Tin‐Based Eco‐Friendly Perovskites for Sustainable Future

Perovskite solar cells (PSCs) have opened a floodgate of diverse research. These highly functional materials have immense potential prospects in renewable energy, optoelectronic applications, to name a few. Most of the perovskites have lead (Pb) as an integral active component. Though the Pb‐based organometallic perovskites have shown outstanding performance (over 25%), significant challenges exist including the presence of Pb, its toxicity as well as the instability that can hamper the use of these devices toward practical applications. There are some instances where a complete absence of Pb has been used to achieve respectable device performance, but they face a challenge from the towering success of their counterparts. To offset the issues, significant efforts are being carried out to employ other alternatives (cations) in conjunction with Pb, without distorting the inherent material properties. To that end, tin (Sn) has emerged as a promising alternative to Pb in achieving benchmark performance. Bringing these Sn PSCs into more prominence is a crucial step in taking low Pb or Pb‐free PSCs to the next level of research and commercialization. In this topical review, the notable advancements are highlighted, overviewed and recommendations are made for prospects toward environmentally friendly Sn‐based perovskite photovoltaics.

Perovskite solar cells (PSCs) have opened a floodgate of diverse research.These highly functional materials have immense potential prospects in renewable energy, optoelectronic applications, to name a few.Most of the perovskites have lead (Pb) as an integral active component.Though the Pb-based organometallic perovskites have shown outstanding performance (over 25%), significant challenges exist including the presence of Pb, its toxicity as well as the instability that can hamper the use of these devices toward practical applications.There are some instances where a complete absence of Pb has been used to achieve respectable device performance, but they face a challenge from the towering success of their counterparts.To offset the issues, significant efforts are being carried out to employ other alternatives (cations) in conjunction with Pb, without distorting the inherent material properties.To that end, tin (Sn) has emerged as a promising alternative to Pb in achieving benchmark performance.Bringing these Sn PSCs into more prominence is a crucial step in taking low Pb or Pb-free PSCs to the next level of research and commercialization.In this topical review, the notable advancements are highlighted, overviewed and recommendations are made for prospects toward environmentally friendly Sn-based perovskite photovoltaics.
commercially available Si, Ga-As, or Cd-Te solar cells. [4]Figure 1 shows the golden triangle for solar cells, and perovskites have already achieved two milestones, namely, efficiency and cost.All the requirements must be simultaneously fulfilled for their widespread acceptance in the renewable energy market (Figure 1).What remains is the shelf-life that will be the deciding factor if the PSCs will survive in the competing market or not.
The increased power conversion efficiencies (PCEs) of these solar cells with different morphologies, structural variations, and device fabrication have given promising results at a lab scale.PCEs now certified by accredited independent laboratories (that include the National Renewable Energy Laboratory [NREL] and Fraunhofer Institute for Solar Energy Systems) give credence to this new, emerging field of thin-film photovoltaics as well as other optoelectronic applications.These PSCs showing the champion performances contained pure Pb, or a high proportion of it, which is a highly toxic metal. [7]In addition to that, their optical bandgap is not quite optimum, with FAPbI 3 having the lowest value of 1.48 eV, which mandates the optimal bandgap to be 1.34 eV for achieving the maximum efficiency of %33%. [8]he reason lead-halide perovskite structure delivers such incredible performance is due to a unique combination of high electronic dimension, the symmetry of the perovskite structure, and electronic arrangement of Pb ions (Pb 2þ ).The lone electron pair in the 6s orbital and the dormant electrons in the 6p orbital of Pb form the valence and conduction bands, respectively, also known as higher occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO). [9]Unfortunately, Pb is quite toxic and upon exposure to moisture, it forms a compound of Pb such as Pb(OH) 2 along with water, which is highly toxic and highly miscible in water.This Pb oxidation on the PbI 2 interface leads to damage to the bond between Pb and I ions.This breakage leads to the unavoidable exposure of the underlying methylammonium iodide (MAI) layer.Water molecules can penetrate this exposed MAI layer, causing an irreversible breakdown of the perovskite due to hydration. [10]If such poisoned Pb-contaminated water gets into the ecosystem or the food cycle, this can impose various toxicological effects on the human body as well as other living species. [11]19][20][21][22] The performance of perovskite-based optoelectronic functional devices can be tuned by using key components such as light-absorbing/photoactive layers, interfacial/buffer layers, electrodes, and encapsulation films. [13,23][30] The ideal attributes of ideal interfacial materials include energy levels well-aligned with those of the adjacent photosensitive layer, good photo/thermal/air/moisture tolerance, [31,32] high charge carrier mobility, and easy film formation capability. [30,33]33][34][35] Due to the outstanding progress made in the field of hybrid and all-inorganic PSCs, the amount of literature available is enormous, and yet certain complex issues need to be addressed.As previously stated, the issue of material stability and the resulting toxicity of Pb necessitate the investigation of alternative potential materials that either offer higher stability and/or reduce toxic byproducts in the event that the fabricated device architecture is damaged.Therefore, a comprehensive review of the latest developments in this thriving field is essential for getting a bird's-eye view of the latest progress and developments.Fortunately, to resolve this issue, another choice of metal has been proposed which is in the same group as Pb, sitting just above Pb and the name of this element is tin (Sn).Pure Sn perovskites, with a bandgap near the ideal value of 1.34 eV, have proven to be a viable alternative to Pb-based ones, which would help in achieving very high-efficiency values as predicted by the Shockley-Queisser limit. [36]In addition to that, it also possesses a good tolerance factor (t), in the perovskite phase, comparable to that of Pb, if not better. [37]Citing the anomalous bandgap effect, [37] it has been found that using certain combinations of Sn and Pb in a mixed cation PSC, the air stability of the fabricated devices could be significantly enhanced. [38]Although Sn is innocuous when compared to Pb, it has its share of problems with its stability. [39][42] In this topical review, we have provided a systematic yet comprehensive overview of the recent progress in organic and inorganic lead-free PSCs, with an emphasis on the Pb-less Sn-based PSCs.The general aspects of a PSC, including its structure, and the materials used to synthesize the same have been elaborated as well.The key factors influencing the stability of conventional Pb-based PSCs have been briefly discussed to lay the ground for further discussion, followed by the usage of Sn to diminish such issues to an extent.Some attention has also been given to the nature of solvents used in the perovskite material synthesis to show the degree of progress made in solvent engineering by making use of solvents other than the regular solvents dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), γ-butyrolactone (GBL), etc.Although DMSO and DMF have provided the best results so far with Pb-halide perovskites, for truly green PSCs, not only should the active material be Pb-free, but also use significantly less-toxic or eco-friendly solvents.Toward the end, we summarize and highlight some of the highly relevant reports on the performance of Pb-free, Sn-based PSCs, and the other prospects in this area.As Sn and germanium (Ge) are two of the most promising alternatives to Pb in the production of future optoelectronic devices, the forthcoming sections will focus on their potential prospects in PSCs.Despite the unmatched performance of Pb perovskites in various electronic devices and their outstanding optoelectronic characteristics, Figure 4 gives an idea of the potential toxicological effects of Pb on human organs.In the upcoming sections, the various degradation pathways responsible for inferior device performance over time will be explained, and how such challenges can be mitigated by  incorporating other alternative metal cations through careful compositional engineering into their photoactive layer.
One of the pillars of PSCs is the structure and shape of the perovskite crystal.Ideally, the structure is supposed to be cubic, which is governed by the empirical parameter otherwise called Goldschmidt's tolerance factor (t).This tolerance factor is a function of the radii of the "A" and "B" site cations and the halide anion, respectively. [43]As the structural integrity of the crystal is directly dependent on its shape, a robust perovskite structure is anticipated to have a t value of 0.8-1, with 1 being perfectly cubic. [44]Substituting the ionic radii values gives us the t values for different ions.This opens an opportunity to substitute different ions to tune the various properties, and this can directly influence the performance characteristics.Hence, the overall performance of the perovskite crystal and the device performance depend on the combinations of multiple parameters attributed to the t.Taking this into account, Pb can be substituted by potential alternative elements such as Sn, Ge, and a few other metals with ionic radii close to that of lead.Table 1 outlines some features of the cations for the perovskite crystal of the form MABI 3 (MA-methylammonium, ionic radius = 1.8 Å and that of iodide ion is 2.2 Å).
Another critical issue that should be considered while designing new perovskites is that if PSCs need to be scaled up for commercial applications, then how they can be processed via a green approach and will have minimal impact on the environment is an important question to consider.To the best of the authors' knowledge and from the literature survey, the highest numbers of lead-halide perovskites have been synthesized inside the glove box to ensure minimal ambient factors affecting the formation of the perovskite and hence affecting the performance metrics of the final product.Although this method is appropriate for laboratory-based research work as per practicability, broad area applications and manufacturing are concerned; the current library of perovskite materials is not practically feasible.Hence, there is a great interest in alternative perovskite materials that can be synthesized outside of the glove box in ambient conditions, without relying on sophisticated arrangements (like cleanroom, glove box, etc.).As the price of a product is directly related to its manufacturing methods, it is thus hoped that readily available and low-cost precursors, in combination with air processability, will be quintessential to the commercial prospects of PSCs.It is thus anticipated from the above research that the material obtained will have the desirable optoelectronic properties.After that, its stability must be engineered to obtain the most suitable candidates for Pb-and glove box-free PSCs.
In the subsequent sections, we will explore some of how the above objectives have been met, with varying degrees of success, making use of Sn as an example or a model system.Although there have been some excellent reviews on this topic such as by Cao et al., [45] Pitaro et al., [46,47] and others [48,49] what sets the current work apart is not just putting the Sn in the spotlight but also different material arrangements that further enhance the viability of Sn as the B-site cation in the perovskite crystal.The review addresses the stability issues arising with the established Pb-based materials, focusing on different degradation pathways.The reason for emphasizing these is that almost all perovskitebased devices tend to suffer from these issues, in varying degrees.Then we transition to discussing Sn-based PSCs as an emerging material and a potential replacement for Pb.Different kinds of synthesis methods, and variability in other cations, with their effect on device performance, have been described.Some discussions pertaining to Ge have also been briefly discussed, as it has been observed to be another material with potential in PSCs.The device engineering aspects have been slightly touched and described as well.Besides, the status and the prospects in this prosperous area are also elaborated.The review concludes by presenting a summary of the various Pb-free Sn-based PSCs based on their performance indices.

