atoms B or B′ in four different classes are shown as octahedra.

Close modal

First, as Sn2+ and Ge2+ share the same nominal oxidation state with Pb, Sn- and Ge-based perovskite materials can attain the same ABX3 stoichiometry. The formed conventional A(Sn,Ge)X3 perovskite structures can adopt either the cubic reference lattice, like their Pb-based counterparts, or slightly distorted ones due to the constituents’ incorporation. Moreover, since Sn, Ge, and Pb belong to the same column of the Periodic Table, they share the same electronic configuration (valency); thus, Sn and Ge-based perovskites are most likely to exhibit similar electronic and optical properties as their Pb-based counterparts. In fact, the remarkable physical and optoelectronic properties of APbX3 perovskites are known to directly relate to the antibonding electronic states: Pb[6s]–X[np] and Pb[6p]–X[np] (with n = 3, 4, and 5 for X = Cl, Br, and I) located at the valence band maximum (VBM) and conduction band minimum (CBM), respectively.21 

A set of popular examples of Sn-based perovskites are CsSnI3, MASnI3, and FASnI3, all of which have direct optical bandgaps in the range of 1.3–1.4 eV.18–20 The electronic structure of these materials shows that the band-dispersion is similar to the Pb-based compounds, while their bandgaps are ideally placed for photovoltaic devices in the near-infrared range.22,23 Yet, a well-known drawback of Sn-based perovskites is the favored oxidation of Sn2+ to Sn4+, leading to a natural p-doping in samples but also to an irreversible degradation of the device performances over a short period.24,25 In addition, the ease of formation of Sn vacancies results in an electrical behavior close to a metal that is far from ideal for the reproducibility, stability, and performance of devices. Therefore, the incorporation of excess Sn2+ in the compounds and the use of large cations are recommended in order to suppress the unwanted defects- or traps-induced recombination.26,27 We note that extra care should be taken when large cations are introduced, usually to avoid disrupting the pristine crystal structure, as the materials can then form one-dimensional networks of octahedra, like in the case of (CH3)3SSnI3.28 Another class of materials with enhanced stability is obtained by adding large ethylenediammonium (en) cations within the ASnI3 perovskite lattice (A = MA, FA) to form the well-known hollow perovskite lattices.29 

Second, classic Ge-based perovskites are CsGeI3, MAGeI3, and FAGeI3, exhibiting direct bandgaps of 1.6, 1.9, and 2.2 eV, respectively.30 Similar to the Sn-based perovskites, Ge-based ones also face stability issues due to the oxidation of Ge2+ to Ge4+. In addition, the octahedra in Ge2+ based perovskites are typically heavily distorted due to the stereochemical expression of the Ge lone pair.30 

Another approach toward stable and performant materials is alloying. Such an alloying strategy has been first explored for the case of mixing Pb with Sn.31 These Pb/Sn mixed alloys have a smaller bandgap compared to the pristine compounds. Recently, this strategy has been expanded to fully Pb-free materials by alloying Ge with Sn, which were found to exhibit higher stabilities and narrower bandgaps than the pristine Sn and Ge-based materials, with CsSn0.5Ge0.5I3, for example, exhibiting an optical bandgap of 1.5 eV.32 Similar to the case of Pb-based materials, the design of 2D/3D heterostructures is another promising recipe for achieving highly efficient and stable Sn-based solar cells. Specifically, forming well-aligned 2D layers over the 3D crystal phase can considerably improve the hysteresis and light soaking of the compounds. As a consequence, the charge carrier collection, device stability, and efficiency improve with respect to the pristine 3D material.33,34 Yet, the device performance of Sn and Ge based perovskites is still far from those of the Pb based counterparts, and this brings forward the indispensability of more endeavors.

Another class of materials that have been derived from the ABX3 perovskite lattice is the so-called “halide double perovskites.” Many compounds have been reported since the early 1970s, often referred to as elpasolites,35 and exhibit large bandgaps particularly prominent for scintillating applications. Over the past few years, a series of new halide double perovskites with relatively low bandgaps have been synthesized as possible Pb-free alternatives for optoelectronics.36–42 These double perovskites keep the conventional 3D perovskite structure, but every two B2+ site cations are substituted by two cations B and B′ with formal oxidation states of +3 and +1, as shown in Fig. 1.36–38,43 The double perovskite structure corresponds to a general chemical formula of A2BB′X6, which is simply obtained by doubling the standard ABX3 perovskite lattice. Halide double perovskites typically crystallize in face-centered cubic Fm3̄m space groups at room temperature and are known to form stable crystals under ambient conditions.37,38,44 To date, the two of the most successful halide double perovskites are Cs2AgBiBr637,38 and Cs2AgInCl6,32 which have attracted much interest for optoelectronic applications as the former exhibits one of the smallest bandgaps within the halide double perovskites class (estimated at 1.95 eV36), and the latter exhibits interesting emission properties, attaining record-high photoluminescence quantum yield (PLQY) values upon appropriate doping.44 Another series of low bandgap double perovskite materials that have been proposed are the ones containing Tl+ instead of Ag+.45–47 Noteworthy, besides halide perovskites, low bandgap oxide perovskites, such as Ba2AgIO6 and Ba2AuIO6, which contain heptavalent iodine at the B′-site, are also explored as promising candidates for optoelectronics.48,49 Another class of materials that have emerged as potentially interesting low bandgap perovskites are the so-called 111-oriented layered double perovskites. Cs4CuSb2Cl12 was first reported by Vargas et al.,50 and there has been an increased interest in the class51 and other lead-free derivatives synthesized, such as Cs4CuIn2Cl12 and Cs4Cd1−xMnxBi2Cl12.52,53

Starting from the A2BB′X6 double perovskite lattice, a different class of materials can be defined by replacing the B′ site with vacancies, and the B-site with a suitably charged metal to maintain charge neutrality. These compounds are known as vacancy ordered double perovskites (A2BX6), such as Cs2SnI6 and Cs2TiBr6. Similar to double perovskites, the A2BX6 lattice can be derived from the conventional perovskites by doubling the ABX3 unit cell along all three crystallographic axes but subsequently removing every other B site cation. This vacancy ordered lattice is shown in Fig. 1, and typically the materials share the same Fm3̄m space group as double perovskites. Interestingly, by exposing the single ABX3 perovskite CsSnI3 to air, it can transform to the vacancy ordered Cs2SnI6 state by the oxidation of Sn2+ to Sn4+.54 Compared to the conventional ABX3, A2BX6 compounds typically possess longer B–X bonds, lower defect density, and better stability under ambient conditions. Despite being made by structurally isolated BX6 octahedra, therefore, exhibiting low structural and electronic dimensionality, some of the materials have surprisingly dispersive electronic bands and low electronic bandgaps. For example, Cs2SnI6 has an optical bandgap of just about 1.3 eV,55,56 very close to cubic CsSnI3, and a dispersive conduction band.57 Moreover, vacancy ordered halide double perovskites with Pt, Pd, Te, Ti, and Zr as the tetravalent atom in the center of the octahedron have been shown to also form stable compounds and exhibit bandgaps within the near infrared.57–62 

Finally, the concept of vacancy ordering can be expanded to cover another type of Pb-free materials that contain two trivalent cations that replace three Pb2+ atoms. The lattice of A3B2X9 can be considered as a tripled ABX3 perovskite structure that contains one vacancy to maintain charge neutrality. In the lattice, as shown in Fig. 1, the vacancies are ordered along the (111) plane, making 2D layers of BX6 octahedra that crystallize usually in a trigonal P3̄m1 space group. These structures have been also referred to as “2D layered perovskite derivatives” and some examples of materials are Cs3Bi2Br9 and Cs3Sb2Br9.63,64 However, within the same A3B2X9 stoichiometry compounds, notably Cs3Bi2I965 form another perovskitoid structure, within which the octahedra are face-sharing typically attaining a hexagonal P63/mmc space-group at room temperature. In the following, we include all A3B2X9 materials that have been employed in optoelectronic applications, as in some cases, materials like Cs3Sb2I9 and Cs3Bi2I965 can attain both structures.66,67 These A3B2X9 Pb-free halide perovskites contain a pnictogen atom as the trivalent cation at the octahedra center (i.e., Sb and Bi) and are known to exhibit excellent air stability with their bandgaps ranging between 1.9 and 2.9 eV.68,69

Overall, we have summarized the structural details of the four typical prototype lattices, as shown in Fig. 1, which have been applied to date in manufacturing Pb-free optoelectronic devices. In Secs. IIIV, we will overview their device performances and characteristics and highlight the emerging trends within each type of the targeted applications: solar cells, light emitters, and photocatalysts.

Perovskite photovoltaics is the fastest ever-growing photovoltaic technology, with great potential to open the door for low-cost and efficient solar cells. This is a direct consequence of the almost perfect electronic and optical properties of Pb-based perovskites for solar cell applications. Starting from 2009, when MAPbI3 was first used as a light-sensitizer with a PCE of 3.8%,5 halide perovskite solar cells have made a major step forward in 2012 by using MAPbI3 films and achieved a PCE of around 10%.70–72 Interestingly, at the same period, the 3D Pb-free perovskite CsSnI3 was successfully used as a hole-conducting layer in an efficient dye-sensitized solar cell.60 To date, the highest published PCE for a Pb-based perovskite solar cell is 25.8%10 (certified 25.5%) obtained from the cooperation between South Korean laboratories from the Ulsan National Institute of Science and Technology (UNIST) and the Pohang Accelerator Laboratory (PAL). Furthermore, it is demonstrated that perovskite solar cells can be efficiently combined with silicon cells in tandem architectures, approaching PCE of 30% and able to overcome the performances of both separated technologies. The current certified record of 29.8% has been achieved by Helmholtz Zentrum Berlin.73 

Yet, Pb-based solar cells face major stability issues, which can be at least partly tackled by considering 2D Pb-based perovskites with optimized cell configurations and device encapsulations.74,75 Similar strategies are not yet fully explored for Pb-free perovskite technologies. Moreover, additives specifically dedicated to Pb-free materials may bring interesting improvements in device stability and performance. For example, SnF2,26 GeI2,76 or organic compounds27,77 dissolved in the precursor solution contribute to the perovskite film quality, reducing surface defects/traps, attaining better crystallinity, and most importantly, completely suppressing Sn2+ oxidation. However, we have to note that the most efficient Pb-free perovskite solar cells to date are not on par with the performance of Pb containing perovskites.

