Low-Temperature Processing Methods for Tin Oxide as Electron Transporting Layer in Scalable Perovskite Solar Cells

2.1.1 Spin-Coating of SnO₂ Inks

The spin-coating technique serves as one of the most conventional methods of SnO₂ growth holding out the advantages of controlling the thickness of deposition and allowing the combination of different chemicals.^{[ 44, 45 ]} Classified among the inks utilized in this method is a precursor ink, e.g., the ink obtained from the sol–gel method, which contains tin chloride salt along with a sulfur source in solvents including ethanol, water, and isopropanol (IPA). In this procedure, ever since the ink is prepared, the desired layer is deposited on the substrate through the spin-coating method. However, as regards the deficient crystallization and the presence of organic and chloride residues in the prepared layer of the precursor ink, posttreatment methods such as thermal annealing or UV-ozone (UVO) are required. Yan et al. first used SnCl₂.2H₂O as tin source to prepare ETL at 180 °C and reached a PCE of 17.2%, thus promoting the rapid development of LT-SnO₂. Calabrò et al.^{[ 46 ]} could achieve 17.3% PCE making use of SnO₂ ETL prepared through spin-coating of SnCl₂.2H₂O precursor ink in ethanol. Besides, Murugadoss^{[ 47 ]} investigated the effect of ethanol and water solvents in SnO₂ precursors and recognized the superiority of ethanol solvent compared to that of water. In this regard, the utilization of water solvent provokes nonuniform coating of SnO₂ layer due to the formation of Sn(OH)₂ nanoparticles. Moreover, some reports^{[ 40, 48-50 ]} refer to the aging of the precursor ink prior to the deposition, which often appears to improve the quality of the SnO₂ layer. Dong et al.^{[ 48 ]} could manage to realize 20.5% PCE utilizing SnO₂ ETL prepared through spin-coating of SnCl₂.2H₂O precursor ink in ethanol aged for 24 h at room temperature.

Researchers resolved the issue of enhancing the crystalline quality and removing the organic and chloride residues into the application of nanoparticle inks. In this method, SnO₂ nanoparticles are primarily synthesized through solution-based processes including hydro/solvothermal,^{[ 51 ]} microwave,^{[ 52 ]} and grinding.^{[ 53 ]} After subsequent rinsing, the resulting nanoparticles are dispersed in solvents such as ethanol, isopropanol, or water and then deposited through spin-coating on the substrate. Miyasaka et al. first used SnO₂ nanoparticles, which is purchased from chemical company, to prepare ETL and achieved a PCE of 13.0%. In the same manner, Zhu et al.^{[ 51 ]} first synthesized SnO₂ nanoparticles using hydrothermal method and then spin-coated nanoparticles dispersed in IPA on C₆₀ as a bilayer ETL in an inverted structure and achieved the PCE of 18.8%. Abulikemu and coworkers^{[ 52 ]} utilized the microwave method to synthesize SnO₂ nanoparticles (with an average particle size of 3 nm). In the microwave method, the synthesis of SnO₂ nanoparticles is progressed more rapidly than that of the hydro/solvothermal method due to the high temperatures by microwave radiation, and besides, it enables the precise monitoring of the reaction parameters, i.e., temperature, time, and pressure. Furthermore, the grinding technique is based on physical ball milling which the latter has been broadly utilized in mechanical alloying processes. In this technique, the ball is employed not only as a mill but also as a means to form a cold welding between the powder particles so as to fabricate the desired alloy. In this light, Singh et al.^{[ 53 ]} similarly utilized this novel procedure to produce SnO₂ nanoparticles. They prepared nanoparticles dispersed in IPA solvent and deposited on the FTO substrate through spin-coating method and could obtain the devices of 16.46% PCE. Moreover, this group could manage to further increase the PCE to 21.09% through the addition of a compact layer of SnO₂ prepared via the precursor ink spin-coating method beneath the grinding SnO₂ layer.

In recent years, researchers have been using a commercially available colloidal solution of SnO₂ nanoparticles, often from Alfa Aesar, to prepare LT-SnO₂ ETL. As indicated in Figure 6, more than half (54.5%) of the LT-SnO₂ research articles reported since 2015 have employed this solution. In the commercial Alfa-SnO₂, a small amount of KOH has been added to the solution for colloidal stability. The presence of this substance triggers the passivation of perovskite/ETL interface, which results in remarkable performance and significant reduction of PSCs hysteresis built upon Alfa-SnO₂ ETL.^{[ 26 ]} The desired layer prepared using the Alfa-SnO₂ is uniform and pinhole free through which Jiang et al.^{[ 54 ]} could fabricate PSCs of 20.54% PCE based on SnO₂ ETL making use of them. In addition, in 2017, they reported PSCs of an active area of 0.07 and 1 cm² rendering PCEs of 21.6% and 20.1%, respectively.^{[ 35 ]} Xu et al.^{[ 55 ]} used DMF and ethanol as Alfa-SnO₂ aqueous diluents to improve the morphology of the ETL layer. The presence of ethanol as the diluent (Water: Ethanol:1: 1) has not only increased the stability dispersion of the solution but also increased its wettability on the substrate, whereas DMF was the opposite and reduced the wettability of the ETL layer and the crystalline quality of the perovskite layer. They compared three different solvents of pure water, water–ethanol, and water–DMF and found that when ethanol was the solvent, SnO₂ films were formed with uniform grain sizes without agglomeration, yielding 18.84 % reached a yield of 16.35% under the same conditions and with water as a diluent solvent.

The reproducibility of the SnO₂ layer process is a requirement for the commercialization and large-scale production of PSCs. Wang and colleagues^{[ 56 ]} used Alfa-SnO₂ aqueous solution to improve ETL quality by adding oxygenated water (H₂O₂). They showed that the addition of H₂O₂ reduced the density of oxygen traps and defects and increased electron extraction, as reported by a yield of 22.15% with a 94% stability improvement after 1000 h.

Nevertheless, part of the challenges of solution-based methods for tin oxide films is the low quality of the crystallinity of the layer, which causes a relative decrease in electron mobility. The utilization of small colloidal nanoparticles of quantum dot (QD) effectively has been successfully addressed this barrier. In 2018, Yang et al.^{[ 29 ]} could synthesize SnO₂ QDs forming energy levels that well-aligned to that of the perovskite layer and demonstrated long-term stability of up to several months. The Xu group^{[ 57 ]} also synthesized SnO₂ QDs using radiowaves in a fast reaction (3 minutes) without additives and reported a PCE of 20.24%.

2.1.2 Chemical Bath Deposition

CBD is a solution-based method consisting of two stages of nucleation and particle growth and is based on the formation of a solid phase of the solution. In the CBD method, the substrate is immersed in a solution containing precursors. This method is influenced by parameters including bath temperature, pH and concentration of the solution, and time which does not cause physical impairs to the substrate. Furthermore, the CBD method demonstrates strong points over other techniques such as lower cost and temperature as well as higher quality and production volume. Barbe et al.^{[ 58 ]} employed a single-stage CBD method with SnCl₄ precursor at 55 °C without requiring annealing in order to prepare the amorphous LT-SnO₂ layer and achieved a PCE of 14.8%. Also, Anaraki et al.^{[ 59 ]} innovated the combined method of spin-coating and CBD technique to produce SnO₂ bilayer ETL in which the spin-coated film played the role of seed in the CBD process. By means of this combined method, they could manage to achieve devices of superior photovoltaic parameters, longer stability, and lower hysteresis exhibiting a PCE of 20.73%. One of the reasons for using CBD method in LT-SnO₂ synthesis is to reduce the agglomeration of SnO₂ precursor, but the use of CBD is limited due to the time-consuming synthesis process. Ko et al.^{[ 60 ]} attributed the cause of agglomeration to the concentration and effect of common ions. They investigated SnO₂ provinces by rapid CBD method at low temperature and control of the precursor concentration. They deposited the SnO₂ film at 70 °C by CBD and reported a PCE of 20.12%. Surprisingly, Seo et al. conducted a two-step CBD method to tune the electronic, physical, and morphological properties of the obtained SnO₂ film by adjusting the pH and time of reaction, thus leading to a breakthrough certified PCE of 25.2% (0.0937 cm²) and 23.0% (0.984 cm²). However, due to large consumption of solution during the preparation, whether this method is suitable for large-scale production remain a challenging problem.

2.1.3 Electrochemical Deposition

Regardless of the trends of the methods mentioned earlier, they are not suitable for large-scale production. Consequently, the short-reaction electrodeposit technique has been regarded in the industry for converting electrical energy into chemical potential. Apart from the low-temperature production capability, this method initiates excellent adhesion between the deposited layer and the substrate and is an economical way to produce a vacuum-free uniform surface coating. Besides, morphology and layer thickness might be controlled through modifying the deposition conditions. The electrochemical deposition method takes place in a three-electrode cell containing a tin chloride precursor solution, e.g., SnCl₂, which includes a platinum counter electrode, an Ag/AgCl reference electrode, and a working electrode as a substrate on which the deposition is directly applied. Therein, the deposition temperature and duration (time) are of paramount importance for layer properties where the best reported temperature was 50 °C not requiring post-deposition annealing.^{[ 61, 62 ]} Chen et al.^{[ 62 ]} could achieve the PCE of 13.88% using this method. Likewise, Lam et al.^{[ 61 ]} recognized the 14.3% PCE under similar conditions through the addition of a PCBM layer between SnO₂ ETL and perovskite.

2.1.4 Solution Combustion

The solution combustion method is based on the exothermic disposition of self-reactions, and the heat is generated through the exothermic redox chemical reaction, although this method is mostly employed for the NiO_x synthesis. In this manner, Liu et al.^{[ 63 ]} utilized different ratios of urea and acetone acetyl (ACAC) as dual-fuel and compared it with the ACAC single-fuel case, in which the urea behaved as an oxidizer. They could produce SnO₂ nanoplates in the single-fuel case and SnO₂ spherical nanoparticles in that of the dual-fuel case and achieve 12.93% PCE using dual-fuel combustion method.

2.1.5 Spray Coating

The utilization of spray pyrolysis has been quite widespread in the preparation of thin films. In this method, the solution droplets (aerosols) are expelled from the spray gun into the hot surface at high velocity and deposited on the surface with desired uniformity and surface coverage rendering thicknesses of even to 10 nm. In comparison to that of spin-coating method, spray pyrolysis provides the controlling and manipulation of the deposited layer and produces a continuous, uniform coating of higher optical properties. Wang et al.^{[ 64 ]} indicated that the use of chloride precursors in the SnO₂ layer deposition by spraying results in undesirable optical properties. To overcome this issue, they employed aqueous SnO₂ nanoparticle ink for ETL deposition and could acquire a PCE of 18.2%. Besides, Taheri et al.^{[ 65 ]} could realize PCSs of 15.4% and 16.7% PCE, respectively, applying a comparison between those of ethanol and aqueous SnO₂ nanoparticle ink precursors as the solution utilized in the spray pyrolysis method.

2.1.6 Hydro/Solvothermal Method

In addition to the other solution-based techniques, the hydro/solvothermal method has been introduced as a deposition applied directly to the substrate. This method might be considered as a set of chemical reactions in a closed chamber in the presence of aqueous or nonaqueous solvent at temperatures beyond the boiling point of the solvent. Moreover, this process also takes place at high pressures and the selected temperature depends on the reaction required to obtain the targeted material. Thermodynamic parameters such as temperature, pressure, and time might enhance the solubility and assist the growth process. The advantageous features of this method include high efficiency, desired controllability, user-and-environmental-friendliness, manufacture of uniform-size-distributed products, and less energy consumption. With the aim of tackling one of the remarkable challenges of PSCs, i.e., low stability, Liu et al.^{[ 66 ]} utilized hydrothermal growth method could obtain nanohierarchical SnO₂ ETLs as compact and mesoporous ETLs, which could prevent moisture penetration and improved the stability of devices.

2.1.7 Print

Printing is a versatile technique that pushes the envelope of laboratory-scale deposition works much further toward large-scale and enables the commercial production of PSCs and modules involving deposition on flexible substrates such as PET and PEN. The foremost printing methods include: Roll-to-Roll Gravure (R2R Gravure), slot-die coating, and inkjet printing. What is more, the noncontact feature of this technique offers privilege of substrate delegation including rigid (glass, FTO, ITO, etc.) and flexible (plastic/metal foil, etc.). Among them, the R2R Gravure method is the most far reaching in the industry. In ETL printing method, the concentration of precursor and the solvent type are of fundamental importance in forming a uniform and pinhole-free layer. Bu et al.^{[ 26 ]} could achieve the PCE of 15.22% through slot-die coating of Alfa-SnO₂ solution in water/IPA solvent (1:1 ratio) on large-area flexible PET/ITO substrate with 16.07 cm² (active) area. In a similar vein, Gong and coworkers^{[ 67 ]} made use of Alfa-SnO₂ solution in water and water/IPA solvent (1:1 ratio) through R2R Gravure printing method on flexible PET/ITO substrate at 0.07 cm² scale and could achieve 11.2% and 16.95% PCE, respectively.

2.2.1 Atomic Layer Deposition

ALD is a vapor-phase vacuum-based pulse technique that could be employed to produce high-quality metal oxide thin mineral films. Principally, ALD is performed in two procedures: thermal-ALD (T-ALD) at relatively low temperatures (<200 °C) and plasma-assisted-ALD (PA-ALD) in the presence of plasma. In the first half of the pulse, both of which are based on the self-limiting reactions between the surfaces edged –OH and the Sn precursor, whereas in the second half, the main reactant in the T-ALD is water (H₂O) and in the PA-ALD is oxygen (O₂).^{[ 68 ]} Moreover, the manufacturing of thin and controllable films of up to monolayer order could be realized through ALD method. Due to the presence of plasma in the PA-ALD method, the heating temperature of the substrate during the process is lower and the plasma employed in which is the reaction outputs of O₂, N₂ and H₂ gases or their combination. Lee et al.^{[ 69 ]} could fabricate uniform and pinhole-free SnO₂ layers at 120 °C using T-ALD method and exhibit a PCE of 18.37%, which could be further elevated to 20.03% through adding a compact TiO₂ layer under that of the SnO₂ layer. Likewise, Wang and coworkers^{[ 70 ]} could realize the PCE of 18.8% and 16.8% utilizing PA-ALD method for SnO₂ deposition layer at 100 °C on rigid and flexible substrates, respectively. In turn, Kuang and coworkers^{[ 68 ]} employed RF with oxygen plasma (i.e., RF inductively coupled O₂ plasma) as a reactant to fabricate the SnO₂ layer through ALD. As a versatile and robust deposition technique, ALD is capable of producing high-quality inorganic metal oxide thin films. In the first half of the cycle, self-limiting reactions take place between the surface leading to -OH groups and the Sn precursor. Subsequently, in the second half of the cycle, the main reactant in T-ALD and PA-ALD is water (H₂O) and O₂, respectively. The utility of an O₂ plasma reactant renders the deposition of SnO₂ thin films at low temperatures possible. Therein, the research group utilized tetrakis(dimethylamino)tin (TDMASn) as a Sn precursor as well as O₂ plasma to cleanse the substrate surface (ITO) prior to the deposition. Besides, after the ETL deposition and right before the perovskite layer deposition, the SnO₂ layer was exposed to O₂ plasma. In conclusion, champion PSCs showing a PCE of 17.5% using SnO₂ deposited by O₂ plasma-assisted ALD at 50 °C and 17.8% at 200 °C were obtained.

2.2.2 Pulsed Laser Deposition

In pulsed laser deposition (PLD), once the laser pulses hit the solid target material, the surface of which becomes so hot that leads to local evaporation of the material from the target as a plasma plume, the so-called ablation. In consequence of the collision of the plume with the substrate surface, a thin layer is formed on the substrate surface.^{[ 71, 72 ]} In the light of PLD application, the scattering of the ablated material from the target is avoided that provides favorable conditions for a vacuum-based unspotted deposition. If the laser ablation takes place in the air, the plasma would instantly collide with the air molecules and cease moving further. Moreover, the ablated atoms and ions react easily with oxygen, water, and other molecules in the air in a way that they chemically transform.^{[ 72 ]} What is more, it is possible to grow a broad range of materials extending from dielectrics, semiconductors, and metals, to superconductors via the PLD method. These materials might consist of a single element or a combination of several elements such as oxides, nitrides, carbides, and sulfides. The most prominent cause behind this versatility lies in the nature of the laser pulse, which allows the stoichiometric transfer of the target material composition onto the formed layer far from the thermal equilibrium and underoptimized conditions.^{[ 73, 74 ]} Besides, the PLD is a physical deposition technique for the preparation of thin films at low temperatures, which avoids the requisite of further thermal posttreatment. Theretofore, this method holds the potential to be applied for SnO₂ deposition on flexible substrates. Chen et al.^{[ 75 ]} utilized the PLD and could achieve devices of rigid and flexible substrates demonstrating PCEs of 17.29% and 14%, respectively.