Instability of the Photoactive Layer in Pb-Based PSCs
Although Figure 1 represents the ultimate objective for any solar cell, it especially becomes challenging for PSCs to attain balance among the three aspects: efficiency, cost, and lifetime.Even though the cost of manufacturing lab-scale PSCs is meager by any industrial standards, it still suffers from low shelf life, even though the efficiencies achieved by PSCs are rising steadily. [34]he most critical issue that hinders Pb-based PSCs from achieving their full potential and commercialization is due to certain factors that drastically impact the performance of the PSCs over time.The debilitating factors are shown in Figure 5.Although  these issues affect various PSCs in different capacities, here we are using a Pb-based perovskite as a reference to highlight not only some of the factors affecting its stability but also a way to address them by compositing with Sn.

Structural
The basis of the crystal structure, t, is the empirical formula that governs the formation of the crystal structure in perovskites. [50]his is a reliable empirical factor that predicts the structure of the crystal based on the ionic radii in the crystal.For a perovskite structure, the value of this factor should ideally be between 0.8 and 1. [51] For high-performance devices, the crystal structure is fundamental because the efficiency is dependent on acute control over the crystallinity of the material.This rule is applied to any kind of perovskite that is synthesized.The very existence of perovskite is defined by the tolerance factor t because any divergence from the range mentioned above will cause the orthorhombic structure of the perovskite into different forms, as illustrated in Figure 6. [52]here have been some studies that have linked temperature with the crystalline structure, such as the one reported by Poglitsch et al., [46] where they investigated the effect of temperature on the crystal structure of MAPbX 3 (X = Cl, Br, I).They found that all the perovskites exhibited cubic structures at higher temperatures; say for MAPbI 3 (α phase), it was >327.4K.At other temperatures, all three perovskites were present in different phases.Similarly, Berhe et al. proposed substituting MA with FA to enhance the tolerance factor to near unity (%0.99), which also increases the temperature stability as well. [53]oping FA with other cations such as that of Cs or Rb could lead to enhanced thermal stability while keeping the tolerance factor within the correct limits. [54]Table 2 shows the effect of temperature on the structure of the MAPbX 3 perovskite. [52]2.Thermal From Section 2.1, it is evident that temperature and crystal structure are closely related, and the effect of temperature influences the crystal structure.This crystal structure affects the overall behavior and performance of the material, as well as the devices made from such materials.As such, anomalies originating from the deterioration of the perovskite and other transporting layers will severely affect the device, reducing its efficiency and shelflife.The lifetime of the PSCs is drastically affected by the ingress of heat into the absorbing and transporting layers, due to either annealing or exposure to external heat.Some earlier studies have shown the rate of degradation of various lead halides, with PbI 2 degrading at a faster rate than that of PbBr 2 , which is a result of the Pb-Br bond being short, and thereby stronger than the Pb-I bond, and so on.[55] As MAPbI 3 is one of the most thoroughly researched perovskite systems thus far, it has been reported that the decomposition temperature of MAPbI 3 lies in the range of 100 and 140 °C.[56] The following reaction has been proposed for the disintegration of MAPbI 3 : [57] CH To further evaluate the effect of temperature on the perovskite, Philippe et al. [56] heated thin films of MAPbI 3 at high temperatures in ultrahigh vacuum (UHV) conditions to remove any effects of moisture and air.The films were characterized based on the compositions of I/Pb and N/Pb ratios to ascertain the level of disintegration in the perovskite.Figure 7 shows the degradation of perovskite as a function of temperature. [56]he study of the effect of temperature on perovskites is vital because the crystal formation of the perovskite and their Figure 6.The Goldschmidt tolerance factor and the crystal structure have a structural relationship.Reproduced with permission. [52]Copyright 2016, American Chemical Society.associated grain boundaries require annealing, which generally occurs at temperatures higher than room temperature.This step is crucial to transforming the state of the perovskite from a solution to a thin film of uniform crystallinity.Apart from that, when the PSCs are in operation, they will be subjected to external heat sources such as the sun.Therefore, understanding the thermal stability of the as-fabricated cells is vital to the long-term prospects of the devices.To ensure that suitable performance devices last long with minimal defects and best performance, encapsulation of the device is extremely important.Encapsulation minimizes the passivation of oxygen, moisture, and dust particles, which can otherwise tamper with the active layer of the device and adversely affect its performance.Encapsulation also acts as a deterrent to adversely influence the device's performance due to a combination of moisture and heat. [58]Some device structures have eliminated the use of an encapsulating layer.To this end, Prasanna et al. [55] measured the thermal stability of FA 0.75 Cs 0.25 Sn 0.4 Pb 0.6 I 3 perovskite with large grains and found that the stability of the device at elevated temperatures in oxygen was good, with the IZO electrode capping acting as the encapsulation.The as-fabricated PSCs retained 95% of their initial working efficiency after aging at 85 °C in the air for 1000 h, which was reproduced on average across eight devices. [54]In another interesting work, at an elevated temperature of 85 °C and relative humidity (RH) of 85%, [27] a polyolefin encapsulant and butyl rubber edge sealant were used, along with on-substrate conductive connections to perform the damp-heat test.At 85 °C in the air for 1000 h and a RH value of 85%, the devices maintained 95% of their initial performance.
In certain mixed cation perovskites existing as MA 0.5 Cs 0.5 Pb 1À x Sn x Br 3 type, the resulting nanocrystals showed excellent thermal stabilities. [58]Under an inert atmosphere of N 2 , thermogravimetric analysis (TGA) measurements were carried out from room temperature (RT) to 800 °C.The collapse of the inorganic frameworks and the organic ligands has been attributed to the major mass losses at around %300 and %600 °C, respectively.Figure 8 shows the field emission scanning electron microscopy (FESEM) images of the synthesized mixed cation perovskites.The elemental analysis was carried out by photoelectron spectroscopy (PES).The numbers 0 and 2 represent the presence of PbI 2 , whereas 1 and 3 represent MAPbI 3, respectively.Reproduced with permission. [56]Copyright 2020, American Chemical Society.
In a recent work done by a group of researchers led by Anita W. Y. Ho-Baillie, [57] a low-cost polymer/glass stack pressure-tight encapsulation technique has been used to not only prevent outgassing of the active material due to thermal stress but also to provide better thermal stability.The devices CH 3 NH 3 (MA) containing mixed systems of cations and halides based PSCs retained over 1800 h of damp heat test and 75 cycles of humidity freeze test exceeding the standard requirement.Three different types of encapsulation layers such as polyiso-butylene-based polymer (PVS 101) (PIB) blanket, polyolefin (PO) blanket and PIB 3 edge-seal, respectively, were employed to analyze the encapsulation effect on the thermal stability of the PSC. [57]3.Moisture Among the stability issues highlighted in Figure 4, moisture ranks alongside thermal as one of the biggest challenges faced by PSCs and is an extremely challenging issue to resolve.The problem with methylamine group (CH 3 NH 3 )-based perovskites is the fact that amine-based salts are hygroscopic.[59] Most MAPbX 3 types of perovskites suffer from moisture-induced degradation issues, in which the CH 3 NH 3 is readily lost via sublimation and only solid PbX 2 is left.[53] So, the presence of such amine groups that make up the crystal structure of the perovskite poses a threat to the entire material becoming hydrophilic, whose performance is easily compromised when exposed to an environment that contains moisture.For PSCs to indeed be acceptable for regular applications, they must be robust enough to withstand moisture and oxygen.So, adequate measures must be put in place to prevent the hydrolysis of the perovskite to its precursors, which is an irreversible degradation process.Contributing to this are heat, UV, and electric fields, which accelerate the process.[60] Figure 9 shows the proposed moistureassisted degradation of MAPbI 3 perovskite.[61] The general chemical reactions taking place in the process can be expressed by the following equations: [61] CH 3 NH 3 PbI 3 ðin presence of moistureÞ (2) On close perusal of the literature, it has been found that not only water is detrimental to the performance and shelf-life of the perovskite material, but the byproducts such as MA, HI, and even the solid PbI 2 are water-miscible. [62]This leaching of PbI 2 from the solar modules into the surrounding water bodies can pose a significant eco-toxicological risk.Even though the quantities of the degraded materials may be small, over time, they can quickly compound to larger values, and the risk of toxicity will increase further.Thus, there has been considerable interest in developing moisture-tolerant perovskites without any degradation in their optoelectronic properties. [63]Although most of the reports list moisture as a significant issue for PSCs, it has been reported earlier that better quality thin films were obtained when MAPbX 3 (X = I and Br) were processed at a higher level of humidity than normal.In this regard, Eperon et al. reported an RH of 50% produced a PCE of over 14% due to the mitigation of excess MA þ ions.In addition to that, the rate of film formation in a controlled moisture environment was faster.There was a compromise with noncontinuous surface morphology but a higher photoluminescence yield (PLQY).There was also a dramatic improvement in the open-circuit voltages (V OC ) and thus, overall device performance.These enhancements may originate from the lowering of trap density, possibly ascribed to the partial solvation of the MA ions and the "self-healing" of the perovskite crystal lattice. [64]The damage to MAPbI 3 films could be avoided if synthesized in a controlled moisture environment.This observation is due to the infiltration of the perovskite lattice during final production. [65]In another interesting study, Yang et al. used selected inorganic anions to create a thin layer of inorganic lead oxysalt (lead sulfate) on the perovskite surface. [64]This type of passivation results in strong chemical bonding with the underlying perovskite layer and provides a much better tolerance to minimize detrimental factors in the environment such as moisture and light.The passivation using lead sulfate (PbSO 4 ) enhanced the PSC lifetime and efficiency. [64]The passivated perovskite layer showed excellent resistance to water ingression.To study the effect of water permeability on the active material, a control consisting of MAPbI 3 was used against the passivated perovskite layer.When dripped in water, the control film deteriorated within 10 s, turning from black to yellow, indicating the decomposition of MAPbI 3 into its precursor hydrates and the yellow-colored PbI 2 .When a drop of water was dropped on the control and passivated devices, a similar color change was observed in the unpassivated devices, whereas the one coated with PbSO 4 remained black for up to 3 min.This shows that Reproduced with permission. [61]Copyright 2014, American Chemical Society.
the lead oxysalt significantly slowed down the movement of water into the perovskite. [64]

Ion Migration
In contrast to their silicon counterpart, PSCs consisting of the active material generally consist of ions, such as the MA (CH 3 NH 3 þ ), Pb 2þ , and I 3À ions in MAPbI 3 , respectively.Ion migration, although not discussed in detail in many papers, also contributes to defects in the perovskite, primarily due to the low activation energy within the perovskite layer. [66]It is to be noted that the effect of ion migration is accelerated when the device is under temperature, light, and external electric field. [67]on migration can influence the composition/morphology (microstructure) of perovskite films by creating pinholes besides damaging the charge transport layers and electrodes. [68]Ion migration is also considered a prime reason for the existence of hysteresis in PSCs. [69]Toward this end, Zhao et al. have shown that under the effect of an electrical bias or light irradiation, hysteresis occurs due to the migration of vacancies in the iodide ion.This is a direct consequence of the changes in the efficiency of the collection of the light-generated charge carriers. [70]For instance, in the case of MAPbI 3 , there can be extrinsic ion migration in addition to intrinsic migration.The extrinsic migration in MAPbI 3 usually involves the transportation of I À ions within the lattice (intrinsic migration), but it also migrates out of the perovskite layer to the metal back contacts.This phenomenon leads to nonradiative recombination at the grain boundaries, thereby affecting the performance of the device. [71]Tress et al. investigated this issue of ion migration in MAPbI 3 perovskite.The hysteresis existing in the device using MAPbI 3 as the active material is ascribed to the build-up of the depletion region around the contacts caused by ion migration.They also reported that hysteresis was found to rise with the device ageing.The primary reasons behind this unusual behavior are due to 1) degradation of MAI, 2) formation of intermediate PbI 2 , and 3) vacancies in the iodide ion. [72]Yang et al. used phenylmethyl ammonium iodide (PEAI) to stabilize the black phase of pure FAPbI 3 . [73]In another work, Yang et al. used a passivation layer comprising PbSO 4 over the perovskite. [64]The resulting ion migration was found to be suppressed when compared to the control device.It was thought to be due to the strong ionic bonds existing between the lead oxysalt and the photoactive layer.However, further investigation is needed to ascertain the role of ion vacancies on degradation.