Table I reports the Pb-free compounds synthesized so far for solar cell applications. Their PCE (in %), open-circuit voltages (in V), and fabrication methods are listed. Among different categories of Pb-free materials, Sn-based ABX3 type perovskites show the highest PCE. This can be understood by the fact that these Sn-based ABX3 type perovskite materials exhibit very similar properties to the Pb-based ones, as discussed in Sec. II. It was reported in 2020 for FASnI3 perovskites that partially substituting FA with ethylammonium (EA) and surface passivation with 1,2-diaminoethane (EDA) can promote the crystallinity and energy band alignments with charge transport layers.76 As a result, the charge carriers show enhanced mobility and longer lifetimes, resulting in an improved PCE with respect to 9% of the pristine case, with 13.24% and 12.64% for forward and reverse scans, respectively. At the beginning of 2021, the solar cell stability was improved and a maximum efficiency of 13.4% was attained.78 The currently certified record efficiency amounts today to 14.6% with negligible hysteresis.79 A remarkable photostability improvement was obtained in 2022 from a synergistic chemical engineering approach, leading to over 1300 h operational stability in N2 with maximum power point (MPP) tracking.80 Similarly, the highest reported PCEs of CsSnI3 and MASnI3 until now are 12.96%81 and 7.78%,82 respectively, although it is proven theoretically that their PCEs are limited to 32.3% by comparison to 30.5% for Pb-based solar cells.83 The much lower stability of Sn-based perovskite solar cells by comparison to Pb based ones even when encapsulated, leading to the exploration of surface passivation strategies.84 For example, Jokar et al.85 found that the addition of SnF2 and (EDA)I2 could improve the PCE and the stability of FASnI3 and that additional 20% doping of nonpolar organic cation, guanidinium (GA+), could increase the PCE from 7.1% to 8.5%. Furthermore, when the material ages after storage in a glove-box environment for 2000 h, the PCE of the same device tends to be 10%. Devices made of the hybrid perovskite (GA, FA)SnI3 remain stable for 1 h under continuous 1-sun illumination and for 6 days in the dark and in air without encapsulation. Very recently, dipropylammonium iodide (DipI) together with the reducing agent sodium borohydride (NaBH4) was shown to prevent the premature degradation of the Sn-based devices. Efficiencies above 10% were achieved with enhanced stability with 5 h in air [60% relative humidity (RH)] at MPP and 96% of the initial PCE after 1300 h at MPP in the N2 atmosphere.80 According to the reported device I–V characteristics, the highest VOC of Sn based perovskite solar cells reported so far is 0.96 V,79 which is still significantly lower than the expected ideal value,83 while the highest VOC of Pb based perovskite solar cells is closer to the physical limit being over 1.3 V.86 

TABLE I.

Solar cell applications of different Pb-free halide perovskites and their power conversion efficiencies (PCE, in %), open-circuit voltages (VOC, in V), and fabrication methods. Boldface denotes state of the art results.