2.2.3 E-Beam Evaporation

E-beam evaporation is a vacuum-based method performed by means of an electron beam. The energy required for evaporation is provided through the transport of energy from the electron beams to the target material. The presence of air molecules in the route of metal vapor transport from the target material to the substrate might decrease the deposition rate and prevent the formation of compact coating, indicating the essentiality of a high vacuum. The E-beam evaporation method has been proven to enable the large-scale and cost-cutting production of LT-SnO₂ with uniform coverage. Thereupon, SnO₂ films prepared through this method demonstrate high electron mobility, desired antireflection, and directional crystallization that takes a leading role in high-efficiency PSCs.^{[ 76 ]} Ma et al.^{[ 77 ]} utilizing LT-SnO₂ prepared through E-beam evaporation method as ETL in PSC could obtain the PCE of 18.2% on a large active area of 1 cm². Since it works at vacuum condition, time and energy cost should be considered for the industrial production.

2.2.4 Sputtering

The sputtering process involves the ablation of target atoms/molecules through the plasma of an inert gas accelerating under the field and deposited on the substrate. In the same manner to that of other vacuum-based physical deposition methods, the sputtering method comprises three steps: ablation of the target material, transporting of atoms/molecules from the target to the substrate, and formation of a thin film on the substrate. In this method, the physical interaction of particles colliding with the target material serves for the ablation of the target material.^{[ 38, 78 ]} Qiu et al.^{[ 38 ]} employed different ratios of reactive O₂ and inert Ar gas and altered the power and time of the deposition for SnO₂ ETL, which ultimately could manage to achieve 20.2% PCE on a small scale of 0.09 cm² and that of 12.03% on a large scale of 22.8 cm².

2.3.1 Aerosol-Assisted Chemical Vapor Deposition

In the AACVD method adapted from CVD, a thin film of solid material is formed on the substrate as a consequence of a chemical reaction in the vapor phase. In this manner, CVD involves of a stream of gas or gaseous compounds containing a precursor chemical composition in a chamber including one or more hot surfaces (substrates). Conventionally, the CVD method exhibits a high reaction temperature in which AACVD is employed as a modification to reduce the deposition temperature. In the AACVD method, the precursor is generated through ultrasonic using a liquid/gas aerosol and subsequently introduced to the substrate surface. Noh et al.^{[ 79, 80 ]} implied that the application of AACVD at low temperatures to produce SnO₂ leads to oxygen vacancies thereof. Notwithstanding, the report demonstrated that the presence of oxygen vacancies improved charge transport in the SnO₂ layer through elevating the density of free carriers. Consequently, they could successfully obtain the 10.2% PCE on a small scale of 0.07 cm² (utilizing PCBM as an interlayer between perovskite and SnO₂ ETL)^{[ 79 ]} and that of 1.4% on a large scale of 1 cm².^{[ 80 ]}

3.1.1 Thermal Treatment (Annealing)

Due to the widespread utilization of solution-based methods to fabricate the SnO₂ layer, thermal decomposition of Sn precursors and the conversion to SnO₂ require thermal annealing.^{[ 40, 82-85 ]} Thermal annealing under air decreases charge transport imbalance in the ETL/perovskite and HTL/perovskite interface, leading to a reduction in J–V hysteresis.^{[ 86 ]} It should be mentioned that the temperature and environment, i.e., atmosphere pressure and humidity, of thermal annealing is of considerable importance. Annealing temperature should be below 200 °C. Although high-temperature annealing (>400 °C) might enhance the film crystallinity, it could result in more oxygen vacancies as well as pinholes and cracks in the film and generating gaps between the ETL and the substrate due to thermal stress and overcrystallization.^{[ 87 ]} This in turn would increase the recombination and reduces the device performance.^{[ 88 ]} In 2017, Jiang et al.^{[ 35 ]} successfully ascertained 21.52% PCE of the devices using low temperature (150 °C) annealing of spin-coated SnO₂ film from a commercial colloidal suspension of SnO₂ nanoparticles as ETL in the planar PSC cell structure. Far from it, the realization of flexible PSCs requires flexible films such as PET or PEN as substrates incompatible with such high-temperature treatment and thus the deposition of the layers on these substrates must be processed at temperatures below 150 °C.^{[ 23 ]} However, some reported performing at higher temperatures (180 °C^{[ 24-26 ]} and 185°^{[ 27 ]} for PET) and (180 and 200 °C^{[ 28, 29 ]} for PEN). The presence of moisture is a prerequisite for the thermal decomposition process; therefore, the annealing process in practice is carried out under medium-humidity air.^{[ 88 ]} Nevertheless, it has been reported that this thermal process is performed under a special atmosphere such as oxygen, nitrogen, alkali gas, and ethanol vapor.

Thermal Annealing Under O₂ Gas

According to the literatures, annealing SnO₂ films under O₂ makes the oxygen in SnO _x films dramatically increase and the oxygen vacancies to reduce leading to more perfect and larger SnO _x rutile crystals. Besides, annealed SnO _x films under O₂ gas have been reported to have lower surface roughness and higher compactness that favors the perovskite growth. The reduction in surface roughness could be attributed to the reduction of the defect density in SnO _x thin films and the migration of Sn and O atoms to the defects during the annealing process.^{[ 89 ]} Jiang et al.,^{[ 89 ]} after which the SnO₂ films were dried at 100 °C under air, evaluated the thermal annealing effect (e.g., 180 °C) under N₂/O₂ gas flow with different O₂ percentages on the performance of the SnO₂ layer in PSC. They could successfully adjust the oxygen content, electron mobility, and trap state density in SnO _x films (Figure 7 ). To investigate the effect of self-passivation, they analyzed the charge transport dynamics (i.e., transport and carrier recombination processes) of SnO _x -based PSCs. The results demonstrated that the SnO _x films annealed under 100% O₂ gas flow presented the best carrier collection performance in PSCs.^{[ 89 ]}

Two-Step Thermal Annealing

Huang et al.^{[ 91 ]} combined the TiO₂ QDs in the anatase phase along with MXene in solution-based with SnO₂ nanoparticle colloid and deposited through spin-coating method. The prepared ETL was then annealed in a two-step process at 150 °C under air (5 min) and N₂ gas (25 min). According to the results, in the two-step annealing, SnO₂ nanocrystals and quantum dots of TiO₂ were formed and grew due to oxidation in the first step of air annealing. In the second step of annealing under N₂ gas, the crystals resumed growing; however, the source of the oxygen element was supplied by –OH functional groups and preformed TiO₂ crystals due to oxygen deficiency in the annealing atmosphere. This, in turn, has led to the formation of defective TiO₂ crystals that inclined to assemble with vicinal SnO₂ crystals and eventually formed a more stable phase of TiO₂/SnO₂ heterojunctions. Conclusively, utilizing a two-step thermal annealing process with the formation of TiO₂/SnO₂ nanoscale heterojunction and the enhancement of ETL conductivity due to the presence of MXene as conductive bonded bridges as a multidimensional conductive network (MDCN).

Huang and coworkers^{[ 92 ]} also employed a versatile two-step annealing process to improve SnO₂ ETL. In the first step, annealing was performed at 150 °C and then the samples were kept in a reaction chamber containing a certain amount of ammonia solution in which the samples were separated from the solution. Consequently, the chamber was sealed and heated for 2 h at a temperature of 120 °C, during which time the layer was exposed to the alkaline gas produced (Figure 8a). Regarding the corrosion caused by alkaline gas during the second step of annealing, the surface of SnO₂ film was polished, and also ─NH₂ group was chemically adsorbed onto the surface of the SnO₂ layer. The surface polishing resulted in the ETL surface smoother. According to the results, the crystallinity quality of the perovskite film has significantly excelled after applying SnO₂ annealing under alkaline gas. The justification behind these effects was presented in two pathways. As shown in Figure 8b, the ─NH₂ groups adsorbed onto the SnO₂ surface would firmly interact with the perovskite halide anions through hydrogen bonding providing germination sites for further nucleation of perovskite crystals by accelerating atoms in the preheating of perovskite film. As viewed by the classical nucleation theory, during the nucleation process, the reduction in volumetric free energy caused by a temperature decrease could merely recompense 2/3 of the surface energy of the nucleus and the rest has to be supplied through the conditions stemmed from local energy fluctuations of the perovskite films preheating. The presence of hydrogen bonds formed between perovskite halide anions and the ─NH₂ group could efficiently increase local energy fluctuations and accelerate crystal nucleation. Furthermore, ─NH₂ groups are of smaller molecular dimensions than that of the perovskite organic molecule which comes to their aid to incorporate into the inorganic perovskite framework and substitute the perovskite organic molecule at the bottom of the film as the annealing process resumes. As a consequence, the rate of perovskite crystals nucleation would be enhanced and a bridge link between SnO₂ and perovskite is constructed. By and large, utilizing a two-step thermal annealing process with polishing and adsorption of ─NH₂ group, they could manage to improve mobility, charge transport in the perovskite/ETL interface, and crystallinity of the perovskite layer with the achievement of a 21.1% PCE.

3.1.2 UV-Ozone Treatment

Since the conventional methods of the preparation of SnO₂ films demand the application of precursor ink (i.e., tin chloride) that requires high temperature, they are not suitable for upscaling. To tackle this issue, UVO posttreatment has been presented as a suitable method that will further overcome the technical rigors of the manufacturing process and production costs and facilitate the large-scale fabrication of SnO₂ films. Generally, UVO treatment is always considered to remove the surface contaminants, improve the wettability, and change the states of surface oxygen. The major wavelengths of the ultraviolet rays radiated from a well-known low-pressure mercury vapor lamp are 184.9 and 253.7 nm. During the process of formation or decomposition of O₃, atomic oxygen (O) having a strong oxidizing ability is generated. Organic compounds can be decomposed by irradiating them with energy stronger than the bond energy. These excited contaminants, or the free radicals of the contaminants formed by photolysis, react with atomic oxygen to form simple molecules such as CO₂, H₂O, N, and O₂, which are removed from the surface.^{[ 94-96 ]} Generally, UVO treatment is applied as pretreatment and posttreatment, before and after the ETL deposition, respectively.

(UV-Ozone/Humidity) Pretreatment

Dong et al.^{[ 48 ]} produced moisture molecules adsorbed onto the surface through the utility of UVO pretreatment under humidity on substrates prior to the SnO₂ layer deposition (Figure 9a). They managed to harness the water adsorbed onto the substrate surface through ambient humidity and UVO pretreatment duration monitoring.

The results of the systematic studies demonstrated that the optimal amount of surface adsorbed water molecules throughout the deposition of SnO₂ nanocrystals using the sol–gel method at room temperature on UVO pretreated substrates facilitates optimal hydrolysis-condensation reactions for SnO₂ regrowth (Figure 9b). Furthermore, UVO pretreatment of the substrate makes for a perfect coverage of SnO₂ of desired morphology and crystallization along with superior optical and electrical properties as well as optimized roughness that provokes a more perfect bonding of the interface with the perovskite layer. The sight of pinholes and cracks will be vanished in UVO pretreated samples, resulting in a thin and compact film of ideal coverage and low roughness. Nevertheless, UVO pretreating of longer than optimized time drives the SnO₂ grains to agglomerate and further surface roughness increment and a defective coating in the films is obtained (Figure 9c,d). Employing the UVO pretreatment, Dong et al.^{[ 48 ]} could manage to simultaneously optimize crystallinity, trap state density, and surface roughness in the SnO₂ ETL employed in PSCs fabricated at room temperature. The structural advantages of the treated SnO₂ film made for further improvements in charge transport dynamics and minimized energy loss in the PSCs that significantly enhanced their PCE (Figure 9f,g).

UV-Ozone Posttreatment

As viewed by the literature, the duration of UVO posttreatment has been reported to vary between 5^{[ 27 ]} and 60^{[ 95, 96, 106 ]} minutes, with that of 15 minutes to be more frequent.^{[ 40, 65, 92, 93, 104, 107-110 ]} UVO posttreatment, through wiping out the organic residues on the surface of SnO₂ film, would lead up to the increased hydrophilicity and wettability of the SnO₂ layer and improved surface adhesion between the SnO₂ layer and perovskite. All but most of the SnO₂-based PSCs reported heretofore are freshly employed with that of a few minute treatments under UVO, which appears as an essential step before the perovskite deposition.^{[ 111 ]} Furthermore, posttreatments of the ETL using UVO have an impact on perovskite growth.^{[ 30 ]} In particular, as regards the decrease in the concentration of oxygen vacancies making a downward change in the Fermi levels, the work function of metal oxides might be elevated upon UVO posttreatment. Surface UVO posttreatment improves the interface properties between the ETL and the perovskite that promotes the interaction between the ETL and the perovskite layer, thereby facilitating expeditious electron injection from the perovskite layer to the ETL.^{[ 87 ]}

In UVO treatment, ultraviolet light is simultaneously generated at two wavelengths of 253.7 and 184.9 nm possessing high energy and the corresponding photon energy of which is 472 and 647 kJ mole⁻¹, respectively. Both photon energies are higher than those of Sn–Cl and O–H bonds having bond energies on the order of 350 and 459 kJ/mole, respectively. Also to be found, they are higher than those of C–C, C–H, and C–O bond energies thereabouts 346, 411, and 358 kJ mole⁻¹, respectively. Therefore, UV light is capable of readily breaking down these chemical bonds so that the reaction perpetuates. Meanwhile, UV light of 184.9 nm wavelength could convert O₂ oxygen molecules to active O₃ ozone molecules, which facilitates the formation of SnO₂, decomposition as well as the oxidation of organic compounds, and the final by-products are released as Carbon monoxide (CO₂) and water (H₂O).^{[ 94 ]} So too did, Liu et al.^{[ 94 ]} studied the deposition of SnO₂ quantum dot colloidal solution to construct the QD SnO₂ layer as ETL. High-temperature sintering is customarily applied to remove contaminants albeit in might lead to some defects in the SnO₂ QD film. Nevertheless, to curb this issue, the research group utilized UVO posttreatment at room temperature to detach ethyl groups from SnO₂ QD film and managed to obtain a PCE of 20.1%.

Furthermore, it has been reported that the surface temperature of the layers throughout UVO upraises to about 60 to 70 °C due to the effect of UV light.^{[ 95, 96 ]} Case in point, Li et al.^{[ 96 ]} spin-coated a commercial Alfa-SnO₂ ink followed by annealing at 50 °C for 5 minutes. Thereby, the layers were treated with UVO for 60 minutes. The results of temperature measurement using an infrared thermometer demonstrated that the temperature of the layers increased by about 60 °C during UVO due to its light radiation. According to them, UVO posttreated SnO₂ layers displayed superior compactness, purity, and transparency.

Moreover, Huang et al.^{[ 95 ]} compared the performance of UV-sintered SnO₂ films with that of 180 °C annealed films in the PSC architecture. Thereby, UV-sintered SnO₂-based PSCs possessed 16.21% PCE, whereas annealed SnO₂-PSCs displayed 11.49% at 180 °C. Likewise, it was reported therein that the UVO treatment process imposes a thermal process in the meantime and that the temperature in the entire UVO process is normally below 100 °C. From the findings of the experiment, in the short periods, the layers were of pinholes which could undermine the hole blocking performance of SnO₂. Besides, it was demonstrated that the –OH bond density increased and then further decreased upon increasing UVO treatment duration. These findings implied that after which the UVO treatment was applied the hydroxyl groups were formed on the SnO₂ surface, and the more –OH groups on the SnO₂ surface the lower the recombination rate. These results are schematically summarized in Figure 10 .

