Engineering Stable Lead‐Free Tin Halide Perovskite Solar Cells: Lessons from Materials Chemistry

Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high‐efficiency lead‐free devices. Despite the encouraging efficiencies of Sn‐PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their first application in 2014, investigations into the properties of Sn‐PSCs have contributed to a growing understanding of the mechanisms, both detrimental and complementary to their stability. This review summarizes the evolution of Sn‐PSCs, including early developments to the latest state‐of‐the‐art approaches benefitting the stability of devices. The degradation pathways associated with Sn‐PSCs are first outlined, followed by describing how composition engineering (A, B site modifications), additive engineering (oxidation prevention), and interface engineering (passivation strategies) can be employed as different avenues to improve the stability of devices. The knowledge about these properties is also not limited to PSCs and also applicable to other types of devices now employing Sn‐based perovskite absorber layers. A detailed analysis of the properties and materials chemistry reveals a clear set of design rules for the development of stable Sn‐PSCs. Applying the design strategies highlighted in this review will be essential to further improve both the efficiency and stability of Sn‐PSCs.


Introduction
Decoupling from carbon to tackle climate change is undoubtedly one of the most important challenges of the 21st century. Solar energy stands as the world's most powerful renewable energy source and as of April 2020, photovoltaics (PV) reached a peak in the United Kingdom of 9.7 GW. At the peak, this amount of energy represented ≈30% of the country's electricity supply, [1] are highly unstable due to the rapid oxidation of Sn 2+ to Sn 4+ , resulting in self-doping and the formation of Sn vacancies. [45] This process results in short carrier lifetimes, reducing both the efficiency and stability of Sn-halide PSCs. [46] Thus far, the three most common approaches to improve the stability of Sn-halide PSCs are composition, additive and interface engineering. In composition engineering, employing large hydrophobic cations to develop quasi-2D, hybrid 2D/3D, or Ruddlesden−Popper phase layered structures, has been shown to improve stability and has resulted in promising efficiencies in both light emitting diodes and PSCs. [37,40,42,[47][48][49][50][51][52] In particular, aromatic phenylethylammonium-based (PEA) cations have been shown to dramatically improve the morphology and crystallization of Sn-halide perovskites by forming ordered Snhalide perovskite grains, as represented by work from Li et al. depicted in Figure 1c,d. [51] Additive engineering with Sn-halides (e.g., SnCl 2 and SnF 2 ) has also been used to prevent the oxidation to Sn 4+ , where such additives have been shown to act as a sacrificial agent [53,54] that modulate the vacancies of Sn 2+ to mitigate self-doping. [55,56] Interface engineering through either perovskite passivation or energy alignment of the charge transport layers represents a promising method to improve the stability and efficiency of PSCs. In the first instance, interface passivation has been shown to protect the perovskite surface, enhance stability, and improve efficiency. [43,50,57,58] On the other hand, modifying the interface between the absorber and the charge transport layers via careful selection of the latter has also been shown to improve both stability and efficiency as a result of better energy alignment. For example, the introduction of indene-C 60 bisadduct as the electron-transport layer by Jiang et al. resulted in improved carrier extraction and thus higher efficiencies of up to 12.4%. [59] In this review, we provide a concise overview of the most recent developments in fabricating stable Sn-halide PSCs as well as a summary of their degradation processes. There is a general consensus in the literature that tin perovskite stability and device PCE are directly correlated. For example, several recent studies have shown that tin perovskite films exhibiting low stability also lead to poor PCE's when integrated into solar cell devices. [60][61][62][63][64][65] As such, this overview is timely and impor-

Degradation of Sn-Halide Perovskites
The difference in stability between Sn(II) and Pb(II) states in metal halide perovskites can be attributed to the inert pair effect; electrons in the Pb 6s orbitals are more tightly bound to the nucleus due to the poor shielding effect of d and f orbitals, thus making the Pb 2+ oxidation state the most stable. However, such effect is much weaker in Sn and hence Sn 2+ can be easily oxidized to Sn 4+ . As briefly introduced above, the presence of Sn 4+ in the perovskite turns an otherwise intrinsic semiconductor into a metallic conductor via unwanted p-doping, even when dopants are found in trace amounts. [66] This detrimental process is known to occur in all stages of perovskite preparation. For example, several groups have shown that SnI 4 is present in the SnI 2 starting material, identifying precursor impurities as a Sn 4+ ingress pathway and highlighting the need of using high quality chemicals for the preparation of Sn perovskites. [30,[67][68][69] Furthermore, Saidaminov et al. and Pascual et al. have found that a solvent commonly employed in the preparation of Sn perovskite films, i.e., dimethylsulfoxide (DMSO), can oxidize SnI 2 in solution to SnI 4 in the presence of HI as catalyst and under the high temperatures typically applied for precursor dissolution (≥≈100 °C). [70,71] Hence, considering alternative solvents or lower processing temperatures is expected to limit Sn 4+ formation. In this context, Di Girolamo et al. recently reported a study assessing an extensive selection of solvents, aiming to avoid the solvent-mediated oxidation of Sn 2+ . A cosolvent system consisting of N,N-diethylformamide and 1,3-dimethyltetrahydro-2(1H)-pyrimidinone was identified as particularly promising, giving PSCs with superior performance than those made with DMSO. [72] In Sn perovskite films, other degradation pathways can also introduce Sn 4+ into the material which can lead to decomposition of the perovskite. Leijtens et al. demonstrated that FASnI 3 is oxidized in the presence of O 2 according to the following reaction [46] + → + + 2FASnI O 2FAI SnO SnI (1) The chemical process above is a general O 2 -driven degradation of organic tin iodide perovskites (the pathway has also been identified in other types of materials, i.e., (PEA) 2 SnI 4 [73] and (PEA) 0.2 (FA) 0.8 SnI 3 [69] ). Other authors have also reported the formation of vacancy-ordered double perovskites (A 2 SnI 6 ) as degradation products of tin perovskite under ambient conditions, [74][75][76][77] although this process may also arise from the solidstate reaction between SnI 4 and AI [69] + → 2AI SnI A SnI 4 2 6 (2) While the O 2 -induced degradation of Sn perovskites described above does account for their high ambient sensitivity, additional decomposition pathways may also contribute to the poor stability of these materials. Lanzetta et al. identified the critical role of SnI 4 , both a precursor impurity and a degradation product, on the degradation mechanism of Sn perovskites. [69] It was found that SnI 4 can evolve into I 2 via the cooperative action of H 2 O and O 2 , as described as follows 4 2 2 (3) Crucially, the authors also demonstrated that I 2 can severely degrade perovskite due to its oxidizing character, giving more SnI 4 + → + ASnI I AI SnI It is pertinent to note that the degradation product SnI 4 can take part again in process (3), resulting in a cyclic degradation mechanism that drives perovskite degradation in the presence of both H 2 O and O 2 (Figure 2a). Hence, this sheds light on the chemical processes that must be mitigated to achieve competitive performances and stability in Sn perovskite optoelectronics.
In agreement with the above mechanism, Sanchez-Diaz et al. recently reported that storage of Sn-halide perovskites under vacuum indeed resulted in an increase of both Sn 2+ /Sn 4+ and I − /I 2 ratios. [79] They highlight that while this process would be expected for samples with and without additives, the use of additives can tackle the issues arising from iodine chemistry. In their work, they report a chemical engineering approach based on the incorporation of dipropylammonium iodide (DipI) together with sodium borohydride (NaBH 4 ), acting as: i) a passivator of surface sites that give rise to I 2 and ii) a reducing agent that reacts with I 2 , respectively. They demonstrate PCEs above 10% and show that their initial efficiency remained unchanged upon 5 h in air (60% RH) at maximum-power-point (MPP). In nitrogen, 96% of their initial PCE was kept after 1300 h at MPP, which is thus far the highest reported values for Sn-halide PSCs. Their findings demonstrate a beneficial synergistic effect when additives are incorporated and highlight the key role of iodide in the performance upon both light soaking and controlling the halide chemistry in Sn-based PSCs. Clearly, understanding the degradation mechanism of Sn-halide perovskite films is key to further developments when deployed in PSCs. Such developments have seen Sn-halide PSCs reach a record PCE of 14.8%, [37] which now makes breaking the 15% barrier in efficiency, the next major target for Sn-halide PSCs. The development of Sn-halide PSC efficiency from 2016 to present is shown in Figure 2b. [78]

Composition Engineering
Sn-halide perovskites, this section will identify the most successful A/X sites and low-dimensional composition routes which have been applied to achieve Sn-halide PSCs with superior stability. Although mixed B-site Sn-halide perovskites are beyond the scope of this review, a notable achievement by Chen et al. in 2019 found that mixing germanium (Ge) into the Sn B-site can produce a stable native oxide layer which encapsulates and passivates the perovskite surface. [80] This route incorporated an all-inorganic perovskite with the composition CsSn 0.5 Ge 0.5 I 3 , and the native oxide passivation demonstrated a novel route to further improve both the stability and efficiency of Sn-halide PSCs.

