A defect is often induced by the fast oxidation of Sn⁺² due to its fast kinetics of nucleation and growth in perovskites; therefore, additives that often reduce the fast oxidation and improve the film morphology (compactness) are required. Sn halides (SnF₂, SnCl₂, SnBr₂, and SnI₂) and several organic molecules have been shown to prevent oxidation and enhance the performance of TPSCs. Furthermore, Sn halide additives are able to compensate for Sn vacancies in the films, improving the film morphology, reducing the likelihood of vacancy formation, and reducing the background hole density ^[1][104].

One of the first reports of reducing agents was implemented in 2012 by Chung et al. ^[2][24] using CsSnI₃ Sn perovskites as an HTM by doping with SnF₂ in dye-sensitized solar cells, which helped in producing a V_OC of 0.42 V and an overall PCE of 0.9%. Following this, in 2014 Kumar et al. ^[3][105] found that incorporating SnF₂ into CsSnI₃ reduces the formation energy of Sn vacancies, leading to less conductivity in CsSnI₃, and as a result, the TPSCs gained a high current density (J_SC) of 22 mA cm⁻². The impacts of SnF₂ doping in FASnI₃ with 20% mole were confirmed with X-ray diffraction (XRD) data that indicated a reduced amount of Sn^{+4 [4]}[42]. This helped the current density to increase by 10 mA cm⁻². In 2018, Xiao et al. ^[5][106] were able to achieve homogeneous crystal growth and uniform film coverage. They demonstrated that SnF₂ can reduce Sn vacancy (V_Sn) concentrations by boosting their formation energy. Following that, Hartmann et al. ^[6][107] studied the electronic structure of CsSnBr₃ and observed that Sn oxidation was inhibited by the addition of 20% mole SnF₂. In 2016, Ma et al. ^[7][26] showed that SnF₂ had the effect of a distinguishable increase in carriers’ lifetime from 0.7 ns to 6 ns. Additionally, hole diffusion length was estimated to increase substantially, as a result of the addition of SnF₂, whereas the electron diffusion length remained unchanged. SnF₂ is commonly used in most tin-based photovoltaic systems for easy optimization of Sn perovskites.

An additional reducing agent commonly used in Sn perovskites is SnCl₂ ^[8][108]. It was used to increase the stability of the Sn-based devices in an HTM free structure. Using X-ray photoemission spectroscopy (XPS) analysis on CsSnI₃ perovskite samples treated with the addition of 10 mol% of SnCl₂, they found that SnCl₂ was present at the perovskites’ surfaces, and that the SnCl₂ layer could act as a dryer to improve the stability of CsSnI₃. Interestingly, after 5 months of storage under a nitrogen environment, the PCE was observed to increase, along with the V_OC and FF. This improvement in the performance over time can be explained by the SnCl₂ doping on the electron-transport layer of the used ETM (i.e., PCBM). In addition, they evaluated different tin halide additives (SnCl₂, SnBr₂, SnI_2, and SnF₂) to see how they could affect HTM-free TPSCs (ITO/CsSnI₃/PC₆₁BM/BCP/Al). Among the tested devices, with the SnCl₂ additive, a PCE of 3.56% was the best, and SnCl₂ resulted in the highest film stability. Performance improvements may be attributable to the enhancement of PCBM ETM crystallization under light illumination.

Hydrazine has long been used in chemical synthesis to prevent oxidation (reducing agent). Additionally, hydrazine’s highly volatile nature makes it an easy agent to be introduced as a reducing atmosphere. Song et al. ^[9][60] introduced hydrazine vapor atmosphere prior to the spin-coating process of the perovskite precursor. The films were formed in hydrazine atmosphere resulting in reduced defects, oxidation, and therefore better performance. Similarly, Kayesh et al. ^[10][109] were able to minimize the concentration of Sn⁺⁴ by 20% and significantly suppress carrier recombination during the fabrication of FASnI₃ perovskite films, by incorporating hydrazinium chloride (N₂H₅Cl) into a single precursor solvent system. A high PCE with significantly enhanced V_OC and pinhole-free FASnI₃ perovskite films were achieved. Li et al. ^[11][110] reported a solution–deposition method for the fabrication of MASnI₃ that included hydrazinium iodide (N₂H₅I) with SnI₂ precursor. A mesoporous TPSC with a PCE of 7.13% was achieved.

Hypophosphorous acid (HPA). In the synthesis of tin-based perovskites, HPA has long been used as a common reducing agent. In most circumstances, HPA is utilized as an assisting reducing agent in antioxidation when powerful agents such as hydroiodic acid (HI) or SnF₂ are present, eventually stabilizing the process. Researchers used HPA as a coordinating agent in the CsSnIBr₂ production process, which allowed them to speed nucleation while restricting Sn⁺² oxidation. Charge carrier density and Sn vacancy levels were lowered as a result of the HPA integration ^[12][111].