3.1.3 Plasma Treatment

One of the utilities of plasma treatment involves surface activation, which offers numerous benefits including higher efficiency, cost-effectiveness, and environmentally friendliness over traditional methods of wet physical and chemical activation. When the plasma comes in contact with the surface, the induced energy transferred by the plasma engenders subsequent reactions on the surface of the material.^{[ 112 ]} In consequence, the wettability of the surface increases and the water contact angle decreases owing to plasma treatment, and thus the surface is rendered more hydrophilic. Besides, it curtails the concentration of C–C or C–H bonds.^{[ 112-114 ]} Each gas type possesses variant characteristics that affect the surface properties of an object within different respects. As regards, neutral or inert argon (Ar) plasma has reportedly displayed no chemical effects on the surface.^{[ 115 ]} The major difference between Ar and O₂ plasma lies in the mechanism of the surface activation, i.e., physical etching and chemical reaction, respectively.^{[ 116 ]} Moreover, the chemical interaction of oxygen-based plasma systems can form strong carbon–oxygen (C–O) covalent bonds of higher polarity than those of carbon–hydrogen (C–H) bonds.^{[ 112, 114, 117 ]}

In recent times, plasma utilization has been acknowledged to be an efficient posttreatment procedure. In particular, the impacts of plasma treatment on metal oxide-based semiconductors to control carrier concentrations associated with oxygen vacancies have enormously attracted research interests.^{[ 118, 119 ]} The reports have demonstrated that the O₂ and N₂ plasma treatment reduced the carrier concentration which was also accompanied by some oxygen vacancies. By comparison, the oxygen vacancies population is significantly higher in Ar plasma treatment. In the case of O₂ plasma, bulk traps are treated through filling the oxygen vacancies even though the surface is damaged by ion bombardment.^{[ 119 ]} O₂ plasma treatment has been more frequently utilized compared to the other gases in both pre- and posttreatment methods, i.e., before and after SnO₂ ETL deposition. In the cases of O₂ plasma utilization as pretreatment, the duration of pretreatment was reported to extend between 3^{[ 68 ]} and 20^{[ 110, 120 ]} minutes in which the 5-minute time was more frequent.^{[ 24, 121, 122 ]} Besides, in the case of O₂ plasma utility as posttreatment and an alternative of thermal annealing, the posttreatment duration was reported to vary between 2.5 and 15 min and likewise, the 5-min time was dominant.^{[ 25, 68, 123-126 ]} In these reports, the main purpose of functioning the posttreatment was to modify the roughness, activate the surface, tune the energy level of the conduction band (E _CB), and enhance the work function of SnO₂ ETL. Luan et al.^{[ 123 ]} applied O₂ plasma to posttreat the surface of trifluoroethanol-incorporated SnO₂ (T-SnO₂). They investigated the effect of O₂ plasma power on PSC performance in which a champion efficiency of 21.68% was achieved at 60 watts. Yu et al.^{[ 124 ]} were of the pioneers that reported the utilization of the Ar/O₂ plasma atmosphere as the major energy source to activate the sol–gel film to form a compact SnO₂ thin film of SnCl₂ precursor at a record rate of ≤5 min. The amorphous SnO₂ thin films employed as ETL in PSC were prepared through anhydrous SnCl₂ spin-coating solution followed by thermal annealing (T-SnO₂) or plasma (P-SnO₂) (Figure 11a). In turn, the T-SnO₂ thin film was prepared slowly under thermal stress of 180 °C temperature for 1 h, whereas the P-SnO₂ thin film was fabricated very rapidly in 5 min under Ar/O₂ plasma atmosphere along with undesired heat (i.e., ≈50 °C) as well as plasma ion bombardment (Figure 11a). Even though the undesired thermal energy is involved to some extent, its effect is negated owing to the degree of oxidation caused by the oxygen flow rate and the duration of the radiation. Therein, the duration of plasma irradiation in the range of 15 to 600 s was examined (Figure 11e). In the semi-oxidized state of the SnO₂ thin film (i.e., <450 s), the remaining Sn⁺² and Cl⁻ ions might incorporate into the perovskite lattice and undermine the PSC performance since Sn⁺² ions are prone to facile oxidization under ambient air.

Also been demonstrated, the formation of metal–oxygen–metal (M–O–M) and hydroxyl metal (M–OH) bonds in P-SnO₂ thin film was higher and lower, respectively, than that of T-SnO₂ in which M–OHs plays the role of shallow trap sites. The widespread presence of M–OH indicated the deficient oxidation of the oxidative network, which reduces the mobility of the SnO₂ thin film, decreases the electron life, and hinders desired electron transport. Furthermore, thermal annealing is given to make for more grain boundaries due to the activation of necking between SnO₂ particles wherein a high density of shallow trap sites is generated owing to network mismatch. Moreover, due to the passivation of the surface stemming from the treatment of inherent defects and the substitution of carbon impurities with oxygen during plasma annealing, the conduction band minimum can be moved upward (Figure 11b).^{[ 124 ]} Therefore, with the use of plasma annealing, the research group was capable of fabricating a denser oxide network compared to that of thin layers oxidized under thermal annealing resulting in supreme electrical conductivity along with lower trap densities. As concluded by these findings, plasma annealing is a cost-effective and efficient fabrication procedure of high-quality thin films for various metal oxides that can be prepared using the sol–gel route.^{[ 124 ]} Also, Shekargoftar et al.^{[ 127 ]} used argon plasma (atmospheric pressure and power density 2.5 W cm⁻², 5 min, <60 °C) for the treatment of SnO₂ films and its performance by thermal annealing (30 min, 180 °C) compared. The O_1S peak of the XPS spectrum of their samples was fitted by three compounds with binding energies of 530.5, 531.6, and 532.9 eV, which correspond to the oxygen contained in SnO₂/SnO, Sn–OH hydroxides, and water adsorbed on the surface, respectively. Thermal annealing was more effective in removing water due to higher temperatures. But the plasma-treated sample showed more Sn–OH hydroxides, which the group attributed to the formation of surface defects by high-energy plasma argon species bombardment. Subbia et al.^{[ 25 ]} reported that compared to PSCs with heat-annealed SnO₂, PSCs with SnO₂ treated with N₂ plasma had higher efficiencies of 20.3% on rigid substrates and 18.1% on flexible PET/ITO substrates. These flexible PSCs were extremely stable since they could maintain 90% of their initial PCE after 1000 cycles. The result demonstrated that deep UV (DUV) radiation employing N₂ and N₂O plasma emissions plays a prominent role in achieving high attribute metal oxide thin films at lower temperatures. In this line, this group^{[ 25 ]} reported a novel strategy for the production of SnO₂ metal oxide semiconductors at near-RT temperatures including low-power inductively coupled radio frequency (RF) plasma. In this method, sol–gel-coated SnO₂ thin films are activated using nitrogen plasma to break down the alkoxy and hydroxy groups allowing the formation of a M–O–M network (Figure 12a). Besides, the performance comparison among the as-deposited/annealed SnO₂ thin films and that of deposited through RF coupled with O₂, N₂, and N₂O plasmas (i.e., AD-SnO₂, OPT-SnO₂, NPT-SnO_2, and NOPT-SnO₂, respectively) and also that of thermally annealed SnO₂ (TA-SnO₂) in PSCs was conducted (Figure 12c). According to the results, the chlorine-containing residues in the thermally annealed samples and treated-under-nitrogen-plasma samples were less than that of the treated-under-oxygen-plasma sample. It is remarkable that the formation of SnO₂ film from Sn metal-halide precursor merely takes place in the presence of N₂ plasma, whereas reactive species of O₂ plasma fared poorly. Studies in this group have represented that the formation mechanism in the presence of ultraviolet (UV) rays involves breaking down the metal alkoxy-hydroxyl bonds to form the M–O–M structure followed by film thickening and compactness. In the described mechanism, the plasma emissions containing DUV photons provide the energy required to initiate the breakage of alkoxy-metal bonds although the RF power is relatively low. In the case of O₂ plasma, the emission spectrum at an operating pressure close to 1 torr does not contain the UV component required to initiate the bond cleaving process, therefore, the produced film retains the original composition of the deposited layer to a greater extent. The favorable outcome of NOPT-SnO₂-based PSCs verifies that DUV emission from N₂ plasma is indispensable for the initial cleavage of metal alkoxy and metal hydroxyl bonds and the resulting formation of SnO₂ thin films.^{[ 25 ]}

3.1.4 Photonic Annealing

The long processing time is a bottleneck for fast and large-scale production, which is a major motivator and financial incentive for the widespread deployment of PSCs. In this regard, intense pulsed light (IPL) annealing holds the convincing potential to significantly enhance the performance and compatibility of the manufacturing process. In this method, broadband light pulses in a short scale of milliseconds and of high intensity, normally xenon flash lamp, are employed for swift and selective annealing of light-absorbing materials. The short duration of this process causes the temperature in the absorbent material to ascent locally devoid of damaging the temperature-sensitive substrates such as flexible plastics. Moreover, the intrinsic feasibility of utilizing this method for large-area production, along with the short processing time as well as its simpatico with the substrates, makes it favorable for roll-to-roll (R2R) manufacturing platforms. Besides, light-based fast annealing strategies in PSCs, in addition to ETL, have been utilized for annealing the absorbent layer.^{[ 39 ]} In a related study, near IR light was employed to anneal the Al₂O₃-MAPbI₃ absorbing layer for 2.5 s and resulted in a PCE of 10.0% in comparison with that of PSC using 45-min thermal annealing yielding 10.9% PCE.^{[ 128 ]} Furthermore, photonic annealing for TiO₂ has been utilized and a 15% PCE was obtained^{[ 129 ]} Likewise, Zhu et al.^{[ 39 ]} utilized this method for annealing SnO₂ ETL films prepared through a solution-based low-temperature process on the FTO substrate. As stated by their findings, IPL annealing triggered the formation of SnO₂ films through the following reaction: (SnCl₄ + 2 H₂O → SnO₂ + 4 HCl). According to their results, IPL annealing has reduced the surface roughness and increased layer compactness. In addition to the hydroxyl groups (–OH), small amounts of chlorine were also observed in IPL annealed SnO₂ films indicating the presence of chlorinated compounds such as SnCl₄. So too did, Ghahremani et al.^{[ 110 ]} applied the IPL method (5 pulses of IPL, each carrying 2.1 kJ energy) for SnO₂ annealing and also made use of this method to anneal the perovskite absorbing layer. Also, Oh et al.^{[ 130 ]} used IPL for 30 seconds (120 pulses of IPL, each carrying 1.84 J cm⁻² energy) for sintering of amorphous SnO₂. It should be noted that before photonic annealing, the ETL layer was dried for 1 min at 100 °C. They reported the IPL annealing reduces hydroxyl groups and increases conductivity and electron mobility. Also, their results showed that IPL annealing can effectively control the ETL/perovskite interface in a short time by regulating intrinsic perovskite (MAPbI₃) properties such as stress and nucleation density. Thus, they improved the PCE from 7.06% (amorphous SnO₂) to 17.68% by using photonic annealing.

3.1.5 Water Treatment

The enhancement of SnO₂ ETL through water treatment has been undertaken in three avenues comprising hydrothermal treatment, water-bath treatment, and water-spin coat treatment. In the first two methods, SnO₂ ETL is initially deposited and thereafter treated with water. However, in the water-spin coat treatment method, water pretreatment is performed prior to the SnO₂ ETL deposition.

Hydrothermal Treatment

SnO₂ film destitute of treatment suffers from poor crystallinity, which leads to low conductivity and inferior charge transport at the interface between the ETL and the perovskite adsorbing layer. Notwithstanding, surfactants are normally added to colloidal SnO₂ solutions to maintain the dispersion of SnO₂ nanoparticles. These surfactant coatings on the surface of SnO₂ nanoparticles typically lay behind high resistance in SnO₂ film and are required to be discarded appropriately. In this line, thermal annealing is a prevailing procedure; however, annealing at low temperatures (<200 °C) fails to perfectly eliminate surfactants and leads to poor PSC performance. Moreover, the majority of surfactants include organic carbon (R) compounds that could be decomposed into gaseous hydrogen fuels through hot water steam: R + H₂O = R(O) + H₂. Therefore, SnO₂ water steam treatment might be capable of eliminating carbonaceous surfactants. Nevertheless, conventional water-gas shift reactions require relatively high-temperature (i.e., >200 °C) treatment steps. However, the thorough abolishment of the surfactants might be met by means of the high vapor pressure of water steams in the hydrothermal process which could proceed completely at low temperatures (≤100 °C). Hot water steam thoroughly annihilates the organic surfactants coated on the surface of SnO₂ nanoparticles facilitating the formation of a ligand-free SnO₂ ETL (Figure 13 ).^{[ 131 ]}

Recently, Liu et al.^{[ 131 ]} exploited a novel low-cost process to prepare high-quality SnO₂ ETLs at low temperatures (100 °C) by integrating spin-coat SnO₂ nanocrystalline solutions with hydrothermal treatment. As reported from their results, the performance of SnO₂ on the rigid ITO/Glass substrate showed that the hydrothermal treated SnO₂ ETL (HT-SnO₂) at 100 °C was furthermore higher in characteristics than that of SnO₂ ETL heat annealed (TA-SnO₂) at 150 °C. They also employed this process to fabricate SnO₂ ETLs on flexible PEN/ITO substrates and achieved an outstanding champion PCE of 18.1% for PSC devices (Figure 13).

Water-Bath Treatment

In the water-bath treatment method, irrespective of the ambient humidity, SnO₂ crystalline films are to be acquired using a sol–gel thin film water bath. The precursor residues in SnO₂ films prepared in ambient air are incapable of being thoroughly hydrolyzed to form SnO₂ and yet thermal annealing was applied. However, during the water-bath treatment process, the extant precursors undergo complete hydrolysis with the aid of large water molecules in the environment along with thermal energy. Thus, SnO₂ films treated with the water-bath method are composed of almost perfect crystalline SnO₂ nanocrystals, which facilitate the rapid extraction of electrons from the perovskite and the minimization of surface defects (Figure 14a). Therefore, the performance of the perovskite device is also remarkably improved.^{[ 121 ]} In this line, Li et al.^{[ 121 ]} applied this treatment on SnO₂ films prepared through spin-coating of SnCl₄.5H₂O/isopropanol solution in the air under diverse humidity range of 0–75% RH followed by thermal annealing at 180 °C and realized PSCs of 19.17% PCE, compared with that of devices fabricated of sol–gel thin films in the absence of water-bath treatment (i.e., 17.59%), possessing high reproducibility. Besides, using this method, they could manage to produce 16.16% PCE for the large-area PSC module with an area of 16.07 cm² (Figure 14b,c).

3.1.6 Sulfur Treatment

The sulfur treatment has been utilized for passivating the interface between ETL and perovskite. Also, solvents or sulfur-containing molecules have been employed directly to modify the perovskite layer. Sulfur-containing compounds extending from thiophene,^{[ 132 ]} thiadiazole,^{[ 133 ]} and thiourea^{[ 134 ]} are capable of forming adducts with PbI₂ (a Lewis acid) in perovskite compound by interacting with a sulfur-bearing Lewis base.^{[ 135 ]} Therefore, this would conduct to the elimination of deep traps and increment in the passivation between grain boundaries and perovskite crystallinity. Sulfur could be directly coordinated with lead atoms in the perovskite thereby interacting with the perovskite valence band, facilitating the efficient extraction of carriers from the perovskite and increasing the PSC efficiency.^{[ 132 ]} Moreover, poor interaction between lead and sulfur (R2C = S· PbI₂) might regulate the perovskite nucleation and crystal growth.^{[ 133, 134 ]}

In the utility of sulfur treatment for the modification of the perovskite/SnO₂ interface, sulfur atoms would make bonding to Pb²⁺ in perovskite and SnO₂ simultaneously and, owing to resultant chemical interaction, lead to improved charge transport at the perovskite/SnO₂ interface. Although complex organic molecules such as heparin sodium^{[ 136 ]} biopolymer are also utilized to facilitate accelerated charge transport in the interface as well as reduce hysteresis, plausible defects in the interconnections of organic molecules constrain the effectiveness of surface trap passivation which is undesired for the performance of the perovskite photovoltaic devices. Furthermore, sulfur mineral atoms at the SnO₂ surface have been reported to hold stronger electrostatic interactions with Pb in the perovskite than that of oxygen atoms in the SnO₂ ETL.^{[ 29 ]} Actually, surface sulfur atoms desist traps in the shallow area and carrier recombination at the interface through filling the iodide vacancy at the interface. In order to apply the sulfur treatment on SnO₂ to functionalize it at relative humidity above 60% (RH), Wang and coworkers^{[ 102 ]} loaded a solution of potassium O–hexyl xanthate in methanol on the SnO₂ surface for two minutes and then spin-coated. Subsequently, after three washes with methanol, the samples were annealed at 130 °C minutes to decompose xanthate (Figure 15a,b). According to the group, the surface of SnO₂ film came to be more hydrophilic owing to sulfur treatment and the decomposition of tin xanthate and the formation of tin sulfide, which stemmed from an affinity change from hydrogen to sulfur bonds. Moreover, sulfur treatment reduced the work function of SnO₂, which resulted in a superior energy band alignment between the perovskite and the ETL. Accordingly, the research group has attained PSCs possessing 18.41% PCE using sulfur treatment (Figure 15c).

To that end, Ai et al.^{[ 137 ]} utilized (NH₄)₂S for SnO₂ ETL sulfur treatment to passivate surface defects. They admixed different concentrations of (NH₄)₂S in SnO₂ precursor colloids and subsequently prepared SnO₂ ETLs using the spin-coating method and annealed at 180 °C. Within SnO₂ ETL, there lie a great deal of lateral Sn bonds, and after applying the (NH₄)₂S to SnO₂ colloids, the surface S atoms would impregnate the surface lateral Sn bonds to form the S–Sn bonds. Therefore, the formation of S–Sn bonds could partially eliminate surface oxygen vacancies. Furthermore, S–Sn surface bonds can harness the moisture diffusion into the cell, as shown in Figure 16 . In addition, the anchor S bonds of the surface Sn–S bonds make bondage with lead atoms in the perovskite crystal process to form Sn–S–Pb. These anchors establish an interconnection from the perovskite directly to the ETL as a pathway for electron transport which thus improves electron extraction. This method reduces the density of surface defects in ETL, thereby removing electron transport obstacles and improving electron mobility and thus conductivity and stability. Conclusively, the research group could manage to achieve a PCE of over 20% making use of this sulfur treatment.