MASnX 3 versus FASnX 3
Initial studies of Sn-halide perovskite solar cells focused on MASnI 3 . However, PCEs of devices based on MASnI 3 have not exceeded 7.78% [81] and the MA cation has proved to be problematic in forming stable Sn-halide perovskites. The poor stability of MASnI 3 perovskites has resulted in most research groups using FASnI 3 due to the higher formation energies of Sn vacancies in this material. This can be attributed to the antibonding coupling between Sn 5s and I 5p orbitals being weaker in FASnI 3 than in MASnI 3 due to the larger ionic size of FA. [44] In addition, the high susceptibility of MASnI 3 to undergo reactions with the Sn-I bond in the presence of ambient gas molecules, forming H-I and Sn-O bonds, causes limitations to stability. [82] Further studies using X-ray diffraction and differential scanning calorimetry (DSC) have shown that MASnI 3 undergoes a phase transition from cubic to tetragonal at 57 °C, limiting its further development. Replacing MA with FA in Snhalide perovskites has since become the most popular A-site in recent advances in Sn-halide PSCs. In addition, FASnI 3 exhibits a greater thermal stability which has been attributed to the enhanced hydrogen bonding between FA cation and the inorganic matrix. [83] Wang et al. [84] studied the degradation mechanism of Sn-halide perovskites and confirmed that FASnI 3 has a better tolerance to O 2 . It was shown that MASnI 3 easily oxidized to Sn 4+ and replacing MA with FA reduced the extent of Sn oxidation. Dang et al. [85] show that in bulk single crystals, MASnI 3 and FASnI 3 are p-and n-type semiconductors respectively, where FASnI 3 exhibited superior stability. In addition to improved stability, the bandgap of FASnI 3 is 1.41 eV, which is wider than MASnI 3 . [86,87] Shi et al. also demonstrated that by varying the growth conditions, the conductivity of FASnI 3 can be tuned from p-type to intrinsic whereas, MASnI 3 only showed strong unipolar p-type conductivity. [44] FASnI 3 has also been reported to exhibit a lower conductivity than MASnI 3 . [84] Other advantages of FASnI 3 include its threshold charge carrier density of 8 × 10 17 cm −3 and charge carrier mobility of 22 cm 2 V −1 s −1 . [86,88] Like lead-based systems, key properties of Sn-halide perovskites are tuneable by employing alternative X-site anions. [89,90] For example, mixed or pure halide compositions of FASnI 2 Br [91] or FASnBr 3 [92] are capable of producing bandgaps of 1.68 and 2.4 eV respectively; this is useful for lead-free tandem solar cell applications. The introduction of bromide into the FASnI 3 lattice has been shown to be beneficial for a number of reasons including: i) reduced carrier density, ii) minimization of leakage current and improved recombination lifetime, and iii) yielding enhanced and more reproducible device performance. [93] It has also been reported that excess bromide (x = 0.33), can result in the formation of pinholes in the perovskite layer, thus reducing device performance. [94] Although the first FASnI 3 PSC achieved a modest efficiency of 2.1%, [39] tuning film morphology via the use of diethyl ether as an antisolvent resulted in a dramatic improvement in efficiency (PCE = 6.22%) with high reproducibility. [40] Ke et al. [95] incorporated a TiO 2 -ZnS cascade electron-transport layer and achieved a PCE of 5.27%. The cascade electron-transport layer was reported to effectively reduce the interfacial charge recombination and facilitate electron transfer. Moreover, these studies demonstrate the key role of morphological engineering and the electron-transport layer in the realization of more efficient and reproducible FASnI 3 solar cells. It is pertinent to note that in Sn-halide PSCs, it is common to employ additives such as ethylenediammonium diiodide (EDAI 2 ) or SnF 2 to further improve stability and PCE; Section 3.2 will discuss the latest advances in additive engineering.   [69] Copyright 2021, Springer Nature. b) Key developments in efficiency for p-i-n Sn-halide PSCs from 2016 to present. Adapted with permission. [78]

CsSnX 3
As an alternative to organic cations, cesium (Cs) can be used to form all-inorganic CsSnX 3 perovskites. It should be noted that CsSnI 3 perovskites exhibit four polymorphs: a yellow 1D double-chain structure (Y), a black 3D perovskite structure (B-γ), a black cubic perovskite phase (B-α) when the Y phase is heated above 425 K, and a black cubic perovskite phase (B-β) after the B-β is cooled to 351 K. [96] At 298 K, CsSnI 3 adopts a distorted 3D perovskite structure which crystalizes in the orthorhombic Pnma space group as reported by Chung et al. [96] CsSnI 3 is p-type and has relatively low carrier density. At room temperature, the black orthorhombic perovskite phase of CsSnI 3 is a direct bandgap semiconductor and, after heating, the photoluminescence and electrical conductivity increase. [96,97] CsSnI 3 exhibits diffusion lengths which are comparable to their Pbbased analogs. [86] The carrier concentration and conductivity of CsSnI 3 depends on the concentration of Sn, I and Cs vacancies as reported by Rajendra Kumar et al. [98] CsSnI 3 perovskites have similar optical and electrical properties to FASnI 3 ; however, they have shown much better thermal stability. [74,96,99] Due to the heavy p-type doping and high hole mobility, CsSnI 3 perovskites were first applied as a hole-transport material (HTM) for dye-sensitized solar cells (DSSCs). [100] Despite its early use in DSSCs as a HTM, CsSnI 3 has an ideal bandgap of 1.3 eV, which is interesting for use in single junction solar cells. In 2012, Chen et al. reported its use as an absorber layer in planar solar cells. [101] Later in 2014, Kumar et al. reported that the addition of SnF 2 in CsSnI 3 is required to reduce: i) Sn vacancies and ii) free carrier density in films. [55] This work has since played a pivotal role in the development and subsequent use of additives to enhance the device performance of Sn-halide PSCs and will be discussed in Section 3.2. Like their hybrid organic-inorganic equivalents (e.g., FASnI 3 or MASnI 3 ), halide substitution can be used tune the bandgap of CsSnI 3 from 1.3 to 1.7 eV of CsSnBr 3 and the crystal structure changes from orthorhombic to cubic, respectively. [102] Despite the low PCE (0.9%) reported for early CsSnI 3 solar cells, [101] devices based on CsSnI 3 quantum dots (QDs) have now achieved 5.03%. This improvement in performance was attributed to: i) a high photocurrent density of 23.8 mA cm −2 and ii) an excellent stability after 90 days. [103] It is worth noting that the CsSnI 3 QDs contained an antioxidant solvent, triphenyl phosphite (TPPi), which may be considered as an additive. With the use of a defect passivation additive, i.e., a thiosemicarbazide, that Li et al. reported the highest PCE for all-inorganic Sn-halide PSCs to date (8.20%). [104] Other notable achievements include the controlled coarsening of perovskite grains using different annealing temperatures. In 2016, Wang et al. [105] utilized coarse-grained B-γ CsSnI 3 perovskite thin films whereby its modulation, together with the optimal PSC architecture, obtained planar heterojunction-depleted B-γ CsSnI 3 PSCs with a PCE of 3.31% without the use of any additives. An improved PCE of 3.56% was later reported using a codeposition approach with SnCl 2 which improved the stability and efficiency in holetransport-layer free CsSnI 3 PSCs. [53] Impressively, the CsSnI 3 PSCs without a hole-selective interfacial layer exhibited a stability 10 times higher than devices with the same architecture using MAPbI 3 . Excess SnI 2 was also reported as another example of composition engineering which suppressed the Sn 2+ vacancies and resulted in a remarkable photocurrent density of 25.7 mA cm −2 and a PCE of 4.81%. [106] These findings illustrate the excellent progress which has been made with allinorganic Sn-halide PSCs. However, it is pertinent to note the performance of such all-inorganic Sn-halide devices still lags behind that of their hybrid organic-inorganic counterparts.