UV/Visible Light
Another fact that is responsible for the degradation of the photoactive layer in the PSCs over time is light.It is an irony that the very objective of the device can get compromised due to the subject of study.The real issue is with the light spectrum, which consists of visible as well as invisible UV light as well. [74]Upon exposure to light as well as the atmosphere, the active material degrades even more rapidly.This degradation results in the perovskite layer breaking down to its precursors such as methylamine (CH 3 NH 2 ), PbI 2 , and I 2 , respectively. [75]The UV radiation in question is not directly responsible for the damage to the device, but it affects the electron transporting layer, which creates a chain of events that, over time, drastically affects the performance of the PSCs.The most efficient system uses titanium oxide (TiO 2 ) as the electron-transporting layer (ETL).Lietjens et al. showed that under AM1.5G illumination (1 sun), the devices exhibited varying levels of decay. [75]The most surprising fact was the rapid decaying of the nonencapsulated devices more than that of the encapsulated devices.It was found that the degradation of the device originated deeper, most likely in the meso-TiO 2 layer. [75]The degradation of the PSCs is twofold, with the TiO 2 and the perovskite layer following different chemical pathways.The degradation of the perovskite under light has been attributed to the following reactions taking place: [76] 2I The generated electron in Equation ( 6) interacts with TiO 2 , which results in the breakdown of the lattice leading to the production of I 2 .The electrons injected into TiO 2 get trapped in unoccupied sites (deep-lying), leading to further deterioration in the performance of the device. [76]Some progress has been made to improve device performance against UV-induced degradation.Ito et al. used Sb 2 S 3 as a surface-blocking layer over TiO 2 in a MAPbI 3 , to study the action of TiO 2 under UV light, and found the layer provided increased stability at the TiO 2 /MApbI 3 interface. [77]Turren-Cruz et al. used inorganic cations like Rb and Cs and discarded MA to create highly crystalline FA-based PSC with a record high PCE of 20.35%, with additional polymer polymethyl methacrylate (PMMA) additive in the interlayers to improve light stability.It resulted in a device that was tested for 1000 h at maximum power point (MPP) tracking in an N 2filled glove box. 7][88]

Tin Is a Potential Replacement for Lead
The Pb toxicity and associated symptoms paint a grim picture, and this makes the replacement of Pb a priority.[91][92][93] Mitzi and co-workers explored the hybrid halide perovskite with a clear focus on the Sn-based systems, based on the earlier seminal work done by Weber in 1978. [94]t was discovered that the electrical properties, such as conductivity, could be significantly improved by tuning the thickness of the perovskite layers.It is to be noted that the Sn 5s band along the (111) orientation (cubic Brillouin region) causes a minimal crossover of the Sn 5s/Sn 5p bands, with the Fermi energy lying between the two bands. [95]However, conductivity is determined by the length of the Sn-I bond (in the case of iodine as the halide) as well as crystal symmetry.The conductivity decreases as the distortion from the ideal octahedral arrangement increases. [96]This high mobility is a special property of tin-halide perovskites, allowing them to maintain longer carrier diffusion lengths.
In addition, the resulting Sn-based perovskite is a direct bandgap semiconductor similar to its Pb counterpart, [97] which means that the maximum valence and maximum conduction band are on the same line in the k space.Bands varied according to the choices of "A" cation (MA), formamidinium (FA), cesium (Cs), oleylamine (OAm), and crystal structure.Bands varied accordingly and its bandgaps are smaller than those of others, largely because of relativistic effects.As masses have relativistic effects with increased mass, effects are multiplied by 3 times the effects of a spin-orbit coupling compared with Sn. [98] The absorption coefficients are high as a direct bandgap material, although the absorption decreases above the bandgap as compared to the Pb analogues [75] compared to basic MAPbI 3 .Table 3 summarizes several key parameters for a range of perovskite compositions based on Sn and Pb. [39,99]

Sn-Halide Perovskites
][102][103] In a particular work, Snaith group members demonstrated a PCE of %6% for MASnI 3 . [99]The total band structure could be controlled for the more visible area of the solar spectrum, as reported by Hao et al., by replacing iodide with bromide anion. [43]The possibility of fabricating MASnI 3 PSCs was further bolstered by Ma et al. when they reported that the addition of SnF 2 as a dopant increased the carrier diffusion length to >500 nm by reducing the background carrier density. [104]Similarly, Liu et al. combined thermal antisolvent and DMSO to increase the coverage of the film and average crystallite for the FA 0.75 MA 0.25 SnI 3 system, resulting in a PCE of over 7%. [77]But Sn-based perovskites, because of their double oxidation status and their changes from Sn 2þ to Sn 4þ , are quite sensitive to moisture and oxygen.Moreover, Sn 4þ , as a p-type dopant, assigns metal nature to the composite semiconductor by employing a self-doping process and thus attacks its intrinsic properties. [104]This oxidation process leads to the instability of optoelectronic systems and poor performance.Different strategies to suppress the oxidation state of Sn 2þ have been explored through adding precursor additives to the solution, atmospheric adjustment, and manipulating perovskite crystal morphology.In another interesting study, Seok et al. used solvent engineering to inhibit Sn 4þ formation and thus improve stability with the SnF 2 -pyrazine complex. [105]hao et al. used an FA and MA mixture with an efficiency of 8.12% to formulate Pb-free Sn iodide PSCs with an inverted structure.The stoichiometry [x(FA) x (MA) 1Àx SnI 3 ] was performed and the molar ratios of SnF 2 were changed and the thickness of the pervasive layers varied.The champion's cell's V OC was 0.61 V.The changes in the performance of the J-V profiles because of variations in specific parameters are demonstrated in Figure 10 and Table 4. [106] In another study, Heo et al. [107] experimented with primarily three halides, F, Cl, and Br, and found that the lattice constants changed when SnX 2 was added as a dopant to the perovskite compound formulation, and this was confirmed by comparing the (202) peaks as shown in Figure 11. [107]The perovskite discussed above is an organic tin-halide perovskite, in this case, CsSnI 3 with SnX 2 (X = Cl, F, Br) doping to prevent further oxidation of Sn.It was reported using density functional theory (DFT) calculations that SnX 2 fulfilled two plausible roles: the first role was controlling the defect and the other being surface passivation.A key to ensuring efficient photovoltaic effects as energy levels are filled up, leading to a reduction in tin vacancy, was the reduction of carrier background levels through the addition of SnF 2 . [108]The effect on the best performance of the CsSnI 3 PSCs by adding SnX 2 is summarized in Table 5. [106] Hypophosphorous acid (HPA) as a reducing agent has been reported to induce stabilization in hydroiodic acid (HI) aqueous solution-based synthesis of perovskites. [109]Even in other acidic media such as hydrobromic acid (HBr), HPA has proven to be quite useful in inhibiting the oxidation of Sn 2þ to Sn 4þ .During the liquid synthesis of octylammonium tin halide [OCTAm) 2 SnX 4 ], HPA was added over 6 times the amount of HBr, to prevent the oxidation of tin.The resulting perovskite showed impressive air and moisture stability and was synthesized outside a glove box.The resulting perovskite (OCTAm) 2 SnBr 4 showed good yellow phosphorescence under UV, which was later incorporated as one of the precursors for the fabrication of white LED.
Another FAMSnI 3 using toluene-purified SnI 2 as one of the precursors witnessed a PCE of 10.7%.Upon exposing the Table 3.Comparison of tin-based perovskites with methylammonium lead iodide perovskite (MAPbI 3 ). [39,99]abricated PSCs to an N 2 atmosphere for about a month, the PCE jumped to 11.06% with remarkable stability.The toluene washing increased the quality of the Sn-based PSCs, which led to their good performance. [110]Jiang et al. used a combination of three reactants, namely, formamidine acetate (FAAc) and ammonium iodide (NH 4 I) along with SnI 2 that replaced formaminium iodide (FAI).The triple reactant method enabled a longer reaction pathway for the growth of Sn-PSC-based thin films.The absence of FAI enabled the growth of larger perovskite crystals, followed by the subsequent annealing process which led to the removal of NH 4 Ac as the by-product.The resulting PSCs gave an excellent PCE of 14.6%. [49]Ac