2,2,2-trifluoroethylamine hydrochloride (TFEACl). In combination with SnF₂, 5 mol% of TFEACl was found to improve and enhance FASnI₃ solar cells ^[13][100]. The work function of perovskite films may be adjusted by adding Cl. Therefore, the perovskite films are better aligned with the charge transport layers. In addition, light soaking stability was found to be improved, which all resulted in improved device performance and charge collection.

Gallic acid (GA).Wang et al. ^[14][112] used the antioxidant GA as an additive together with excess SnCl₂. GA was found capable to form a complex with SnCl₂ that is evenly distributed on perovskite grains. The characteristics of the GA can be derived from the aromatic ring’s hydroxyl groups (–OH), which can donate electrons and absorb oxygen by hydrogen atoms. The SnCl₂ layers present atop perovskite grains were expected to result in a wider bandgap compared to the bulk. After 1000 h of air exposure, unencapsulated GA-based devices preserved more than 80% of their initial PCE, which is one of the highest reports. Moreover, the solar cells with GA exhibited a high PCE of 9.03%.

Ascorbic acid (AA). It is a simple but effective additive that inhibits the oxidation of Sn⁺² also regulates its film crystallization and creation, and can be utilized to build polymer-stabilized Pb/Sn binary PSCs ^[15][113], enhancing the optoelectronic quality of dual, perovskite films greatly. The resulting MA_0.5FA_0.5Pb_0.5Sn_0.5I₃ film’s photogenerated carrier lifetime (183 ns) demonstrates this. As a result, MA_0.5FA_0.5Pb_0.5Sn_0.5I₃ treated with AA achieved a high PCE of 14.01% and a higher stability than the control device employing the SnF₂ additive, outperforming it. This research proposes a novel method for improving the performance and obtaining more stable Pb/Sn-PSCs.

There are many surface modifiers for TPSCs that are applied using different methods and can be applied before or after the perovskite layer deposition. Controlling the surface terminations can majorly affect the stability and morphology ^[16][17][18][59,114,115].

One of the common examples is the introduction of antioxidant-carrying 4-fluorobenzohydrazide (FBH) on top of FASnI₃ perovskite films ^[19][116]. The C=O group in such modifier was observed to interact with Sn⁺² and promote the formation of largely oriented perovskites. Additionally, it was found that FBH results in the reduction in Sn⁺⁴ by the hydrazide group. According to the performed density functional theory calculation, the oxygen absorption barrier is increased after the FBH modification, resulting in a delay in the oxidation process. As a result of such, the interface modifier (capping layer) in the PCE increased from 8.34% to 9.47%.

Similarly, a dense layer of Al₂O₃ as a buffer layer separating perovskites and HTL can prevent degradation from moisture ^[20][117]. Cetyltrimethylammoniu bromide (CTAB) doped zirconium oxide (ZrO_x) can also act in a similar manner ^[21][118]. In general, the switch from Pb to Sn affects the morphology severely due to the higher Lewis acidity of Sn⁺² compared to Pb^{+2 [22]}[50]. Therefore, one of the major goals when making Sn films is to achieve compact and pinhole-free thin films.

Cations play a major role in Sn halide perovskites thin films in regard to lattice strain engineering. Nishimura et al. ^[23][119] investigated the relationship between lattice strain in tin-based perovskite films and TPSCs efficiency. They prepared tin-based Q_x(FA_0.75MA_0.25)_1−xSnI₃ perovskites, where Q is various cations with different ionic radii such as Na⁺, K⁺, Cs⁺, BA⁺, and ethylammonium (EA⁺). The link between actual measured lattice strain and solar performance was explored. As the tolerance factor approached unity, the lattice strain decreased (measured by the Williamson hall plot of XRD data). As the lattice strain decreased, the performance of the Sn perovskites was enhanced. EA_0.1(FA_0.75MA_0.25)_0.9SnI₃ with the lowest lattice strain yielded the best performance, because carrier mobility increased as lattice strain decreased. These lattice strains would disrupt carrier mobility and reduce solar cell performance ^[23][119]. The lowest lattice strains were found for Cs-0.1 and EA-0.1 and provided the highest mobility of about 43 cm² V⁻¹·s⁻¹; however, in the case of Na-0.1, the lattice strain was found to be higher and therefore the mobility was down to 4.6 cm² V⁻¹·s⁻¹.

Sun et al. ^[24][120] added a bi-linkable reductive cation (i.e., formamide (FM)), into FASnI₃ to function as molecular glue for improving the stability and performance of TPSCs by the formyl group (–CHO) and amine group (−NH₂). They revealed that the NH₂ and C=O groups in FM are capable of interacting with FA⁺ and Sn⁺² through hydrogen bonds and Lewis acid–base coordination, respectively. This resulted in a greater grain size, preferred orientation, lower defect density, and better film stability. The TPSC device based on 10% of FMI resulted in a 40% increased PCE from 5.51% to 7.71% with notable enhanced stability, retaining its initial PCE after one year in N₂ without encapsulation.

Jokar et al. ^[25][17] used guanidinium cations (GA⁺) as an additive with at least 1% of ethylenediammonium diiodide (EDAI₂) to form a FASnI₃ films, and this resulted in the remarkably improved performance of TPSCs. A high PCE of 8.5% was achieved and increased to 9.6% after 2000 h of storage in a glove box. Additionally, the resulting perovskite operated for almost an hour under continuous illumination and for six days in air without encapsulation ^[25][17].