Mixed A-Site Sn-Halide Perovskites
Mixed cations in the A site first became popular in lead-based perovskites, resulting in higher stability and PCEs. [107][108][109] As expected, mixed A-site Sn-based perovskites have also been successful. Mixed cations MA and Cs were introduced by Liu et al. [110] however, the PSCs suffered low PCEs which was mainly attributed to a poor open-circuit voltage of 0.22 V. Zhao et al. [111] later introduced mixed MA and FA cations, and showed that the optical properties of Sn-based perovskites were influenced by the ratios of the mixed cations. This was evident by a blue shifted emission of MASnI 3 following the incorporation of an FA cation. Moreover, these findings showed that the bandgaps of (FA) x (MA) 1−x SnI 3 can be shifted between 1.26 and 1.36 eV; increasing FA content resulted in a wider bandgap. In addition, the mixed MA and FA cation perovskite showed a reduction in charge carrier recombination and improved film morphology. As such solar cells based on (FA) 0.75 (MA) 0.25 SnI 3 , showed a remarkably high open-circuit voltage of 0.61 V and a maximum PCE of 8.12%. Aside from monocations such as Cs + and FA + , Ke, and Spanopoulos et al. introduced an ethylenediammonium (EDA) dication to FASnI 3 and MASnI 3 , sometimes labelled as "hollow" hybrid halide perovskites. [42,112,113] Although EDA is too large to fit into the perovskite crystal structure, it does so by the partial displacement of monocations. Usually, changes in the A-site cation only result in small changes in the bandgap, which is typically attributed to structural distortion. However, the incorporation of EDA can readily tune the bandgap of FASnI 3 from ≈1.3 to and ≈1.9 eV with increasing EDA content. This wide level of bandgap tunability was attributed to changes in the electronic band structure due to the emergence of Sn-I Schottky vacancies. [42] This also resulted in improved film morphology, reduced background carrier density and increased carrier lifetime, resulting in significantly enhanced stability and photovoltaic performance. Propylenediammonium (PN) and trimethylenediammonium (TN) have also been used to produce hollow, FASnI 3 -based perovskites, where solar cells exceeded 5% in PCE for both types of materials. [114]

Low-Dimensional Sn-Halide Perovskites
Recently, layered 2D perovskites have attracted significant attention due to their improved moisture resistance. The superior stability of such materials is due to the hydrophobic behavior of the long organic chains which improves the shelf life of Sn-based perovskites. [49,115] The generic formula for layered 2D perovskite can be expressed as (RNH 3 ) 2 (A) n−1 MX 3n+1 (n = integer), and these are typically referred to as Ruddlesden-Popper 2D perovskites, as depicted in Figure 3a. [86,115] One of the most prominent features of Ruddlesden-Popper 2D perovskites is the RNH 3 component, which is typically a primary aliphatic or aromatic alkylammonium cation and acts as a spacer between the perovskite layers. The cations are represented by A and M (Sn), while the anions are X, which forms the perovskite framework. The first layered 2D Sn-based perovskite solar cells were based on (BA) 2 (MA) n−1 Sn n I 3n+1 and showed improved device performance and stability compared to their 3D equivalents. [49] Interestingly, the choice of solvents play a crucial role on layered 2D perovskites, which can be oriented parallel to the substrate when DMSO is used or perpendicular when DMF is used ( Figure 3b). In addition, it was found that the film morphology was improved, and the doping level was lowered after adding triethylphosphine to the precursor solution. In terms of photovoltaic performance, the champion devices exhibited a PCE of 2.5%. [49] It was also reported that the bandgaps of layered 2D Sn-based perovskites can be tuned from 1.83 to 1.20 eV by increasing the number of layers from n = 1 to ∞ (3D). [49] These early findings have played a pivotal role in the development of layered 2D Sn-based PSCs. Another common 2D Sn-based perovskite incorporates the PEA cation as an organic spacer layer. [47,116] [115] Copyright 2016, American Chemical Society (https://doi.org/10.1021/acs.chemmater.6b00847, further permissions for the excerpted materials should be directed to the ACS). b) Different film growth orientations for layered 2D Sn-based perovskites. Reproduced with permission. [49] Copyright 2017, American Chemical Society. c) PEA high orientation of Sn-based perovskite and JV curve for champion device. Reproduced with permission. [116] Copyright 2017, American Chemical Society.
protected by a PEA spacer have been shown to be significantly more stable than their 3D equivalents. Similar to other layered 2D Sn-based perovskites, by adjusting the ratio of PEA, the orientation of the perovskite domains can be modified. This manipulation of orientation was first realized by adding 20% PEA where in addition to improved stability, the highly oriented film resulted in a PCE of 5.94% in FASnI 3 PSCs (Figure 3c). [116] Recently, a similar strategy was employed to yield exceptionally high PCEs of 14.03% in Sn-halide PSCs; by employing a fluorine-functionalized version of PEA (4-fluoro-phenylethylammonium), Yu et al. obtained high-quality 2D/3D perovskite films with additional hydrophobic protection at grain boundaries, which results in lower Sn 2+ to Sn 4+ conversion and hence high device performance. [37] The field of lead-free perovskites for light-emitting diodes (LEDs) has also benefited from using Ruddlesden-Popper phases as the active layer; 2D Sn-based perovskite LEDs were first reported by Lanzetta et al. These devices employed (PEA) 2 SnI 4 as the emitter layer due to its superior emission properties and stability relative to MASnI 3 . [47] This early work has paved the way to highly emissive 2D Snbased perovskites with photoluminescence quantum yields above 20%, such as those based on thienylethylammonium (TEA), [117,118] and bright 2D Sn perovskite LEDs. [73,119] Other low-dimensional structures of hybrid Sn iodide perovskites were explored by Stoumpos et al., where cations such as guanidinium (GA + ) and acetamidinium (ACA + ) were investigated. [120] It was found that these cations were too large to stabilize the perovskite structure [121] but rather form 1D nonperovskite structures known as "perovskitoids"; however, these structures are beyond the scope of this review.

Additive Engineering
This strategy involves the use of additives in the precursor solution for Sn-halide perovskite film deposition to: i) compensate Sn(II) vacancies in the lattice; ii) serve as antioxidants to prevent the oxidation of Sn(II) ions; or iii) regulate the crystallization process and optimize the film morphology; as illustrated in Figure 4. It is worth noting that some additives could perform multiples roles simultaneously.

Sn(II) Vacancy Compensator
The oxidation of Sn 2+ into Sn 4+ causes the formation of Sn(II) vacancies in the perovskite lattice, and consequently p-dopes the tin perovskite. [56,122] To tackle this, a useful strategy is to introduce additives to compensate these Sn(II) vacancies. The most commonly used additives as Sn(II) vacancy compensators in tin-based perovskite solar cells are tin (II) halides, which have additionally been shown to sequestrate Sn 4+ in solution. [123] Kumar et al. first added SnF 2 into the precursor for CsSnI 3 deposition. [55] The carrier density of the CsSnI 3 films made with 20% SnF 2 (>10 17 cm −3 ) was reduced by approximately two orders of magnitude in comparison to pristine CsSnI 3 (Figure 4a). Solar cells prepared with 20% SnF 2 additive showed a high photocurrent density of more than 22 mA cm −2 and a device PCE of 2.02%; devices prepared without SnF 2 performed poorly in comparison. Besides SnF 2 , other tin halides have also been used as Sn(II) compensating additives. Marshall et al. used an excess of SnI 2 in the precursor for CsSnI 3 synthesis. [124] They found a 10% surplus of SnI 2 increased the short-circuit current density (J sc ) and open-circuit voltage (V oc ) by up to 30% in devices based on the following architecture: ITO/CuI/CsSnI 3 /C 60 /BCP/Al (ITO ≡ indium tin oxide; BCP ≡ bathocuproine). Moreover, the addition of SnI 2 resulted in a boost in PCE from 0.75% to 1.5% along with an improvement in stability. In particular, unencapsulated devices based on CsSnI 3 films prepared using stoichiometric precursors displayed a 70% decrease in their PCEs within ten days of storage in a glovebox and stopped working completely after 30 minutes exposure in ambient atmosphere. In contrast, devices based on CsSnI 3 films made with 10% excess of SnI 2 only lost 10% of their initial PCEs after ten days of storage in a glovebox and still retained 10% of the PCE even after being exposed in air for 14 h. The same group also compared the use of 10 mol% of SnI 2 , SnF 2 , SnBr 2 , and SnCl 2 additives [53] for the synthesis of CsSnI 3 for hole-transporting-layer-free devices ITO/CsSnI 3 / PC 61 BM/BCP/Al. Of the four additives, devices with 10% SnCl 2 demonstrated the best photovoltaic performance, achieving a champion PCE of 3.56%. Importantly, the unencapsulated devices with 10 mol% SnCl 2 additive showed very good stability, with 70% of the initial PCE being retained on average after ≈7 h continuous illumination in ambient air (ambient stability enhancement upon Sn(II) halide addition shown in Figure 4b).
The use of Sn(II) halide additives in the precursor has been shown to be a highly effective in boosting the efficiency and stability of tin perovskite and has now become a general strategy (commonly used together with other additives) for the fabrication of efficient and stable tin halide perovskite solar cells. [59,125]