CategoryCompoundsPCE (%)VOC (V)Fabrication methodReference
ABX3 MASnI3 7.78 0.66 Spin-coating 82  
7.13 0.486 Two-step thin film 87  
   deposition method  
6.4 0.88 Spin-coating 25  
5.44 0.716 Spin-coating 88  
5.23 0.68 Spin-coating 24  
3.89 0.38 Spin-coating with SnF2 89  
   additive and hydrazine  
   vapor treatment  
3.15 0.46 Spin-coating with SnF2 additive 90  
2.14 0.45 Spin-coating with solvent bathing 91  
 1.94 0.25 Spin-coating with SnF2 additive 92  
 1.86 0.273 Low-temperature vapor assisted  93  
   solution deposition process  
 1.7 0.38 Thermal co-evaporation 93  
MASnI2Br 5.48 0.77 Spin-coating 24  
MASnIBr2 5.73 0.82 Spin-coating 24  
MASnBr3 4.27 0.88 Spin-coating 24  
1.12 0.498 Vapor deposition 94  
MASnBr0.5I2.5 1.05 0.18 Spin-coating 95  
MASnIBr1.8Cl0.2 3.1 0.38 Drop casting 96  
MA0.9Cs0.1SnI3 0.51 0.49 Vapor assisted solution  97  
  deposition process  
 0.3 0.20 Spin-coating 98  
(FA0.9EA0.1)0.98EDA0.02SnI3 13.2 0.84 Spin-coating with GeI2 additive 76  
FA0.98EDA0.01SnI3 12.2 0.70 92 days of storage in N2: 80% PCE 99  
PEA0.15FA0.85SnI3 + NH4SCN 12.4 0.94 Spin-coating with SnF2 additive 100  
FA0.9EA0.1SnI3 11.75 0.65 Spin-coating with GeI2 additive 76  
PEA0.15FA0.85SnI3 7.1 0.78 Spin-coating with SnF2 additive 100  
FASnI3 + 4-(aminomethyl) 10.9 0.69 Spin coating 101  
piperidinium     
FASnI3 10.1 0.63 Spin coating with FAI, SnI2, and SnF2  102  
   additive  
 13.4 0.81 Spin-coating with phenylhydrazine 78  
 14.6a 0.96 Spin-coating with adducts 79  
 10.6 0.66 Stability recorda 80  
FASnI3 9.03 0.72 Spin-coating with GeI2 additive 76  
 6.75 0.58 Spin-coating with Sn powder additive 103  
 6.6 0.48 Spin-coating with SnF2 additive 104  
 6.48 0.553 Spin-coating with SnF2 additive 105  
 6.22 0.48 Spin-coating with SnF2 additive 106  
 5.27 0.38 Spin-coating with diethyl ether 107  
    dripping and SnF2 additive  
 4.8 0.32 Spin-coating with pyrazine  107  
   mediator and SnF2 additive  
 3.12 0.31 Spin-coating with SnF2 and HPA  108  
 2.10 0.24 additive Spin-coating 109  
 8.92 0.63 Spin-coating with SnF2 additive 105  
FASnI3 + 2,3-diaminopropionic  7.23 0.52 Spin-coating with SnF2 additive 110  
acidmonohydrochloride      
(2,3-DAPAC)     
FASnI2Br 1.72 0.47 Spin-coating 109  
Br-doped FASnI3 5.5 0.41 Spin-coating with pyrazine mediator  111  
   and SnF2 additive  
FA0.8MA0.2SnI3 1.4 0.24 Spin-coating solvent engineering 98  
FA0.25MA0.75SnI3 4.49 0.48 Spin-coating with SnF2 additive 104  
FA0.5MA0.5SnI3 5.92 0.53 Spin-coating with SnF2 additive 104  
FA0.75MA0.25SnI3 8.12 0.61 Spin-coating with SnF2 additive and  104  
   chlorobenzene dripping  
 9.06 0.55 Spin-coating + SnF2 additive  112  
    and chlorobenzene  
   dripping + post-annealing  
FA0.75MA0.25SnI3 CsSnI3  7.2 0.55 Spin-coating with SnF2 additive  113  
+ phthalimide (PTM)   and hot antisolvent   
   treatment and solvent vapor annealing  
 3.31 0.46 Spin-coating + antisolvent 114  
 2.60 0.37 Spin-coating + SnF2 additive 115  
 10.1 0.64 Two-step spin-coating + chlorobenzene  77  
   as antisolvent  
CsSnI3 + thiosemicarbazide  8.20 0.63 Passivator assisting sequential  84  
(TSC)   vapor deposition  
CsSnI3 4.81 0.38 Spin-coating with SnI2 additive 116  
 3.83 ⋯ Spin-coating with SnF2  117  
   and piperazine additive  
 3.56 0.50 Spin-coating with SnCl2 additive 118  
 3.31 0.52 Spin-coating and annealing 119  
 2.76 0.43 Spin-coating with SnI2 additive 120  
CsSnI3 2.02 0.24 Spin-coating with SnF2 additive 121  
FA0.75MA0.25Sn0.95Ge0.05I3     
 1.83 0.17 Spin-coating with SnF2 additive and  89  
   hydrazine vapor treatment  
 1.70 ⋯ Solid-state reaction 32  
 1.66 0.20 Spin-coating with SnF2 additive 81  
 0.90 0.42 Sequential evaporation and  121  
   subsequent annealing  
  7.90 0.45 Spin-coating + GeI2 and SnF2  115  
    additives  
 FA0.75MA0.25Sn0.95Ge0.05I3 4.48 0.42 Spin-coating + antisolvent 114  
 Native oxide passivated  7.11 0.63 Solid-state reaction 32  
 CsSn0.5Ge0.5I3     
 CsSn0.5Ge0.5I3 3.72 0.48 Solid-state reaction + N2 32  
 CsSnI2.9Br0.1 1.76 0.22 Spin-coating with SnF2 additive 122  
 CsSnI2Br 1.67 0.29 Spin-coating with SnF2 additive 81  
 CsSnIBr2 3.20 0.31 Sequential evaporation and  122  
    subsequent annealing  
 CsSnIBr2 1.56 0.31 Sequential evaporation and  81  
    subsequent annealing  
 CsSnBr3 3.04 0.37 Spin-coating with SnF2 additive and  89  
    hydrazine vapor treatment  
  2.10 0.41 Spin-coating with SnF2 additive 123  
 CsSnBr3 0.95 0.41 Spin-coating with SnF2 additive 81  
 CsSnI3 (QR) 0.55 0.45 All vapor deposited with SnF2 additive 124  
  13.0 0.86 Spin-coating 81  
 CsSnBr3 (QR) 10.5 0.85 Spin-coating 81  
 CsSnCl3 (QR) 9.66 0.87 Spin-coating 118  
 MAGeI3 0.68 ⋯ ⋯ 125  
 MAGeI3 0.20 0.150 ⋯ 125  
 MAGeI2.7Br0.3 0.57 0.449 Spin-coating 125  
 CsGeX3 4.94 0.51 Quantum rods 126  
 CsGeI3 0.11 0.074 ⋯ 127  
 {en}MASnI3 6.63 0.43 Spin-coating with SnF2 additive 128  
 {en}FASnI3 7.14 0.48 Spin-coating with SnF2 additive 29  
 {en}FA0.78GA0.22SnI3 9.60 0.619 Spin-coating with SnF2 additive 85  
A2BB′X6 Cs2AgBiBr6 + carboxy-chlorophyll 3.11a 1.04 Spin-coating 129  
derivative (C-chl)     
Cs2AgBiBr6 2.84 1.06 Spin-coating 130  
Cs2AgBiBr6 2.43 0.98 Spin-coating 131  
(Cs0.9Rb0.1)2AgBiBr6 2.23 1.01 Spin-coating with antisolvent 132  
 1.44 1.04 Spin-coating 132  
 1.26 1.02 Spin-coating with antisolvent 131  
 1.37 1.12 Sequential vapor deposition 133  
Cs2NaBiI6 1.52 0.99 Spin-coating 134  
Cs2NaBiI6 0.42 0.47 One-step hydrothermal process 135  
A2BX6 Cs2SnI6 0.96 0.51 Thermal evaporation, annealing, and phase change 136  
Cs2SnI6 0.86 0.52 Spin-coating 137  
Cs2SnI5Br 1.47 0.37 Two-step thin film deposition method 138  
 0.47 0.25 Chemical bath deposition 139  
Cs2SnI4Br2 1.60 0.44 Two-step thin film deposition method 138  
Cs2SnI4Br2 2.03 0.56 Two-step thin film deposition method 138  
Cs2SnI2Br4 1.08 0.58 Two-step thin film deposition method 138  
Cs2SnIBr5 0.002 0.57 Two-step thin film deposition method 138  
Cs2TiBr6 3.28 1.02 Two-step vapor deposition 140  
Cs2TiBr6 2.26 0.89 Two-step vapor deposition 140  
Cs2SnI3Br3 3.63a 0.70 Two-step thin film deposition method with Z907 dye 141  
A3B2X9  1.64 0.81 Deposition and homogeneous transformation 63  
 0.42 0.67 Deposition and homogeneous transformation 142  
 0.39 0.81 Thermal evaporation, spin-coating, and annealing 143  
 0.36 0.65 Solvent engineering 144  
MA3Bi2I9 0.31 0.51 Spin coating 145  
MA3Bi2I9 0.26 0.56 Spin-coating 63  
MA3Bi2I9Clx 0.19 0.35 Spin-coating 146  
 0.12 0.68 Spin-coating 146  
 0.11 0.72 Solvent engineering 147  
 0.08 0.69 Spin coating with gas-assisted 148  
 0.07 0.66 Spin coating 149  
 0.053 0.84 Spin-coating 150  
MA3Sb2I9 0.003 0.04 Spin-coating 146  
 2.77 0.70 Spin-coating with pyrene/HI + chlorobenzene additive 151  
MA3Sb2I9 2.46 0.69 Spin-coating with perylene/HI + chlorobenzene additive 151  
MA3Sb2I9 2.25 0.63 Spin-coating with HI + chlorobenzene additive 151  
MA3(Sb0.6Sn0.4)2I9 1.89 0.62 Spin-coating with HI additive 151  
 2.04 0.62 Spin-coating with HI additive 152  
 0.62 0.75 Spin-coating with chlorobenzene dripping 153  
 0.5 0.89 Spin-coating 154  
CsBi3I10 2.80a 0.57 Spin-coating with chlorobenzene dripping 153  
CsBi3I10 0.40 0.31 Spin-coating 155  
Cs3Bi2I9 1.09 0.85 Spin-coating 146  
Cs3Bi2I9 0.02 0.26 Spin-coating 155  
Cs3Sb2I9 0.84 0.60 Spin-coating with HI additive 152  
Rb3Sb2I9     
Cs3Sb2I9 <1 0.30 Dual annealing 152  
Rb3Sb2I9     
(NH4)3Sb2I9 0.66 0.55 Spin-coating with toluene dripping 156  
(NH4)3Sb2I9 0.51 1.03 Spin-coating with chloroform dripping 157  
CategoryCompoundsPCE (%)VOC (V)Fabrication methodReference
ABX3 MASnI3 7.78 0.66 Spin-coating 82  
7.13 0.486 Two-step thin film 87  
   deposition method  
6.4 0.88 Spin-coating 25  
5.44 0.716 Spin-coating 88  
5.23 0.68 Spin-coating 24  
3.89 0.38 Spin-coating with SnF2 89  
   additive and hydrazine  
   vapor treatment  
3.15 0.46 Spin-coating with SnF2 additive 90  
2.14 0.45 Spin-coating with solvent bathing 91  
 1.94 0.25 Spin-coating with SnF2 additive 92  
 1.86 0.273 Low-temperature vapor assisted  93  
   solution deposition process  
 1.7 0.38 Thermal co-evaporation 93  
MASnI2Br 5.48 0.77 Spin-coating 24  
MASnIBr2 5.73 0.82 Spin-coating 24  
MASnBr3 4.27 0.88 Spin-coating 24  
1.12 0.498 Vapor deposition 94  
MASnBr0.5I2.5 1.05 0.18 Spin-coating 95  
MASnIBr1.8Cl0.2 3.1 0.38 Drop casting 96  
MA0.9Cs0.1SnI3 0.51 0.49 Vapor assisted solution  97  
  deposition process  
 0.3 0.20 Spin-coating 98  
(FA0.9EA0.1)0.98EDA0.02SnI3 13.2 0.84 Spin-coating with GeI2 additive 76  
FA0.98EDA0.01SnI3 12.2 0.70 92 days of storage in N2: 80% PCE 99  
PEA0.15FA0.85SnI3 + NH4SCN 12.4 0.94 Spin-coating with SnF2 additive 100  
FA0.9EA0.1SnI3 11.75 0.65 Spin-coating with GeI2 additive 76  
PEA0.15FA0.85SnI3 7.1 0.78 Spin-coating with SnF2 additive 100  
FASnI3 + 4-(aminomethyl) 10.9 0.69 Spin coating 101  
piperidinium     
FASnI3 10.1 0.63 Spin coating with FAI, SnI2, and SnF2  102  
   additive  
 13.4 0.81 Spin-coating with phenylhydrazine 78  
 14.6a 0.96 Spin-coating with adducts 79  
 10.6 0.66 Stability recorda 80  
FASnI3 9.03 0.72 Spin-coating with GeI2 additive 76  
 6.75 0.58 Spin-coating with Sn powder additive 103  
 6.6 0.48 Spin-coating with SnF2 additive 104  
 6.48 0.553 Spin-coating with SnF2 additive 105  
 6.22 0.48 Spin-coating with SnF2 additive 106  
 5.27 0.38 Spin-coating with diethyl ether 107  
    dripping and SnF2 additive  
 4.8 0.32 Spin-coating with pyrazine  107  
   mediator and SnF2 additive  
 3.12 0.31 Spin-coating with SnF2 and HPA  108  
 2.10 0.24 additive Spin-coating 109  
 8.92 0.63 Spin-coating with SnF2 additive 105  
FASnI3 + 2,3-diaminopropionic  7.23 0.52 Spin-coating with SnF2 additive 110  
acidmonohydrochloride      
(2,3-DAPAC)     
FASnI2Br 1.72 0.47 Spin-coating 109  
Br-doped FASnI3 5.5 0.41 Spin-coating with pyrazine mediator  111  
   and SnF2 additive  
FA0.8MA0.2SnI3 1.4 0.24 Spin-coating solvent engineering 98  
FA0.25MA0.75SnI3 4.49 0.48 Spin-coating with SnF2 additive 104  
FA0.5MA0.5SnI3 5.92 0.53 Spin-coating with SnF2 additive 104  
FA0.75MA0.25SnI3 8.12 0.61 Spin-coating with SnF2 additive and  104  
   chlorobenzene dripping  
 9.06 0.55 Spin-coating + SnF2 additive  112  
    and chlorobenzene  
   dripping + post-annealing  
FA0.75MA0.25SnI3 CsSnI3  7.2 0.55 Spin-coating with SnF2 additive  113  
+ phthalimide (PTM)   and hot antisolvent   
   treatment and solvent vapor annealing  
 3.31 0.46 Spin-coating + antisolvent 114  
 2.60 0.37 Spin-coating + SnF2 additive 115  
 10.1 0.64 Two-step spin-coating + chlorobenzene  77  
   as antisolvent  
CsSnI3 + thiosemicarbazide  8.20 0.63 Passivator assisting sequential  84  
(TSC)   vapor deposition  
CsSnI3 4.81 0.38 Spin-coating with SnI2 additive 116  
 3.83 ⋯ Spin-coating with SnF2  117  
   and piperazine additive  
 3.56 0.50 Spin-coating with SnCl2 additive 118  
 3.31 0.52 Spin-coating and annealing 119  
 2.76 0.43 Spin-coating with SnI2 additive 120  
CsSnI3 2.02 0.24 Spin-coating with SnF2 additive 121  
FA0.75MA0.25Sn0.95Ge0.05I3     
 1.83 0.17 Spin-coating with SnF2 additive and  89  
   hydrazine vapor treatment  
 1.70 ⋯ Solid-state reaction 32  
 1.66 0.20 Spin-coating with SnF2 additive 81  
 0.90 0.42 Sequential evaporation and  121  
   subsequent annealing  
  7.90 0.45 Spin-coating + GeI2 and SnF2  115  
    additives  
 FA0.75MA0.25Sn0.95Ge0.05I3 4.48 0.42 Spin-coating + antisolvent 114  
 Native oxide passivated  7.11 0.63 Solid-state reaction 32  
 CsSn0.5Ge0.5I3     
 CsSn0.5Ge0.5I3 3.72 0.48 Solid-state reaction + N2 32  
 CsSnI2.9Br0.1 1.76 0.22 Spin-coating with SnF2 additive 122  
 CsSnI2Br 1.67 0.29 Spin-coating with SnF2 additive 81  
 CsSnIBr2 3.20 0.31 Sequential evaporation and  122  
    subsequent annealing  
 CsSnIBr2 1.56 0.31 Sequential evaporation and  81  
    subsequent annealing  
 CsSnBr3 3.04 0.37 Spin-coating with SnF2 additive and  89  
    hydrazine vapor treatment  
  2.10 0.41 Spin-coating with SnF2 additive 123  
 CsSnBr3 0.95 0.41 Spin-coating with SnF2 additive 81  
 CsSnI3 (QR) 0.55 0.45 All vapor deposited with SnF2 additive 124  
  13.0 0.86 Spin-coating 81  
 CsSnBr3 (QR) 10.5 0.85 Spin-coating 81  
 CsSnCl3 (QR) 9.66 0.87 Spin-coating 118  
 MAGeI3 0.68 ⋯ ⋯ 125  
 MAGeI3 0.20 0.150 ⋯ 125  
 MAGeI2.7Br0.3 0.57 0.449 Spin-coating 125  
 CsGeX3 4.94 0.51 Quantum rods 126  
 CsGeI3 0.11 0.074 ⋯ 127  
 {en}MASnI3 6.63 0.43 Spin-coating with SnF2 additive 128  
 {en}FASnI3 7.14 0.48 Spin-coating with SnF2 additive 29  
 {en}FA0.78GA0.22SnI3 9.60 0.619 Spin-coating with SnF2 additive 85  
A2BB′X6 Cs2AgBiBr6 + carboxy-chlorophyll 3.11a 1.04 Spin-coating 129  
derivative (C-chl)     
Cs2AgBiBr6 2.84 1.06 Spin-coating 130  
Cs2AgBiBr6 2.43 0.98 Spin-coating 131  
(Cs0.9Rb0.1)2AgBiBr6 2.23 1.01 Spin-coating with antisolvent 132  
 1.44 1.04 Spin-coating 132  
 1.26 1.02 Spin-coating with antisolvent 131  
 1.37 1.12 Sequential vapor deposition 133  
Cs2NaBiI6 1.52 0.99 Spin-coating 134  
Cs2NaBiI6 0.42 0.47 One-step hydrothermal process 135  
A2BX6 Cs2SnI6 0.96 0.51 Thermal evaporation, annealing, and phase change 136  
Cs2SnI6 0.86 0.52 Spin-coating 137  
Cs2SnI5Br 1.47 0.37 Two-step thin film deposition method 138  
 0.47 0.25 Chemical bath deposition 139  
Cs2SnI4Br2 1.60 0.44 Two-step thin film deposition method 138  
Cs2SnI4Br2 2.03 0.56 Two-step thin film deposition method 138  
Cs2SnI2Br4 1.08 0.58 Two-step thin film deposition method 138  
Cs2SnIBr5 0.002 0.57 Two-step thin film deposition method 138  
Cs2TiBr6 3.28 1.02 Two-step vapor deposition 140  
Cs2TiBr6 2.26 0.89 Two-step vapor deposition 140  
Cs2SnI3Br3 3.63a 0.70 Two-step thin film deposition method with Z907 dye 141  
A3B2X9  1.64 0.81 Deposition and homogeneous transformation 63  
 0.42 0.67 Deposition and homogeneous transformation 142  
 0.39 0.81 Thermal evaporation, spin-coating, and annealing 143  
 0.36 0.65 Solvent engineering 144  
MA3Bi2I9 0.31 0.51 Spin coating 145  
MA3Bi2I9 0.26 0.56 Spin-coating 63  
MA3Bi2I9Clx 0.19 0.35 Spin-coating 146  
 0.12 0.68 Spin-coating 146  
 0.11 0.72 Solvent engineering 147  
 0.08 0.69 Spin coating with gas-assisted 148  
 0.07 0.66 Spin coating 149  
 0.053 0.84 Spin-coating 150  
MA3Sb2I9 0.003 0.04 Spin-coating 146  
 2.77 0.70 Spin-coating with pyrene/HI + chlorobenzene additive 151  
MA3Sb2I9 2.46 0.69 Spin-coating with perylene/HI + chlorobenzene additive 151  
MA3Sb2I9 2.25 0.63 Spin-coating with HI + chlorobenzene additive 151  
MA3(Sb0.6Sn0.4)2I9 1.89 0.62 Spin-coating with HI additive 151  
 2.04 0.62 Spin-coating with HI additive 152  
 0.62 0.75 Spin-coating with chlorobenzene dripping 153  
 0.5 0.89 Spin-coating 154  
CsBi3I10 2.80a 0.57 Spin-coating with chlorobenzene dripping 153  
CsBi3I10 0.40 0.31 Spin-coating 155  
Cs3Bi2I9 1.09 0.85 Spin-coating 146  
Cs3Bi2I9 0.02 0.26 Spin-coating 155  
Cs3Sb2I9 0.84 0.60 Spin-coating with HI additive 152  
Rb3Sb2I9     
Cs3Sb2I9 <1 0.30 Dual annealing 152  
Rb3Sb2I9     
(NH4)3Sb2I9 0.66 0.55 Spin-coating with toluene dripping 156  
(NH4)3Sb2I9 0.51 1.03 Spin-coating with chloroform dripping 157  
a