3.1.7 Self-Assembled Monolayer Treatment

In the self-assembled monolayer (SAM) treatment, the insertion of an interlayer in the perovskite/ETL interface would make for the energy-level adjustment, the surface energy modification, and the enhancement of the films or substrates affinity. Moreover, this treatment has been utilized as a surface modification method in organic solar cells^{[ 138 ]} and dye-sensitized solar cells.^{[ 139 ]} SAM treatment is also reported to impress beneficial effects upon the interface modification and this is accomplished through manipulating the energy levels by creating a dipole moment at the interface.^{[ 140 ]} Also to be found, it could also passivate the inorganic surface states using chemical bonds thus regulating surface electrical states. Furthermore, SAM treatment could exert influence on the growth process of organic semiconductors and make for various morphologies. Accordingly, SAM treatment has been employed as a streamlined procedure to optimize the SnO₂ ETL/perovskite interface in planar PSCs.^{[ 141, 142 ]} In general, SAMs molecules consist of three parts: anchoring, terminal, and spacer groups. The anchoring group can interact with the modified interface level and affect electronic performance and ETL specifications. The terminal group with perovskite layer can have interfacial chemistry and thus affect the quality of perovskite adsorbent film and carrier dynamics PSCs. The spacer group connects the anchoring and terminal groups and affects the attenuation rate of the transport.^{[ 143 ]}

Si-Based Self-Assembled Monolayer Treatment

Yang et al.^{[ 142 ]} had utilized the Si-based SAM treatment to modify the perovskite/SnO₂ ETL interface in which 3-aminopropyltriethoxysilane (APTES) SAM was employed as an interlayer for the modification of the interface between the SnO₂ ETL/perovskite layer. To do so, the prepared SnO₂ compact films were immersed in APTES solution in isopropanol (5 mM), then washed with isopropanol, and finally dried under N₂. The fabricated SAM layer enhanced the contact surface at the interface and improved the morphology and crystallization of perovskite films. Moreover, through the formation of dipoles, the functionalized APTES SAM could manage to give onto the tuning of energy band alignment, increment of the built-in potential, and reinforcement of the driving force to segregate the photo-produced carriers and thus accelerating the charge extraction (Figure 17b). With regard to the multifunctional character of APTES SAM, alkyl chains acted as an electrical insulation barrier thus intercepting the backward electron transport from the electrodes to the perovskite layer and remarkably reducing recombination processes. Likewise, the terminal groups passivated the trap states at the perovskite surface through hydrogen bond interactions (N–H…I) (Figure 17a) thus reducing the charge accumulation and recombination resulting from them. In conclusion, the result of the research works was the achievement of a high PCE of 18% as well as low hysteresis for planar PSC using the surface treatment of SnO₂ with APTES SAM.

Zwitterion Self-Assembled Monolayer Treatment

Zwitterion is a molecule that contains an equal number of positively- and negatively charged functional groups thereupon neutral. Choi et al.^{[ 148 ]} reported a planar PSCs with zwitterion SAM-modified SnO₂ ETL of thermal stability and high efficiency. Therein, the zwitterionic compound, 3-(1-pyridinio)-1-propane sulfonate (NDSB-201), was utilized to surface modify SnO₂ ETL. Hydroxyl groups on the surface of SnO₂ chemically react to form bonds with zwitterion molecules containing anions such as–COO⁻, –SO³⁻, and –SO⁴⁻. Consequently, the zwitterion SAM treatment of the SnO₂ ETL leads to the formation of surface dipoles, the shifting of the SnO₂ ETL work function, the inhibition of electron transport, and the abatement of charge recombination (Figure 18 ). Besides, zwitterion passivates perovskite Pb–I antisite traps using its positively charged atom and improves the thermal stability of the PSC. In culmination, this research group managed to obtain PSCs possessing PCE of 20.91% and 21.43% using Zwitterion modification of SnO₂ ETL prepared by spin-coating of tin chloride precursor solution and CBD, respectively.

Zhao et al.^{[ 149 ]} also used another material called diethylenetriaminepentakis (DETAPMP) to modify SnO₂ ETL surface. The phosphate group present in DETAPMP reacts with SnO and passively forms the SnO₂ defect by forming an ester bond. In addition, the ammonium group in DETAPMP balances the PbI₃ ⁻ charge and passivates the PbI₃ ⁻ defects. This group could reduce charge recombination and increase charge transport by improving the SnO₂ ETL layer zwitterion modification by DETAPMP, increasing the PCE from 17.74% to 20.02%. Also, by changing the absorber layer from MAPbI_3−x Cl _x to (FAPbI₃)_0.95(MAPbBr₃)_0.05, they were achieved a yield of 21.65%.

Choline Chloride Self-Assembled Monolayer Treatment

Naturally occurring, choline chloride is a plant photosynthetic promoter substantively reverberating the plant performance increment. In this line, choline chloride increases electron transport in photosynthesis in thylakoid membranes, thereby conducting the synthesis of adenosine triphosphate (ATP) in chloroplasts. Inspired by this biological substance, Yan and their coworkers^{[ 150 ]} made use of choline chloride SAM in PSC (Figure 19a,c). Supposedly, its performance in PSC is analogous to that of plant photosynthesis promoter in which improved electron transport and enabled ETL to extract electrons more efficiently. Furthermore, the utilization of this material in the ETL/perovskite interface serves to passivate oxygen vacancy defects at the ETL surface. Besides, choline chloride SAM anchors on SnO₂ ETL to impose a chemical interaction with the perovskite film (Figure 19b), which remarkably reduces the oxygen vacancies at the interface and improves electron transport, carrier lifetime, and open-circuit voltage (V _OC) of PSC. Besides, SnO₂ ETL modified with choline chloride SAM treatment could better the charge extraction, reduce recombination in the ETL/perovskite interface, and improve the quality of ETL and perovskite films.^{[ 150 ]}

3.1.8 Alkali Metal Treatment

In recent years, a great deal of research studies has demonstrated that doping with an alkali metal, especially potassium, imposes a positive effect upon PSCs performance.^{[ 105, 153-156 ]} Thereby, the addition of compounds including potassium halide (e.g., KI, KBr) to perovskite precursors make them incorporate potassium ions into the perovskite. As a consequence of potassium ions incorporation, the crystalline quality and light absorption of perovskite increase and the amount of trap states decreases.^{[ 153 ]} Likewise, Bu et al.^{[ 26 ]} examined the commercial colloidal solution of Alfa-SnO₂ and realized that a small amount of KOH was added to the solution for colloidal stability. In consequence of this addition, the perovskite/ETL interface would be passivated which, in turn, outstanding performance and a significant reduction in the hysteresis of PSCs based on Alfa-SnO₂ ETL was observed. Likewise, Wang et al.^{[ 156 ]} applied a very thin layer of KCl as a buffer layer between the SnO₂ ETL and the perovskite (Figure 20a). According to their results, since the MA⁺ cations and I⁻ anion take the possibility to immigrate and move in the perovskite and also, due to the smaller size of K⁺ cation and Cl⁻ anion than that of MA⁺ and I⁻, the formers could easily diffuse and move into the absorbent layer throughout thermal annealing of the perovskite layer. According to the Goldschmidt tolerance factor, KPbI₃ and KPbCl₃ are incapable of forming a stable perovskite structure since the size of the K⁺ cation (1.33 Å) is even smaller than that of the rubidium cation (1.48 Å). Therefore, the K⁺ cation is inclined to reside and move at the interstitial site in the perovskite, whereas the Cl⁻ anion might either fill the halide vacancy states or remain in the interstitial position, as shown in Figure 20b. What is more, several trap states have proven to be formed during perovskite crystallization, particularly at the interface and grain boundary of the perovskite layer in which the diffusivity of the K⁺ cation and the moving Cl⁻ anion should passivate these trap states and decorate the surfaces and grain boundaries as well. In this end, the research team was able to achieve PSCs of 19.44% PCE utilizing potassium treatment of SnO₂ ETL prepared by spin-coating the tin chloride precursor solution (Figure 20c).^{[ 156 ]}

Correspondingly, Bu et al.^{[ 26 ]} investigated the impact of K⁺ cation through the deposition of potassium-containing compounds between SnO₂ ETL and perovskite(Figure 21a). On the contrary to preceding findings, they supposed that K⁺ possessed a much more effective share than Cl⁻ anion in passivation, thereupon, nonhalide compounds such as KOH and KCH₃COO were employed to assert the claim and obtained analogous results with that of potassium halide. In consequence, they realized that the presence of K⁺ cations in the SnO₂ ET/perovskite interface remarkably impressed upon the germination and growth of the perovskite layer. Moreover, potassium treatment leads to the formation of larger grains, a smoother surface, and the removal of PbI₂ impurities in the perovskite layer. Besides, they reported that, given the strong ion bond of KBr, K⁺ cations hold the potential to easily react with Br ions through substitutional reaction to form KBr at the surface of the perovskite film. Thereby, KBr-rich interfaces could passivate the halide vacancies of the interface and enhance the PSC performance along with hysteresis abatement. Moreover, the strong KBr dipole preferentially binds to PbX₂ (Figure 21b) and serves as the nuclei for perovskite formation. Besides, Br-leaching of the perovskite film could provide the ground for the formation of I-rich perovskite, thereby expanding the perovskite crystal network by more incorporation of I⁻. This might explain the manner in which PbI₂ impurities in the perovskite layer was removed. In conclusion, the group could obtain PSCs possessing PCEs of 20.50%, 17.18%, and 14.89% using potassium treatment for slot-die printed SnO₂ ETL on the rigid, small-size flexible (0.16 cm²), and large-size flexible (16.07 cm²) substrates, respectively.^{[ 26 ]}

Recently, Huang et al.^{[ 105 ]} have also systematically reported the influence of alkali metals (i.e., Li, Na, K, Rb, and Cs) in modifying the interface of PSCs. To modify the SnO₂ films, chloride salt solutions of each alkali metal were spin-coated on the surface of SnO₂ and then annealed at 150 °C. Based on the results therein, the surface roughness increased upon the increasing radius of alkali metal, and the conductivity of SnO₂ films in the presence of Na was higher than that of the other alkali metals owing to the higher conductivity of this element. The performance of PSCs based on SnO₂ ETL treated with alkali metals involving Li, Na, and K with PCEs of 18.29%, 18.57%, and 18.67%, respectively, was higher than that of PSCs built upon untreated SnO₂ ETL possessing 17.84% PCE. This is since alkaline metals incorporation has led to a decrement in the density of trap states as well as an increment in the mobility of the SnO₂ layer. However, PSCs based on SnO₂ ETL treated with alkaline metals including Rb and Cs, representing PCEs of 17.85% and 17.83%, respectively, fared poorly to demonstrate significant results compared to that of PSCs based upon untreated SnO₂ ETL.^{[ 105 ]}

3.1.9 Halogen Treatment

SnO₂ film, much the same as other metal oxides (e.g., TiO₂, ZnO), bears oxygen-induced bulk and surface defects that might frustrate their electronic and charge transport properties.^{[ 150 ]} The presence of halogens such as chlorine on the surface of SnO₂ ETL not only reduces surface defects in the ETL/perovskite interface but also constructs a bridge for electron transfer from the perovskite film to that of SnO₂ ETL.^{[ 120, 157 ]} Ren et al.^{[ 120 ]} subjected the SnO₂ film into 1,2-dichlorobenzene solution as a source of chlorine under UVO for the chlorine treatment of SnO₂ ETL. The 1,2-dichlorobenzene solution is of high reactivity Cl radicals, which could enable deeper passivation, thus by bonding the chlorine anion to Sn on the SnO₂ ETL surface.^{[ 120 ]} For bromine treatment of SnO₂ ETL, Liu et al.^{[ 157 ]} placed the SnO₂ film in a solution of methanol containing 1-butyl-3-methylimidazolium bromide as a source of bromine in a nitrogen glove box. Thus, they were able to increase the PSC efficiency from 16.8% to 18.8% with bromine treatment.^{[ 157 ]}

3.1.10 Amine-Based Treatment

In recent years, the amino acid (HOOC-R-NH₂) utility has been reported as a buffer layer in the ETL/perovskite interface for TiO₂ and ZnO ETL-based PSC devices.^{[ 147, 158 ]} The role of amine-based compounds has a close analogy to that of “linker” between ETL and perovskite. Furthermore, amine-based compounds could induce favorable growth of perovskite crystals and thus increase perovskite crystallinity.^{[ 34 ]} However, modification of SnO₂ ETL using the buffer layer of amine-based compounds and their role in manipulating the crystal lattice has been rarely reported.

Generally speaking, annealing of the SnO₂ layer for decreasing oxygen vacancies is performed in ambient air, which inevitably leads to the adsorption of oxygen on the SnO₂ nanocrystalline film.^{[ 86 ]} The physical adsorption of these ambient oxygen molecules metamorphoses to chemisorption on the surface of SnO₂ through effectively extracting intrinsic electrons from the SnO₂ conduction band during the annealing process promoting the formation of O²⁻ on the surface. Therefore, a band bending and an energy barrier are formed between the perovskite and SnO₂ interface resulted in a significant reduction in SnO₂ conductivity. Due to the negative O²⁻ adsorbed charge, further charge recombination at the ETL/perovskite interface hinders the transport of electrons produced in the perovskite layer into SnO₂ and reduces the device efficiency.^{[ 159, 160 ]} On one side, a cation (i.e., positive charge) is required to compensate for this effect and tackle the issue. On the other side, it has been reported that the substitution of A-site ions in perovskite with multifunctional guanidinium cation would enhance the stability and performance of the device.^{[ 161 ]} Besides that, as a large organic cation, the positive charge of guanidinium forms a strong bond with O²⁻ whereas the ammonium in guanidinium could form a firm hydrogen bond with iodide in the perovskite (Figure 22a,c). Consequently, the application of guanidinium as a linker in the SnO₂ ETL/perovskite interface can curb the energy barrier of charge transport stemmed from O²⁻ between SnO₂ and perovskite.^{[ 162 ]} To accomplish guanidinium treatment, Yu et al.^{[ 162 ]} initially spin-coated the guanidinium chloride solution on SnO₂ film and then annealed it at 80 °C. In consequence, guanidinium treatment enhanced the wettability of the SnO₂ layer in light of the formation of a strong hydrogen bond (N─H… I⁻) between the amino groups of the guanidinium interlayer ions and the perovskite precursor iodide ions, as depicted in Figure 22c.

Although SnO₂ offers numerous merits as ETL it suffers from certain drawbacks common in metal oxides whatsoever including surface oxygen vacancies and containing surface hydroxyl groups making for trap states adjacent to the valence band.^{[ 104 ]} According to the literature,^{[ 104, 163 ]} two types of hydroxyl groups on the metal oxide surface have been reported involving terminal-hydroxyls (OHT), which binds to one metal site of basic properties, and bridge-hydroxyls (OHB), which binds to two acidic metal sites. The presence of OHT on the hydroxylated surface creates deep energy states in the bandgap (Figure 23a), which behave as traps that result in the production of nonradiative recombination and energy loss. Thereupon, diminishing the defect sites on the SnO₂ surface is of paramount importance since they could engender nonradiative recombination in the SnO₂/perovskite interface. However, adjusting the SnO₂ energy level through doping allows the enhancement of electron extraction from the perovskite.^{[ 104, 164, 165 ]} Inspired by the thought of fluorine-doped SnO₂ (FTO) preparation through interaction between tin(IV)tetrahydroxide (Sn(OH)₄) and ammonium fluoride (NH₄F), surface treatment of SnO₂ with NH₄F could alleviate defect sites and regulate the Fermi level of SnO₂ thin films. Moreover, this treatment could annihilate hydroxyl groups from the SnO₂ surface and also dope it with fluoride ions (Figure 23b).^{[ 104 ]} It is possible that the weakly acidic ammonium cation of NH₄F reacts with OHT on the SnO₂ surface producing ammonia gas and water vapor and replacing the fluoride anions at the defect sites, which regulates the SnO₂ energy level.^{[ 104 ]} In this manner, four situations in the treatment of the hydroxylated surface with NH₄F would take place: 1) The first and most obvious is the reaction between the ammonium cation and the OHT group since OHT is alkaline and inclined to dissociate as an ─OH anion. The removal of OHT surface hydroxyl groups relates to their basic nature as well as their readiness to react with the NH₄ ⁺ cation, on the contrary, doubly-bonded OHB removal is more challenging. Thus, removing the OHT hydroxyl groups deletes the trap states which in turn enhances the V _OC of PSCs. 2) In the second plausible reaction with the OHT hydroxyl group, a fluorine atom is replaced through anion exchange and terminal fluorine (FT) is formed. As singly-bonded fluorine, FT possesses a lower electron density share of Sn compared to that of doubly-bonded substitutional fluorine. Therefore, FT brings about a higher positive electrostatic potential to bind to the electron and in consequence, OHT trap states are also eliminated once fluoride is exchanged. 3) The third scenario is the replacement of bridge oxygen with fluorine which is of minor effect on the band structure as trap states are not made thereof. 4) In the latter case, in which fluorine is attached to an oxygen bridge, the electronic structure of SnO₂ is remarkably influenced by n-type doping. Fluorine contributes directly to shallow states under the conduction band minimum and the work function of SnO₂ is reduced through NH₄F treatment.^{[ 104 ]} Theretofore, Jung et al. could manage to elevate the PCE of PSC from 22.4% and 20.7% to 23.2% and 21.8% making use of NH₄F treatment of SnO₂ layer prepared by CBD method or spin coat of the commercial solution of SnO₂ nanoparticles, respectively.^{[ 104 ]}

Glycine representing the C₂H₅NO₂ chemical formula has been employed by Du et al.^{[ 34 ]} as a buffer layer between SnO₂ and the perovskite layer to control perovskite crystal growth. Figure 24a represents the chemical structure of glycine, which consists of the carboxylic acid and the amino group. When deposited on SnO₂, the glycine carboxylic acid group might react with hydroxyl groups on the SnO₂ surface, whereas its outer amine group assists in perovskite formation. Moreover, the presence of glycine would facilitate electron transport at the SnO₂/perovskite interface and obstruct carrier recombination thereof to a great extent (Figure 24b). Figure 24c exhibits the PSC energy level for each layer employed in its architecture. It is also demonstrated that charge carrier extraction and transport could be improved through proper alignment of the energy band at the interface. In this light, glycine treatment could considerably enhance the grain size and quality of the perovskite film as well as the charge extraction/transport at the perovskite/SnO₂ interface. Thereupon, Du et al.^{[ 34 ]} managed to increase PSC efficiency from 18.40% to 20.68% using SnO₂ ETL glycine treatment.