Solvents and secondary solvents can play a major role in the formation of pinholes in thin films, especially thin films that contain organic materials ^[26][121]. One common way to produce a compact Sn film is by different solvents, such as dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) ^[27][28][122,123]. Post-treatment with antisolvents also plays a crucial role in the quality of tin-based perovskite films ^[28][29][123,124].

It was reported that when perovskite films are formed without any anti-solvent dripping, the precursor is far from supersaturated until the solvent totally evaporates and, as a result, the nucleation site density is rather low, resulting in a flower-like film with limited surface coverage ^[28][123]. Therefore, different antisolvents (diethyl ether (DE), toluene (TL), and chlorobenzene (ClB)) were tested. The size of pinholes in the FA_0.75MA_0.25SnI₃ films dripped by DE is less than that of pinholes in the film dripped by TL, but the quantity of pinholes is greater. In the case of ClB, the film has a uniform surface with full coverage and clear grain features. One reason for this could be related to its high boiling point of 131 °C, which has a slower evaporation rate, which extends the crystal development period during the thermal annealing process.

Hao et al. ^[4][42] investigated the DMSO function as a the Lewis base solvent to adjust the crystallization rate of MASnI₃ perovskite by Lewis acid–base interaction. When DMSO molecules react with Lewis acid SnI₂, they form the SnI₂-3DMSO intermediate adduct, which effectively slows down the interaction between MAI and SnI₂, resulting in an enhanced MASnI₃ film. Similarly, the generation and orientation of FASnI₃ perovskites was controlled when poly-ethylene-co-vinyl acetate (PEVA) was introduced into the anti-solvent ^[30][125]. At the grain boundaries, the C=O groups of PEVA molecules cause a complexation between Lewis acid–base interaction and Sn⁺². This results in larger grains and lowered surface defects of FASnI₃, leading to enhanced devices’ performance.

In fact, using different solvents incorporated with different additives as well as cations with different radii often has a direct influence on the dimensionality of the perovskite films, which is on its own is a major research and development direction. Lower dimension perovskites seem to be more stable than three-dimensional ones, so these are expected to improve tin-based perovskites’ stability ^[31][126]. The three-dimensional structure of perovskites could be decreased to two- or one-dimensional by substituting the bulky organic ammonium ions at the A-site in the perovskite lattice or by inserting 2D materials in the precursor solution ^[32][33][127,128]. 2D perovskites seems to have exceptional optoelectronic properties and therefore may make them excellent photovoltaic materials ^[34][129]. 2D perovskite reduces moisture as well as oxygen from going inside the film ^[35][36][130,131]. It also can reduce defects, resulting in a low amount unwanted self-doping ^[37][132]. Many researchers are currently investigating 2D TPSCs to increase their stability based on the benefits of lower dimension perovskites.

Similarly, low-dimensional Sn perovskites have become a topic of interest in TPSCs due to their ability to improve device performance and stability. In 2017, Liao et al. ^[38][133] incorporated phenylethylammonium (PEA) into FASnI₃ perovskites and they achieved perpendicularly oriented, low-dimensional Sn perovskite films with remarkably enhanced stability and a PCE of 5.9%. In 2020, a report by Liang et al. ^[39][134] utilizing indene-C₆₀ bisadduct (ICBA) as an ETM found that their Sn-based perovskite (PEA_x FA_1−xSnI₃) with PEA incorporation formed a low-dimensional perovskite with reduced defect concentrations, which resulted in a high V_OC of 0.94, a record PCE of 12.4%, and better stability (shelf stability of 3800 h).

There are many additives that were reported in recent years to enhance the performance of TPSCs ^{[40][41][42][43][44]}[55,72,135,136,137]. However, here (Table 1) thwe researchers ssummarize most of the very recent and noticeable additives and correlated outstanding device performance of lead-free TPSCs that were reported during the last two years. Table 2 provides a list of the most recent and highly performing surface/interface modifiers that were applied in lead-free TPSCs. Table 3 provides a list of most recent highly performing mixed Pb-Sn perovskites with Pb ≤ 50% and it includes devices with both perovskite additives and surface/interface modifiers. The table also provides structure and reported stability.

It is noteworthy to mention that the reasons behind the enhancement of the performance of the devices listed in the tables are often related to the same reasons. Here, the rwesearchers briefly list the reasons behind the enhancement in general, which is important for further future consideration and development to achieve even higher performance: (i) oxidation, (ii) reduced defects, (iii) controlled crystallization, (iv) morphology (compactness and pinholes, strain relaxation), (v) charge diffusion and extraction (mobility, carriers density, energy levels, recombination), (vi) built-in electric field (gradient vertical perovskite growth), (vii) better choice of cations (reduced or eliminated MA⁺) (iix) hydrophobicity, and (ix) passivation of the acidic and hygroscopic surface of the commonly used PEDOT:PSS HTL (or alternative or HTL or SAMs).