Perovskite
It is important to observe the emergence of a new class of hybrid perovskites (2D and 3D), which is designated as the (R-NH 3 ) 2 A nÀ1 M n X 3nþ1 formulation, in which R is an alkyl group (e.g., octylamine, nonylamine, etc.), with X being the halide. [111]hese 2D hybrid perovskites offer an alternative to the purely 3D ABX 3 type of perovskite structures.The reason that lowdimensional perovskite networks are emerging because these materials offer better control over quantum confinement, which is challenging to design in 3D bulk materials such as in ABX 3 -type perovskites. [112]side from the purported improvement in performance, lowdimension perovskites such as 2D, 1D, or 0D are more robust to desorption and moisture than their 3D counterparts. [113]As the tolerance factor of the standard ABX 3 perovskite can only accommodate small organic and inorganic cations in the "A" site of the perovskite, [114] such issues can be resolved in low-dimensional perovskites.This leads to even more significant variability in the fitting of inorganic as well as organic molecules of different chain lengths, thereby giving several possibilities for structural and compositional arrangements.This feature is quite attractive from an organic chemistry viewpoint because a plethora of organic molecules that fit the description can be tailored to fit in the active site, thereby giving a new class of perovskites altogether.As mentioned previously, the use of octylammonium or nonylammonium as the "A" site cation led to unusual optoelectronic properties, as well as ambient stability. [115]igure 12 shows some salient features of the as-prepared (OCTAm) 2 SnBr 4 perovskite.By changing the molar ratios of the halides, the optical properties can be controlled.For example, the emitted color of the light under UV can be varied from shades of yellow to red, by varying the concentrations of bromide and iodide in the aqueous media as shown in Figure 13d. [116]he prior works also demonstrate that HPA did not play a pivotal role in the perovskite stabilization nor did it improve the quantum yield, but that the total air and humidity stability of HPA (%240 h, as reported) was achieved for a long time. [117]n a similar aspect, the self-oxidation of tin can be suppressed by hydrazine, in the presence of SnF 2 .Although SnF 2 alone is good enough to mitigate some of the problems of tin, the resulting devices produce no positive and reproducible electrical characteristics. [118]From the device modeling perspective, Baig et al. reported a MASnI 3 PSC with a high PCE of 18.71%.It used Cd 1Àx Zn x S as the ETL, while the absorber material served as the HTL.The best results were obtained with a thickness of 500 nm and a doping concentration of 1 Â 10 16 cm À3 . [119]