Heretofore, tremendous efforts have been devoted to eliminate the defects and make molecular amendments in the ETL/perovskite interface using organic or inorganic interlayers.^{[ 166 ]} In comparison with that of inorganic materials, organic materials, principally those of small molecules, present well-defined structural advantages, multifunctional anchoring groups, desirable surface coverage, as well as high reproducibility.^{[ 81, 167 ]} Sonmezoglu et al.^{[ 81 ]} utilized the small organic molecule 2-methylbenzimidazole (MBIm) to modify the perovskite and ETL interface in which, owing to its peculiar structure, it is capable of making interactions with Lewis acid defects in the perovskite through the N atom, whereas the methyl chain is hydrophobic. The extant of these functional groups render supreme characteristics for well-ordered passivation. Furthermore, surface treatment triggers enhancement in the perovskite crystal quality as well as decreasing the density of trap-states in the interface and enabling faster electron transport through energetic disturbance minimization.^{[ 81 ]} Figure 25a illustrates a schematic of the treatment of SnO₂ layer with small MBIm molecules. MBIm could represent an efficient passivation performance through donating lone electron pairs uncoordinated Pb²⁺ atoms, which operate as electron traps and attenuate charge transport efficiency. The formation of a coordinate or dative-covalent bond with the beneath surface of the perovskite layer makes the electron valence density to delocalize from the surface in contact with Pb into the bulk. Consequently, this feature improves miscibility with the perovskite layer, which indicates the enhanced perovskite crystallization and a superior passivation effect. This “anchoring effect” associated with MBIm treatment allows the reduction of recombination at the interface and the improvement of V _OC. Therefore, Sonmezoglu and coworkers could successfully elevate the PCE of PSCs from 19.46% to 21.57% using the SnO₂ layer treatment with MBIm (Figure 25b).^{[ 81 ]}

The presence of surface energy bond bending in SnO₂, due to high oxygen vacancy, causes undesirable accumulation and recombination of electrons in the ETL interface and perovskite layer. Also, the deep-level defects (Pb_I, Pb_i, IP_b) in solution-processed polycrystalline perovskite films intensify the uncoordinated and nonradiative energy band recombination. Sun^{[ 168 ]} and colleagues used the amphoteric linking molecule p-amino benzenesulfonic acid (ABSA) to modify the interface between SnO₂ ETL and the perovskite layer. Sulfonic acid groups in ABSA, especially the active S–OH group, obtain a stable sulfonate by passivating the undercoordinated Sn ions, which causes the undercoordinated Sn or O ions to become inactive. Pass the SnO₂ film level. In addition, electron-rich alkaline amino groups can passivate the perovskite layer by providing single electron pairs, defect V _I, and deep-level I-Pb antisite defects. The amino may also interact with the iodide ion to stabilize its position. ABSA makes electron loss difficult by passively activating undercoordinated Sn ions. Following the lower electron loss, the modified SnO₂ has a higher N_D and a smaller eV_surface, which reduces the energy band bending and energy band barrier. Thus, it both reduces recombination at the interface and helps increase conductivity. The team was able to increase the efficiency of LT-SnO₂-based PSCs from 18.02% to 20.32% after using ABSA.

3.2.1 SnO₂ and Organic Materials

Organic compounds of fullerene and its derivatives extending from C₆₀, phenyl-C₆₁-butyric acid methyl ester (PCBM), and C₉ represent desirable charge extraction as well as the ability to passivate the interfacial defects of the perovskite layer/ETL. A great deal of research studies has been paid on the modification of SnO₂ surface using compounds of fullerenes or graphene to overcome the related issues of this layer and thus improve the performance of the device. Among the most remarkable advantages of utilizing organic compounds between the perovskite layer and SnO₂ ETL is the coupling in the perovskite/organic layer interface as well as passivation in both the perovskite/organic layer and organic/SnO₂ layer interface. The use of organic compounds increases the extraction and transport of electrons from the perovskite absorbing layer into the electrode by passivating the SnO₂ film surface and optimizing the E _CB, which makes for reducing the recombination at the perovskite/ETL interface as well as significant hysteresis abatement.

Zhu et al.^{[ 51 ]} could enhance electron extraction by inserting the C₆₀ layer between that of the perovskite and the SnO₂ layer in the inverted structure PSC. They prepared the SnO₂ layer through sol–gel and hydrothermal methods and observed that the performance of the layer prepared by the latter was superior. In respect of both the E _CBs of the SnO₂ and C₆₀, 4.5 and 3.9 eV, respectively, and the E _CB difference between those of the SnO₂ and the perovskite (about 0.6 eV), which might lead to undesirable charge accumulation at the interface, the presence of C₆₀ in between could be profitable. What is more, the remarkable effect of the C₆₀ layer made for a perfect hysteresis elimination and subsequent PCE enhancement from 15.05% to 18.82%. Furthermore, Mahmood et al.^{[ 169 ]} realized that SnO₂ nanosheets prepared through the electrospray technique demonstrated higher performance than that of the hydrothermal method in a planar PSC structure. The SnO₂ nanosheets synthesized by high electrospray possessing high mesoporosity resulted in reduced hysteresis, increased charge collection efficiency, and improved environmental stability. Therein, intending to optimize the interface between SnO₂ and perovskite layer, they employed a C₆₀ layer to alleviate the recombination and enhance the band energy alignment. Therefore, utilizing the C₆₀ interlayer, they could increase the efficiency of planar PSCs based on electrospray-prepared SnO₂ ETL from 15.69% to 20.20%. Zhao et al.^{[ 170 ]} used a low-temperature, open-air plasma process for deposition of SnO _x thin films. They optimized the deposition conditions and parameters and fabricated a SnO _x thin film on substrates up to 100 cm² without any post-annealing. Also, Zhong et al.^{[ 171 ]} applied a fullerene derivative with a carboxylic group (C₆₀ pyrrolidine tris-acid, CPTA) between the SnO₂ ETL and perovskite interface. The presence of CPTA as an interface modification agent increased conductivity and electron transport and, consequently, the efficiency of PSC on rigid glass/ITO and flexible PEN/ITO substrates elevated from 16.91% to 19.14% and from 15.35% to 18.36%, respectively. Moreover, the result of this work involved the comparison between that of CPTA and PBCM as organic compounds in which the former demonstrated higher performance. Hang et al.^{[ 172 ]} modified PCBM ester by a small amount of the bathophenanthroline (Bphen), which could stabilized the C₆₀-cage via the formation of much more thermodynamically stable charge-transfer complexes against the stress of natural light. Huang et al.^{[ 173 ]} compared CPTA and 6,6-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM)-modified SnO₂ as bilayer ETLs in PSCs. CPTA and PC₆₁BM have a favorable energy band edge alignment to perovskites, so enhanced electron injection rate. They reported that the interfacial tailored chemical bond between the SnO₂ surface and carboxylic acids of CPTA leads to interface dipoles. Due to the presence of interface dipoles, the chemisorption of CPTA fullerene functions acts as a recombination suppressor in both the SnO₂/perovskite interface and perovskite. In contrast, the physisorption of PC₆₁BM fullerene exhibits relatively high recombination rates both in the SnO₂/perovskite interface and within the perovskite. Likewise, Liu and coworkers^{[ 174 ]} utilized a fullerene-derived compound, i.e., C₉ as the passive layer between the perovskite layer and the SnO₂ layer. The consequent effective passivation of oxygen vacancy defects on the SnO₂ surface resulting from this compound inhibited charge recombination by forming a Lewis adduct between Sn and that of the C₉ lateral hydroxyl groups. Besides, surface modification with C₉ increases the current thoroughly owing to the negligible barrier against charge extraction and strong electron demand. In a similar manner, the utilization of C₉ in addition to surface passivation improved the quality of the perovskite film through the grain size enlargement as well as diminishing the grain boundaries and the crystallinity enhancement. Also, Chen et al.^{[ 175 ]} used PMMA:C₆₀ composite as an interlayer between perovskite and SnO₂ layer and could realize PCEs of 18.99%, 19.82%, and 21.41%, through reducing trap sites in PSCs based on different absorbents of MAPbI₃, CsFAPbI₃, and CsFAMAPbI₃, respectively. They made use of PTABr to modify perovskite to increase grain size and diminish grain boundaries in the absorbent layers.

Delgado et al.^{[ 176 ]} utilized C₆₀ interlayers deposited through solution processing and vacuum-based thermal evaporation methods at the interface between SnO₂ and the perovskite (MAPbI_3−x Cl _x ) layer and optimized the thickness of this layer. In the optimized conditions, C₆₀ passivated the trap states at the SnO₂/perovskite interface reducing the charge carrier accumulation at the ETL/Perovskite interface, enhancing the charge extraction, facilitating the charge transport, as well as increasing the fill factor. Eventually, the reported champion efficiency of PSC therein was 18.9% with 10 nm C₆₀ thickness. Tian and coworkers^{[ 177 ]} have also employed yet another derivative of fullerene, i.e., amine-functionalized group, 2,5-diphenyl C₆₀ fulleropyrrolidine (DPC₆₀) for passivating the interfacial defects between perovskite and SnO₂ ETL. Furthermore, the hydrophobicity of the DPC₆₀ layer suppressed heterogeneous nucleation and boosted the crystallinity of the perovskite film and the stability of the cells. Tsarev et al.^{[ 178 ]} deposited an optimized thickness of a low-cost organic n-type semiconductor, perylenetetracarboxylic dianhydride (PTCDA), via thermal evaporation as an interlayer between perovskite/SnO₂ for planar PSCs. Hu and coworkers^{[ 179 ]} have presented a passivating agent for the limitation of the interfacial carrier recombination and hysteresis employing an ultrathin composite layer of PMMA: PCBM. Besides the inverted and planar structured PSCs, Yu et al.^{[ 37 ]} have employed C₆₀ as an interlayer between the perovskite and the SnO₂ layer in the all-perovskite tandem structure. C₆₀ interlayer acts as an effective electron collecting layer along with a SnO_2–x (0 < x < 1) layer rendering an ambipolar carrier transport property for the presence of a large density of Sn²⁺ which this approach induces ohmic contacts with both wide and narrow bandgap perovskites as well as low contact resistivity. Moreover, the fullerene layer avoids the formation of a reverse-biased diode between the two subcells, which would enhance the PCEs of small-area tandem cells (5.9 mm²) and large-area tandem cells (1.15 cm²) to 24.4% and 22.2%, respectively, representing high light stability.

He's group^{[ 180 ]} used polyethylenimine ethoxylated (PEIE) in the SnO₂ ETL structure as composite (SnO₂:PEIE) and bilayers (PEIE/SnO₂ and SnO₂/PEIE) structures, that PEIE is a polyelectrolyte containing simple aliphatic amine groups. By comparing these three ETL structures, they found that the presence of PEIE layer under SnO₂ layer has the best performance. As the underneath PEIE layer, by adjusting the surface energy and work function of the top SnO₂ layer, improved the nucleation process of perovskite layer and reduced the energy-level mismatch between ETL and perovskite layer. Accordingly, they succeeded to achieve PCE of 21.44% using PEIE/SnO₂ bilayer ETL compared to pure SnO₂ (18.42%), SnO₂/PEIE bilayer (10.52%), and SnO₂:PEIE composite (19.06%) ETLs. Li et al.^{[ 181 ]} by adding tris(pentafluorophenyl) boron (TPFPB) as interfacial modification layer between SnO₂ ETL and perovskite layer formed a compact perovskite layer and decreased defects on SnO₂ film. They reported that the presence of the interfacial TPFPB layer improves the interface performance and enhances the crystallinity of MAPbI₃, and thus the fabricated device with TPFPB modification shown fast charge transfer and low trap state density. By optimizing the amount of TPFPB, they enhanced PCE from 16.92% to 19.41%, which resulted from the weak chemical bonds between TPFPB and the top and underneath layers, i.e., the F and B atoms in TPFPB forming H–F, N–B bonds with SnO₂ and perovskite (MAPbI₃), respectively. Gao et al.^{[ 182 ]} demonstrated to regulate the interface ETL/perovskite by imidazole bromide functionalized graphene quantum dots (I-GQDs). Their results showed the incorporation of the functional groups on the surface of I-GQDs at the interface can improve the conductivity and eliminate surface defects of SnO₂ ETL to achieve a better E_CB alignment between SnO₂/perovskite. The presence of I-GQD interlayer enhanced the interfacial charge carrier transfer and inhibited the interface charge recombination. Gao group^{[ 183 ]} modified the SnO₂-based ETL in PSCs by insertion of fused-ring organic semiconductors (FROSs) layer on the SnO₂ layer. They used three types of FROSs (IDT-I, IDT-T, and IDDT-T), which include indacenodithiophene (IDT) or indacenodithienothiophene (IDDT) as the bridging donor moiety and 1,3-diethyl-2-thiobarbituric or 1,1-dicyromethylene-3-indanone as the strong electron-withdrawing units. IDT-T and IDT-I include one IDT backbone with two 1,3-diethyl-2-thiobarbituric and two 1,1-dicyromethylene-3-indanone electron-withdrawing end groups, respectively. In comparison, IDDT-T include an IDDT ore with two 1,3-diethyl-2-thiobarbituric terminal groups. The interface decorated by FROSs enhances the charge extraction and transportation at the interface between ETL/perovskite layer.

3.2.2 SnO₂ and Inorganic Materials

In addition to organic compounds, inorganic materials along with SnO₂ have been utilized as ETL in PSCs, which could better the morphological properties, tune the band alignment, and enhance the V _OC in PSCs.^{[ 184, 185 ]} Case in point, the application of TiO₂ in combination with SnO₂ as ETL promotes more perfect segregation of photogenerated carriers and electron transport due to the increased charge extraction ability and the hindrance of carrier recombination in the ETL/perovskite interface. Liu et al.^{[ 184 ]} utilized SnO₂ to passivate TiO₂ surface aimed at the fabrication of PSCs based on carbon electrode with the CsPbBr₃ perovskite material. By inserting a SnO₂ layer between the TiO₂ layer and that of the perovskite, they could elevate the efficiency from 8.12% to 8.79%. Furthermore, they could successfully achieve a PCE of 6.9% for the module based on TiO₂/SnO₂ bilayer ETL with an active area of 1 cm². Correspondingly, Deng et al.^{[ 186 ]} could realize the PCE of 15.14% utilizing the TiO₂/SnO₂ bilayer layer as ETL in the planar structured PSC. Besides, they could manage to increase the efficiency to 16.74% through the modification of TiO₂ with HI and by the optimization of the conditions, a further PCE enhancement of up to 18.05% was achieved.^{[ 185 ]} Li et al.^{[ 84 ]} employed TiO₂, SnO₂, and TiO₂/SnO₂ bilayer as ETL in planar structured PSCs and obtained PCEs of 14.53%, 17.21%, and 19.11%, respectively. In like manner, Xie et al.^{[ 187 ]} could achieve the PCEs of 13.15%, 16.7%, and 18.85%, respectively, wherein TiO₂, SnO₂, and TiO₂/SnO₂ bilayer was employed as ETL in the planar PSCs.