Antioxidant
One of the key reasons for the instability of tin-based perovskite is the oxidation of Sn(II) ions. As such, a straightforward strategy to tackle this is to use antioxidants to prevent the oxidation process. Song et al. used hydrazine vapor to treat MASnI 3 , CsSnI 3 and CsSnBr 3 film during the preparation process ( Figure 4c). [126] The hydrazine treatment significantly reduced Sn 4+ content in the films and improved the performance of devices based on these perovskite materials. In addition to hydrazine vapor, its salts have also been explored as reducing additives for tin perovskites. Kayesh et al. added hydrazinium chloride N 2 H 5 Cl directly in the precursor for FASnI 3 thin films and found that the addition of 2.5 mol% N 2 H 5 Cl in: i) reduced concentration of Sn 4+ content from 7.3% to 4.9% in the FASnI 3 film, ii) suppressed carrier recombination, and iii) improved the coverage of the films. [129] The devices with 2.5 mol% N 2 H 5 Cl displayed an improved PCE of 5.4% and retained 65% of the initial PCE after 1000 h storage in a N 2 glove box. In contrast, devices without the N 2 H 5 Cl additive showed PCE of 2.5% which rapidly dropped and led to device failure after 850 h. Li et al. studied trihydrazine dihydriodide (THDH) as a reducing agent in the FASnI 3 precursor employing a low purity 99% SnI 2 source. [130] They found that addition of 3 mol% THDH was capable of significantly reducing the Sn 4+ content in the FASnI 3 films from 35.9% to 9.1% and increasing the PL lifetime from 0.18 to 3.51 ns, resulting in a high PCE of 8.48%. Hydrazine monohydrobromide has also been used as a reducing additive for FASnI 3, [131] leading to improved photovoltaic performance of 7.81%. Wang et al. employed a phenylhydrazinium halide additive in FASnI 3 PSCs as a combination of the reducing power of the hydrazinium group and the hydrophobic character of the phenyl group, achieving a high PCE of 11.4%. [132] This strategy was found to passivate defects in the material, yielding remarkable device shelf lives of over 110 days and providing the perovskite with self-healing properties (PCE recovery) after air exposure. The same group achieved very high PCEs of 13.4% and good stability (≈5000 h shelf life in N 2 ; ≈350 h under continuous AM1.5 illumination in N 2 ) by using the same type of additive. [133] It is noteworthy that closely related hydrazide-based antioxidants (4-fluorobenzohydrazide) have also been employed recently in FASnI 3 PSCs by He et al., delivering PCEs above 9% in a wide O 2 concentration range during processing, i.e., 9.47% at 0.1 ppm O 2 and 9.03% at 100 ppm O 2 , as well as a high operational stability (93% PCE retention in encapsulated devices after 600 h in air under continuous AM1.5 illumination). [134] This illustrates a promising pathway to facilitate the processing of high-efficiency Sn-halide PSCs under ambient conditions.
Although hydrazine compounds have shown promising results as additives for improving both the photovoltaic performance and stability of tin perovskite solar cells, hydrazine is a strong reducing agent and could cause the over reduction of the precursor. [126] The safety concerns about these chemicals also make them less favorable for industrial use. Therefore, other mild and safer antioxidants have been explored. One of the most promising candidates is the Sn(0) metal. Tin metal can work as a reducing agent to reduce Sn 4+ , and the reaction produces Sn 2+ which can serve as Sn(II) vacancy compensators as discussed in the previous section. Gu et al. added tin metal powder into FASnI 3 solution made from 99% purity SnI 2 to reduce Sn 4+ ions to Sn 2+ in the precursor. [135] A PCE as high as 6.75% was achieved with a V oc of 0.58 V, a J sc of 17.5 mA cm −2 , and a fill factor (FF) of 0.66, and the PCE retained 90% of its original value after 860 h storage in a glovebox. Nakamura et al. introduced Sn(0) nanoparticles formed in situ in the FA 0.75 MA 0.25 SnI 3 precursor solution as a reducing additive. A compound, 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (TM-DHP), was added to the precursor solution and reacted with SnF 2 to form Sn(0) nanoparticles which then worked as a Sn(IV) scavenger. Sn(IV) content in the bulk of the perovskite film was significantly reduced with this method. With further surface passivation treatment and device architecture optimization, a high PCE of 11.5% was achieved, with no decrease after 50 days storage in inert condition without encapsulation. [136] Other mild reductants have also been explored as reducing additives for tin perovskite solar cells. Li [55] Copyright 2014, Wiley-VCH. b) Evolution of absorbance at 500 nm for CsSnI 3 films with 10 mol% of SnI 2 , SnBr 2 , SnCl 2 , or SnF 2 , and with no tin halide additive, illustrating improved stability with the tin(II) halide additives. Reproduced with permission. [53] Copyright 2016, Springer Nature. c) Schematic showing the possible mechanism of hydrazine vapor reaction with Sn-based perovskite materials. Reproduced with permission. [126] Copyright 2016, American Chemical Society. d) High-resolution XPS spectra (Sn 3d) and e) PL time decay trace of CsSnIBr 2 perovskite thin films w/o HPA additive, illustrating reduced Sn(IV) content and longer lifetime in the film after the introduction of HPA as a reducing additive. d,e) Reproduced with permission. [127] Copyright 2016, Royal Society of Chemistry. f) Schematic illustration of hydroxybenzene sulfonic acids as additives improving morphology of FASnI 3 films by suppressing the formation of SnCl 2 aggregates and forming an antioxidant outer layer. Reproduced with permission. [128] Copyright 2019, Wiley-VCH. g) SEM images of MASnI 3 perovskite films without (left) and with 15% EDA (right) as additive, illustrating improved film morphology with the introduction of EDA in the films. Reproduced with permission. [112] Copyright 2017, American Chemical Society.
hypophosphorous acid (HPA) in 2016 as additive for CsSnIBr 2 perovskite solar cells. [127] As a gentle reducing agent, HPA can prevent the oxidation of Sn 2+ to Sn 4+ (Figure 4d) and consequently reduce the vacancy density in the perovskite film and increase charge carrier lifetime (Figure 4e). As a result, the HPA addition significantly improved the V oc and FF of the perovskite solar cell devices, and almost doubled the PCE to an average of 3.0%. Moreover, these devices showed no loss in PCE after 77 days storage in ambient conditions (with encapsulation). In addition, the devices were held at their MPP under 1 sun illumination for 9 h and retained 98% of their PCE at 200 °C. Tai et al. introduced hydroxybenzene sulfonic acids as antioxidant additives into FASnI 3 precursor solution in the presence of excess SnCl 2 . [128] The additives used in this work were phenol-sulfonic acid (PSA), 2-aminophenol-4-sulfonic acid (APSA), and the potassium salt of hydroquinone sulfonic acid (KHQSA). The -SO 3 group coordinated with Sn 2+ ions, resulting in the uniform distribution of SnCl 2 at grain boundaries, while the -OH group worked as O 2 scavenger, forming in-situ encapsulation of the perovskite grains (Figure 4f). The stability of solar cells made with tin perovskite films treated with the hydroxybenzene sulfonic acid additive was greatly improved. In particular, unencapsulated devices showed a >80% retention of PCE after more than 500 h exposure to air in the dark. In contrast, devices based on films without the additive degraded rapidly, losing 80% of the initial PCE within 48 h. Meng et al. recently used a volatile reducing liquid formic acid as the additive for FASnI 3 perovskite. [137] The formic acid served as a reducing agent suppressing the oxidation of Sn 2+ and its volatile nature facilitates its escape during annealing process, thus no residual solvent remained in the film and the crystallinity of the film was not sacrificed due to the introduction of the additive. A maximum PCE of 10.37% was achieved for devices composed of films processed with 50 mol% of formic acid additive. Moreover, 95% of the original efficiency was retained after 200 h of operation under light soaking with encapsulation.