New Materials and Synthesis Methods of Tin Halide Perovskites
Colloidal synthesis methods also have been reported by researchers as potential candidates for the synthesis of perovskite nanoplatelets, under ambient conditions with good shelf life and stability.The use of oleylamine (OAm) has been reported to aid in the formation of nanoparticles.OAm is also less expensive than the more commonly used pure alkylamines.Being a liquid at room temperature, OAm can be readily used in the synthesis Table 4. Photovoltaic performance of (FA) x (MA) 1Àx SnI 3 -based devices. [106]rovskite V  ).Reproduced with permission. [106]Copyright 2017, Wiley-VCH.
of NPs via the hot injection method. [120]Although reported back in 2013, it is only in the past few years that this compound has been seriously considered for the formulation of nanoparticles.Interestingly, OAm is an electron donor at higher temperatures, which makes the hot injection method appropriate for the use of OAm and the synthesis of NPs as such. [121]recurring problem in the synthesis of NPs is their final yield, which is quite low, given the quantities of the precursors used.The scaling-up operation becomes a challenge, as the quality of NPs is sacrificed at the cost of scaling. [107]This issue has been actively managed using OAm.It has been reported previously by Du et al. that hexagonal CuS nanosheets, hexagonal ZnS nanowires, orthorhombic Bi 2 S 3 , and Sb 2 S 3 nanowires were prepared in the presence of OAm, and the yields were on the order of grams. [122]Based on this work, Lian et al. designed a 2D perovskite crystal without Pb for solar cells.For the first time, nanoplatelets of colloidal 2D Cs 3 Bi 2 Br 9 have been synthesized with hot injection methods.With excellent air stability for up to 20 days, the results were encouraging, with no observable changes to the absorption spectrum of perovskite.The synthesis protocol for the same is illustrated in Figure 14. [123]gure 11.a-c) Phase and d-f ) air stabilities of CsSnI 3 films containing a,d) SnF 2 , b,e) SnCl 2 , and c,f ) SnBr 2 as additives.Samples (a-c) were stored for 100 h in a N 2 atmosphere, and (d-f ) samples were removed from a N 2 atmosphere and kept in the open air for 1 h.Reproduced with permission. [107]opyright 2018, American Chemical Society.
Table 5.The effect of SnX 2 on the photovoltaic performance of CsSnI 3 PSCs. [106]ditive The Cs 3 Bi 2 Br 9 air stability can be caused by several of the following factors.The first is the very stable crystalline structure of the perovskite Cs 3 Bi 2 Br 9 .Leng et al. demonstrated that the crystal Cs 3 Bi 2 Br 9 began to lose weight at 500 °C, while its mass started to fall at 200 °C.The high-temperature limit is, therefore, a good reason why the perovskite crystal is temperature stabilizing, without any loss in material yield. [116]Second, on the surface the oxidized stage BiOBr provided the Cs 3 Bi 2 Br 9 NCs with good passivity. [124]Third, combining oleic acid and OAm in the hot injection synthesis ligands leads to the development of a pair of alkylammonium carboxyl ions that closely bind to the surface of the NC and thereupon create an obstacle to the environmental factors that are reported by De Roo et al. [125] In the synthesis of Cs 3 Sb 2 Br 9 nanocrystals (NCs), Gan et al. [126] used similar chemical pathways and reported changes in the crystal structure as a result of changes in hot injection process temperature, time of nucleations, amount of OAm, and the ligand responsible for the accurate control of the formation of the NCs.The pictures of TEM in Figure 15 show morphological variations in the crystal structure. [126]As stated previously, the Cs 3 Sb 2 Br 9 NCs showed excellent air stability, and the absorption in ambient conditions showed only a slight variation over 30 days. [126] completely different approach was adopted by Hong et al. for the synthesis of the perovskite material without using any wet chemistry techniques. [127]They employed mechanochemical methods such as ball milling.All the precursor salts were taken and ground together at set speeds and temperatures, without a glove box, and the results reported were shown to be on par with the same perovskite materials synthesized using wet chemistry techniques. [128]Pal et al. reported earlier the use of mechanochemical techniques for synthesizing Pb-based perovskites. [129]Hong et al. reported synthesizing varieties of Sn-based inorganic perovskites. [128]The novelty of this technique is its speed, less stringent requirements, and the potential to scale the product up to kilogram levels with good yields.As a ball mill does not require many optimizations before its use, this simplifies the overall synthesis procedure.Critically, metastable perovskites have even been achieved under environmental conditions in a mechanochemical manner with great quality while avoiding additives or thermal treatment (Table 6).The structure of a typical planetary ball mill used and the Sn-based perovskites developed as a consequence is shown in Figure 16. [128] newly developed class of perovskites known as double perovskites has also shown very promising results in terms of stability and scalability.Han et al. have proposed a relatively simple hydrothermal process for synthesizing Sn-based double perovskites, which was used to fabricate a photodetector. [130]igure 17 shows the simple steps in the fabrication of the photodetector and the as-formed structure of the double perovskite.The most attractive feature of this method is its relative ease and processability under ambient conditions.Reproduced with permission. [116]Copyright 2019, Royal Society of Chemistry.
The most fascinating aspect of the perovskite, contrary to þ2 oxidation, is the presence of Sn in its þ4 state.The crystals of Cs 2 SnX 6 have an ordered vacancy with a variable defect structure.That is because half of the sites (Sn) are empty in the crystal structure.Following a drop-casting method, Wang et al. fabricated a uniform dense film.The  and e) PL spectra produced by the (OCTAm) 2 SnBr 4 after halide exchange.Reproduced with permission. [116]opyright 2019, Royal Society of Chemistry.f ) HRTEM image of a single FFT pattern.Reproduced with permission. [123]Copyright 2018, Royal Society of Chemistry.
following equation shows the reaction mechanism during the synthesis: The techniques described show clearly, and following exposure to humidity, light, and high temperatures, the resulting material demonstrated extraordinary stability and therefore was a potential candidate for optoelectronic applications.The grain sizes are clearly in the order of a few micrometers in the SEM images.The degree of good crystallinity of the synthesized material, with a truncated octahedron morphology, is further proved.The measurement of TGA shows that both Cs 2 SnI 6 and Cs 2 SnBr 6 decomposition temperatures are quite high, respectively, at 313 and 439 °C.These are far more severe temperatures than traditional organic-inorganic perovskites (80 °C for iodides of methylammonium plum and 150 °C for FAPbI 3 ).These results determine perovskites' temperature stability (Figure 18).
Remarkable stability against moisture, temperature, and light was witnessed in the as-synthesized perovskites.The properties of the perovskite were stable in the air for 1 month under sunlight for 1 week and at a high temperature of 100 °C for 1 week, respectively.This further reveals the strength of the synthesis protocol and the endless possibilities that can be achieved by tuning in a range of optoelectronic devices and components utilizing the composition of precursors.
A simple benchtop chemistry involving an equimolar (1:1) mixture of CsI (259.81 mg) and SnI 2 (372.52 mg) in a vial is presented in another wet chemical synthesis as suggested by Eduardo Lomez-Fraguas et al.A black suspension, removed at 750 rpm for several hours, was obtained after 1 mL of acetone was added to the mixture to ensure the full dispersion of precursors.The precipitated black crystals of Cs 2 SnI 6 were obtained after keeping the reaction vessel in ambient conditions, to enable evaporation of the remaining acetone. [123]ue to the vast array of elemental compositions available for fine-tuning of the cations leading to modifications in the optoelectronic nature of the resulting perovskites, in addition to bandgap tuning and high-efficiency performance, a multitude of device architectures such as single or multijunction devices OAm volumes (mL).Reproduced with permission. [126]Copyright 2019, Taylor & Francis Publishers.can be made.This makes Pb-halide perovskite such an exceptional material for use in solar cells and has been emphasized in the literature throughout.The primary concern with a lead that acts as a huge drawback for the solar cell is the solubility of Pb in water.Although the replacement of Pb with Sn to lower its toxicity has its benefits, there is another toxic aspect of PSCs that is not generally touched upon.It is the solvent that is used in the facile synthesis of the active material.The production of highquality perovskite nanomaterials is usually done regularly with organic solvents such as N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), etc. and low-exposure antisolvents such as di/chlorobenzene, etc. [131] The reason for using such solvents despite knowing their toxic nature is in their ability to give good quality material postsynthesis and the nature of the solvents allowing them to dissolve a large variety of organic and inorganic precursors for forming the solution.DMF and DMSO were also able to form adducts in the perovskite precursor solution. [132]evertheless, some thought must be given to the solvent, and to a lesser extent, the antisolvents as well.Combining a low-toxic solvent with a Pb-free perovskite precursor to create a decent perovskite material with good performance would be nothing short of a remarkable achievement.Galagan et al. addressed the large-scale fabrication of PSCs using fewer toxic solvents.Although the perovskite used in this case is Pb-based, in theory, this method could also be employed for Pb-free perovskites.Due to certain legal issues, gamma-butyrolactone (GBL) was used by them to study the solubility of the precursors.Although PbI 2 and MAI have extremely low solubility in GBL individually, their mixture is soluble. [133]However, Galagan et al. used a mixture of DMF, 2-methylpyrazine (2-MP), and pyrrole (P). [133]2-MP acted as a cosolvent with DMSO and was successful in material synthesis.Along with that, N,N-diethylformamide (DEF), which is less toxic than DMF and is absent from the carcinogenic, mutagenic, or toxic for reproduction (CMR) list. [134]It has also been considered as the solvent of choice for PSK preparation, and, in their work, the low device efficiency was attributed to DEF forming additional adducts with the perovskite precursor.When the Table 6.Summary of the mechanochemical synthesis of the Sn halide perovskites. [128]