The utilization of TiO₂ is not merely limited to beneath the SnO₂ layer as the TiO₂/SnO₂ bilayer ETL, and its utility above the SnO₂ as the structure of SnO₂/TiO₂ bilayer ETL has also been reported. Martinez-Denegri et al.^{[ 85 ]} achieved efficiencies of 14.2%, 8.6%, and 14.9%, respectively, using TiO₂, SnO₂, and SnO₂/TiO₂ bilayer as ETL in planar structured PSC. Hu et al.^{[ 188 ]} made use of SnO₂/TiO₂ and improved the PCE from 18.09% to 20.50% in light of cascade-aligned energy levels of ITO/SnO₂/TiO₂/perovskite, multicomponent for the charge transporting layers including ETLs and HTLs and the reduced defects in the perovskite film. Wu's group^{[ 189 ]} designed five LT-processed ETLs (SnO₂, TiO₂, TiO₂:SnO₂ composite, TiO₂/SnO₂ bilayer, and TiO₂:SnO₂/SnO₂ bilayer) and found TiO₂:SnO₂/SnO₂ bilayer is optimized ETL structure with PCE of 22.04%. In this optimized design, there was TiO₂:SnO₂ layer as the bottom layer that contained the small TiO₂ and highly conducting big SnO₂ nanoparticles for making a dense film to avoid the short circuit, and increasing the conductivity of ETL, adjusting the CB level for reducing the charge accumulation and series resistance of the cell, respectively. A top SnO₂ layer was used to facility electron transport and to create a hydrophilic surface for deposition of high-quality perovskite films. The TiO₂:SnO₂ nanoparticle suspension used for fabricating a TiO₂:SnO₂ nanocomposite was prepared by ball grinding of the TiO₂ and SnO₂ powder mixture in ethanol at room temperature without adding any surfactant and the top SnO₂ layer was spin-coated from Alfa-SnO₂ nanoparticle water suspension. Rao et al.^{[ 190 ]} used TiO₂ compact layer between FTO substrate and SnO₂ layer, and they improved the PCE by reducing the trap-assisted recombination with the incorporation of KI and KC₆H₅BF₃, respectively, into perovskite film.

In addition to TiO₂, MgO has also been recognized as a promising candidate to use as ETL along with SnO₂. Ma et al.^{[ 191 ]} by adding a thin layer of MgO between the substrate and SnO₂, could substantially increase the PSC efficiency from 16.43% to 18.23%. Likewise, Dagar et al.^{[ 108 ]} were able to increase PSC performance by employing a thin layer of MgO to the SnO₂ layer, therein making use of MgO, SnO₂, SnO₂/MgO bilayer, and SnO₂/SnO₂ bilayer as ETLs in planar PSCs. Also Wang et al.^{[ 192 ]} used MgO layer between the SnO₂ and perovskite (CsPbIBr₂) to passivate the undesirable recombination and thereby V _oc increases. The top MgO layer provided a better qualify perovskite formation and reduced the interface vacancy defects and also its tunneling effect and better band alignment with perovskite effectively blocked the hole carriers and accelerated the electron carriers to the electrode. Yan and coworkers^{[ 193 ]} realized the highest V_oce of 1.23 V for PSCs based upon SnO₂/ZnO bilayer using a ZnO layer of optimized thickness on the SnO₂ layer. The presence of the ZnO layer possessing a conduction band position between SnO₂ and the perovskite layer makes improvements in the extraction and transport of carriers and reduces recombination as well. Therefore, utilizing ZnO, SnO₂, and SnO₂/ZnO bilayer as ETL in planar PSCs. To diminish the energy loss in MAPbI₃-based planar PSCs, Noh et al.^{[ 194 ]} proposed the utilization of SnO₂/ZnO bilayer ETL owing to its befitting energy band matching at the interface of MAPbI₃. Besides, thermal annealing lowered the Fermi level of the bottom ZnO (not-annealed) layer in the bilayer ETL and enhanced the superior film quality, suppressed the trap density, and reduced the charge recombination at the interface of ETL/perovskite making for PCE increment up to 20.43%. Yang et al.,^{[ 195 ]} as to the lower work function of ZnO compared to that of SnO₂ curtailing the energy losses, have employed ZnO/SnO₂ bilayer as ETL. With this bilayer ETL, a high iodine content CsPbI₂Br film was formed on which a regular crystal grain, as well as full coating coverage and a more perfect crystal matching between perovskite and SnO₂, was realized.

Since the conduction band of In₂O₃ is higher than that of Sn-doped In₂O₃ (ITO), Wang^{[ 196 ]} has utilized In₂O₃/SnO₂ bilayer ETL and enhanced the charge transport from perovskite to the ETL. Whereas SnO₂ layers yielded 21.42% efficiency, and the obtained ETL represented high uniformity, compactness, as well as low-trap-density and elevated the PCE further to 23.24%. Dagar et al.^{[ 107 ]} also applied an Al₂O₃ layer on the SnO₂ layer as the ETL bilayer in the planar PSCs. Nevertheless, Al₂O₃ had previously been employed as a scaffold, a thin layer of 50 nm particles was deposited therein. The effect of this layer on the shunt and series resistance (R _sh and R _s) and series as well as charge extraction facilitation made for enhanced efficiency. Therefore, they could realize the PCE elevation from 15.3% in PSCs based on SnO₂ ETL to 20.1% in that of PSCs based on SnO₂/Al₂O₃ bilayer ETL. Furthermore, another method of the hole blocking as well as preventing the recombination on defective surface coverage is by far the utilization of a WO _x layer between the electrode and SnO₂. By applying this, Wang et al.^{[ 197 ]} managed to increase the PCE of SnO₂ ETL-based PSCs from 17.67% to 20.44%. Sun and coworkers^{[ 198 ]} modified ITO using a ZrO₂ interlayer between ITO and SnO₂ prepared through ultraviolet (UV) treatment at room temperature. The effects of the interlayer on the performance of MAPbI_3−x Cl _x -based PSCs could reach to champion efficiency of 19.48% with negligible J−V hysteresis (more than reference 15.56%). The improved performance has resulted from the reduced trap states, the promoted charge extraction at the ITO/SnO₂ interface, and the suppressed carrier recombination at the ITO/SnO₂ interface.

Lin et al.^{[ 199 ]} utilized SnO₂/SnO₂ bilayer ETL to diminish the hysteresis of PSCs based on LT-SnO₂ ETL in which the lower SnO₂ layer was prepared through spin-coating of chloride precursor and that of the upper layer was prepared through the spin-coating of colloidal nanoparticle ink. Zhang et al^{[ 200 ]} introduced an ultrathin and compact SnO _x amorphous layer derived from SnCl₄ at the interface of SnO₂/perovskite or FTO/SnO₂ to form SnO₂/SnO _x bilayer ETL or SnO _x /SnO₂ bilayer ETL, respectively, to enhance the electron coupling between layers, passivate the trapping defects, and optimize the energy-level alignment. As a result of the increased interface electron collection and reduced interface recombination, the planar PSC with pretreated SnO₂ nanocrystal ETL exhibited boosted efficiency from 16.3% to 18.6%. Besides, posttreated SnO₂ nanocrystal ETL made the efficiency increment from 16.3% to 17.3% which these results indicate that both treatment methods could improve the performance of the PSCs to great effect. Likewise, Sun et al.^{[ 201 ]} could manage to fabricate a PSC based on SnO₂/SnO _x bilayer ETL by inserting an amorphous SnO _x layer via a tin chloride precursor ink spin-coated on the SnO₂ nanocrystalline layer. Also Kang's group^{[ 202 ]} used SnO _x /SnO₂ bilayer ETL for ohmic contact improvement and recombination decreasing in the ETL/perovskite interface to achieve the PCE of ≈20.39% for lab scale and ≈14.93% for large-area PSCs (active areas of ≈3.55 cm²). Moreover Choopun's group^{[ 203 ]} used SnO₂/SnO _x bilayer ETL and compared with SnO₂ ETL in PSCs. Guo et al.^{[ 204 ]} used the SnO₂:InCl₃ as a top layer on SnO₂, because the Cl ions can well passivate the SnO₂ surface defects and perovskite grain boundaries by Cl ions diffusion into the perovskite layer and Pb–Cl bonds formation with Pb ions in the perovskite, furthermore, In can improve the SnO₂ electron mobility. Hereupon, the PCE increased because of the lower interfacial trap density, larger perovskite grain size, and higher ETL electron mobility. Ye et al.^{[ 205 ]} fabricated several types of SnO₂-based ETLs by pure, low-, and high-amount NH₄Cl-doped SnO₂ (L-NH₄Cl:SnO₂ and H-NH₄Cl:SnO₂). Based on their results, they found the best performance for L-NH₄Cl:SnO₂/H-NH₄Cl:SnO₂ bilayer as an ETL. Also Song et al.^{[ 206 ]} designed and used a SnO₂/NH₄Cl:SnO₂ bilayer as an ETL in PSC structure. They reported the top layer showed a better energy-level alignment with perovskite and reduced the alkalinity of ETL/perovskite interface to avoid perovskite degradation and hereupon enhanced electron extraction and interfacial stability. Also the underneath SnO₂ retained the capability of efficient carrier transport to avoid charge accumulation.

Shi et al.^{[ 207 ]} obtained the optimized PCE of 21.28% in the PSC through the application of a two-step deposition of SnO₂ following UV-assisted thermal annealing for SnO₂/SnO₂ bilayer ETL. Thereafter, they could also manage to passivate oxygen vacancy defects through the deposition of the small multifunctional molecule pentetic acid (i.e., DTPA) on the surface of the ETL bilayer and the consequent formation of a chemical reaction and bonding between the DTPA carboxylic groups and the undercoordinated Sn on the surface of the SnO₂ bilayer ETL. What is more, the presence of hydrophilic DTPA on the ETL surface allowed the homogeneous perovskite nucleation via reducing Gibbs-free energy in the DTPA-SnO₂ complex. Furthermore, the formation of coordination of DTPA carboxylic groups with undercoordinated Pb ions made for the passivation of perovskite.

Chen et al.^{[ 208 ]} used the Lanthanide ions (La³⁺,Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺, Yb³⁺) doped WO _x nanorods were prepared by the hot solvent method for perovskite crystal quality improving and charge recombination hindering by interface traps/defects reducing between the ETL and perovskite. As their result, the La:WO _x /SnO₂ bilayer ETL showed the best PCE of 21.42% with high stability in compared to SnO₂-based devices (19.08%). Zhuang et al.^{[ 209 ]} used rubidium halides (RbF, RbCl, and RbI) as the top layer of SnO₂ to form SnO₂-based bilayer ETLs, based on their efficiency (21.12%, 20.37%, and 20.32%), RbF showed the best performance. They also used RbF to prepare the SnO₂:RbF composite ETL and compared its performance with the SnO₂/RbF bilayer ETL. According to their achievements, in SnO₂:RbF composite ETL, F and Sn have strong interaction and contributing to the improved electron mobility of SnO₂ and charge transport, and in SnO₂/RbF bilayer ETL, Rb⁺ cations actively escape into the interstitial sites of the perovskite lattice to inhibit ions migration and reduce nonradiative recombination, which dedicates to the improved V _oc. Also, they noted that F⁻ stays in place and does not enter the perovskite. Finally, they were able to increase the PCE to 23.38% by modifying the perovskite via p-methoxyphenethylammonium iodide (CH₃O-PEAI).

Zhou's group^{[ 210 ]} has reported using the diboron (B₂Cat₂) molecule as the interfacial modifier between SnO₂ and perovskite. Based on the SnO₂/B₂Cat₂ bilayer ETL, they obtained a higher E_F on the surface of SnO₂, resulting from a better energy matching at ETL/perovskite interface and a higher PCE of 22.04%. They showed SnO₂/B₂Cat₂ bilayer possesses some Sn³⁺ species, acting as electron donors to obtain a more n-type nature of ETLs. And speculate that the formation of surface diboron–oxygen Lewis pair induces a reducing state of diboron complexes, which further enables spontaneous electron redistribution for the formation of Sn³⁺–O^−•species on the surface of SnO₂Chen et al.^{[ 211 ]} introduced two-dimensional Bi₂O₂Se nanoflakes to modify the surface of SnO₂ thin film with high charge mobility. They synthesized Bi₂O₂Se nanoflakes by facile composited molten salt method. SnO₂/Bi₂O₂Se bilayer ETL provides a smoother and more hydrophobic surface for larger perovskite crystal formation. The PCE of PSCs based on SnO₂/Bi₂O₂Se bilayer ETL compared with PSCs based on SnO₂ ETL was improved to 19.06% from 16.29%.

MXenes are 2D mineral sheets of transition metal carbides and nitrides, which have been highly interested owing to their metal conductivity, low electrical resistance, high hydrophilicity, and structural diversity.^{[ 212 ]} The general formula of MXenes is M _n+1X _n T _x , in which M represents an early transition metal, X is carbon and/or nitrogen, n = 1–3, and T _x stands for the termination functional groups (O, OH, and/or F).^{[ 91, 212, 213 ]} Highly transparent Ti₃C₂T _x MXene with high electron mobility also possesses excellent metallic conductivity and superior thermal stability.^{[ 214 ]} Due to the rich functionalized surface groups of Ti₃C₂T _x MXene, the work function of Ti₃C₂T _x MXene ranges from 1.6 to 6.25 eV.^{[ 214 ]} Wang et al.^{[ 214 ]} modified surface between FTO and SnO₂ by inserting a thin layer of Ti₃C₂T _x MXene, which led to well energy-level alignment, enhanced electron mobility, and efficient charge transfer dynamics. MXene/SnO₂ bilayer ETL led to a significantly reduced nonradiative recombination and enhanced charge extraction. In addition, surface of MXene/SnO₂ bilayer ETL has a more hydrophobic and smoother morphology, beneficial for the growth of a perovskite film with low trap density which achieved an enhanced efficiency of 20.65%. Zhang group^{[ 215 ]} by using aluminum acetylacetonate (Al(acac)₃) as a cross-linking agent, which is used for interface layer between SnO₂ and perovskite could achieve high reported PCE (20.87%) for flexible PSCs up to now. It was due to the well-matched energy levels and improved grain size and crystallinity of the perovskite and also Al(acac)₃ is similar to the spiderweb which can act as an effective barrier. Based on their achievements, the oxygen moiety of the carbonyl group of Al(acac)₃ can interact with Pb atoms of perovskite and forming a Lewis adduct to passivate defects at the grain surface. Also, adhesive interaction of the Al(acac)₃ can produce a compact and protective structure to resist water infiltration and mechanical bending, improving the overall stability of flexible PSCs and enhancing bending resistance. So that attributed to the Al(acac)₃ super bending resistance, more than 91% of the original PCE.

3.2.3 SnO₂ and Organic/Inorganic Materials

He et al.^{[ 216 ]} designed a sandwich-like architecture using an ultrathin bandgap tunable carbon quantum dots (CQDs) layer inserted between ultrathin SnO₂ bottom layer and SnO₂ top layer like SnO₂/CQDs/SnO₂. The bottom ultrathin SnO₂ layer passivates the defects of FTO and reduces the carrier recombination, while the CQDs layer enhances the optical transmission of ETLs, accelerates the carrier transport process, and improves the hole-blocking ability, resulting in the PCE of 20.78% (boosted from 17.46%) which is hysteresis free. Prasad’S and Ng's groups^{[ 217 ]} introduced an optimal amount of organic cross-linker, 2,20—(ethylenedioxy)bis (ethylammonium iodide) (EDAI) for defect passivation in the bulk of mixed cation perovskite and ETL consisted of SnO₂ quantum dots (SnO₂ QD) or SnO₂ nanoparticles (SnO₂ NP). By this approach due to the generation of defects and electrically insulating properties of EDAI, the hysteresis is reduced, efficiency enhanced, and stability improved. Therein, they employed a SnO₂ QD/SnO₂ NP/PMMA: PCBM multilayer as an ETL in PSCs which yielded the PCE of 19.5% for 0.06 cm² active area and 12–18% for the larger area devices from 0.15 to 0.85 cm² active area.

3.3.1 Dopants

Among the initiatives for modifying the LT-SnO₂ ETL is the application of elemental dopants in the SnO₂ crystal lattice. The elements employed as dopants are principally cationic metal elements of different capacities (Li, Al, Ga, La, Ta, Nb, Y, Yb, and Gd)^{[ 41, 218-226 ]} and anionic halogens including F, Cl.^{[ 165, 227, 228 ]} Broadly speaking, the presence of dopants in SnO₂ could enhance its electrical, optical, and structural properties; however, their mechanisms of action in SnO₂ appeared to be intricate. Table 4 summarizes the performance and precise parameters of PSCs based on LT-SnO₂ ETL modified via elemental doping. Besides, the doped elements in LT-SnO₂ along with their improved properties have been depicted in Figure 27 .