Crystallization Regulator
The performance and stability of tin perovskite solar cells critically depend on the ability to control the morphology of the photoactive layer. This is sometimes achieved by twostep deposition methods; however, due to the poor stability of Sn 2+ , this approach is more commonly employed in Pb-halide perovskites. [138][139][140][141][142] Notably, the crystallization of Sn-halide perovskites is much faster than that of their Pb-halide counterparts. It is therefore important to discuss routes which look into better control of the crystallization dynamics. Highly crystalline domains with compact uniform film morphology are required to reduce permeable pathways for oxygen and moisture; both O 2 and H 2 O are known to degrade Sn 2+ and lower stability. [143] As such, a number of additives have been used to regulate the crystallization in the perovskite layer and improve the morphology of the films. Ke et al. added EDA in the FASnI 3 perovskite precursor and demonstrated that the EDA cation was incorporated into the 3D perovskite structure despite its big size. [42] The FASnI 3 film based on 10% EDA was found to be considerably smoother with less pinholes than the pristine film. However, it is interesting to note that the bandgap of the perovskite increased after the incorporation of EDA cations. The addition of 10% EDA pushed the PCE of the FTO/c-TiO 2 /m-TiO 2 /FASnI 3 /PTAA/Au devices to 7.14%, and greatly improved their stability. The unencapsulated FASnI 3 device with 10% EDA retained 50% of its initial efficiency after 1 h under constant illumination in air at room temperature while the control devices stopped working after 20 min. The same group later applied the EDA additive strategy to MASnI 3 and CsSnI 3 systems and achieved similar improvements in performance and film morphology (Figure 4g). [112] Jokar et al. compared bulky organic cations butylammonium (BA) and EDA as additives for FASnI 3 films and found that addition of 0.5-5% EDAI 2 effectively resulted in highly packed, uniform and pinhole-free FASnI 3 films. [50] BAI worked in a similar way but less effectively. The best device performance was obtained with 1% EDAI 2 addition. The fresh FASnI 3 -1% EDAI 2 devices delivered a best PCE of 7.4%, and strikingly instead of deteriorating, the device performance continuously increased during storage in a glove box to a maximum PCE of 8.9% after over 1400 h and only slightly degraded after 2000 h.
As mentioned earlier, one obstacle to high quality tin perovskite films is the fast crystallization process during film deposition; this is problematic for the growth of uniform films. [144] Therefore, to obtain homogeneous tin perovskite films compatible with high PCE and stability, better control of the crystallization process is required. Wu et al. successfully slowed down the crystallization of FASnI 3 perovskite by introducing π-conjugated Lewis base molecules, 2-cyano-3-[5-[4-(diphenylamino)phenyl]-2-thienyl]-propenoic acid (CDTA), in the precursor solution. [145] During film fabrication, these molecules coordinated with Sn 2+ ions which significantly slowed down the nucleation and crystallization rate of FASnI 3 perovskite. As a result, compact films with a reduced number of pinholes and larger grains were obtained with 0.2% CDTA addition. The films also showed a preferable growth orientation along (h00) planes, which is beneficial for charge carrier transport. A PCE of 10.17% was achieved for ITO/PEDOT:PSS/ FASnI 3 /C 60 /BCP/Ag solar cell devices with 0.2% CDTA as additive. Moreover, this device exhibited good stability; 97% of PCE was maintained after 400 h continuous light soaking with encapsulation. Meng et al. employed an alternative additive, namely pentafluorophen-oxyethylammonium iodide (FOEI), to control the crystallization process of FASnI 3 perovskites. [146] The authors found that the nucleation started at the solution-air interface and the FOEI additive successfully regulated the crystallization by reducing the surface energy at such interface. As a result, highly (h00) oriented and smooth FASnI 3 films were obtained, showing long charge-carrier lifetimes. Devices based on the FASnI 3 -FOEI films delivered a certified PCE of 10.16% with high operational stability. Encapsulated devices showed no drop in PCE after 500 h of operation under light soaking. Recently, Li et al. reported the use of ionic liquids, namely n-butylammonium acetate, in perovskite precursor solutions, yielding PCEs over 10% and high device shelf stability (≈1000 h under N 2 /dark storage) in p-i-n 2D/3D Sn-halide PSCs. [147] This strategy relies on the coordination of the ionic additives to Sn 2+ and halide ions in solution, which facilitates a controlled crystallization. This in turn leads to highly compact, pinhole-free perovskite films with strong preferential orientation, as opposed to more disordered nucleation in absence of the ionic liquid. By exploiting the coordination chemistry of Sn 2+ , Jiang et al. recently reported a novel synthetic approach for perovskite Sn precursors; SnI 2 ·(DMSO) x adducts prepared by the oxidation of metallic Sn by I 2 in DMSO lead to well-dispersed Sn 2+ complexes in the precursor solution, as opposed to the aggregated nanostructures obtained by using conventional SnI 2 precursors. [38] The use of the adduct-based synthetic route is shown to significantly improve carrier diffusion length in Snhalide perovskites. Consequently, solar cells based on 2D/3D FASnI 3 showed enhanced J sc and achieved high PCEs of 14.6% and 96% efficiency retention after storage in N 2 /dark conditions for 100 days.
As discussed earlier, bulky organic cations such as PEA have been widely incorporated in tin perovskite films to form 2D/3D phases. In this structure, the out of plane orientation of the crystal is considered more favorable for both charge transfer and surface passivation of the 3D component by the 2D layer. Extensive research efforts have revealed that inclusion of thiocyanate-based additives in the precursor solution can regulate the orientation of the crystals. Kim et al. added formamidinium thiocyanate (FASCN) into quasi-2D PEA-FASnI 3 tin-based perovskites and found that it enhanced the parallel orientation of (h00) planes with respect to the substrate. The FASCN additive also improved the morphology of the film and significantly suppressed the oxidation of Sn(II) due to its interaction with Sn 2+ . An improved PCE of 8.17% was achieved for the device containing the FASCN additive, with over 90% retention after 1000 h in a nitrogen-filled glove box. [61] Wang et al. used an alternative thiocyanate additive, NH 4 SCN, as a crystallization regulator for the preparation of PEA 0.15 FA 0.85 SnI 3 2D/3D perovskites. [148] With the addition of NH 4 SCN, the nucleation and crystallization growth processes were separated and a 2D layer formed at the surface of the film with a parallel preferred orientation (vs substrate), leading to a 2D-quasi-2D-3D hierarchical structure. The thin 2D surface layer worked as a protecting layer against oxidation of Sn(II). The devices with 5% NH 4 SCN showed a high PCE of 9.41%, which remained unchanged after 600 h storage in a glove box.
Ji et al. [149] showed that the crystallization dynamics and crystal orientation of MASnI 3 can be tuned by using ethylammonium bromide (EABr) in the film precursor solution. The EABr additive was also found to reduce the oxidation of Sn 2+ . This led to PSCs with efficiencies of 9.59% that maintained 93% of their initial efficiency after 30 days in a N 2 glovebox without encapsulation. More recently, Nasti et al. [150] demonstrated a solvent engineering approach to deposit compact Snhalide perovskite films from DMSO-free solutions. Despite its role as crystallization regulator, DMSO solvent has been shown to facilitate the undesirable Sn 2+ to Sn 4+ oxidation of Sn-halide perovskites. The authors solved this issue by replacing DMSO by 4-(tert-butyl) pyridine, a solution additive capable of forming stable complexes with dissolved SnI 2 salts. Sn-halide perovskite films deposited from pyridine were shown to exhibit an order of magnitude lower defect density, leading to the highest PCEs for devices processed from DMSO-free solutions (7.3%). For a more detailed overview of the crystallization dynamics in Snhalide PSCs, we recommend the detailed review by Cao et al. [151]