The highest value of PCE for each category of Pb-free perovskites, as highlighted in a bold font.

Ge-based perovskite solar cells have achieved lower efficiencies than Sn-based ones (see Table I). Ge2+ oxidation under ambient air conditions causes structure instability, which is similar to the outcome of the Sn2+ oxidation in Sn-based perovskites. As an alternative, perovskite solar cells using alloys of Ge and Sn are developed, showing promising PCEs and better stabilities than their pure Ge equivalents due to the native-oxide passivation of surfaces. The energy band structure behavior of Sn/Ge perovskite compounds is completely different from the Sn/Pb alloys that show bandgap bowing.158 Indeed, their bandgaps (R3m space group) are linearly increasing with respect to the concentration of Ge.159 For instance, the CsSn0.5Ge0.5I3 has a bandgap (1.50 eV) in between CsSnI3 (1.31 eV) and CsGeI3 (1.63 eV) and allows photo-absorption across the visible light region.32 As a result, a PCE of 7.11% is obtained for CsSn0.5Ge0.5I3 in comparison to the 3.72% attained without forming a native-oxide layer and the 1.7% of the pure CsSnI3. As discussed previously, mixing halides might lead to chemical inhomogeneities in the conventional perovskite compounds. However, mixed-halide CsSn1−xGexI3−yBry compounds are easier to synthesize than the single-halide CsSn1−xGexI3 compounds and are superior in terms of optical response in the visible light range, according to Chang et al.160 Among them, the CsSn0.5Ge0.5I2Br is the best choice. To date, the highest PCE of Sn/Ge alloys amounts to 7.9%, with FA0.75MA0.25Sn0.95Ge0.05I3 based solar cells fabricated by passivating and reducing trap densities with GeI2 and SnF2 additives.115 More importantly, the PCE retains 91% of its initial value for 500 h under 1-sun illumination in air. Although the results of Sn/Ge mixed perovskites are encouraging, they are still far from the expected maximum PCE value. For instance, the theoretical predicted PCE of CsSn0.5Ge0.5I3 is as high as 24.20%.161 

Double perovskites and vacancy ordered double perovskites are very stable under ambient conditions. Yet, the electronic bandgaps of the synthesized double perovskites range from 2 to 3.4 eV and thus are too large for efficient light harvesting in single-junction photovoltaics. The highest PCE obtained for a double perovskite, 3.11% obtained for Cs2AgBiBr6, is, nevertheless, a decent achievement for a large bandgap semiconductor.129 Doping with In, a common strategy for highly efficient light emitters discussed below, could also improve the attained photovoltaic performance as recently shown by Schade et al.162 More generally, their stability combined with their optoelectronic properties, including photoconversion at high energy, light emission, and charge transport, might be more attractive for other optoelectronic applications.

Meanwhile, the highest PCE obtained for a vacancy ordered double perovskite is 3.63% for the mixed-halide Cs2SnI3Br3.141 Since the Sn atoms used in vacancy ordered double perovskites correspond to the +4 oxidation state, these perovskite structures are the most stable among Sn based perovskite materials under ambient conditions.

Although Pb-free A3B2X9 2D perovskite derivatives are quite stable in air, their achieved PCE remained much lower than 1% without improvement for a long time. A breakthrough was obtained in 2018 for Pb-free 2D perovskite based on Sb dimers with the help of additives and antisolvents treatment. A continuum, smooth, and pinhole-free morphology of MA3Sb2I9 can be formed, thanks to fast heterogeneous nucleation, yielding a record PCE of 2.77%.151 In the same year, a PCE of 2.80% was achieved for MA3(Sb1−xSnx)2I9 by heterovalent substitution when x is 0.4, contrasting to a PCE of 0.62% when x is 0.153 By Sn4+ substitution, the bandgap is reduced from the pristine bandgap value of 2 eV close to the optimum value of 1.55 eV, and the electronic conductivity is changed from p-type to n-type, leading to a better band alignment with the selected contact layers. All the above discussions point out that chemical engineering, additives, antisolvent treatment, and hydrophobic charge transport layers are the guides to further improve the PCEs of double and 2D Pb-free perovskites.

To establish a connection between the types of Pb-free materials and their application in solar cells, we calculate the percentage of each type of Pb-free materials that have been reported in solar cell applications. Figure 2(a) highlights the contribution of the conventional single ABX3 (pink color), double A2BB′X6 (cyan color), vacancy ordered double A2BX6 (gray color), and 2D layered A3B2X9 Pb-free perovskites (orange color) by the inner ring of the sunburst chart, where their percentages are listed at the inner blank spaces, which are 62%, 7%, 9%, and 22%, respectively. In addition, the contributions of different types of B site cations are highlighted by the outer ring of the sunburst chart, where the corresponding percentages are summarized at the outer blank spaces. In addition, the contribution of A site cations and X site anions is represented by the 3D pie charts, where their corresponding percentages are summarized next to each of them.