were dissolved in 1 mL of methyl-acetate in various ratios (1:1.9, 1:2, 1:2.2, 1:2.4,1:2.5, and 1:2.6) and sonicated overnight at room temperature.Before using the thin films for device fabrication, they were heated to 110 °C for 2 h.PCEs of 1.12% and 1.62% were obtained when Spiro-OMeTAD and P3HT were used as hole-transporting materials, respectively.The optoelectronic properties of the MBI films are depicted in Figure 19. [134]The stability was also evaluated without encapsulation for 840 h under 1 sun illumination (35 days).BiI 3 has a lower solubility in water than PbI 2 , which improves overall stability and reduces the device's toxicity. [134]n this regard, to address the oxidizing issue of Sn-based perovskites, a volatile reducing solvent, liquid formic acid (LFA), was used. [135]Unlike other solid reducing agents, it evaporates quickly leaving behind no residue in the FASnI 3 film.The Sn 2þ was effectively suppressed by incorporating LFA in the precursor (Figure 20a-d).Although DMSO was the primary solvent, it still shows that Sn can be stabilized using other, less toxic materials, such as LFA and solid oxalic acid (SOA).Figure 20e shows the surface morphology and the XRD patterns of the material.The films produced with LFA and its varied compositions were pinhole-free and had large grain sizes.The slight enhancement of the FASnI 3 film's XRD peak at 24.4°upon the addition of 50 mol% LFA solvent could have resulted from a change in the surface energy of the FASnI 3 crystals at that particular facet (102, 24.4°). [135]gure 19.a) Absorbance and PL spectra.The insets on the right and left sides depict the MBI solution and film, respectively.b) PL steady-state measurements, and c) time-resolved photoluminescence decay of various HTLs at a 525 nm excitation wavelength.Reproduced with permission. [134]Copyright 2019, Nature Publishers Group.LFA, e) XRD patterns of FASnI 3 films exposed to various concentrations of LFA, and f ) H-NMR of a FASnI 3 with 50% LFA dissolved in deuterated DMSO.Reproduced with permission. [135]Copyright 2020, American Chemical Society.
To demonstrate that LFA was superior in terms of the microstructure of the FASnI 3 , SOA was added to the precursors in various concentrations ranging from 2% to 10% mol%, which had a diminishing effect on the perovskite's grain size.Because SOA is a solid reducing agent, it may have hampered the formation of the Sn-I-Sn framework, resulting in grain sizes less than 200 nm.Additionally, the addition of SOA created numerous pinholes in the films.The presence of residual SOA was confirmed by the 1 H NMR resonance signals at δ = 6.6-7.8 attributed to the -COOH group following the dissolution of the FASnI 3 -10% SOA film in DMSO-d 6 . [135]he performances of devices fabricated with liquid and solid reducing agents were compared.The device's cross section is depicted in Figure 21a.Experiments with various concentrations of LFA demonstrated that 50 mol% was sufficient to inhibit Sn self-oxidation.The perovskite absorber layer became thinner as LFA was added, most likely due to a decrease in the viscosity of the perovskite precursor solution.The pristine FASnI 3 sample and the SOA-added sample exhibit very similar device performance, whereas the LFA-added sample produces significantly higher values, as illustrated in Figure 21b.Stability-wise, the LFA-added devices demonstrated an impressive level of stability.After 200 h of operation under 1 sun, the FASnI 3 with 50% LFAbased device retained %95% of its initial efficiency, which has been attributed to fewer defects that further reduced the routes for ion migration. [96,136]Additionally, as illustrated in Figure 21d, the reproducibility of the LFA-based devices is excellent, with significantly lower standard deviations in device PCEs.This is most likely because LFA suppresses Sn 2þ , making it a promising material for future Pb-free PSCs.
In another novel one-step synthesis method, to improve the interaction between Sn and I 2 and to give better dispersion in the precursor media, DMSO was used.The process led to a highly coordinated SnI 2 •(DMSO) x adduct that gave a higher diffusion length of around 290 AE 20 nm and superior film quality.A PCE of 14.6% was achieved by this method, which further shows the versatility of using Sn and the novel techniques that can be employed to tune up the performance of PSCs. [129]y substituting formamidinium iodide (FAI) with 4-fluorophenethylammonium bromide (FPEABr), Yu et al. [100] identified two key improvements-the first being the fact that the inclusion of FPEABr reduced the natural tendency of Sn to change its oxidation state from divalent to tetravalent and the second being a decline in defect density, which led to a 57.89% enhancement in PCE from 9.38% to 14.81% in the champion device. [100]One of the most novel aspects of this approach lies in the fact that, unlike flat-out FASnX 3 devices that are purely 3D, the devices fabricated using FPEABr create 2D/3D hybrid Sn PSCs.The best result was obtained when FASnI 3 was incorporated with 10% FPEABr, and the resulting 2D-3D PSCs had a bandgap of 1.43 eV, while the inverted PSCs had a good fill factor of 70.76% and a certified PCE breaking 14.81%. [100]Figure 22 shows the device geometry and its comparison with a pure FASnI 3 device.Reproduced with permission. [135]Copyright 2020, American Chemical Society.
To further tackle the self-oxidation issues of Sn, a surface engineering approach was employed by Zhou et al. [137] in which the self-formed Sn 4þ ions were removed resulting in dedoping the perovskite surface while enhancing the other properties.The FA 0.75 MA 0.25 SnI 3 film was first coated with 3 nm of FACl (Figure 23).The reaction between FACl and the Sn 4þ (from SnI 4 ) results in a SnI 4 •xFACl complex which was confirmed by XRD profiles.No detrimental effect of this complex was found on the performance of the fabricated PSCs (confirmed using TGA).Upon depth profiling by X-ray photoelectron spectroscopy (XPS) it was found that with increasing depth, the concentration of Sn 4þ decreased from 23.3 mol% on the top surface to a minimum of 3.2 mol% deeper within the film.Following this method, the champion device delivers a high PCE of 14.7%. [137]n another study by Zou et al, [138] the inclusion of 4-fluorobenzylammonium iodide (FBZAI) as an additive to FA 0.98 EDA 0.01 SnI 3 :GeI 2 led to higher crystallinity while increasing carrier lifetime. [138]The added benefit of this inclusion was an inhibition in the oxidation of Sn from þ2 to þ4 state.The optimized surface passivation effect was observed when 2% FBZAI was incorporated into the perovskite precursor.At this concentration, the highest photoluminescence intensity with the longest carrier lifetime was obtained.The resulting PSCs had the best PCE of 13.85% which retained 95% of its performance for 160 days when stored unencapsulated (Figure 24). [138]n 2þ , being thermodynamically stable in an acidic medium, implies that incorporating some mild acid can inhibit the selfoxidation of Sn, and in this regard, formic acid (FAc), acetic acid (HAc), caffeic acid (CA), and ascorbic acid (AA) have led to the formation of good quality Sn perovskite films.[139] By using formamidine acetate (FAAc) as a Sn precursor, a higher degree of structural stability was obtained with a reduced Urbach energy value.This stability emerged from a coordination ability of the C═O-N.The optimized results were obtained with a PCE of 12.43%, and 94% of this value was retained after aging for 2000 h in an N 2 glove blox.[139] Attempts to minimize the photovoltage losses arising between the perovskite and the C 60 , a well-known electron transporting material, have led to the use with 2D/3D FASnI 3 with 10% FPEABr, respectively.Reproduced with permission. [100]Copyright 2021, Wiley-VCH.
of a fullerene derivative, indene-C 60 bisadduct (ICBA). [140]Still, the energy mismatch arising from using ICBA drastically affects the PCE.To address this issue, the solvent and annealing temperature were optimized.Solvent engineering was effectively utilized (chlorobenzene/1,2,4-trichlorobenzene (CB/TCB = 10/1, v/v)) for the preparation of PEA 0.15 FA 0.85 SnI 3 .In addition to that, the annealing temperature was raised from 70 to 100 °C, which led to the PSCs displaying a V OC of 1.01 V, which approaches the famed Shockley-Queisser limit and is one of the highest reported so far. [140]n a similar note, different regioisomers of diethylmalonate-C 60 bisadduct (DCBA) were used as the ETL in PEA 0.15 FA 0.85 SnI 3 . [141]The regioisomer trans-3-based device delivered the best performance with a high PCE of 14.58%, with the other regioisomers such as trans-2, trans-4, and e films giving lower values.Figure 25 shows the molecular structures of the four regioisomers that were used for this study. [141]