Coadditives and Other Approaches
Another obstacle to the realization of homogeneous tin perovskite films lies in the nonuniform distribution of tin(II) halide additives within the film. As mentioned in the previous section, the inclusion of Sn(II) halides such as SnF 2 as additives in tinbased perovskites has been employed as a general strategy to achieve efficient and stable tin perovskite solar cells. However, during film deposition, the Sn(II) halide is prone to aggregation, resulting in poor uniformity of the perovskite film. This in turn is detrimental to the performance and stability of tin perovskite solar cells. To overcome this challenge, the use of coadditives has been investigated. Lee et al. introduced pyrazine in FASnI 3 perovskite together with SnF 2 as additives. [41] The addition of pyrazine into the precursor solution effectively prevented the formation of plate-like aggregates of SnF 2 , resulting in a smooth and dense FASnI 3 perovskite layer without phase separation. A PCE of 4.8% was achieved, and encapsulated solar cells maintained 98% of the initial efficiency following 100 days storage under ambient conditions. Hydroxybenzene sulfonic acid was found to function in a similar way to pyrazine. Tai et al. also found the phase separation in FASnI 3 films is due to the aggregation of SnCl 2 additive. [128] However, with the addition of the hydroxybenzene sulfonic acid, aggregation of SnCl 2 was significantly reduced, and the uniformity of the perovskite film was considerably improved. Kayesh et al. [152] used 5-ammonium valeric acid iodide (5-AVA) as a coadditive for the fabrication of FASnI 3 -based perovskite films. The 5-AVA additive was shown to influence crystal growth of the perovskite through hydrogen bonding of this molecule with I − and SnI 6 4− octahedra. This resulted in pinhole-free homogenous films with increased stability due to lower Sn 4+ content. Solar cells based on such layers yielded PCEs of 7% and maintained their efficiency under 1 sun illumination at MPP for 100 h. In 2019, Cao et al. [153] introduced an ammonium hypophosphite (AHP) additive which effectively suppressed the oxidation of Sn 2+ and assisted with grain growth of FASnI 3 . This subsequently resulted in improved device stability and performance. For example, unencapsulated PSCs achieved a PCE of 7.34% and exhibited shelf-life stability of 1000 h where 70% of the initial PCE was retained. Recently, Wang et al. found that another antioxidant, i.e., gallic acid (GA), could also work as a coadditive with SnCl 2 to restrict the aggregation of SnCl 2 by forming a SnCl 2 −GA complex. [63] The FASnI 3 perovskite solar cell devices with 1 mol% GA addition exhibit an average PCE of as high as 8.32%, and strikingly maintained ≈80% of the initial PCE after storage in air without encapsulation for 1000 h. Liu et al. [154] showed that structural imperfections in Sn-halide perovskites can be healed using a biocompatible chelating agent, namely, 2-guanidinoacetic acid (GAA). The use of this additive was shown to effectively passivate deep-level defects, release lattice strain, improve structural toughness, and enhance carrier transport. Solar cells composed of perovskite layers with GAA exhibited an impressive PCE of 13.7% with enhanced operational stability. Specifically, the GAA additive was shown to extend the lifespan of unencapsulated Sn-halide PSCs with devices retaining 93% of their initial PCE for over 1200 h upon storage in a N 2 atmosphere.
All-inorganic Sn-halide PSCs have recently gained attention as a promising approach to mitigate the thermal instability of the commonly employed FA cation in hybrid systems. Recently, Zhang et al. [155] reported on inorganic Sn-PSCs based on CsSnI 3−x Br x perovskite films comprising a multifunctional dimethyl ketoxime (C 3 H 7 NO, DMKO) additive. The DMKO antioxidant additive reduced Sn 4+ to Sn 2+ , making the perovskite film more resistant to oxidation. Additionally, the authors reported that the electron-rich oxime group (=NOH) can interact with Sn 2+ ions and help limit defect formation, leading to low defect densities. The authors reported on all-inorganic Sn-halide PSCs with 11.2% efficiency with a record open-circuit voltage of 0.75 V. The devices exhibited a shelf-life stability of 1000 h, retaining 80% of their initial PCE (in unencapsulated devices).
Surface reconstruction of Sn-halide perovskites was recently introduced by Li et al. [156] The approach involved spin coating a 6-maleimidohexanehydrazide trifluoroacetate (MHATFA) salt (in IPA) onto FASnI 3 to passivate the defective surface. Notably, ethanediamine dihydroiodide (EDADI) was also used as an additive in this work. MHATFA treatment transformed the p-type to n-type perovskite surface which induced an extra built-in electric field, thus inhibiting charge-carrier recombination and improved the V oc from 0.59 to 0.69 V. Specifically, the reductive hydrazide and carboxyl groups alleviated Sn(IV), homogenizing surface potential and prolonging carrier lifetime. The devices delivered a PCE of 13.64% and retained over 75% of the original PCE after 1000 h of illumination under low oxygen conditions (<50 ppm). This strategy demonstrated a new type of surface reconstruction and is thus a promising route for the future development of high-performance Sn-halide PSCs.
Chemothermal de-doping is an alternative coadditive strategy which has recently been employed by Zhou et al. [157] to reduce surface Sn(IV) self-doping. After perovskite film deposition, FACl was introduced via controlled thermal evaporation (0.05 nm s −1 ). The resulting films were then annealed at 100 °C for 20 min to initiate surface de-doping and removal Sn(IV) states. The authors reported impressive solar cell PCEs (14.7%) with good stability. Here the devices retained 92% initial PCE following storage in N 2 environment for 1000 h. Clearly, the chemothermal surface de-doping, presents a significant advance in the field and offers a potential route to high-performance and stable Sn-halide PSCs.
An alternative approach, referred to as dual-site passivation, was recently introduced by Jiang et al. [158] Here an ethylenediammonium dibromide (EDABr 2 ) additive was employed to prevent Sn 2+ oxidation at both the surface bulk of the perovskite. The authors also demonstrated that the choice of halide salt was important with for example EDABr 2 showing a greater degree passivation of Sn I antisite defects/deep-level traps as compared to EDAI 2 . Moreover, champion Sn-halide PSCs incorporating EDABr 2 gave PCEs of 14.23% which maintained 93% of the initial efficiency following storage in N 2 for 4000 h. Table 1 below represents recent significant advances in the stability of Snhalide PSCs using additive engineering.

Interface Engineering
Sn-based PSCs still show relatively low PCEs mainly due to their low V oc , [95,160] with values typically ranging from 0.4 V to 0.9 V and still far from those in Pb-based PSCs. [144] One reason for this arises from the Sn element in the perovskite layer leading to excessive carrier recombination, as discussed earlier. The use of a suboptimal device architecture with mismatched energy levels between charge transport layers (CTLs) and Sn-halide PSC can also lead to a low V oc . CTLs play a crucial role in charge separation and extraction in PV devices. Compared with Pb-based perovskites, the reported conduction band (CB) and valence band (VB) edge values of Sn-based PSCs show larger variation due to easy oxidation of Sn 2+ during sample preparation/characterization. [40,86,161,162] Hence, it is particularly important to understand the role of individual layers and optimize the energy band alignment between CTLs and Sn-halide perovskites.

Hole-Transport Layer (HTL)
The first Sn-based PSCs were fabricated with mesoporous device architecture by Hao et al. and Noel et al., as reviewed above. [30,31] Spiro-OMeTAD was commonly used as the HTL, and Li-TFSI and tert-butylpyridine (TBP) were required as dopant materials to improve conductivity. [176] However, the Li salt is sensitive to moisture, and TBP can dissolve the perovskite, inducing degradation. Ke et al. developed a dopant-free HTL material, tetrakis-triphenylamine (TPE), via a facile synthetic route. [177] TPE exhibits intrinsically high hole mobility, and the best-performing Sn-PSCs incorporated with TPE achieved an efficiency of 7.23%. This is attributed to suitable band alignment between TPE and Sn-PSCs; the highest occupied molecular orbital (HOMO) of TPE shallower than those of commonly used HTLs including spiro-OMeTAD and PTAA.
In inverted device architectures (p-i-n), PEDOT:PSS is a commonly used HTL. Liu et al. developed a new HTL with a tuneable work function by introducing poly(ethylene glycol) (PEG) into PEDOT:PSS. [162] PEG is proposed to interact with PSS via hydrogen bonding and reduce the surface dipole in the HTL, thus lowering its work function and mitigating the energy level mismatch between FASnI 3 and PEDOT:PSS. The best-performing device achieved an efficiency of 5.12% with high stability and reproducibility. Liu et al. reported on the use poly[tetraphenylethene 3,3′-(((2,2-diphenylethene-1,1-diyl) bis(4,1-phenylene))bis(oxy))bis(N,N-dimethylpropan-1-amine) tetraphenylethene] bromide salt (PTN-Br) as a hole transporter between FASnI 3 and PEDOT:PSS. This strategy was found to effectively passivate the trap states, leading to a high efficiency of 7.94% device with improved UV stability. [178] These performance enhancements were, in part, attributed to the suitable HOMO energy level of PTN-Br.