FIG. 2.

Sunburst charts showing the use of different Pb-free halide perovskite classes (conventional ABX3 in pink, double A2BB′X6 in cyan, vacancy ordered double A2BX6 in gray, and 2D layered A3B2X9 in orange) in the fabrication of solar cells (top), LEDs (middle), and photocatalysts (bottom). In each chart, the contribution of different perovskite classes is highlighted by the inner ring, and their percentages are summarized at the inner blank spaces; the contribution of different B site cations in each class is highlighted by the outer ring with the same color as the inner ring, and their percentages are summarized at the outer blank spaces. The corresponding distribution in terms of different A site cations and X site anions is shown in the 3D pie charts (right panels). Percentages below 1% are not included.

FIG. 2.

Sunburst charts showing the use of different Pb-free halide perovskite classes (conventional ABX3 in pink, double A2BB′X6 in cyan, vacancy ordered double A2BX6 in gray, and 2D layered A3B2X9 in orange) in the fabrication of solar cells (top), LEDs (middle), and photocatalysts (bottom). In each chart, the contribution of different perovskite classes is highlighted by the inner ring, and their percentages are summarized at the inner blank spaces; the contribution of different B site cations in each class is highlighted by the outer ring with the same color as the inner ring, and their percentages are summarized at the outer blank spaces. The corresponding distribution in terms of different A site cations and X site anions is shown in the 3D pie charts (right panels). Percentages below 1% are not included.

Close modal

Among all Pb-free compounds, most materials contain Sn, with 90% of the cubic perovskites and 82% of the vacancy ordered double perovskites being Sn-based compounds. This is in line with the favorable PCEs of Sn based perovskite solar cells being the most promising to date. Among all Sn-based compounds used for solar cell applications, 89% of Sn compounds are materials with Sn in the +2 oxidation state, which is consistent with the fact that stability remains the main issue in this field. Therefore, the still rare (5%) use of either pure Ge (5%) or mixtures of Ge and Sn might become the next step in the quest for stable Pb-free perovskite solar cell applications. Among A2BB′X6 double perovskites, the ones based on B = Bi and B′ = Ag (including doped derivatives) are almost the sole Pb-free candidates to date that have been tested for solar cells; those based on B = Bi and B′ = Na have merely achieved a PCE of 0.4%. In addition, in 2D layered perovskites, most of the B site cations are either Bi (57%) or Sb (40%), with a small percentage (3%) of Bi–Sb mixtures. In addition, from the 3D pie chart indicative of the halide species, as for Pb-based perovskites, most compositions used for solar cell applications are iodides (74%), followed by bromides (16%) and mixed I–Br compounds (9%). In contrast, the Cl and mixed Br–Cl compounds are rarely used (less than 1%). Finally, organic (57%) and inorganic (41%) A-site cations are almost equally used, with a small amount (2%) of organic–inorganic mixtures.

Over the past decades, thanks to their intrinsic properties, including high PLQY, Pb-based metal halide perovskites have also gained great interest for LED applications. For example, Zhou et al.163 achieved a record-high PLQY of 94.6% with MAPbX3 (X = Cl, Br, I) nanocrystals embedded in polyvinylidene fluoride composite films. However, achieving large external quantum efficiencies (EQE) in operating LED devices requires more, especially good carrier transport and carrier injection from the electrical contacts toward the active zone. Once these conditions are fulfilled, it is considered nowadays that perovskite-based LEDs should overcome EQE of 20% starting from high PLQY, thanks to photon recycling and efficient optical outcoupling.164 Green LEDs are the best performing devices, leading to a maximum EQE of 28.1% in 2021.165 Using non-perovskite matrices was also successful for Pb-based green emitters leading to stable operation over 50 h with an EQE of 15%.166 This solution has the disadvantage of putting strong practical limits on the current injected into the active zone (1.2 mA cm−2 in Ref. 166) but may provide some guidance for the design of Pb-free emitters. Pb-based red LEDs emitting at 627 nm with an impressive EQE of 20.3% (starting from a PLQY of 88%) were obtained recently, although the operational stability remains limited to 1 h and the EQE drops very quickly for current densities above 1 mA cm−2. The performances of blue Pb-based LEDs are more limited leading, for example, to an EQE of 12.3% starting from a PLQY over 90%,167 or a sky-blue emitting device with an EQE of 13.8% but driven by a high voltage of above 4 V.168 Finally, it shall be mentioned that demonstrations of single layer Pb-based white light LED do exist,169 although with room for improvement. A promising EQE of 1.2% (starting from a PLQY of 85%) with a color rendering index (CRI) of 93 was, for example, obtained in 2020 using CsPbCl3 quantum dots doped with Sm3+.170 It is now clear that Pb-based perovskites are nowadays strong competitors in the field of LEDs, with remarkable EQE demonstrations for monochromatic red, green, or blue optical sources and even promising single layer white-light emitters. However, challenges are still remaining for commercialization including among others, highly efficient blue electroluminescence, long device lifetime, toxicity, and bioavailability of lead.171 

From the perspective of LED engineering, the quality of the interfaces is of utmost importance as mentioned above. This additional constraint might hinder the practical application of most Pb-free perovskite materials to LEDs, despite exhibiting attractive PLQY. So far, most of the experimental data reported on Pb-free materials to illustrate light emission capabilities are related to the PLQY. This interesting piece of information shall be handled with care since it might not guarantee LEDs with attractive performances. We already saw indeed in Sec. III on solar cells that interface issues are limiting the performances of Pb-free perovskite devices. It shall be further pointed out that current densities flowing through operating LED devices may exceed by one order of magnitude the current densities observed in solar cells. This may put even more stringent requirements on the Pb-free material quality and their interfaces with carrier transporting layers. In this Perspective, we, nevertheless, limit our overview of Pb-free perovskite performances to PLQYs, leaving aside EQEs mainly due to the lack of extensive data in the literature.

As it seems Pb-free materials can hardly compete with Pb-based ones for solar cells due to their low PCEs, this might be also the case for LED applications. Their high tunability across the entire visible light spectrum might be nevertheless one comparative advantage.163,172–174 Narrowband emission, usually observed at low temperature, is very often attributed to intrinsic free excitonic transitions. Such transitions can be influenced by lattice parameters and thermal expansion mismatches between perovskites and charge transport layers. An additional broadband emission at low energy is also observed in some cases. This broadband emission is believed to have good potential for white light LEDs. Self-trapping excitons (STEs) are very often claimed to be at the origin of this broad redshifted, but extrinsic or defect-related emission mechanisms are still not ruled out.

Among the four classes of Pb-free perovskites, double perovskites and vacancy ordered double perovskites show promising PLQYs as well as high intrinsic thermodynamic stabilities and low carrier effective masses. One of the major challenges is their wide and/or indirect bandgaps, which makes them less suitable for optoelectronic applications in the visible region. For this reason, homovalent and heterovalent co-doping have been developed. Cs2AgBiCl6 and Cs2AgInCl6 have become the reference materials for realizing white light LEDs for Pb-free materials. In comparison to pure Cs2AgBiCl6, the higher stability of Na-doped Cs2(Na1−xAgx)BiCl6 was attributed to the easier formation of a [NaCl6]5− octahedron compared to a [AgCl6]5− one, but this, in turn, results in a larger bandgap.175 It is also found that the minor luminescence of the as-prepared Cs2NaInCl6 nanocrystals (dark STEs) can convert into a bright yellow emission (attributed to bright STEs) by Ag+ doping.176 Two physical mechanisms have been proposed to explain the enhanced light emissions.177 First, [NaCl6]5− and [AgCl6]5− octahedra can break the parity-forbidden selection rule for the direct optical transition hence stimulating effective photoluminescence (PL) emission. Second, the presence of Ag+ can mitigate the effect of lattice vibrations on PL quenching. Sb3+, In3+, and Mn2+ doping are also popular choices for tuning the electronic properties. For example, the Sb- and In-doping of Cs2AgBiBr6 induce opposite variations of the bandgaps, leading to an increase (decrease) for In3+(Sb3+) doping. It has to be noted that Sb3+ doping produces the smallest bandgap within this class, with a value of 1.86 eV for Cs2Ag(Bi0.625Sb0.375)Br6. In addition, these doped systems are only stable when In3+ and Sb3+ doping ratios are within the 0%–75% and 0%–37.5% ranges, respectively.178 

Substituting or doping double perovskites that contain Bi (or Sb) with In results in an interplay between direct and indirect bandgaps,32,155,179 which could be promising for the luminescence of perovskite phosphors toward white-light emission. In contrast, the bandgaps of Cs2AgInCl6 double perovskites have no significant difference before and after Mn2+ doping.180 However, with Mn-doping as high as 1.5%, the weak PL of undoped Cs2NaBiCl6 has been enhanced to emit a new orange-red PL (from 525 to 700 nm). The PLQY increases by an order of magnitude, namely, from 1.6% up to 16%. This enhanced emission has been attributed to the near-UV light absorption of [BiCl6]3− octahedra in the host lattice and the energy transfer from Bi3+ to Mn2+ activators via the spin-forbidden 4T16A1 transition.181 This 4T16A1dd transition is expected to interact less with nonradiative trap states in the host lattice, accounting for the enhanced PL intensity and millisecond long lifetime.154 As reported in the literature,182 the internal quantum efficiency of Cs2(Ag0.4Na0.6)InCl6: 1% Bi phosphor can be further increased from 89.9% to 98.4% and 98.6% by co-doping 1% Ni and 1% Ce, respectively. The physical mechanisms at the origin of light emission depend on the nature of the perovskite host. Specifically, for Mn-doped Cs2(Na0.75Ag0.25)BiCl6, the distinct energy-transfer channel from Mn2+ ion guest to STEs perovskite host has been proposed to result in the dominant Mn2+ emission,175 which is similar to pure Cs2AgInCl6.180 In contrast, for Mn-doped Cs2(Na0.4Ag0.6)In0.95Bi0.05Cl6, the efficient energy transfer from broadband STEs host to Mn2+ guest dd transitions would explain the high PLQY.183 In addition to the above two mechanisms, for the Mn-doped (C6H18N2O2)PbBr4, the tentative explanation for the ultrabroad band warm light emission relies on a simultaneous enhancement of STE emission and Mn2+ emission.184 