Improvement of Electrical Properties

In the case of the electrical properties improvement, the dopants make for the desired energy band alignment between SnO₂ and the perovskite through the energy position shifting of the E _CB.^{[ 40, 229 ]} The cationic and anionic dopants enhance the conductivity or mobility of SnO₂ through increasing the diffusion coefficient and the density of the free charge carriers, respectively.^{[ 229, 230 ]} Moreover, the enhancement of these electrical properties of SnO₂ leads to reduced resistance, current leakage, and nonradiative charge recombination at the ETL/perovskite interface.^{[ 221, 231, 232 ]} Consequently, the extraction and transport of electron charge at the perovskite/ETL interface are upgraded.^{[ 40 ]}

Liu and coworkers^{[ 221 ]} have for the first time utilized Nb⁵⁺ as a dopant in the SnO₂ structure owing to its ionic radius close to Sn⁴⁺ to adjust the electrical properties of the SnO₂ layers as well as the energy band alignment between SnO₂ and the perovskite. The results demonstrated that Nb⁵⁺ doping increased the conductivity of SnO₂ ETL and made for a downward shift of E_CB position of SnO₂ toward that of the perovskite E_CB. Hence, this facilitates the injection of electrons from the perovskite into SnO₂ accelerating the charge transport and reducing nonradiative recombination. Accordingly, they could successfully improve the PCE of PSC devices from 18.06% to 19.38% through the Nb⁵⁺ doping in SnO₂ ETL.

Bai et al.^{[ 229 ]} proposed SnO₂ doping with Sb for the efficient development of inorganic ETLs. Thereby, the doping of SnO₂ with the Sb has resulted in the significant electrical conductivity enhancement due to the increase in the electron carrier concentration. The free electrons overflow has partially pervaded the surface traps and caused the Fermi-level energy position to shift upwards (Figure 28 ). In consequence, the optimized energy band alignment between SnO₂ and perovskite stemming from the change in Fermi-level shifting reduced the charge recombination and thus sustained the electron recombination lifetime and improved the V _OC. As a result, the group could manage to increase PSC efficiency from 15.7% to 17.7% through the doping of Sb into SnO₂ ETL. Moreover, the further investigation of this group indicated that the highest conductivity of Sb-doped SnO₂ occurs in the presence of a certain amount of Sb dopant (i.e., 3–5 mol%) in which the concentration of charge carriers, as well as mobility, was at an elevated level. However, the excessive amount of Sb dopant higher than the optimized would bring about the increment in the effective activation energy of the donor and a rapid decrement in carrier mobility leading to intensified disorder or scattering.

Enhancement of Structural Properties

With regard to the utility of dopants for structural properties improvement, their presence in the SnO₂ crystal lattice would increase the lattice constant which in turn increases the grain size and diminishes the crystal aggregation.^{[ 41, 220 ]} Hence, these phenomena induce the formation of a layer of uniform, dense, and pinhole-free coverage^{[ 220 ]} impacting upon the perovskite deposition and leading to an increase in the size of perovskite grains as well as a grain boundary decrease in the perovskite layer.^{[ 165 ]} Furthermore, it is observed that cationic dopants of capacities less than Sn⁴⁺ (e.g., Li⁺, La⁺, Zr³⁺, Eu³⁺, and Nd²⁺),^{[ 41, 231, 233-235 ]} as well as anionic dopants (i.e., F⁻, Cl⁻),^{[ 165, 227, 236 ]} could passivate the oxygen vacancy defects. Therefore, in the company of these dopants, a reduction in interfacial defect or trap density is realized.^{[ 236 ]} What is more, it has been specifically reported that SnO₂ doped with halogen element fluorine represents superior lattice matching with that of the FTO substrate owing to its high resemblance with FTO.^{[ 227 ]}

Gong and coworkers^{[ 165 ]} produced chlorine-doped SnO₂ nanocrystals (Cl-doped SnO₂) (Figure 29a). They recognized that in consequence of the Cl halogen making the surface of Cl-doped SnO₂ ETL more hydrophobic (Figure 29c), the grain size and crystallization of perovskite have been remarkably enhanced which, in turn, led to a significant reduction in the charge trap density and recombination of perovskite film. Besides, more expedited charge extraction and transport are brought about by the establishment of the desired band alignment between the perovskite and the ETL (Figure 29b). Accordingly, via doping SnO₂ with chlorine, the group could successfully increase the PCE of PSCs from 15.07% to 18.1% (Figure 29c).

Song et al.^{[ 76 ]} could also improve the electrical conductivity of ETL via doping LT-SnO₂ ETL with Zn (Figure 30a,b). This might have reduced the distance between the Fermi level and the E_CB of ETL (Figure 30c) and have enhanced its conductivity through the formation of zinc interstitials (Zn_i) or oxygen vacancies (V _O). Notwithstanding, semiconductor conductivity could be enhanced by the reduction in defect density. Besides, due to the presence of Zn²⁺ dopant, the UV-ray absorption of ETL increased which alleviated the perovskite layer impairments under UV light. Moreover, the results demonstrated that the SnO₂ crystal lattice size came to have a significant increment with zinc doping which implies the larger the lattice size the lower the defect density. Therefore, the doping of Zn²⁺ in SnO₂ reduces the resistance of ETLs. Furthermore, Zn²⁺ doping elevated the E _CB of the ETL, leading to a more perfect electron transport as well as reducing the E_CB difference between the perovskite and that of the ETL, resulting in a V _OC decline mitigation. On that account, the group could manage to boost the PCE of PSCs from 18.95% to 20.16% through doping SnO₂ with Zn (Figure 30d,e). Thereupon, they could also fabricate a flexible PSC based upon Zn doped-SnO₂ ETL exhibiting a 15.25% PCE which retained 84% of its initial efficiency after 100 bending cycles (Figure 30f,g).

Improvement in Optical Properties

Dopants usually change the electrical properties of SnO₂. Nevertheless, some of them, such as La,^{[ 41 ]} Nb,^{[ 237 ]} Y,^{[ 219 ]} and Cl,^{[ 165 ]} enhance the transparency of the SnO₂ layer through the elevation of the bandgap,^{[ 219 ]} reducing the crystalline aggregation^{[ 41 ]} and/or smoothing the surface.^{[ 237 ]} Thus, these dopants increase the photocurrent in PSCs by improving the optical properties of SnO₂ ETL.^{[ 41, 219, 237 ]} Xu and coworkers^{[ 41 ]} employed La-doped SnO₂ as a blocking layer within planar PSCs. The inferior surface morphology of LT-SnO₂, especially the presence of pinholes after the low-temperature annealing process, confines its application in the PSCs. Therefore, to surmount the SnO₂ drawbacks, the group-doped lanthanum (La) in the SnO₂ layer, which has resulted in the reduced aggregation of SnO₂ crystals and construction of a uniform film. Moreover, it was observed that La doping affects the crystallization of SnO₂ during the annealing process and improves the morphology of the SnO₂ layer. Besides, La doping makes for an enhancement in the conductivity and upward shifting of the E _CB of the ETL, which in turn leads to electron extraction and V _OC increment as well as recombination and hysteresis abolishment. In consequence, the group could manage to advance the performance of PSCs from 14.24% to 17.08% PCE via doping SnO₂ with the La.

Yang et al.^{[ 219 ]} reported that SnO₂ doped with yttrium (Y-doped SnO₂) has significantly reduced hysteresis and elevated PSC performance (Figure 31a). This facilitated the formation of well-aligned SnO₂ nanosheet arrays of a higher homogeneous distribution, which has served the more beneficial perovskite diffusion and the better interface between perovskite and ETL. Furthermore, the Y-doped SnO₂ nanosheet arrays demonstrate higher conductivity as well as facile and firm charge extraction and transport (Figure 31b). Ever since the ETL bandgap is ascended by the Y doping in SnO₂, the optical transparency of the layer increases as well (Figure 31c). What is more, owing to the more perfect homogeneous distribution of Y-doped SnO₂ nanosheet arrays, light scattering is also reduced in order that the light absorption of perovskite film using Y-doped SnO₂ ETL is approximately higher than that of the pristine SnO₂ ETL throughout the visible spectrum. This could be attributed to the enhancement of the perovskite/ETL interface and the superior deposition of the perovskite material. This improved light absorption property of the perovskite film built upon the Y-doped SnO₂ ETL might enable the perovskite absorbent layer to harvest light more efficiently and thus render a higher J _SC in PSCs based on Y-doped SnO₂ ETL. Conclusively, by employing the doping of SnO₂ with Yttrium, the group could boost the PCE of PSCs from 13.38% to 17.29%.

3.3.2 LT-SnO₂-Based Composites and Complexes

The utilization of SnO₂-based nanocomposites in the other solar cell configurations including dye sensitized solar cells (DSSCs) has been already suggested.^{[ 240 ]} In the whole course, to improve the performance of LT-SnO₂ ETLs, both organic materials such as graphene,^{[ 241 ]} carbon nanodots,^{[ 242 ]} and nanotubes,^{[ 243 ]} as well as inorganics such as Bi₂O₂S,^{[ 244 ]} KCl,^{[ 245 ]} and MXene,^{[ 91 ]} have been utilized to form a complex with SnO₂. The application of these complexes renders several advantages like reduced recombination at the interface, elevated charge transport, increased stability of the precursor ink, and higher wettability. In this section, the research literature on the application of SnO₂ ETL composite and complex is surveyed in three categories including SnO₂/carbon composites, SnO₂/inorganic composites, and SnO₂-based complexes which their working mechanisms and performances in PSCs based on LT-SnO₂ are thoroughly discussed. Eventually, the performance and precise PV characteristics of PSCs based on LT-SnO₂ composites and complexes are summarized in Table 5 .

SnO₂/Carbon Composites

Wang and coworkers^{[ 242 ]} obtained carbon nanodots (NDs) using the hydrothermal method and applied them within SnO₂ ETLs (Figure 32a). The results demonstrated that owing to the presence of carbon NDs, the density of trap states was reduced and the mobility in SnO₂ film was enhanced. The schematic illustration of the electron transport from the perovskite layer to the SnO₂: carbon NDs composite layer is represented in Figure 32b,c. Therein, resulting from the reduction of defect and resistance in SnO₂: carbon NDs ETL and the interaction between carbon NDs and FA or MA cations in the perovskite layer, electrons could be extracted and transported more facilely by means of carbon NDs in ETL. Therefore, through the formation of SnO₂: carbon NDs composites, this group could successfully increase the PCE of LT-SnO₂-based PSCs from 18.54% to 20.03% (Figure 32d).

Tang et al.^{[ 243 ]} utilized carbon nanotubes (CNTs) in SnO₂ ETLs and designated high-performance, hysteresis-free planar PSCs. Under the utility of SnO₂ and CNTs composites, the conductivity of ETL has been significantly improved, and the trap states density has been reduced leading to a more effective electron extraction and transport. Therein, they could elevate the PCE of LT-SnO₂-based PSCs from 17.9% to 20.33% by making the composites of SnO₂:CNTs. Hui et al.^{[ 246 ]} employed red carbon quantum dots (QDs) enriched by carboxylic acid and hydroxyl functional groups for synthesizing the composites with SnO₂ and the preparation of ETL. The results showed that by making composites of SnO₂ with red carbon QDs, the quality of perovskite films was improved due to the reduction in Gibbs-free energy of SnO₂ surface owing to the thereby increased hydrophilicity. This was remarkable in the way that the crystal phase of perovskites grown on SnO₂: red carbon QDs composite ETL was purer and more uniform than that of the perovskites grown on SnO₂ ETL. Moreover, mobility has been significantly enhanced in this composite compared to that of the pristine SnO₂, which leads to improved charge extraction and transport. Conclusively, through the application of SnO₂ with red carbon QDs composite, they could manage to increase the efficiency of LT-SnO₂-based PSCs from 19.15% to 22.77%.

Employing a furnace aerosol reactor, Kouhnavard and coworkers^{[ 247 ]} converted 2D graphene oxide (GO) nanosheets into 3D crumpled structures, then utilized them to make a composite with SnO₂ and further improve the performance of PSCs based on SnO₂ ETL. They demonstrated that the application of SnO₂ with crumpled-GO composite ETL enhanced the charge transport in ETL and reduced charge recombination in the perovskite layer. Likewise, Zhou et al.^{[ 248 ]} applied the SnO₂:graphene QDs composite as the LT-ETL in the PSC. The results indicated that this composite presented higher electron mobility, a more perfect film coverage, and desired energy level compared with that of SnO₂, which facilitated and alleviated the charge transport and recombination, respectively. By optimizing the size and concentration of graphene QDs, they improved the conductivity, the film coating uniformity, and the Fermi-level energy adjustment of the ETL with perovskite which better electron transport and reduced charge recombination at the interface. In consequence, benefitting from SnO₂ with graphene QDs (with 0.5% wt. of graphene QDs) composite ETL, the group could successfully boost the efficiency of LT-SnO₂-based PSCs from 16.80% to 19.60%. In the same manner, Pang et al.^{[ 249 ]} employed SnO₂ with Graphene QDs composited as ETL in the architecture of planar PSCs. There again, the results demonstrated that the incorporation of the graphene QDs between SnO₂ and perovskite could enhance the perovskite film quality, expedite the electron transport, and diminish the charge recombination. Consequently, they could increase the PCE of LT-SnO₂-based PSCs from 18.60% to 21.10% making use of SnO₂ with graphene QDs composite. Zhang et al.^{[ 250 ]} used graphdiyne (GDY) to prepare the SnO₂ ETL layer. GDY has a new carbon allotrope with a combination of sp and sp² and a completely π-conjugated and rigid structure. By maximizing collocation between SnO₂ and perovskite, GDY accelerates electron extraction and improves interface as well as passivates surface defects. Thus, they succeeded to achieve PCE of 21.11% using SnO₂:GDY composite.

Chen and coworkers^{[ 241 ]} utilized a carbonitride-modified SnO₂ nanocomposite, i.e., SnO₂/graphitic carbon nitride (g-C₃N₄) quantum dots nanocomposite, as the ETL in the PSC structure. g-C₃N₄ possesses high conductivity and chemical stability, the capability to improve traps, and corrosion resistance. As such, they exhibited that the high-conductive g-C₃N₄ could effectively passivate oxygen-vacancy-reduced-traps through modifying the electron density distribution around the SnO₂ crystal unit, thereby increasing charge transport within the bulk and at the interface. Furthermore, besides the high conductivity, this nanocomposite offers extra desirable electrical properties such as optimal energy-level alignment. Therefore, they could manage to elevate the efficiency of LT-SnO₂-based PSCs from 20.21% to 22.13% with the utility of making SnO₂:g-C₃N₄ nanocomposite. Cao et al.^{[ 251 ]} also used sulfur-doped g-C₃N₄ nanoparticles (S doped-g-C₃N₄) to prepare the SnO₂:S-doped g-C₃N₄ nanocomposite, which increased electron mobility and conductivity of ETL layer. In addition, by passivation of the ETL/perovskite interface via the interaction of sulfur atoms in S-doped g-C₃N₄ with undercoordinated lead (Pb) ions in perovskite films, the surface recombination of the carriers was significantly reduced. Therefore, using this strategy, the PCE increased from 18.98% to 20.33%.

Luan et al.^{[ 123 ]} employed 2,2,2-trifluoroethanol (TFE) in SnO₂ ETL and increased its mobility through the preparation of a composite. They showed that this structure has dramatically decreased the density of trap states at the interface and thorough the perovskite absorbent which in turn has remarkably diminished the charge recombination. Therein, by making a composite of SnO₂ with TFE, the group realized the efficiency enhancement of LT-SnO₂-based PSCs from 19.17% to 20.92%. Huang and coworkers^{[ 252 ]} fabricated and developed highly efficient PSCs using SnO₂ and tetramethylammonium hydroxide (TMAH) composites as LT-ETL. The results indicated that TMAH has led to conductivity enhancement as well as charge transport enhancement and accordingly, they could manage to increase the efficiency of LT-SnO₂-based PSCs from 18.14% to 21.05% using this composite structure.

Bi et al.^{[ 253 ]} using Girard's Reagent T (GRT) incorporating molecules with several functional groups to modify SnO₂ nanoparticles. Using SnO₂: GRT nanocomposite, they were able to increase the PCE of PSCs from 19.77% to 21.63% through the following three factors: 1) Improving the electrical properties and quality of SnO₂ film by inhibiting nanoparticle agglomeration through strong chemical interactions between GRT and SnO₂ 2) facilitate vertical growth and grain size increasing of perovskite crystals by improving wettability and reducing the surface roughness of the SnO₂ layer, and 3) reduction of surface defects through the effective passivation of the quaternary ammonium cation and chloride in GRT for anionic and cationic defects at the surface of the perovskite film.

Zhang et al.^{[ 254 ]} used monosodium glutamate (MSG) to prepare the SnO₂:MSG composite. MSG is known as gourmet powder and includes certain functional groups such as carboxyl group, amino group, and Na⁺ cation ion. As a food additive, MSG is a nontoxic, low-cost, and abundant reserves, and more importantly, MSG can be dissolved directly in water. They reported that due to the interaction between electrical dipole and strong crosslinking MSG, the ETL/perovskite interface is optimized, charge transfer is accelerated, and interface defects are reduced. Accordingly, this group succeeded to increase the PCE from 16.56% to 17.71%. Huang et al.^{[ 255 ]} used polyethylenimine ethoxylated (PEIE) to make the SnO₂: PEIE nanocomposite. PEIE is an organic polymer with a simple aliphatic amine group that can effectively smoothen the surface of the substrate (such as ITO) and reduce its work function. In addition, the presence of PEIE improves the conductivity and mobility of SnO₂ film. Finally, this group was able to increase the PCE of PSCs from 18.74% to 20.61% by using SnO₂: PEIE composite.