Electron-Transport Layer (ETL)
ETLs play a crucial role in electron transport from perovskite layer to the cathode. TiO 2 is the most used ETL material in the conventional device architecture due to its good electrical properties and chemical stability. However, TiO 2 -based Sn-PSCs suffer from low V oc due to the excessively deep energy levels, and severe charge recombination at interfaces. [144] Ke et al. have developed a cascade device structure based on a TiO 2 / ZnS/FASnI 3 heterojunction. [95] Devices based on this heterojunction were found to exhibit 5.27% PCE. The ZnS interlayer was found to: i) improve electron transport from perovskite to TiO 2 and ii) act as an energy barrier reducing the interfacial back-recombination. These observations were attributed to a favorable level alignment between FASnI 3 and ZnS.
Inorganic metal oxides such as ZnO and SnO 2 have also been used as alternative ETL materials. [169] ZnO has been a promising alternative to TiO 2 due to its similar bandgap (3.2 eV) and higher electron mobility offering improved electron extraction. However, ZnO is hygroscopic by nature and  [157] reacts with weak acids and bases, which may lead to degradation of perovskite under ambient conditions. [179] SnO 2 has been shown to be a successful ETL candidate in Pb-based perovskites due to its wider bandgap (3.8 eV), and deeper conduction band which affords better energy level alignment. In addition, SnO 2 is less hygroscopic and requires lower deposition temperature (<200 °C) than TiO 2 . However, tin perovskite solar cells comprising solely SnO 2 typically show a low V oc attributed to energy losses associated with a large conduction band offset (CBO). [180] Yang et al. have developed an interfacial engineering strategy to improve the performance of SnO 2 -based tin perovskite solar cells. Their strategy involved inserting a thin layer of C 60 pyrrolidine tris-acid (CPTA) between SnO 2 and perovskite layer. As a result, a PCE of 7.4% was achieved with an enhanced V oc of 0.72 V. The presence of CTPA is very important because: i) it improves the electron extraction and ii) the conduction band minimum (CBM) of CPTA is much higher than the CBM of SnO 2 , leading to a smaller CBO. Therefore, CPTA can act as an energy barrier for holes at SnO 2 /perovskite interface, reducing the charge recombination. [181] Yokoyama et al. have reported the use of niobium oxide (Nb 2 O 5 ) as an alternative ETL in FASnI 3 PSCs. [180] Compared to SnO 2 and TiO 2 , Nb 2 O 5 has a smaller CBO (+0.2 eV) with the Sn-based perovskite layer (Figure 5a), leading to an improved V oc from 0.27 to 0.42 V thereby offering a promising route toward high V oc in n-i-p Sn-halide PSCs. Inverted, p-i-n Sn-PSCs have been shown to exhibit higher PCEs and stability as compared to those based on conventional, n-i-p architecture. This is mainly due to more favorable band alignment and suppressed hysteresis. Fullerene and its derivatives are the common choice as ETLs for p-i-n architectures due to better matching of the energy levels and a reduced defect density at ETL/perovskite interface. The use of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) offers suitable band alignment with tin perovskites, which is key to achieving fast electron extraction. In addition, PCBM can also be introduced at the ETL/perovskite interface as a passivation layer, often improving voltage and reducing hysteresis. [182][183][184] An alternative ETL material, C 60 , is typically deposited via evaporation and it helps passivate the charge traps at both surfaces and grain boundaries of perovskite layer. In addition, the lowest unoccupied molecular orbital (LUMO) of C 60 matches closely with the CBM of FASnI 3 , allowing a favorable electron transfer from the perovskite to the fullerene, while the HOMO level of C 60 lies below the valence band maximum (VBM), acting as a hole blocking barrier. [185] Low open-circuit voltage remains a major challenge in p-i-n Sn-PSCs. A possible reason for this is the deep LUMO of PCBM and C 60 , which limits the maximum attainable photovoltage. In 2020, Jiang et al. introduced indene-C 60 bisadduct (ICBA) as the ETL. [59] Compared with PCBM, ICBA has a shallower conduction band energy level, enabling a larger maximum attainable voltage. The use of ICBA was also proposed to suppress iodide doping of the ETL caused by the perovskite layer, which reduces interfacial recombination. Favorable energy level alignment and reduced interfacial recombination led to a device PCE of 12.4%, with an impressive V oc of 0.94 V. [59] (Figure 5b). Lee et al. compared C 60 , PCBM and ICBA as ETLs for PEA 0.1 FA 0.9 SnI 3 solar cells. Among these ETLs, ICBA has the highest lowest unoccupied molecular orbital (LUMO), which leads to a minimal energy offset with the CB of the Sn perovskite. [174] As a result, Sn-PSCs with ICBA demonstrated a high V oc of 0.651 V, which is 0.161 and 0.248 V higher than those achieved with PCBM and C 60 , respectively (Figure 5c). However, ICBA-based solar cells display a slightly lower J sc as compared to PCBM and C 60 . Such differences are proposed to arise from poorer electron transport in ICBA, which can be attributed to its lower electron mobility (6.9 × 10 −3 cm 2 V −1 s −1 ) relative to that in PCBM (6.1 × 10 −2 cm 2 V −1 s −1 ) and C 60 (1.6 cm 2 V −1 s −1 ). [186] This work highlights the importance of tailoring the energetics at the perovskite/ETL interface and opens ways of further improving PCEs in Sn-PSCs.

Other Interlayers
Modification of CTL/perovskite heterojunctions with chemical agents (often referred to as interlayers) offers an attractive route to enhance device performance. Low-dimensional perovskite materials have been mixed with 3D perovskites in the precursor solution to improve the device performance, as discussed earlier, and have also been used as interlayers to improve film morphology, charge transport and stability. [48] Chen et al. introduced an ultrathin interlayer of phenylethylammonium bromide (PEABr) at the PEDOT:PSS/perovskite interface. [187] This interlayer passivates trap states and allows the growth of higherquality FASnI 3 , leading to higher quality film morphology and hence better charge transport. As a result, a device PCE of 7.05% was achieved with good stability and negligible J-V hysteresis. A similar strategy was adopted by Ran et al., where a so-called 2D/3D perovskite bulk heterojunction was grown from a solution-processed FAI layer treated with sequentially evaporated PEAI and SnI 2 ; [188] this was shown to enhance surface coverage and morphology. Additionally, a LiF interlayer was introduced between ITO and PEDOT:PSS in order to improve hole extraction, leading to a device PCE of 6.98% with improved stability. By adding both SnF 2 and SnCl 2 in the precursor solution, Liu et al. reported the growth of an amorphous triple-halide (F − , Cl − , I − ) perovskite interlayer on top of polycrystalline CsFASnI 3 films. [159] The amorphous perovskite layer was shown to block oxygen and moisture, leading to high PCEs in p-i-n devices (>10%) and impressive operational stabilities, i.e., ≈95% PCE retention after 1000 h of operation at MPP under 100 mW cm −2 illumination.
Ding et al introduced an ultrathin layer of poly(methyl methacrylate) (PMMA) at the PEDOT:PSS/(PEA) 0.2 (FA) 0.8 SnI 3 perovskite interface. This resulted in a significant increase in J sc , i.e., from 16.2 to 22.9 mA cm −2 , and hence in PCE, from 6.5% to 10.0% relative to control devices (Figure 5d, top). [172] PMMA is proposed to reduce interfacial charge recombination (Figure 5d, bottom) and to induce a more favorable distribution of 2D perovskite phases. Similarly, Liu et al. has reported a pretreatment by spin coating n-propylammonium iodide (PAI) on PEDOT:PSS. [189] The PAI layer acts as an interfacial grain template for perovskite film growth, and induces a highly crystalline FASnI 3 film with preferential orientation along the (100) plane. As a result, the device efficiency improved from 7.93% to 11.78% with V oc increasing from 0.53 to 0.73 V. This is mainly attributed to suppressed trap states in the templatedgrowth perovskite film, which also leads to an improved device stability, maintaining 95% of its initial efficiency after operating at MPP under 100 mW cm −2 for 1000 h.