Table II presents, for the four material classes, the Pb-free perovskite compositions synthesized to date for LED applications, together with their emission peak positions (in nm), the light emission colors, the full-width half maxima (FWHM, in nm) of the emissions, and the PLQYs (in %). We note that the highest PLQY of about 93% has been achieved with Sb-doped Cs2KInCl6.185 Apart from double perovskites, vacancy ordered double perovskites also appear as promising candidates for LED applications. The highest PLQY among them is obtained for Te-doped Cs2SnCl6, with 95.4%.186 Replacing Sn4+ with Te4+ is proposed to lead to the generation of new defect levels above band edges, which promote an exciton transition nearby [TeCl6]2− octahedra, thereby initiating STEs emission of broad yellow–green luminescence.186 This Te3+ triggered STEs emission is also hypothesized in the A site organic perovskite (NH4)2SnCl6, whose PLQY increases from less than 0.05% before Te3+ doping to 83.5% after doping.187 Noteworthy, the good stability against water of the Te-doped Cs2SnCl6 makes it particularly suitable for underwater lighting applications.186 

TABLE II.

Overview of various Pb-free halide perovskites used for LED applications. The emission peak positions (in nm), light emission colors, full-width half maxima (FWHM, in nm) of the emissions, and PLQYs (in %) are indicated. Films, powder, nanocrystals (NCs), or more rarely nanoplatelets or nanocages are used. Boldface denotes state of the art results.

CategoryCompoundsEmission peak (nm)ColorFWHM (nm)PLQY (%)CommentReference
ABX3 MASn(Br/I)3 667–945 ⋯ ⋯ <5.3 Films 188  
CsSn(Br/I)3 667–887 ⋯ ⋯  Films 54  
CsSnBr3 670 Dark red 56 2.1 Nanocages 189  
CsSnBr3 672 Red 54 9.1 Films 190  
CsSnI3 950 Infrared  3.8 Films 191  
CsBr: Eu2+ 440 White 31 32.8a NCs 192  
A2BB′X6 Cs2AgBiCl6 395 ⋯ 68 6.7 NCs 193  
610 ⋯ 200 Powder 175  
Cs2AgBiBr6 465 ⋯ 82 0.7 NCs 193  
Cs2(Na0.75Ag0.25)BiCl6 610 ⋯ 160 45 Powder 175  
Cs2(Na0.4Ag0.6)InCl6: Bi3+ 552 White 40.9 86.2 Powder 177  
Cs2(Na0.4Ag0.6)InCl6: 5.49% Ho3+ 490 White  60.5 Powder 194  
Cs2Ag(In0.875Bi0.125)Cl6 585 White ⋯ 70.3 Powder 195  
Cs2NaBiCl6 730 ⋯ ⋯  Powder 181  
Cs2NaBiCl6: Mn2+ 590   15 Powder 181  
Cs2Na0.995Bi0.995Mn0.01Cl6 590 Orange–red ⋯ 11.4 Powder 181  
Cs2Na0.987Bi0.987Mn0.026Cl6 590 Orange–red ⋯ 12.4 Powder 181  
Cs2Na0.969Bi0.969Mn0.062Cl6 590 Orange–red ⋯ 15.1 Powder 181  
Cs2AgInCl6 560 White ⋯ 1.6 NCs 180  
⋯ ⋯ ⋯ NCs 196  
Cs2AgBi0.085In0.915Cl6 600 White 200 34 Powder 197  
Cs2AgIn0.9Cr0.1Cl6 1010 Near-infrared 180 23.5 Powder 198  
Cs2NaInCl6: 10% Ag 535 Yellow ⋯ 31.1 NCs 176  
Cs2Ag0.4Na0.6InCl6 ⋯ ⋯ ⋯ 22 NCs 196  
Cs2AgInCl6: 0.9% Mn2+ 632 White ⋯ 3–5 Powder 199  
Cs2AgInCl6: 0.5% Mn2+ 620 Orange ⋯ NCs 180  
Cs2AgInCl6: 1.5% Mn2+ 620 Orange ⋯ 16 NCs 180  
Cs2AgInCl6: Mn2+ 630 ⋯ ⋯ Powder 181  
Cs2NaInCl6: 1% Sb3+ 445 Blue  82 Powder 185  
Cs2KInCl6: 5% Sb3+ 495 Green  93a Powder 185  
Cs2NaEuCl6 593 Red ⋯ 35 Powder 200  
Cs2NaTbCl6 548 Green ⋯ 56 Powder 200  
A2BX6 Cs2SnCl6: 2.75% Bi3+ 455 White 66 78.9 Powder 201  
Cs2SnCl6: 1.16% Bi3+ 457 White 63 68.3 Powder 201  
Cs2SnCl6: 0.11% Bi3+ 454 White 65 67.6 Powder 201  
Cs2SnCl6: Sb3+ 602 White 101 37 Powder 202  
Cs2SnCl6: Ce3+ 455  80 6.57 NCs 203  
Cs2SnCl6: Te3+ 580 Yellow–green <100 95.4a Powder 186  
(NH4)2SnCl6: 0.5% Te3+ 590 Orange 127 83.5 Powder 187  
Cs2SnI6 643–742 ⋯ 75 28 Nanoplatelets 204  
(NH4)2SnCl6 590 ⋯ 127 <0.05 Powder 187  
Cs2ZrCl6 ⋯ ⋯ ⋯ 31 Powder 205  
Cs2ZrCl6: Bi3+ 456 Blue 63 50 Powder 205  
A3B2X9 Cs3Bi2Br9 410 Blue 48 19.4 NCs 206  
Cs3Bi2Cl9 393 Blue 59 26.4 NCs 206  
Cs3Sb2Br9 410 Blue 41 46 NCs 63  
Cs3Sb2Cl9 370 ⋯ 52 11 NCs 63  
Cs3Sb2I9 560 ⋯ 56 23 NCs 63  
Cs3Sb2Br9 408 Violet ⋯ 51.2 NCs 207  
FA3Bi2Br9 437 Blue 65 52 NCs 208  
MA3Bi2Br9 430 Blue 62 12 NCs 209  
MA3Bi2(Cl, Br)9 422 Blue 41 54.1a NCs 210  
CategoryCompoundsEmission peak (nm)ColorFWHM (nm)PLQY (%)CommentReference
ABX3 MASn(Br/I)3 667–945 ⋯ ⋯ <5.3 Films 188  
CsSn(Br/I)3 667–887 ⋯ ⋯  Films 54  
CsSnBr3 670 Dark red 56 2.1 Nanocages 189  
CsSnBr3 672 Red 54 9.1 Films 190  
CsSnI3 950 Infrared  3.8 Films 191  
CsBr: Eu2+ 440 White 31 32.8a NCs 192  
A2BB′X6 Cs2AgBiCl6 395 ⋯ 68 6.7 NCs 193  
610 ⋯ 200 Powder 175  
Cs2AgBiBr6 465 ⋯ 82 0.7 NCs 193  
Cs2(Na0.75Ag0.25)BiCl6 610 ⋯ 160 45 Powder 175  
Cs2(Na0.4Ag0.6)InCl6: Bi3+ 552 White 40.9 86.2 Powder 177  
Cs2(Na0.4Ag0.6)InCl6: 5.49% Ho3+ 490 White  60.5 Powder 194  
Cs2Ag(In0.875Bi0.125)Cl6 585 White ⋯ 70.3 Powder 195  
Cs2NaBiCl6 730 ⋯ ⋯  Powder 181  
Cs2NaBiCl6: Mn2+ 590   15 Powder 181  
Cs2Na0.995Bi0.995Mn0.01Cl6 590 Orange–red ⋯ 11.4 Powder 181  
Cs2Na0.987Bi0.987Mn0.026Cl6 590 Orange–red ⋯ 12.4 Powder 181  
Cs2Na0.969Bi0.969Mn0.062Cl6 590 Orange–red ⋯ 15.1 Powder 181  
Cs2AgInCl6 560 White ⋯ 1.6 NCs 180  
⋯ ⋯ ⋯ NCs 196  
Cs2AgBi0.085In0.915Cl6 600 White 200 34 Powder 197  
Cs2AgIn0.9Cr0.1Cl6 1010 Near-infrared 180 23.5 Powder 198  
Cs2NaInCl6: 10% Ag 535 Yellow ⋯ 31.1 NCs 176  
Cs2Ag0.4Na0.6InCl6 ⋯ ⋯ ⋯ 22 NCs 196  
Cs2AgInCl6: 0.9% Mn2+ 632 White ⋯ 3–5 Powder 199  
Cs2AgInCl6: 0.5% Mn2+ 620 Orange ⋯ NCs 180  
Cs2AgInCl6: 1.5% Mn2+ 620 Orange ⋯ 16 NCs 180  
Cs2AgInCl6: Mn2+ 630 ⋯ ⋯ Powder 181  
Cs2NaInCl6: 1% Sb3+ 445 Blue  82 Powder 185  
Cs2KInCl6: 5% Sb3+ 495 Green  93a Powder 185  
Cs2NaEuCl6 593 Red ⋯ 35 Powder 200  
Cs2NaTbCl6 548 Green ⋯ 56 Powder 200  
A2BX6 Cs2SnCl6: 2.75% Bi3+ 455 White 66 78.9 Powder 201  
Cs2SnCl6: 1.16% Bi3+ 457 White 63 68.3 Powder 201  
Cs2SnCl6: 0.11% Bi3+ 454 White 65 67.6 Powder 201  
Cs2SnCl6: Sb3+ 602 White 101 37 Powder 202  
Cs2SnCl6: Ce3+ 455  80 6.57 NCs 203  
Cs2SnCl6: Te3+ 580 Yellow–green <100 95.4a Powder 186  
(NH4)2SnCl6: 0.5% Te3+ 590 Orange 127 83.5 Powder 187  
Cs2SnI6 643–742 ⋯ 75 28 Nanoplatelets 204  
(NH4)2SnCl6 590 ⋯ 127 <0.05 Powder 187  
Cs2ZrCl6 ⋯ ⋯ ⋯ 31 Powder 205  
Cs2ZrCl6: Bi3+ 456 Blue 63 50 Powder 205  
A3B2X9 Cs3Bi2Br9 410 Blue 48 19.4 NCs 206  
Cs3Bi2Cl9 393 Blue 59 26.4 NCs 206  
Cs3Sb2Br9 410 Blue 41 46 NCs 63  
Cs3Sb2Cl9 370 ⋯ 52 11 NCs 63  
Cs3Sb2I9 560 ⋯ 56 23 NCs 63  
Cs3Sb2Br9 408 Violet ⋯ 51.2 NCs 207  
FA3Bi2Br9 437 Blue 65 52 NCs 208  
MA3Bi2Br9 430 Blue 62 12 NCs 209  
MA3Bi2(Cl, Br)9 422 Blue 41 54.1a NCs 210  
a

The highest value of PLQY for each category of Pb-free perovskites, as highlighted in a bold font.