You et al.^{[ 256 ]} reported that they used a biological polymer of heparin potassium (HP) to modulate the arrangement of SnO₂ nanocrystals and to fabricate the SnO₂:HP composite. Heparin is a linear sulfated polysaccharide containing ≈2.7 sulfate groups per saccharide, which can be coordinated with Pb atoms of perovskite. High negative charge density of HP leads to stabilization of SnO₂ nanocrystals in dispersion. Functional groups in HP can bind perovskites to well-arranged SnO₂ nanocrystals, leading to vertically aligned crystal growth. Thus, using SnO₂:HP composite, they managed to achieve PCEs to 23.03% (with rigid glass/ITO substrate) and 19.47% (with flexible PET/ITO substrate). Zhang et al.^{[ 257 ]} used poly vinylpyrrolidone (PVP) to fabricate the SnO₂:PVP composite. They reported that by placing of PVP chains between SnO₂ nanoparticles, the accumulation of SnO₂ nanoparticles is inhibited, and the morphology of SnO₂ film is improved. PVP reduces leakage current at the interface by suppressing ETL/perovskite interface defects and thus improves PSC performance. According to their results, charge transfer at very high PVP concentrations is reduced due to poor PVP conductivity. As a result, it is necessary to optimize the PVP concentration to achieve the desired efficiency (optimal PVP concentration = 2 mg mL⁻¹). Using SnO₂:PVP compound.

SnO₂/Inorganic Composites

Song et al.^{[ 240 ]} prepared SnO₂:ZnO nanocomposites with different ZnO/SnO₂ ratios as low-temperature ETL to use in planar PSCs and compared this composite ETL with those of pristine ZnO and SnO₂. The resulted composite exhibited higher temperature stability than that of pure ZnO since the number of hydroxyl groups and acetate ligands remaining on the surface has been decreased. Moreover, the results implied a high increase in electron extraction and charge recombination resistance at the perovskite/ETL interface in PSCs based on SnO₂:ZnO composite ETL in comparison with that of PSCs based on pristine ZnO and SnO₂ ETLs. Accordingly, this group could successfully achieve a PCE of 14.3% for planar PSCs based on SnO₂:ZnO and 13.9% and 12.4% for PSCs based upon pristine ZnO and SnO₂ ETLs, respectively. What is more, they also employed the mesoporous Al₂O₃ layer between the composite ETL and the perovskite layer to further enhance the efficiency and performance of SnO₂:ZnO ETL-based PSCs and realized a 15.2% PCE for SnO₂:ZnO ETL-based mesoporous PSCs.

In the application of other inorganic material, Zhu et al.^{[ 245 ]} utilized the SnO₂:KCl composite as a LT-ETL in planar PSCs to simultaneously passivate the defects at the ETL/perovskite interface as well as the perovskite film boundaries owing to the infiltration of K⁺ ions from the ETL into the perovskite. Therefore, by the alternative SnO₂:KCl composite ETL to that of SnO₂ ETL in planar PSC structure, they managed to elevate the PCE from 20.2% to 22.2%. Lv et al.^{[ 258 ]} used the SnO₂:CdS QD composite as ETL in the PSC structure. Adding CdS QD to SnO₂ has three advantages: 1) ETL electrical conductivity improvement, 2) charge transfer acceleration due to better energy-level matching between ETL and perovskite, and 3) passivation of Pb defects in the ETL/perovskite interface by the coordination reaction of S²⁻ ions in ETL. Therefore, with aid of SnO₂:CdS QD composite, they increased the PCE of PSCs from 18.27% to 20.78%.

Chen and coworkers^{[ 244 ]} prepared and employed SnO₂:Bi₂O₂S composite as a LT-ETL in the fabrication of planar PSCs and compared its performance with that of SnO₂ ETL. Therein, Bi₂O₂S nanoparticles served to alleviate the recombination and increase charge transport as well as PSC stability by passivating oxygen vacancy defects throughout the SnO₂ film. Besides, SnO₂:Bi₂O₂S composite film enhanced perovskite crystalline quality owing to the adjustment of ETL morphology and interface. Thereupon, according to the results, the replacement of SnO₂ ETL with that of SnO₂:Bi₂O₂S composite ETL in the architecture of planar PSCs resulted in an efficiency increment from 14.61% to 17.13%. Furthermore, they could manage to boost the PCE to 20.14% through the alteration of the MAPbI₃ absorbent layer to Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45. Through the combination of Bismuthene with SnO₂, Xue et al.^{[ 259 ]} developed a LT-SnO₂-based composite ETL and compared its performance with that of SnO₂ ETL in PSC. As regards the lower surface roughness and hydrophobicity of SnO₂:Bismuthene composite layer than that of the pristine SnO₂, it served to increase the grain size of the perovskite layer and the energy bands in this composite shifted to higher levels compared with that of SnO₂. Besides, the utilization of Bismuthene has led to an improvement in the optical transmittance and electrical conductivity of ETL and decreased the interfacial resistance between the perovskite/ETL. Moreover, SnO₂: Bismuthene composite has significantly assisted in the combination of a lattice-matching attribute of adjacent lattice-spacing between Bismuthene and SnO₂ crystals owing to its lower interfacial mismatch than that of the pristine SnO₂. They demonstrated that the PCE of PSCs increased from 18.55% to 19.38% by the alternative application of SnO₂:Bismuthene composite ETL to the pristine SnO₂ ETL. Qian et al.²³⁷ optimized the electron transport property of SnO₂ by adding the titanium diisopropoxide bis (acetylacetonate) (TiAcAc) agent molecule in a solution of SnO₂ nanoparticles and making the SnO₂:TiAcAc composite. The TiAcAc molecule has a TiO₄ ⁴⁻ nucleus, a functional group –C=O, and long alkene groups. According to their report, the TiO₄ ⁴⁻ nucleus regulates the electrical properties of the SnO₂ layer and the long alkenes act as stabilizer to prevent the accumulation of nanoparticles and electronic glue among SnO₂ nanoparticles and increase its homogeneity and conductivity. The remained –C=O factor groups at the ETL surface also make a strong bond with Pb²⁺ and improve the intimacy interface between ETL and perovskite. Their results demonstrated increase the efficiency of PSCs from 19.00% to 20.4% by using SnO₂:TiAcAc composite as ETL.

Yang and coworkers^{[ 213 ]} utilized Ti₃C₂ MXene in LT-SnO₂ ETL to benefit from its metal high conductivity. The results showed that the presence of Ti₃C₂ MXene metal nanosheets in this composite ETL leads to an enhancement in charge extraction and transport, electron mobility increment, and reduced electron transport resistance at the ETL/perovskite interface and thus leads to the increased photocurrent. Therefore, compared with that of the PSCs based on SnO₂ ETL yielding a PCE of 17.23%, the efficiency of PSCs based on SnO₂:Ti₃C₂ composite ETL was elevated to 18.34%. Huang et al.^{[ 91 ]} fabricated a composite of SnO₂, MXene, and the anatase phase of TiO₂ QDs through the ad-mixing of MXene to SnO₂ solution and two-step annealing at low temperatures of 150 °C under air and N₂. In the first and second step of annealing under the air and N₂ atmosphere, respectively, SnO₂ and TiO₂ anatase crystals were produced and kept growing. In the subsequent annealing in N₂, a more stable phase and then nanoscale TiO₂/SnO₂ heterojunctions were formed. At last, the 2D MXene sheets performed as conductive bridges bonding these TiO₂/SnO₂ heterojunctions at the edge region of the sheet and constructed the final multidimensional conductive network (MDCN) structure. Also, in comparison of the performance of this composite ETL with that of SnO₂ ETL in the PSC configuration, the presence of 2D MXene as conductive bonded bridges made for an enhancement in electrical conductivity of the layer and the ETL/perovskite interface modification (Figure 33b,c). In the case of PCE enhancement, the efficiency of PSCs boosted from 16.83% to 19.14% in consequence of replacing SnO₂ ETL with SnO₂:MXene-based TiO₂ QDs composite ETL (Figure 33a).^{[ 91 ]}

Yang et al.^{[ 260 ]} used N, S co-doped MXene QDs to prepare the SnO₂: N, S co-doped MXene QD composite. According to their results, the presence of this composite as ETL led to an increase in the nucleation rate of perovskite crystals during the initial spin-coating process and the formation of an intermediate phase of perovskite after antisolvent treatment, which achieved high quality perovskite crystals during the annealing process. Thus, they succeeded to achieve a PCE of 23.30%. Tao et al.^{[ 261 ]} developed an efficient ETL using SnO₂:PMo₁₂ composite for printable HTL-free mesoporous PSCs in which PMo₁₂ is the representative of Keggin-type polyoxometalate (POM) H₃PMo₁₂O₄₀. POMs are a large group of metal-oxo cluster compounds extensively employed in a broad range of applications. Among their outstanding features is their ability to perform as shallow electron traps, effectively segregating the photogenerated excitons thereof and thus enhancing the efficiency and performance of the solar cells. The utilization of SnO₂:PMo₁₂ composite ETL making use of the synergistic effect of PMO has resulted in electron mobility enhancement, more expedited electron extraction, and downward shift of the conduction band, which facilitates the injection and transport of electrons from the perovskite to the ETL and desirably diminishes the charge recombination. Therein, the results of this group demonstrated that the PCE of mesoporous PSCs has been raised from 9.8% to 12.6% utilizing the SnO₂:PMo₁₂ composite rather than that of the SnO₂ compact layer.^{[ 261 ]}

Jung et al.^{[ 262 ]} used SnO₂:AlO₆ nanocomposites as ETLs and achieved PCE of 21.04%. By synthesizing amorphous AlO₆ and SnO₂ (SnO₂:AlO₆) nanocomposite films from aluminum chloride hexahydrate precursor, they formed an amorphous AlO₆ octahedra in the amorphous SnO₂ network, which is clearly different from Al-doped crystalline SnO₂. According to their achievements, the SnO₂:AlO₆ nanocomposite increases perovskite grain size, decreases perovskite grain density, stress, and also internal and surface nonradiative recombination. In addition, they reported that the SnO₂:AlO₆ nanocomposite exhibits desirable properties such as high conductivity, fast charge extraction, and excellent recombination resistance. Also, the SnO₂:AlO₆ ETL formed a better energy band alignment with the perovskite band structure with the upward shift of its Fermi level.

3.3.3 Sn-Based Ternary Metal Oxides

Ternary metal oxides (TMOs) are another group of oxide semiconductors that have attracted plenty of research interests due to their tunable optical and electronic properties. The majority of TMOs employed in PSCs are prepared at high temperatures which hinders their further development toward low-cost production and practical applications. In recent years, several low-temperature Sn-TMOs such as Zn₂SnO₄,^{[ 43, 274-278 ]} ZrSnO₄,^{[ 279 ]} SrSnO₃,^{[ 232 ]} Sn _x In_1−x O (TIO)^{[ 280, 281 ]} and their utilities in PSCs as LT-ETLs have been reported. Sn-based TMOs have been examined in PSCs owing to their better electronic properties along with the transparency and potential chemical stability compared with that of SnO₂. These TMOs possess improved electron mobility, high chemical site densities, and tunable band structures.^{[ 282 ]} Aside from PSCs, Sn-TMOs have largely been employed in other solar cell types including dye-sensitized solar cells.^{[ 283 ]} The performance and PV characteristics of PSCs based on Sn-TMOs as LT-ETLs are outlined in Table 6 .

In SnO₂ ETL-based PSCs, Baek et al.^{[ 280, 281 ]} using sputtering of BaO and In₂O₃, applied a thin layer of amorphous Sn–In–O alloy, i.e., Sn _x In_1−x O (TIO; Sn fraction: >50 atomic percent). By varying the Sn ratio fraction, they observed that with the increase of Sn fraction, the conduction band minimum (CBM) energy level ascended, and thus the distance between the Fermi level and the conduction band increased (Figure 34c). As such, the layer resistance is increased because of the electron concentration decline. The bandgap, as well as the transparency of the films, was elevated when Sn increased (Figure 34c). This group initially investigated the different Sn ratios and realized that the most optimal Sn ratio fraction was 92% (Figure 34a). Thereafter, they further improved the performance of the ETL via the addition of TiCl₄ treatment in which the optimized Sn ratio was obtained to be 77% at that point and could acquire a PCE of 14.88% (Figure 34b).^{[ 280 ]} In another study, the same group monitored the effect of oxygen partial pressure (PO₂) throughout the sputtering of ETL along with TiCl₄ treatment and reported a PCE of 15% in which an optimal oxygen atmosphere and the Sn ratio fraction of 83% was acquired (Figure 34d,e).^{[ 281 ]}

Noh et al.^{[ 279 ]} synthesized the ternary semiconductor composition of Sn-TMO-based ETLs including crystalline ZrSnO₄ nanoparticles as the ETL in the structure of PSCs. Due to the propitious optoelectronic properties of ZrSnO₄ nanoparticles extending from high transparency, desired energy level, and electron configuration, ETLs based on which have been proposed as promising candidates for optimal electron transfer and diminishing of charge recombination at the perovskite/ETL interface. In this group, an investigation on the structure and optoelectronic properties of ZrSnO₄ nanoparticles prepared at three temperatures of 75, 55 °C, and room temperature was conducted. Their results implied that the resultant devices using ZrSnO₄ nanoparticles synthesized at room temperature achieved the highest PCE of 16.76%. Guo et al.^{[ 232 ]} employed the SrSnO₃ ternary composition with the orthorhombic ABO₃ perovskite structure as the ETL in the PSC architecture and could realize a PCE of 16.9%. Furthermore, they could manage to enhance the conductivity and mobility of this structure by yttrium (Y) doping and boosted the efficiency to 19%.

Among the Sn-TMOs, the Zn₂SnO₄ compound has received the most attention as ETL. This is largely due to the fact that the Zn₂SnO₄ is an n-type ternary semiconductor metal oxide exhibiting analogous similar properties, as the large bandgap (E _g = 3.8 eV), with that of the anatase TiO₂ (E _g = 3.2 eV). Besides, the electron mobility of this material is 10–30 cm² V⁻¹ s⁻¹. What is more, its charge injection and electron diffusivity are much faster than that of TiO₂-based photo-anodes. Notwithstanding, its large bandgap promotes a reduction in photo-bleaching as well as a reduction in the recombination rate of electron-triiodide. Moreover, it exhibits excellent chemical stability against organic solvents and alkaline/acidic solutions.^{[ 43, 275, 284 ]} Nevertheless, the synthesis of Zn₂SnO₄ nanoparticles is quite challenging and requires a relatively high temperature (i.e., over 200 °C) to crystallize since, in comparison with those of binary oxides including SnO₂ and ZnO, both Sn and Zn ions need to be regulated during the reaction.^{[ 43 ]} In general, the synthesis temperature of Zn₂SnO₄ is significantly dependent upon the type of Zn precursor complex. Typically, Zn₂SnO₄ nanoparticles are prepared using a strong base such as NaOH through the intermediate phase of Zn(OH)₆; however, the conversion of Zn(OH)₆ to crystalline Zn₂SnO₄ requires a high reaction temperature (>200 °C). Therefore, several research groups attempted to decrease the reaction temperature by manipulating the Zn complex precursor using amino and/or carbonate-based inorganic materials. Notwithstanding these efforts, high temperature (>150 °C) applied to form the Zn₂SnO₄ nanoparticles of a uniform morphology results in the accumulation, irregular formation, and diverse-sized particles. Nevertheless, Shin and coworkers^{[ 43 ]} could manage to obtain Zn₂SnO₄ nanoparticles possessing the uniform morphology at low temperatures (i.e., 90 °C) using the Zn-hydrazine complex precursor (Figure 35c). They also investigated various Zn to hydrazine (Zn/N₂H₄) ratios at different temperature ranges. As shown in Figure 35a,b, at low concentrations of hydrazine, the increase of acidity of the reaction medium leads to the formation of SnO₂ nanoparticles. Whereas in high concentrations of hydrazine, the basicity raising of the reaction medium leads to the formation of Zn₂SnO₄ nanoparticles albeit the secondary phase of ZnSn(OH)₆ is formed as well. Nonetheless, then again in the optimized ratio of Zn/N₂H₄ and through a mild alkaline medium, the pure phase of Zn₂SnO₄ was obtained. Therein, Shin et al.^{[ 43 ]} could achieve a PCE of 15.3% (Figure 35d) via the spin-coating of Zn₂SnO₄ nanoparticles dispersed in 2-methoxy ethanol as ETL on a flexible PEN/ITO substrate. Moreover, in yet another similar study, could realize a PCE of 17.1% employing the multilayer ETL fabricated through the Zn₂SnO₄ nanoparticle deposition on the Zn₂SnO₄ quantum dot layer. Thereafter, by the substitution of the absorbent layer, i.e., MAPbI₃ with that of MAPb (I_0.9Br_0.10)₃, they could further raise the efficiency to 17.6%.^{[ 278 ]}