Perspectives and Outlook
This review provides a concise summary of the evolution of Sn-halide PSCs, from early developments to the latest stateof-the-art approaches. Particularly, we describe how materials chemistry has made this technology increasingly competitive to the point of even outperforming well-established technologies such as amorphous silicon. [38] Overall, the attainable solar cell PCE in Sn-halide perovskite can be directly linked to their chemical stability (and that of their Sn precursors) due to the large impact Sn 4+ states have on device performance. Based on this, we next provide a point-by-point list of the most promising future directions to design highly efficient and stable Sn-halide PSCs.
i) FA is the most common choice as the A-site cation, being a requirement for high efficiencies. The partial substitution of FA by PEA (or more recently, its fluorinated derivative) has repeatedly been shown to lead to outstanding stability and performance, being found in two of the few reported examples with PCE > 14%. [37,38] We therefore expect this strategy to become widespread in future high-efficiency Sn-halide PSCs. Devices based on mixed A-site cations such as FA 0.75 MA 0.25 or fully inorganic Cs A-sites have achieved PCEs of 14.7% and 11.2%, respectively. More importantly, these approaches have seen stabilities surpassing 1000 h in the past year highlighting the potential of A-site cation engineering including the use of mixed-cation systems. The generation of iodine as a degradation product (in Sn perovskites) is a major problem in the field and must be addressed to improve stability. Specifically, iodine can directly react with the tin perovskite leading to rapid degradation of the perovskite (Figure 2). Strategies to improve the tolerance of Sn perovskites to I 2 need be developed and implemented in order to boost performance and stability of devices. It is pertinent to note that the X-site of perovskite allows compositional tuning beyond iodide-only recipes for higher stability and performance; while not abundantly applied, mixed-halide systems show superior optoelectronic properties and represent an effective way to improve Sn-halide PSCs, [94,163] and can be found in state-of-art devices (e.g., PCE > 14% reports). [37,38] We hypothesize that the inclusion of Br or Cl in perovskite can mitigate the I 2 -driven degradation pathways illustrated in Figure 2. This is most likely due to the weaker reducing strength of HBr and HCl relative to HI. In fact, the inclusion of bromide in either a PEA [154] or Sn-halide [155] sources have recently started to appear in the literature. However, further research is required to elucidate the role of halide-sources on the chemical and physical properties of Sn perovskites. Such knowledge will be crucial to the development of strategies to stabilize the Sn PSCs.
ii) A broad range of additives has been tested to make Sn-halide PSCs more stable and efficient; SnF 2 is typically employed in high-performance PSC precursor solutions. Additionally, other additives such as crystallization regulators (e.g., SCN salts, EDA or ionic liquids) and antioxidants (e.g., metallic Sn powder) are becoming an increasingly frequent pathway toward high efficiency/stability and will continue to play an important role in the future of Sn-halide perovskite photovoltaic research. Based on latest developments, two promising avenues will be the use of additives with specific iodine-scavenging activity (see Figure 2) and the development of perovskite doping strategies (e.g., molecular dopants or electrical doping) [191][192][193] to fine tune the electronic properties in these materials.
iii) Mitigating V oc loses is critically important to the realization of high performance of Sn-halide PSCs. The main reason behind the deficit in V oc is nonradiative recombination which is due to high defect density in the bulk perovskite film and/ or mismatched energy level alignment at the interfaces. Both additive and interface engineering strategies (or combination of the two) provide promising routes to address the challenges posed by low V oc . For example Chen et al. [194] recently reported on the mitigation of V oc losses via simultaneous suppression of bulk and interface nonradiative recombination. This was achieved through the utilization of an EABr additive which was found to reduce the defect density (bulk and surface). Other strategies discussed include passivation (usually via additive or interface engineering), [154,158,190] or by replacing the commonly used electron-transport layer PCBM with ICBA. [38,59] iv) Sn precursor quality is of utmost importance to obtain reasonable optoelectronic properties, device performance and stability in Sn-halide perovskites. [69] Recent findings have pinpointed SnI 2 ·(DMSO) x adducts as a promising route to obtain more favorable morphologies. [38] Alternatively, commercial SnI 2 purification (e.g., via vacuum sublimation, recrystallization) represents a readily applicable method to overcome reproducibility issues in most labs. The quality/purity of Sn-based precursors vary dramatically within the field and a variety of suppliers are reported in the literature. In addition, some research groups synthesize and purify their own Sn-sources. It is also not uncommon to find studies that neglect to report the finer details of precursors used. In particular, it is essential that purity and the company (i.e., supply source) used to purchase the material are noted to further improve the reproducibility of Sn-halide PSCs.
v) Inverted, p-i-n device architectures are the most common structure used for high efficiency Sn-halide PSCs. Their dominance in Sn-halide PSCs stems from p-i-n architecture favorable band alignment and suppressed hysteresis relative to n-i-p devices. In fact, reduced hysteresis is a key advantage of p-i-n architectures across both the Pb-halide and Sn-halide PSCs. [195,196] Highly promising improvements in photovoltaic parameters have already been reported by carefully selecting the charge transport layers in these architectures for better band alignment; although expensive, ICBA is currently a hotspot in the Sn-halide perovskite field due to the much higher V oc it yields. [37,38,59] We anticipate future advances from the synthesis of low-cost ETLs with higher-lying LUMO levels versus typically used PCBM. With regard to HTLs, it has been shown recently that highly-selective p-type contacts yield higher ambient stability in Sn-halide perovskites. [69] In particular, the efficient withdrawal of perovskite free holes by the HTL can reduce Sn 4+ states back to Sn 2+ and improve stability in accordance to the mechanism proposed in Figure 2. This points toward an important design rule to mitigate material decomposition, which we expect to provide novel HTLs for next-generation Sn-halide PSCs. This is particularly important considering that the current top-choice HTL material (i.e., PEDOT:PSS) is known to cause long-term stability issues in PSCs; [197] hence, new alternatives will be needed for Sn-halide perovskite devices to achieve high operational stability under harsher conditions (e.g., damp heat).
vi) Pb-halide PSCs have benefited from the use of interfacial engineering to achieve better carrier management and fewer losses.  We expect similar strategies to be directly applicable to both charge transport layer/Sn-halide perovskite interfaces. For example, sandwiching the perovskite active layer between passivation interlayers (e.g., PMMA) [172] will not only improve device performance but can also introduce additional functionality to mitigate degradation (i.e., hydrophobicity to stop water-induced degradation pathways). Employing novel nanomaterials that can extract charges from the solar cell more efficiently are also interesting routes to further improve device performance. We recently incorporated the novel nanomaterial, phosphorene nanoribbons (PNRs), in Pb-halide PSCs and demonstrated more effective hole extraction. [200] Given this was the first evidence of how the intrinsic properties of PNRs were beneficial to optoelectronic applications, their employment in Sn-halide PSCs and beyond opens up further research avenues. [201] Other potential avenues which are yet to be applied in Sn-halide PSCs to improve stability, are bilayer electrode evaporation [202] or carbon based top contacts. [203] Metal top contacts such as Ag are known to react with halides from the perovskite layers that diffuse to form AgI, which effects the overall stability of the device. [204] Carbon-based electrodes have been shown to work without underlying transport layers therefore making them an interesting option for Sn perovskite solar cells. [203,205] This is particularly attractive as organic ETL or HTLs relatively low thermal stability. Therefore, substituting common metal top contacts with carbon should further improve long-term stability of Sn-halide PSCs. Recently, we developed a method to improve the crystallization and stability of both MA-based as well as pure FAPbI 3 perovskites using solvent vapor annealing. [206][207][208] These approaches involved controlling the rate at which the solvent vapor passes through a chemical vapor deposition (CVD) reactor to allow specific control on crystallization and nucleation dynamics of the perovskite film. In addition to producing efficient and devices as the end product, the high degree of control over the crystallization would be a significant advantage if applied in tin Sn-halide perovskites and thus should be explored.
Lastly, we emphasize that this list of strategies are compatible with each other. We therefore envisage future breakthroughs in Sn-halide PSCs not to be the result of a single modification, but a combination of them: novel holistic approaches will drive forward the already inaugurated new golden age of Sn-halide PSCs.
Luis Lanzetta is a Postdoctoral Fellow in the KAUST Solar Center at King Abdullah University of Science and Technology. He obtained his Ph.D. in chemistry at Imperial College London in the group of Prof Saif A. Haque, working on the synthesis and characterization of lead-free perovskites for use in photovoltaic and light-emitting applications. His research focuses on molecular doping approaches for halide perovskite semiconductors and the study of degradative process in nanostructured hybrid materials for the design of stable energy conversion applications. Saif A. Haque is a Professor of Chemistry at Imperial College London. He is a physical chemist with a particular interest in nanomaterials, electronic materials, photochemistry, solar energy conversion, and renewable energy. His research is currently addressing the functional characterization and development of solar cell absorbers and devices based upon solution processable hybrid inorganic-organic semiconducting materials, inorganic metal chalcogenides, quantum dots and perovskites. Prior to becoming a professor, he was a Royal Society University Research Fellow at Imperial between 2005 and 2013.