The statistical overview of different classes and compositions of Pb-free perovskites explored to date for LED applications is also shown in Fig. 2(b). Among all Pb-free compounds synthesized for LED applications, 51% are double perovskites and 21% belong to the vacancy ordered double perovskites. The percentages of conventional perovskites and 2D layered perovskites are relatively smaller, with 11% and 17%, respectively. Unlike in conventional perovskites where the B site is always Sn, in double perovskites, B site cations are often mixtures of two or three types of cations, such as Ag–Bi (15%), Na–Bi–Mn (15%), and Ag–In–Mn (15%). In the 2D layered category, only pure Bi-based or pure Sb-based compositions have been reported, with almost an equivalent proportion. Regarding halides, it appears that Cl (83%) and Br (13%) based materials have widely been explored in the context of LED applications, whereas alloys or iodine compositions are rarer. This is quite different from solar cell applications where the iodides are dominant (74%) due to their lower bandgaps capable to harvest low-energy photons. Finally, inorganic cations are by far the most frequent choice for the A site (89%) in LED applications because of their comparatively higher PLQYs, better stability against heating, and slightly lower hygroscopicity. In addition, no organic–inorganic mixtures for the A site are reported so far.

The main requirements for a promising photocatalytic material are suitable band alignments and bandgaps, strong light absorption, high chemical stability, efficient charge carrier transport, and good operation in strong acidity and/or alkali environments.211–213 Over the past few years, the potential of Pb-based halide perovskites has been investigated as a catalyst for photocatalytic hydrogen (H2) evolution, CO2 reduction reaction, and various organic synthesis or chemical reactions.212,214

The performance of a complete water splitting system can be evaluated by introducing the solar-to-hydrogen (STH) conversion efficiency.215 STH has reached nowadays values of 19% close to the theoretical limits (30% under light concentration),216–218 but progress is needed to significantly lower the cost of the systems and enhance their operational stability.219 Naturally, halide perovskites appear as low-cost alternative electrical power sources needed for water splitting, thanks to their excellent photovoltaic performances, but single-junction cells made of efficient perovskite materials for photovoltaic (PV) do not usually produce enough voltage to drive simultaneously the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) in photocatalytic water splitting. This is not the case for perovskite in tandem solar cell configurations with silicon that exhibit nice STH values up to 18.7% but rely on classical electrode materials.220 On the other hand, Pb-free perovskite solar cells are still far from reaching the efficiency and operational stability under the ambient condition of their Pb-based counterparts and are thus not the best candidates so far as electrical power sources to be included in complete water splitting systems.

Interestingly, perovskite materials can also directly participate in the chemical reactions at the electrodes. This was first shown in 2016 when MAPbI3 showed promising performance for photocatalytic water splitting.221 The H2 evolution rate was measured at 57 µmol g−1 h−1 with a splitting efficiency of 0.81%. MAPbI3 is stable at a specific saturation solution where [I] ≤ [H+] and pH ≤ −0.5. In 2018, Wang et al.222,223 proposed a heterostructure of MAPbI3/TiO2/Pt and MAPbI3/Ta2O5/Pt with significant enhancement in the H2 evolution rate of 89-fold and 52-fold over MAPbI3/Pt. From that perspective, Pb-free perovskites may play a more interesting role since the constraint of being an efficient photovoltaic material no longer applies.

In the same objective of reducing the use of fossil fuel, the CO2 reduction has also a very important role to play and it has witnessed significant progress by using perovskite materials. For instance, in 2017, Xu et al.224 used CsPbBr3 nanocrystals (NCs) as novel photocatalysts for CO2 reduction at a rate of 23.73 µmol g−1 h−1 and up to 29.78 µmol g−1 h−1, with graphene oxide to collect the charge of the CsPbBr3 NCs. Although Pb-based perovskites are potential candidates for photocatalysts, they are also somehow limited by their low chemical stability.

To ensure the long-term stability of photocathode and photoanode and to avoid Pb contamination during the pre- and post-processing, Pb-free perovskites have been investigated as alternative solutions.19 Among the technical specifications for an effective photocatalyst, a relatively wide optical bandgap lying in the visible range is ideal for promoting the photocatalytic reaction. Therefore, double perovskites and the 2D perovskite derivatives are of particular interest for such applications. In addition, the high ambient stabilities of these Pb-free materials, compared to conventional 3D perovskites (and especially with respect to Sn2+ compounds) makes them even more promising. In fact, first-principles calculations225 predict that most of the Bi/Ag double perovskites have both bandgaps and energy levels that are suitable for photocatalytic water splitting. On the other hand, in order to promote the CO2 reduction reaction, further chemical engineering is needed to tune the bandgaps of double as well as 2D layered perovskites. For example, the synthesized all-inorganic Pb-free double perovskite Cs2AgBiX6 NCs have their indirect bandgaps decreasing from 2.56 eV when X = Cl to 1.82 eV when X = I.226 As a result, the Cs2AgBiI6 NCs shows the best photoreduction activity with a CO yield of 18.9 µmol g−1 under visible light irradiation (λ ≥ 420 nm, 300 W Xe lamp) within 3 h. Similarly, the bandgap of the 2D layered perovskite Cs3Bi2X9 NCs decreases from 3.08 to 2.01 eV as halide X goes from Cl to I.227 The highest CO yielding speed is 54 µmol·g−1 when X = Br0.5I0.5, compared to the 48 µmol g−1 when X = Cl0.5Br0.5 and 11 µmol g−1 when X = I under visible-light irradiation (λ ≥ 420 nm, 300 W Xe lamp) for 3 h. The suitable band structure, wide light absorption range, large photocurrent, and small impedance of Cs3Bi2(Br0.5I0.5)9 contribute to its greater activity in gas–solid interface than in the majority of the liquid-phase CO2 reduction systems.

Table III lists photocatalytic applications of different types of Pb-free perovskites and the yielding speed (in μmol g−1 h−1) concerning CO2 reduction into CO and/or CH4 and H2 evolution. In double perovskites, the Cs2AgBiBr6 nanoplatelets show the best performance for CO2 reduction.228 Within 6 h, these nanoplatelets have a total electron consumption eightfold higher than that of Cs2AgBiBr6 NCs, namely, 255.4 vs 30.8 µmol g−1. This large increase can be explained by the anisotropic confinement of charge carriers and the in-plane long diffusion length in nanoplatelets.229–231 In order to further improve H2 production, Jiang et al.232 have synthesized successfully a composite of Cs2AgBiBr6 supported on nitrogen-doped carbon materials (N–C). This heterostructure Cs2AgBiBr6/N–C has a speed almost 20-fold faster than the pure Cs2AgBiBr6. In 2D layered perovskite photocatalysts, their surface activities are strongly dominated by the defects/traps. For example, the Cs3Sb2Br9 NCs exhibit the highest CO yield of 127.5 µmol g−1 h−1 to date.233 One explanation is the existence of Sb on their surfaces, which leads to a great improvement of the reactivity. This is in contrast to the nonreactive surfaces of the Pb based perovskites.

TABLE III.

Overview of the various Pb-free halide perovskites used for photocatalytic applications. Boldface denotes state of the art results.