Tin-Germanium Hybrid PSCs
Although a lot of work has been done to make use of Sn as a viable alternative to Pb for working PSCs as has been highlighted in the preceding sections, however, some other materials offer exciting outcomes in this regard.Germanium (Ge), for instance, is another group IV-A element with an electronic configuration quite similar to Pb and Sn.It is surprising, though, that there does not exist a significant body of work using Ge in PSCs.and c) XPS depth profiles of the pristine and dedoped films.Reproduced with permission. [137]Copyright 2022, Elsevier Inc.  and c) J-V curve of the champion PSCs.Reproduced with permission. [138]Copyright 2022, Wiley-VCH.
Nonetheless, some attempts have been made to bridge this knowledge gap through both experiments and simulations, some of which are going to be highlighted in the forthcoming sections.
An important parameter to remember while designing a PSC is the bandgap, which, in turn, is highly dependent on the material of interest.Toward this end, Cheng et al. demonstrated that by combining Sn with Ge, it is possible to alter the bandgap to a large extent.In their work, by creating 2D mixed Ge-Sn PSCs, they have shown that by increasing the amount of Sn, a linear decrease in the overall bandgap was achieved.By employing the (PEA) 2 Ge 1Àx Sn x I 4 , the bandgap was controlled from 2.13 to 1.95 eV (Figure 26). [142]he outcomes of this article show these classes of perovskites are a promising material for emerging photovoltaic applications.
In 2D perovskites, owing to the quantum confinement effect, the bandgap is slightly higher than that of 3D perovskites. [143]n a similar note, the mechanical effects of tensile strain on 2D perovskites were studied via simulation using DFT first principles by Sarkar et al.By employing CH 3 NH 3 Sn (1Àx) Ge x I 3 (0 ≤ x ≤ 0.5) as a lead-free perovskite template, the stoichimetry was varied, and 2%, and 5% tensile biaxial strain was applied to CH 3 NH 3 SnI 3 , CH 3 NH 3 Sn 0.75 Ge 0.25 I 3 , and CH 3 NH 3 Sn 0.50 Ge 0.50 I 3 perovskites, respectively.In the study, the valence band maximum was found to be almost fixed, but the conduction band minimum shifted upward, resulting in an enlargement of the bandgap.This has been attributed to the increase in the applied strain, which lengthened the bonds leading to lower charge interaction.As such, the bandgap increased.Figure 27 shows the band structure plots of the perovskites under various conditions of strain. [144]y increasing the bandgap, the current density J SC decreased, although the PCE increased slightly.These materials make for an interesting study, where strain-engineered optoelectronics come into the picture.Although simulated, the results indicate that 2D perovskites like these can be used as potential materials for PSCs.
As Sn has the issue of getting oxidized from 2 þ to 4 þ on ambiance exposure, this affects the performance of the perovskite. [145]n addition to that, owing to the rapid crystallization of the tinhalide PSCs, the grain size is reduced by three orders of magnitude, from a micrometer in the case of lead-halide PSCs to a few hundred nanometers. [146]This raises the recombination rate at the grain boundaries for tin perovskites.To minimize such issues, doping with germanium halide has been suggested.Ito et al. reported using GeI 2 where Ge 2þ oxidizes into Ge 4þ creating a thin protective layer of GeO 4 that engulfs the Sn perovskite crystals, resulting in increased efficiency and better environmental stability. [147]By adding other organic ligands to the A-site like ethylenediammonium di-iodide, the rate of crystallization was found to have lowered significantly, resulting in slower crystal formation.Pinhole formation was suppressed as the crystal formation time was increased, resulting in much better uniformity in the formed films.This method has been used successfully by Kumarudin et al. to suppress charge carrier recombination on the surface. [148]Combining both these techniques, Nishimura et al. used GeI 2 doping in conjunction with ethylammonium iodide (EAI) to synthesize GeI 2 -doped (FA 1Àx EA x ) 0.98 EDA 0.01 SnI 3 perovskite with x varying from 0, 0.05, 0.1, and 0.2, respectively. [149]Upon oxidation of tin-based PSCs, the Sn 4þ state is generated, which tends to exhibit p-type behavior (Figure 28a,b).From Figure 28c, the shift in the Fermi energy level (E f ) can be observed, which is attributed to the incorporation of EAI into the perovskite lattice, effectively pushing it Figure 25.Molecular structures of the four regiosiomers (gray-carbon; red-oxygen; white-hydrogen).Reproduced with permission. [141]Copyright 2022, Wiley-VCH.
toward the conduction band.The shift arising from the addition of EAI may be due to the generation of additional free electrons in the material, which could have happened due to the passivation of traps/defects. [149]erformance evaluation of the as-fabricated PSCs on the addition of EAI was evaluated using an inverted structure of FTO/PEDOT:PSS/GeI 2 -doped Sn-perovskite/C 60 /BCP/Ag/Au, which were then tested in ambient conditions.The increment in open-circuit voltage V OC led to higher efficiency.As outlined before, the shifting of the E f directly affected the charge carrier extraction, in this case, making it more efficient, thereby yielding better device performance.The champion device showed a V OC of 0.65 V with a moderate PCE of 11.75%. [113]Some of the various lead-free perovskite systems along with their photosensitive layer, deposition methods, and reported performance metrics are summarized in Table 7.