Yielding of main product (μmol g−1 h−1)
CategoryCompoundsCOCH4H2Reference
ABX3 DMASnI3   0.64 240  
DMASnBr3   241  
DMASnBr3 + 10% triethanolamine   11 241  
+ 1 wt. % Pt     
DMASnBr3 + 10% triethanolamine   234  
+ 3 wt. % Pt     
DMASnBr3@C3N4-33% + 10% triethanolamine   1730a 234  
+ 3 wt. % Pt     
A2BB′X6 Cs2AgBiBr6 (NCs) 0.92 0.11  242  
Cs2AgBiBr6 (washed NCs) 2.35 0.16  242  
Cs2AgBiBr6 (bulk) 0.37 0.02  242  
Cs2AgBiI6 (NCs) 6.3   226  
Cs2AgBi(Br0.5I0.5)6 (NCs) 3.93   226  
Cs2AgBiCl6 (NCs) 4.54   226  
Cs2AgBiBr6@C3N4-10% ∼1.8 ∼0.2  243  
Cs2AgBiBr6@C3N4-82% ∼0.66 ∼1.54  243  
Cs2AgBiBr6 (nanocubes) 3.8 1.3  228  
Cs2AgBiBr6 (nanoplatelets) 28.1a 14.5a  228  
Cs2AgBiBr6/N–C   380a 232  
Cs2AgBiBr6   20 232  
Cs2AgBiBr6/RGO   48.9 244  
Cs2AgBiBr6 (bulk)   0.077 245  
Cs2AgBiBr6 (defect-rich)   0.406 245  
Cs2AgBiBr6/Pt   0.733 245  
Cs2AgBiBr6/Mo3S132−   2.47 245  
A3B2X9 Cs3Sb2Br9 127.5a   233  
Cs3Sb2I9 (microblocks) 2.4   246  
Cs3Sb2I9 (microclusters) 1.7   246  
Cs3Sb2I9 92.8 2.9 10.4 247  
Cs3Bi2Cl9 21.01   248  
Cs3Bi2Br9 26.95   248  
Cs3Bi2(Br0.5I0.5)9 18   227  
Cs3Bi2(Cl0.5Br0.5)9 16   227  
Cs3Bi2I9 7.76 1.49  249  
Rb3Bi2I9 1.82 1.70  249  
MA3Bi2I9 0.72 0.98  249  
Cs3Bi2I9 (QDs)   32.21 250  
Cs3Bi2I9@NH2-UiO-66   141.87 250  
MA3Bi2I9   169.21a 251  
Cs3Bi0.6Sb1.4I9   926 252  
Yielding of main product (μmol g−1 h−1)
CategoryCompoundsCOCH4H2Reference
ABX3 DMASnI3   0.64 240  
DMASnBr3   241  
DMASnBr3 + 10% triethanolamine   11 241  
+ 1 wt. % Pt     
DMASnBr3 + 10% triethanolamine   234  
+ 3 wt. % Pt     
DMASnBr3@C3N4-33% + 10% triethanolamine   1730a 234  
+ 3 wt. % Pt     
A2BB′X6 Cs2AgBiBr6 (NCs) 0.92 0.11  242  
Cs2AgBiBr6 (washed NCs) 2.35 0.16  242  
Cs2AgBiBr6 (bulk) 0.37 0.02  242  
Cs2AgBiI6 (NCs) 6.3   226  
Cs2AgBi(Br0.5I0.5)6 (NCs) 3.93   226  
Cs2AgBiCl6 (NCs) 4.54   226  
Cs2AgBiBr6@C3N4-10% ∼1.8 ∼0.2  243  
Cs2AgBiBr6@C3N4-82% ∼0.66 ∼1.54  243  
Cs2AgBiBr6 (nanocubes) 3.8 1.3  228  
Cs2AgBiBr6 (nanoplatelets) 28.1a 14.5a  228  
Cs2AgBiBr6/N–C   380a 232  
Cs2AgBiBr6   20 232  
Cs2AgBiBr6/RGO   48.9 244  
Cs2AgBiBr6 (bulk)   0.077 245  
Cs2AgBiBr6 (defect-rich)   0.406 245  
Cs2AgBiBr6/Pt   0.733 245  
Cs2AgBiBr6/Mo3S132−   2.47 245  
A3B2X9 Cs3Sb2Br9 127.5a   233  
Cs3Sb2I9 (microblocks) 2.4   246  
Cs3Sb2I9 (microclusters) 1.7   246  
Cs3Sb2I9 92.8 2.9 10.4 247  
Cs3Bi2Cl9 21.01   248  
Cs3Bi2Br9 26.95   248  
Cs3Bi2(Br0.5I0.5)9 18   227  
Cs3Bi2(Cl0.5Br0.5)9 16   227  
Cs3Bi2I9 7.76 1.49  249  
Rb3Bi2I9 1.82 1.70  249  
MA3Bi2I9 0.72 0.98  249  
Cs3Bi2I9 (QDs)   32.21 250  
Cs3Bi2I9@NH2-UiO-66   141.87 250  
MA3Bi2I9   169.21a 251  
Cs3Bi0.6Sb1.4I9   926 252  
a

The highest value of product yielding rate for each category of Pb-free perovskites, as highlighted in a bold font.

Although 3D perovskites are not the most popular for photocatalytic applications, several breakthroughs are still made by using the high hydrophobic dimethylammonium (DMA = CH3NH2CH3+) as the A site cation. In 2021, Romani et al.234 have successfully integrated DMASnX3 with the graphitic carbon nitride (g-C3N4), forming the DMASnBr3@g-C3N4. Thanks to the efficient transport of charge carriers, its highest rate of H2 production in deionized water reaches 1730 µmol g−1 h−1, which is much higher than the 6.0 µmol g−1 h−1 of pure DMASnBr3 and 2.0 µmol g−1 h−1 of pure g-C3N4. The small differences in interfacial energy between CBM of DMASnBr3 and H+/H2 reduction potential and between VBM of g-C3N4 and triethanolamine oxidation potential are proposed to contribute to a large nonadiabatic charge transfer between DMASnBr3 and g-C3N4,235 and in turn the high photocatalytic yield.

It should be mentioned that apart from the CO2 reduction reaction and H2 production, the double and 2D layered perovskites also show potential for more complex reactions involving the transformation of one molecule into another (for clarity they are not listed in Table III).236 For example, the Cs2AgBiBr6 NCs can degrade 97% of toxic NO gas within 30 min and maintain stability in four runs of photocatalytic reaction.237 An almost complete degradation (∼98%) of Rhodamine B is obtained by Cs2AgBiBr6 photocatalysis during 120 min under continuous irradiation.238 Cs2AgInCl6 particles degrade ∼98.5% of the water-insoluble carcinogen Sudan Red III in just 16 min and have good stability for five cycle operations.236 In 2020, (MAxCs1−x)3SbBr9 was used for the first time for the activation of C–H bonds, with improved photocatalytic performance, thanks to the substitution of MA by Cs.239 This unique effect of the A site cation tuning is proposed to stem from the octahedron distortion induced by the A cation, which changes not only the electronic properties of the X anions but also the electron transfer from molecules to Br sites.

The summary of various types of Pb-free materials employed so far for photo-catalytic applications is further reported in Fig. 2(c). Double (45%) and 2D layered perovskites (41%) are predominant. The remaining 14% are conventional single ABX3 Pb-free halide perovskites that have been solely used for the photocatalysis of H2. Obviously, vacancy ordered Pb-free halide perovskites have not yet been explored for photocatalysis. In the conventional and double perovskites, all the B site cations are Sn and Ag–Bi, respectively. Instead, in 2D layered perovskites, 66% of the explored site B cations are based on pure Bi, 27% on pure Sb, and the remaining 7% on a mixture of both cations. As the material needs to absorb visible light efficiently, metal halides based on Br (54%) and I (33%) are most often used for photocatalytic applications. As for LEDs, most of the A site cations employed in Pb-free perovskites used as photocatalysts are inorganic (81%). To be noted, compositions with mixed organic-inorganic A site cations have not been reported so far.

This Perspective reports on the state-of-the-art of Pb-free halide perovskite semiconductors for optoelectronic applications, focusing on solar cells, LEDs, and photocatalysts. Clearly, Sn-based halide perovskites are the most explored and performant for photovoltaic applications. On the other hand, Bi, Ag, and Sb atoms are predominant in the composition of photocatalysts, whereas for LEDs many more metallic cations have been explored with the prevalence of alloys.

For solar cell applications, the PCE achieved to date for any of the double perovskites A2BB′X6, vacancy ordered double perovskites A2BX6, or 2D perovskite derivatives A3B2X9 is below 4% and thus far away from the 25.8% achieved with Pb-based halide perovskites.10 Tin-iodides ASnI3 compositions are the most promising with a current record reaching 14.6%.79 They also allow room for improvements given that the predicted theoretical limit for thick layers under AM1.5G illumination for stannates is as high as 32.3%.83 The materials are less stable than their Pb-based counterparts when subjected to ambient conditions due to the fast Sn2+ oxidation. Yet, a remarkable improvement was achieved recently with over 1300 h of operational stability in N2, thanks to chemical engineering by combining the addition of a secondary ammonium salt with that of an effective reducing agent.80 In addition, Sn/Ge mixed perovskite absorbers also demonstrate improved stability, but their PCE needs to be significantly enhanced. Noteworthy, due to their low electronic bandgap, the Sn-based iodide perovskites can become cornerstone materials and pave the way for tandem perovskite/perovskite solar cells or applications based on low bandgaps.253,254

For LED applications, Pb-free halide perovskite materials, in particular, A2BB′X6 double perovskite and A2BX6 vacancy ordered double perovskite show great promise, especially for white light emission. Indeed, their PLQYs for the white color emission are higher than 78% and those for other colors are remarkably high with some of them higher than 93%. More precisely, 93% for the green emission when B = In/Sb mixture and B′ = K in A2BB′X6 and 95.4% for the yellow–green emission when B = Sn/Te mixture in A2BX6. Noteworthy, these attractive PLQYs are just one of many features needed to make efficient LEDs. Interface related issues are anticipated to be one of the key factors that limit the potential of lead-free perovskites for photovoltaic applications.

Finally, the Pb-free halide double perovskites and 2D layered perovskites are also promising in the field of photocatalysis due to their large electronic bandgap and stability.211 For instance, Zhou et al. synthesized Cs2AgBiBr6 nanocrystals that were shown to be stable for more than three weeks in a low polarity medium under light-soaking and 55% relative humidity.242 Double perovskites with B = Bi and B′ = Ag have the highest rate of H2 evolution that reaches 380 µmol g−1 h−1 when supported on carbon doped with nitrogen.232 More recently, an impressive hydrogen evolution rate over 1700 μmol g−1 h−1 was achieved with the use of the ABX3 DMASnBr3 and g-C3N4, thanks to the favorable alignment of the interfacial energy levels. Regarding CO2 reduction, the highest rate reported so far with Pb-free halide perovskite catalysts amounts to 127.5 μmol g−1 h−1 and uses the layered Cs3Sb2Br9 material. This type of development is still in its infancy and Pb-free metal halide perovskites show great potential as catalytically active materials, especially in the form of nanocrystals such as nanoplatelets, which offer desirable features such as exposed facets.

This project received funding from the European Union’s Horizon 2020 Research and Innovation Program [Grant Agreement Nos. 862656 (DROP-IT) and 861985 (PeroCUBE)]. G.V. acknowledges funding from the Chaire de Recherche Rennes Metropole project. J.E. acknowledges financial support from the Institut Universitaire de France.

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article.

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