Summary and Outlook
Pb-halide-based perovskites have shown unparalleled performance in a short period, from the conception of its idea to implementation in devices.As lead is a dense and heavy metal followed by its 2þ oxidation state which allows it to form a 3D symmetrical structure is a cornerstone for perovskites to be used for photovoltaic applications.Its intrinsic electrical merits arise due to its strong coupling between its 6s and 5p antibonding orbitals.Its meteoric rise, however, has been plagued by the inherent toxicity of lead, which poses potential long-term hazards to consumers and the environment.To replicate the quantum conditions of Pb mentioned above, elements possessing ns 2 lone pairs, such as Sn 2 þ , Ge 2þ , Bi 3þ , and Sb 3þ , are preferred candidates that can create octahedral with halogen anions such as F, Cl, Br, and I, respectively.Of these, Sn and Ge have proven to be viable candidates, although Ge being more expensive is slightly prohibitive.In a nutshell, lead-free perovskites hold immense potential for the fabrication of various optoelectronic devices.There are some obvious challenges, such as stability, good electrical properties, and scalability.Nevertheless, several ways have been developed to address the abovementioned critical issues, as highlighted in this review.
The emergence of low-dimensional perovskites as a viable counterpart to conventional 3D perovskites will give better quantum confinement, easier synthesis, and better control over crystal respectively.Reproduced with permission. [144]Copyright 2020, Elsevier.
Table 7. Summary of various lead-free perovskites along with their deposition technique and performance metrics.structure and morphology.Although the lead-free perovskites of Sn exhibit great promise, self-oxidation is a bottleneck for its use.
Reducing agents, such as hypophosphorous acid (HPA) and doping with SnX 2 , can prevent this problem to a large extent, as has been reported in the review.Being a weak acid with a strong reducing nature, HPA should still be handled with care because it is corrosive to the eyes and skin and can cause inflammations and blisters.Given this case, there have not been any reports of HPA being toxic to the environment and is thus generally safe to use, with proper Personal Protective Equipment. [121,150]The film coverage and uniformity can be enhanced further by using colloidal synthesis methods, which employ ligands such as long-chain alkylamines and oleic acids that create a rigid network over the perovskite film, reducing the passivation of air and moisture into the active material.Double perovskites make use of defects in the form of vacancies to give even better performance than tin perovskites and may quite possibly be one of the best alternatives for tin-based perovskites.The use of Cs leads to inorganic lead-free perovskites, which have appreciable optical properties and are hoped to be useful in a range of optoelectronic functional materials and devices.Devices, especially solar cells, made up of such materials have demonstrated competitive performance indicating the underlying beneficial properties of electronic, optical, chemical, and physical nature.Lead forms, Pb 2þ which in turns forms a balanced, symmetrical, 3D cubic structure imparts a range of fascinating properties to the perovskite material.The material's inherent instability and toxicity make lead-halide PSCs a controversial material for potential applications.The barriers to commercializing these PSCs need to be addressed progressively.Using solvents other than the established ones to synthesize perovskites must be given enough attention to impart an overall green chemistry nature to the process.The silver lining here is the use of relatively benign materials such as tin-based materials.The role of lead-free materials can speed up this process, although it comes at a cost of diminished performance over long periods.The study of it will be quite some time before the lead-free perovskites, in all their configurations, catch up to leadhalide perovskites in terms of performance, but more experimentation and theoretical studies are required to attain such goals.

Figure 1 .
Figure 1.The quintessential or golden triangle for solar cells.

Figure 2 .
Figure 2. Crystal structure of a triclinic MAPbI 3 perovskite.Organic cation methylammonium (green, yellow, and pink), metal cation Pb (red), and halide I (purple), respectively; the unit cell is represented by a green line.

Figure 3 .
Figure 3. Schematic diagram showing an OIHP (center) and its host of potential applications.

Figure 4 .
Figure 4. Potential toxicological effects of Pb on human organs.Source: World Health Organization.

Figure 5 .
Figure 5. Factors affecting the performance of lead-based PSCs.

Figure 7 .
Figure 7. I/Pb (red) and N/Pb (black) ratios are plotted as a function of temperatures.The elemental analysis was carried out by photoelectron spectroscopy (PES).The numbers 0 and 2 represent the presence of PbI 2 , whereas 1 and 3 represent MAPbI 3, respectively.Reproduced with permission.[56]Copyright 2015, American Chemical Society.

Figure 13 .
Figure 13.a) 2D phase halide perovskites under room and UV light, b) Photoluminescence (PL) spectrum, c) powder XRD patterns of various precursors,d) photographs of the tin halide perovskites, and e) PL spectra produced by the (OCTAm) 2 SnBr 4 after halide exchange.Reproduced with permission.[116]Copyright 2019, Royal Society of Chemistry.

Figure 14 .
Figure 14.a,b) Crystal structure, c) XRD pattern, d,e) TEM images of Cs 3 Bi 2 Br 9 at various magnifications, d) insets show an enlarged view of (c), andf) HRTEM image of a single FFT pattern.Reproduced with permission.[123]Copyright 2018, Royal Society of Chemistry.
Jain et al. used a nontoxic solvent to fabricate a stable Pb-free Bi-based PSC.They constructed the devices using methyl-acetate solution-processed (CH 3 NH 3 )Bi 2 I 9 (MBI) films.MAI and BiI 3

Figure 21 .
Figure 21.a) SEM cross-sectional view of the fabricated device; b) J-V profiles for FASnI 3 , FASnI 3 -2% SOA, and FASnI 3 -5% LFA based PSCs; c) External quantum efficiency (EQE) and integrated J SC of the champion PSCs; and d) histogram showing the photovoltaic efficiency of PSCs under reverse scan.Reproduced with permission.[135]Copyright 2020, American Chemical Society.

Figure 23 .
Figure 23.a) Scheme showing the chemical route for dedoping of the Sn perovskite film, b) comparison of TGA analyses between SnI 4 and SnI 4 •xFACl,and c) XPS depth profiles of the pristine and dedoped films.Reproduced with permission.[137]Copyright 2022, Elsevier Inc.

Figure 28 .
Figure 28.a) X-ray diffractograms of GeI 2 -doped FA 0.98 EDA 0.01 SnI 3 perovskites with varying ratios of EA, b) UV-vis spectra of GeI 2 -doped FA 0.98 EDA 0.01 SnI 3 with inset showing the absorption spectra from 800 to 1000 nm,and c) schematic diagram of the energy bands in the inverted Sn halide PSCs.Reproduced with permission.[149]Copyright 2020, Elsevier.

Table 1 .
Some widely used cations in perovskite composition.